The relationship between the growth and development of flowers and environmental conditions

The so-called environment refers to everything that exists in the space around plants, such as climate (such as temperature, light, water, etc.), soil, biological factors and other factors. Each of these factors is called an environmental condition (or factor). However, not all environmental conditions are related to plants. We need to study and understand only those environmental conditions that are directly or indirectly closely related to the life activities of flowers at different times or places. Such environmental conditions are also called ecological conditions, or ecological factors. In nature, various ecological conditions do not exist in isolation. They influence and restrict each other, and form a specific ecological environment in a comprehensive way, which affects plants.

   From the previous chapters, we already know that different flowers have different growth and development laws. This internal law is called ecological habit. It is formed by the long-term comprehensive influence of flowers on a specific ecological environment. It can be inherited and belongs to genetic characteristics. On the other hand, flowers can produce various reactions and adaptability to changes in the environment, which is often called ecological adaptability. In other words, changes in environmental conditions can have a great impact on the growth and development of flowers. The ecological habits and ecological adaptability of flowers constitute a contradictory but dialectical relationship between flowers and the environment, which is often called an ecological relationship.

   Studying and understanding the growth and development rules of flowers and their relationship with environmental conditions is the basis for us to introduce and cultivate flowers. In addition, it is also to be able to regulate and control the growth and development of flowers to achieve the purpose of serving mankind.

1. Flowers and Temperature

   Temperature is one of the main factors affecting plant distribution, but it often combines with water to determine the distribution boundaries of plants. Other conditions such as sunshine length and soil type also play an important role in plant distribution. Temperature is one of the most important factors affecting plant growth and development, restricting the growth and development speed of plants and all physiological and biochemical changes in the body.

     There is a limit to the temperature at which flowers can maintain their nutrient content, and the temperature at which they can grow is within an even smaller part of this range.

  1. Growth and temperature of flowers

   The effect of temperature on flower growth is a comprehensive effect that affects various metabolic processes through enzymes. Temperature affects photosynthesis, respiration, transpiration, absorption of water and mineral elements, transportation and distribution of substances, etc.

  1. Three base point temperatures

   The increase in plant volume and weight is growth. Each flower has a temperature range for growth. When the temperature exceeds the minimum temperature required for growth, the growth speeds up until the fastest growing temperature exceeds this temperature. As the temperature rises further, the growth rate drops rapidly. When the temperature reaches the upper limit, the growth stops. The fastest growing temperature is called the optimum temperature. The minimum temperature, optimum temperature and maximum temperature for growth are usually called the three base points of growth temperature. Different flowers have different three base point temperatures, and some of them are very different.

   It should be noted that the optimum temperature for growth is often not the most suitable for the healthy growth of plants. Because the optimum temperature for growth is above the optimum temperature for photosynthesis (when the net photosynthetic rate is the largest), the organic matter consumed by plants at the optimum temperature for growth is more than that at the optimum temperature for photosynthesis, so the accumulation is less. When the temperature exceeds the optimum temperature for photosynthesis, it is not conducive to the accumulation of nutrients in the plant. In production practice, when cultivating healthy seedlings, a temperature higher than the optimum temperature for growth, the so-called "coordinated optimum temperature", is often required. At this temperature, although the plant grows slightly slower, it is stronger. When the temperature rises again after exceeding the optimum temperature for growth, various metabolic processes are affected, resulting in a rapid decrease in the growth rate. Among them, the greatly enhanced respiration and the rapid decrease in the net photosynthetic rate are one of the main reasons for the rapid decrease in the growth rate.

   It is also pointed out that the same flower has different requirements for the optimal temperature at different growth and development stages. For example, the most suitable growth temperature for spring-sown annual herbaceous flowers is lower for seedlings than when the seeds germinate, higher for vegetative growth than for seedlings, and even higher for reproductive growth.

   Temperature also has a significant effect on the differentiation of flower buds of some flowers. For example, the differentiation of flower buds of annual herbaceous flowers sown in autumn is greatly affected by low temperatures, which can promote the differentiation of flower buds. This is the phenomenon of vernalization.

   Temperature can also affect the color of some flowers. For example, in the blue and white petunia varieties, at a high temperature of 30-35℃, the petals are completely blue or purple; at 15℃, they are white; in the temperature range between the above two, the flowers are blue and white.

   By understanding the relationship between flower growth and temperature, we can understand why the same kind of flowers cultivated in the open field in different regions have different sowing periods, vegetative growth periods, flowering periods, etc.; why foliage plants planted in Hainan are taller or grow faster than those in Guangzhou; why greenhouses are necessary to grow tropical flowers in temperate regions, and so on.

   The relationship between temperature and growth and development is very useful in regulating the flowering period of flowers. For example, we often use low temperature treatment to promote the differentiation and flowering of flower buds of autumn-sown annual grass flowers. Another example is that we generally use temperature control to control the opening time of flowers: within a certain temperature range, increasing the temperature can promote the growth of plants and the growth and development of flower buds, making the flower buds open earlier; while lowering the temperature can delay the growth and development of flower buds, thereby delaying their opening, and prolonging the duration of flower opening. Another example is that increasing the temperature accelerates the growth of plants, so that they can reach the size of flowering earlier.

  2. Temperature difference between day and night

   The normal growth of flowers also requires a certain temperature change between day and night (higher daytime temperature and lower nighttime temperature). This phenomenon is called the temperature cycle phenomenon of growth. This is also the result of flowers adapting to the natural temperature that is higher during the day and lower at night. Usually, the temperature difference between day and night for tropical flowers should be 3~6℃, for temperate flowers it should be 5~7℃, and for desert plants it should be more than 10℃. When cultivating flowers in a temperature-controlled greenhouse, it is necessary to pay attention to lowering the night temperature. Lower night temperatures are beneficial to plant growth. The reason can be explained as follows: lower night temperatures can reduce the consumption of organic matter by reducing respiration. In continental climate areas, such as the northwest, Xinjiang and Inner Mongolia, the temperature difference between day and night is greater, and the larger and better bulbs produced in many places are largely related to this. However, it should be noted that if the temperature difference between day and night is greater, growth will be inhibited.

  3. Accumulated temperature

   Plants not only need a certain temperature to start growing and developing, but also need a certain total temperature to complete their life cycle. This is also true during a certain growth and development period. We call the higher temperature value that is effective for plant growth and development (in terms of days, the daily average temperature minus the lowest temperature of growth) the effective temperature. The sum of the effective temperatures of a plant at a certain stage or throughout its life cycle is called the effective accumulated temperature. The calculation method can be taken as an example of the rose variety "Kuhongchou". After pruning, the side buds of the buds start to grow until the other buds open. If the daily average temperature is 20℃ for 91 days and the low temperature limit of its growth is 5℃, then the effective accumulated temperature required for this bud from the beginning of growth to flowering is K=(20-5)×91=1 365℃.

   In the production of ornamental flowers, we are very concerned about the regulation of flowering period. We have mentioned before that increasing the temperature can promote flowering and lowering the temperature can delay flowering, which can also be explained by effective accumulated temperature. For example, the effective accumulated temperature of the rose variety "Ku Hong Chou" from the beginning of growth to flowering is 1,365°C (measured by the author from September to November 1993). If the temperature is increased, the number of days required to reach this effective accumulated temperature will be less, that is, flowering will be earlier, and vice versa. In Guangzhou, the temperature in summer is significantly higher than that in winter, so the time required for rose buds from the beginning of growth to flowering is dozens of days different in summer than in winter.

  3. Relationship between soil temperature and air temperature

   The above mentioned are all about air temperature. We should also pay attention to the relationship between air temperature and soil temperature. Compared with air temperature, soil temperature is relatively stable. The deeper from the soil surface, the smaller the temperature change. The difference between root temperature and soil temperature is not big, so the root temperature change is also small. Although roots are generally not cold-resistant, some perennial flowers that overwinter often have their aboveground parts frozen, but the roots can survive normally. This is because the soil temperature changes less than the air temperature, and the soil temperature in winter can be higher than the air temperature.

  (II) Harm of low temperature on flowers and their cold resistance

   The previous discussion was about the growth temperature of flowers. If the lowest or highest temperature for growth is exceeded, the flowers will naturally or be forced to dormant, and will be damaged or even die. For example, some tropical foliage plants will be forced to dormant and suffer from cold damage when cultivated outdoors in Guangzhou in winter. The change of natural climate is not subject to people's will, and the highest and lowest temperatures in different regions are different, and even vary greatly. Therefore, when flower introduction is quite common, it is necessary to understand the impact of high and low temperatures on flowers and their heat and cold resistance.

1. According to the cold resistance of flowers

   The cold tolerance of flowers refers to their ability to withstand the lowest temperature. Although different flowers have different cold tolerances due to their different origins, we can still roughly divide them into the following three categories:

  (1) Cold-resistant flowers

   It is native to frigid and temperate zones, including most perennial deciduous woody flowers, evergreen coniferous ornamental trees of the Coniferaceae family and some deciduous perennials and bulbous grasses. It can tolerate temperatures as low as -10°C and can naturally overwinter in the open field in most parts of the north. Such plants include roses, crape myrtles, lilacs, forsythia, hostas, daylilies, hollyhocks, cypresses, wisteria, ginkgo, etc.

  (2) Semi-hardy flowers

   It is native to warmer temperate regions, including some autumn-sown annual grass flowers, biennial grass flowers, perennial perennial grass flowers, deciduous woody plants and evergreen tree species. It can safely overwinter in the open field in the Yangtze River Basin. In North China, Northwest China and Northeast China, some need to be buried in the soil to prevent cold overwintering, some need to be wrapped with grass for protection overwintering, and some need to enter cold rooms or cellars for overwintering. Most of their roots will not be frozen in frozen soil, and the above-ground parts of perennial grass flowers will wither; the above-ground parts of woody flowers cannot tolerate the severe winter cold in the north or are afraid of the cold wind from the north, so wind barriers need to be set up for protection; autumn-sown annual grass flowers and biennial grass flowers have a certain degree of cold resistance, but because they do not shed their leaves in winter, they need to enter cold beds or low-temperature greenhouses. This type of flowers includes peony, plum blossom, pomegranate, oleander, large-leaf boxwood, magnolia, five-needle pine, pansy, snapdragon, dianthus, aster, tulip, some ornamental bamboos, etc.

  (3) Flowers that are not cold-resistant

   Including spring-sown annual grass flowers and a considerable number of evergreen perennial and woody flowers native to tropical and subtropical regions, they cannot tolerate temperatures below 0°C, and some cannot even tolerate temperatures around 5°C or higher. Other species and in other regions must enter greenhouses or sheds in winter. Most cacti, succulents, and foliage plants are not cold-resistant flowers.

   2. According to the degree of frost damage to flowers caused by low temperature

The damage to flowers caused by low temperatures can be divided into two types: freezing damage and chilling damage (chilling damage) according to the degree of low temperature:

  (1) Frost damage

   It refers to the damage caused to flowers by freezing point (0℃) and low temperature below. The threshold temperature of frost damage varies depending on the type of flowers and the duration of low temperature experience. Different flowers have obvious differences in structure and adaptability, so their frost resistance is different. Flowers that are not cold-resistant are prone to death due to frost damage. Due to the different speeds at which the temperature drops below the freezing point, there are two different freezing modes: extracellular freezing and intracellular freezing.

• Extracellular ice

When the temperature gradually drops below the freezing point, ice first forms in the intercellular spaces near the cell walls, causing the intercellular water concentration to decrease, and absorbing water into the cells with higher water concentration, causing the ice crystals in the intercellular spaces to continue to grow, and the water in the cells to continue to flow to the outside, eventually causing severe dehydration of the protoplasm, resulting in protein denaturation and irreversible gelation of the protoplasm. Dehydration and denaturation of the protoplasm is the basic cause of damage caused by intercellular ice formation. The second is intercellular ice formation, where the mechanical pressure of the enlarged ice crystals on the cells causes the cells to deform. Furthermore, when the temperature rises suddenly and the ice crystals melt, the cell walls can easily absorb water to return to their original state, but the protoplasm absorbs water more slowly and may be torn and damaged. Intercellular ice formation can generally be tolerated by overwintering flowers, and they can continue to grow as usual when the temperature slowly rises to the corner freeze.

• Intracellular ice formation

When the temperature suddenly drops below 0℃, or frost suddenly falls, the water in the cell membrane, cytoplasm, and vacuole freezes at the same time as the intercellular ice forms. This is called intracellular ice. Intracellular ice directly damages the protoplasm, destroys the fine structure of the protoplasm, and causes fatal damage. What you don’t understand is that the situation of ultra-low temperature liquid nitrogen preservation of flower materials such as pollen and stem tip tissue is different. These materials are quickly put into liquid nitrogen (-196℃), and the water in the tissue is vitrified before it has time to freeze. When the material is taken out of the liquid nitrogen and quickly thawed, it can maintain its original vitality.

   The first frost in autumn is called the first (early) frost; the last frost in the following spring is called the final (late) frost. The number of days from the first frost to the final frost of the following year is called the frost period, and the remaining days are called the frost-free period. The number of days in the frost-free period varies greatly from place to place. Spring is the time for budding, and autumn is often the time for maturity, so the first and final frosts are the most harmful to flowers. Frost damage is generally rare in the Pearl River Delta area of ​​Guangdong.

  (2) Cold damage (cold damage)

   It refers to the damage to flowers caused by low temperatures above 0℃. Flowers that are not cold-resistant and are native to tropical and subtropical areas will be forced to hibernate, suffer damage, or even die when the temperature drops to 0~10℃ (depending on the species, etc.). These flowers cultivated in the Pearl River Delta region of Guangdong are prone to cold damage in winter, and will die when the temperature reaches the cold death point of life.

   The root cause of cold damage is generally believed to be damage to the cell membrane system, which leads to metabolic disorders, such as the decline or cessation of photosynthesis, reduced stomatal conductance, reduced root water absorption capacity, obstructed leaf material transport, reduced synthesis capacity, etc. In appearance, there may be scars on the leaves, the leaves turn dark red or dark yellow, the young branches and leaves wilt, dry up and fall off, etc. Over time, or when the temperature reaches the cold death point of life, the plant will die.

   Different species of flowers are not cold-resistant, but their resistance to cold varies. Seedlings are more vulnerable than mature plants. A sudden drop in temperature is more harmful to plants than a slow drop in temperature or a long period of low temperature. For example, foliage plants such as spiderwort and shade-loving flowers will be seriously damaged by temperatures of about 8°C.

   Although the cold resistance of flowers is determined by genetics, flowers can improve their adaptability and resistance through other means, such as low temperature acclimation, treatment with chemical substances, and taking some cultivation and management measures such as applying more K fertilizer and reducing watering before the arrival of low temperatures.

   3. The harm of high temperature to flowers and their cold resistance

   Exceeding the maximum temperature for flower growth will cause damage to the flowers. The main physiological changes are: respiration is greatly enhanced, making the plant "hungry", and the synthesis of organic matter cannot keep up with consumption; transpiration loss is accelerated under high temperature, water balance is destroyed, stomata are closed, photosynthesis is blocked; plants are forced to hibernate; body temperature rises, protein denaturation, metabolic dysfunction, etc. On the appearance of the plant, there may be burnt necrotic spots or patches (burn rings) and even leaf fall, male sterility, and inflorescence, ovary, flower and fruit shedding, etc., and the plant will die after a long time or when the temperature reaches the thermal death point of life. High temperature causes the inflorescence, ovary, flower and fruit to fall off, etc., and the plant will die after a long time or when the temperature reaches the thermal death point of life. High temperature causes the stems (trunks), leaves, fruits, etc. of flowers to be damaged, which is usually called burns, and the burn wounds are easily attacked by diseases.

   Heat resistance refers to the ability of flowers to endure the highest temperature. Different flowers have different heat resistance due to their different origins. Generally, higher plants can withstand temperatures of around 45°C, and some cacti can withstand temperatures of 60°C, so it is rare for flowers to be directly killed by heat in natural high temperatures.

   Generally speaking, the heat resistance and cold resistance of flowers are related. Flowers with weak cold resistance have strong heat resistance, while flowers with strong cold resistance have poor heat resistance. Among all kinds of flowers, aquatic flowers have the strongest heat resistance, followed by cacti and spring-sown annual grass flowers, as well as hibiscus, oleander, crape myrtle and other plants that can bloom continuously in summer, and most foliage plants native to tropical areas; autumn-sown annual grass flowers and some alpine flowers native to tropical and subtropical areas such as fuchsia have poor heat resistance.

   When analyzing the heat resistance of flowers, we need to pay attention to the local climate conditions of the flowers' origins and cannot apply them mechanically. When we study the origin of a certain flower, we will find that although the area near the equator is a tropical area with no obvious four seasons, the sunshine time in summer here is shorter than that in the temperate and subtropical zones, and it also has an oceanic climate, so the local maximum summer temperature is often lower than other regions. However, it is rare in tropical rainforests. Therefore, some flowers native to tropical areas often cannot withstand the summer heat in most areas, cannot grow and bloom normally, or are forced to hibernate. Poor management may even lead to death, so cooling and heatstroke prevention measures are needed.

2. Flowers and light

   The relationship between light and flower growth and development is manifested in three aspects: light intensity, light length and light quality.

   1. Effect of light intensity on flower growth and development

   Light is a necessary condition for plant survival. Without sunlight, there will be no green plants. The influence of light intensity on plant growth and development is mainly reflected in photosynthesis. The relationship between light intensity and photosynthesis has been detailed in Chapter 4. Each flower has its own light saturation point and light compensation point. Each flower cannot be placed in an environment with light intensity lower than its light compensation point for a long time. Placing flowers in a place higher than their light compensation point but close to the compensation point for a long time is also very unfavorable to their growth and development.

   The intensity of sunlight in nature varies depending on the geographical location, terrain height and cloud cover. The main pattern of its changes is: it weakens with increasing latitude and increases with increasing altitude; the light is strongest in summer and weakest in winter; the light is strongest at noon and weakest in the morning and evening. Light can be divided into direct light and scattered light. The former is the light that the sun directly projects onto the ground in parallel rays, and the latter is the light that the sunlight diffuses from the sky to the ground through air molecules, faeries, water droplets and other substances, and the light intensity is lower. On a sunny day, direct light accounts for about 63% of the light on the ground, and scattered light accounts for about 37%. On cloudy days, plant leaves can still use scattered light for photosynthesis.

   Each flower has its own origin and environmental conditions. The light intensity in different origins and environmental conditions varies greatly. As a result of long-term adaptation, each flower has its own suitable range for light intensity. Despite this, we can still roughly divide flowers into three categories based on their different requirements for light intensity:

  1. Negative flowers

   Most of these flowers are native to tropical rain forests, or distributed under trees on the shady side of high mountains and in dark caves. They have lower light compensation points and light saturation points. In cultivation, negative flowers must be shaded appropriately and not exposed to strong sunlight. Generally, the chloroplasts in the mesophyll cells of higher plants are flat ovals. In weak light, the flat side faces the light to increase the absorption of light; in strong light, the narrow side faces the sunlight and moves to the side wall of the cell to avoid being killed by strong light. Negative flowers grow in weak light for a long time. In order to absorb more light, the chloroplasts have been dispersed in the cytoplasm with the flat side facing the light, and no longer have the characteristics of turning and shifting. Therefore, if negative flowers are placed in strong light, the chloroplasts will be killed by the strong light, and the leaves will turn white, scorch, and fall off. In severe cases, the plant will die.

   Since the indoor light is mainly scattered light with low light intensity, the flowers that can be used for indoor decoration are mainly negative flowers. Negative flowers are mainly foliage plants, and a few flowering plants, such as ferns, arrowroots, araceae, orchids, gloxinia, African violets, etc. Of course, negative flowers cannot be placed in places where the indoor light is too weak. The light intensity of the placement must be above its light compensation point to avoid direct sunlight. Generally, they are placed in places with stronger indoor light, and windows are usually the places with the strongest indoor light.

  2. Positive flowers

   These flowers are native to tropical and temperate plains, or the southern slopes of plateaus and sunny rocks of high mountains. They have high light compensation points and saturation points, and they must be allowed to receive sufficient sunlight when cultivated. If light is often insufficient, photosynthesis will be reduced, and the plants will grow poorly, such as thin branches, elongated internodes, pale and dull leaves, inability to bloom or poor flowering, small and not bright flowers, and weak fragrance. If there is a serious lack of light, the nutrients will be exhausted and die. Positive flowers include most flowering and fruiting plants and a few foliage plants. Although these flowers can withstand direct strong sunlight in summer, the strong light will cause a sharp increase in temperature, which may cause damage to the flowers. This has been described in the previous section.

  3. Neutral flowers

   Most of these flowers are native to tropical and subtropical regions. In their native places, due to the high water vapor in the air, part of the ultraviolet rays are absorbed by the mist, thus weakening the intensity of light. The light intensity requirements of these flowers are between those of sexual and positive flowers. They are not very shade-tolerant and afraid of strong direct sunlight in summer. They usually need sufficient light, but need appropriate shade when the light is strong. Neutral flowers include azalea, camellia, gardenia, daylily, bamboo palm, fuchsia, platycodon, columbine, eight fairy, and some coniferous evergreen trees.

   There are a few special flowers that can adapt to a wide range of light intensity. For example, the Malabar chestnut can grow in full sun or in the shade. Those grown in full sun have higher light compensation points and saturation points, while those grown in shade sheds have lower light compensation points and saturation points. However, if a plant that has been grown in full sun is suddenly moved indoors, it will drop leaves or even die. This is because the plant cannot adapt to the sudden change in light and the light compensation point has not yet dropped.

   Light intensity can also affect the leaf color of some flowers. For example, under strong light, more chlorophyll is synthesized in red mulberry and nandina domestica, making the leaves green. Under strong light, part of the chlorophyll is destroyed and replaced by carotene, making the leaves orange.

   Light intensity also has something to do with the time when the buds open. For example, half-flowered lotus and oxalis must bloom under strong light, tuberose, seaweed jasmine and evening primrose need to bloom in the evening and have a stronger fragrance, epiphyllum only blooms at night, and morning glory and flax only bloom in the morning light of the day.

   Light intensity can also affect the color of certain flowers. For example, purple flowers are formed by the presence of anthocyanins, which can only be produced under strong light and are not easily produced under scattered light.

  2. Effect of light length on flower growth and development

   The length of sunshine hours on Earth varies with latitude and season. The alternation of light and darkness within 24 hours a day is called the photoperiod, which refers to the theoretical sunshine hours from sunrise to sunset in a day, rather than the actual number of hours of sunshine, which is related to the frequency of rainfall and the amount of clouds and fog. Located in the northern hemisphere, taking the northern hemisphere as an example, the higher the latitude (that is, the further north), the longer the sunshine in summer, and the shorter the sunshine in winter. Therefore, the number of sunshine hours in the north varies greatly from season to season. For example, Harbin has only 8 to 9 hours a day in winter, while it can reach 15.6 hours in summer. The difference between seasons in the south is smaller. For example, Guangzhou has 10 to 11 hours of sunshine a day in winter, and only 13.3 hours in summer.

   Different plants have different origins and have adapted and responded to the local changes in the photoperiod. The phenomenon of plants responding to the length of daylight is called photoperiodism. For example, flowering, leaf fall, dormancy, the formation of underground storage organs, etc. all have photoperiodism. The most important and most extensively studied one is the photoperiod of plant flowering.

   The length of daylight has a significant effect on flower bud differentiation and flowering in many plants. Plant responses to photoperiods are generally divided into three categories:

  1. Long-day plants

   Only under long light conditions (generally more than 12 to 14 hours) can flower buds form and bloom normally. If this condition is not met, flowering will be delayed or not bloom. For example, gladiolus is a long-day plant. In winter greenhouse cultivation in the north requires high temperatures and electric lighting to extend the light time in order to bloom. Long-day plants account for half of all plants.

  2. Short-day plants

   Under shorter light conditions (generally less than 12 to 14 hours), flower bud formation and flowering are promoted, otherwise flowering will be delayed or not blooming. For example, chrysanthemums and poinsettias are typical short-day plants, and flower bud differentiation and flowering will only occur when the daylight becomes shorter in autumn. Spring and autumn have the same short days, but the temperature during the short-day period in spring is still low, and short-day plants usually have not reached the flower maturity stage, so it has nothing to do with flowering. The temperature in autumn is higher, which is suitable for plant growth and development, so the sunlight at this time can affect plant flowering. Short-day plants account for about 26% of all plants.

   After understanding the photoperiod phenomenon, it is easy to understand the geographical distribution and seasonal distribution of these plants. At the same latitude, long-day plants mostly bloom in late spring and early summer, while short-day plants mostly bloom in autumn, which are all adapted to the sunlight conditions at the time. In low-latitude areas, there are no long-day conditions, so only short-day plants are distributed; in high-latitude areas, because plants can only grow during the long-day period, there are many long-day plants distributed here; in mid-latitude areas (i.e. temperate zones), there are both long-day and short-day conditions, so both long-day and short-day plants can survive. These are all adapted to the sunlight conditions during the growing season in the place of origin.

  3. Day-neutral plants

   This type of plant is not sensitive to the length of sunshine hours and can bloom all year round as long as the temperature is suitable, such as roses, hibiscus, African violets, gerbera, etc. Day-neutral plants account for about 24% of all plants and are mainly produced in tropical regions.

   Since the flower bud differentiation and flowering time of long-day plants and short-day plants are obviously affected by the length of daylight, the length of light is often artificially controlled to control their flowering in production. For details, please read the chapter "Flowering Period Regulation of Flowers".

   3. Effects of light quality on the growth and development of flowers

   Light quality refers to the components of the solar spectrum with different wavelengths. According to measurements, the wavelength range of sunlight is mainly between 150 and 4000 mm, of which visible light (red, orange, yellow, green, and purple light) has a wavelength between 380 and 760 mm, accounting for 52% of all solar radiation; invisible infrared light accounts for 43%, while ultraviolet light accounts for only 5%. Light quality has a certain effect on the growth and development of flowers. It is also important to understand that the composition of light changes significantly throughout the year, such as less ultraviolet light in spring than in autumn, and more ultraviolet light in summer at noon.

   The red light is the most absorbed by chlorophyll in sunlight and has the greatest effect. Yellow light is second, and the assimilation efficiency of blue-violet light is only 14% of that of red light. However, in the scattered sunlight, red and yellow light account for 50-60%, and in direct light, the intensity of red scattered light is always lower than that of direct light, so the photosynthetic products are not as much as direct light.

   Red light can accelerate the development of long-day plants and delayed short-day plants, while blue-violet light can accelerate the development of short-day plants and delayed long-day plants. Generally, there are more ultraviolet rays on the mountains, which can promote the formation of anthocyanins, so the colors of alpine flowers are brighter. In the glass greenhouse, the entry of ultraviolet rays is reduced, and the alpine flowers are not so bright.

   Generally speaking, flowers cultivated under long light waves have longer internodes and thinner stems, while flowers cultivated under short light waves have shorter internodes and thicker stems. This is also important for cultivating strong seedlings and determining planting density.

4. Flowers and Water

   Without water, there is no life. Of course, without water, there is no plant. In plants, most of the material is water. The water content of tissues and cells with active growth and metabolism is generally 70-80%, and some are more than 90%. If their water content is less than 60%, it may lead to death. Each cell is a water reservoir. When the cell is full of water (that is, the cell maintains tension), the plant's branches and leaves will stand upright and stretch; if the cell lacks water and loses fullness, the plant's stems and leaves will droop, and this situation is called wilting.

   In addition to maintaining the tension of cells, water has other important functions. For example, the living protoplasm in the cells depends on water to survive; water is the raw material for photosynthesis and some other metabolic processes; the circulation of nutrients and other compounds in the plant body is the result of water movement in the vascular tissue; water can also protect plants from potential damage caused by temperature changes.

   Terrestrial plants absorb a large amount of water from the soil, but only a very small part (1-5%) of the absorbed water is used for plant metabolism, and the rest is lost to the body in the form of gas, which is called transpiration.

   1. Transpiration

   Transpiration is the process by which water in a plant is lost from the body to the outside of the plant in a gaseous state through the surface of the plant. In a normal growing plant, about 99.9% of the total transpiration is transpired through the leaves.

   There are two ways of leaf transpiration: one is transpiration through the cuticle, called cuticle transpiration; the other is transpiration through stomata, called stomatal shade transpiration. The transpiration of the cuticle of shade-loving and wet-loving plants is very strong, often exceeding stomatal transpiration; the transpiration of the cuticle of shaded leaves can also reach 1/3 of the total transpiration; the transpiration of the cuticle of young leaves can be as high as 1/3 to 1/2 of the total transpiration. However, except for the above situation, for mature leaves of general plants, cuticle transpiration only accounts for 3~5% of the transpiration. Therefore, stomatal transpiration is the main form of transpiration in general plants.

   Transpiration has important physiological significance. It is a major driving force for plants to absorb and transport water, especially tall plants. Without transpiration, the passive water absorption process of the roots cannot occur, and the higher parts of the plants cannot obtain water. In addition, the rising liquid flow caused by transpiration can distribute the nutrients entering the roots to various parts of the plant to meet the needs of life activities. In addition, transpiration can reduce the temperature of the plant body and leaves.

   The amount of water lost by transpiration per unit leaf area of ​​a plant within a certain period of time is called the transpiration rate or transpiration intensity. Transpiration is actually divided into two steps: first, the water in the mesophyll cell wall around the intercellular space and the substomatal cavity evaporates, then the water vapor molecules diffuse to the diffusion layer of the leaf surface through the substomatal cavity and stomata, and then diffuse into the air from the diffusion layer. The vapor pressure difference between the inside of the leaf (i.e. the substomatal cavity) and the outside world (caused by different water molecule concentrations) restricts the transpiration rate. When the vapor pressure difference is large, the transpiration rate is fast, and vice versa. Therefore, any external conditions that affect the vapor pressure difference inside and outside the leaf will affect the transpiration rate.

   The relative humidity of the air is closely related to the transpiration rate. As the plant is constantly transpiring, the relative humidity of the cavity below the stoma will not reach 100%, but the water in the strong cell wall of the mesophyll cells is constantly converted into water vapor, so the relative humidity of the cavity below the stoma is not low. According to measurements, when the relative humidity of the air is between 40% and 80%, the relative humidity of the cavity below the stoma of a normal leaf is about 91%, which ensures the smooth progress of transpiration. However, when the relative humidity of the air increases, the air vapor pressure also increases, the vapor pressure difference (water molecule concentration difference) inside and outside the leaf becomes smaller, and the transpiration rate decreases. Therefore, the relative humidity of the atmosphere directly affects the transpiration rate.

   Since the relative humidity of the air is affected by factors such as light, temperature, and wind, light, temperature, and wind also affect the transpiration rate. For light, it is the main factor that affects the opening and closing of stomata. Except for many succulent plants, the stomata of most plants are open during the day and closed at night. On the other hand, as the intensity of light increases, the air temperature and leaf temperature rise, thereby increasing the vapor pressure difference inside and outside the leaves, which increases the transpiration rate. Within a certain range, the increase in temperature accelerates the evaporation process of water molecules from the cell surface and the diffusion process of water vapor molecules through the stomata, promoting transpiration. When the wind is not too strong, the wind can blow away the water vapor outside the stomata, making the diffusion layer thinner or even disappear, reducing the external diffusion resistance, and accelerating transpiration. Therefore, transpiration is particularly strong in dry, hot, sunny, and windy weather.

   Understanding transpiration is of great significance to the production of flowers. Plants can only grow normally when the water they absorb is enough to compensate for the water lost by transpiration. As transpiration increases, the water absorption capacity of the roots must also increase. How to meet the water needs of flowers in production is something we will introduce in detail below and later. For example, when flowers are transplanted or planted, in addition to minimizing the damage to the root hairs to ensure the water absorption function of the roots, we must also try to reduce transpiration so that the plants can resume growth faster and reduce the possibility of wilting and death. There are several ways to reduce transpiration. One of them is to reduce the transpiration area, such as removing some leaves when transplanting, which is very common when transplanting large trees. Another way is to avoid external conditions that promote transpiration as much as possible, such as not transplanting at noon when the sun is strong, choosing to transplant on rainy days, and shading the plants after transplanting or placing them in a shaded and humid place. Another good way is to artificially increase the air temperature, such as spraying water or mist on the leaves, which is particularly effective when cutting young branches.

  2. Water absorption by roots

   The main location of root water absorption is the root hair area at the root tip. There are two forms of root water absorption: active water absorption and passive water absorption.

  1. Active water absorption

   Active water absorption refers to the phenomenon that plants absorb water due to the physiological activities of the root system itself. Active water absorption can be seen from the phenomena of "water spitting" and "wound flow". In an environment where the soil is moist, the soil temperature is high, and the air humidity is high, in the evening or morning, you can see water droplets spitting out of the water holes at the tip or edge of the leaf of an intact plant. This phenomenon is called water spitting. If the stem of the plant is cut off near the ground, droplets will soon flow out of the wound. This phenomenon is called wound flow. Active water absorption is the main reason for plant water absorption only when transpiration is weak, and its mechanism is relatively complicated, so I will not discuss it here.

  2. Passive water absorption

   Passive water absorption refers to water absorption by the roots caused by transpiration of branches and leaves. Substances can move spontaneously from high-concentration areas to low-concentration areas, which is diffusion. When leaves are transpiring, leaf cells are short of water, the water concentration decreases, and the water column in the trachea is dragged upward, resulting in insufficient water in the roots, and the root cells absorb water from the soil. The process of water entering the root system is called osmosis, which is a special form of diffusion. Root cells usually accumulate more soluble substances than in the soil solution. Due to transpiration, the water content of root cells decreases again, making the water concentration in the soil solution higher than that of root cells, so water in the soil can enter the root system. However, the solutes in the root cells will not diffuse into the soil solution. This is because there is a cell membrane in the cells, which is similar to a semipermeable membrane that can only allow water to pass through but not solutes. The same is true for water movement between cells.

   Understanding this concept can also explain fertilizer "burn" and damage caused by high concentrations of salts in the soil. As long as the concentration of water in the soil solution exceeds the concentration of water in the root cells, water will continue to enter the root system. If too much chemical fertilizer is applied or there are too many salts in the soil, the solute concentration in the soil solution outside the roots will be greater than that inside the roots, so the water will flow in the opposite direction, and the water in the root cells will be separated from the root cells. In severe cases, the plant will die. Therefore, if excessive amounts of chemical fertilizers are applied by mistake, fertilizer burn will occur, and water should be irrigated immediately. Irrigation can wash away the fertilizer and also help to wash the fertilizer and salts in the root layer.

   Passive water absorption is caused by transpiration, so usually when the water supply is sufficient, the plant's water absorption rate and transpiration rate are exactly the same. The greater the transpiration, the greater the water absorption.

   Among various external conditions, atmospheric factors mainly affect the passive water absorption of plants through transpiration. Soil factors directly affect the active water absorption of plants, but passive water absorption is also affected to a certain extent by soil factors, especially the concentration of the soil solution mentioned above. Of course, the soil must contain available water, which is a prerequisite.

   Soil temperature also has a great influence on root water absorption. For example, low temperature increases the viscosity of water itself and reduces the diffusion rate; the viscosity of protoplasm increases, making it difficult for water to pass through protoplasm; and respiration slows down, affecting active water absorption.

   The aeration of the soil also has a great impact on the water absorption of the root system. Poor soil aeration leads to a lack of O 2 and an excessively high CO 2 concentration. In a short-term environment of lack of O 2 and high CO 2, the root cell respiration can be weakened, affecting active water absorption; after a long period of time, the cells will perform anaerobic respiration, produce and accumulate ethanol, and the root system will be poisoned and injured, absorbing less water. Because the flowers are waterlogged, they show signs of water shortage.

  (III) Flower requirements for soil moisture (soil humidity)

   Water conditions in nature usually appear in different states such as rain, snow, hail, and fog. The amount and duration of these conditions vary greatly from region to region. Due to their different origins, various flowers have lived in different water conditions for a long time, forming different ecological habits and adaptation types. According to their different requirements for soil moisture, flowers can be roughly divided into the following five types:

  1. Drought-tolerant flowers (xerophytic flowers)

   Originated from quite arid conditions such as deserts, dry grasslands, dry and hot hillsides, they have strong drought tolerance. For example, many succulent plants of the Cactaceae and Crassulaceae, as well as aloe vera, agave, etc., are adaptable to arid environments: the plants have developed water-storing parenchyma tissues and can store a large amount of water in their bodies. Some of these plants have a very thick cuticle on the surface, and there are several layers of thick-walled cells under the epidermis. The stomata are few and often closed, and are deeply sunken in the tissues, which can reduce water consumption. The leaves of cacti have degenerated into thorns, which greatly reduces transpiration. This type of flowers should avoid soil with too much water, poor drainage or frequent moisture, otherwise the root system will be damaged and diseases will easily infect, causing root rot and stem rot and death. When cultivating, you should pay attention to the principle of watering the soil "prefer dry to wet".

  2. Semi-drought-tolerant flowers

   These include some flowers with leathery or waxy leaves and a lot of hair on the leaves, such as camellia, rubber fig, white magnolia, geranium, and clematis, as well as some flowers with needle-like or flaky branches and leaves, such as asparagus, as well as pine, cypress, and cedar plants. In the cultivation and management, the principle of "dry and water thoroughly" can be followed during the growing season.

  3. Mesozoic flowers

   Most flowers belong to this category. They require more soil moisture than semi-drought-tolerant flowers, but they cannot keep the soil wet for a long time. The principle of watering this type of flowers during the growing period is "alternate dry and alternate wet", watering when the soil moisture content is about less than 60% of the field water holding capacity.

  4. Moisture-resistant flowers (warm-weather flowers)

   This type of flower originates from the most humid environment on land, such as under forests in humid areas, valley wetlands, riverbed low-lying land, swamp soil, etc. In these places, not only is the soil humid, but the air humidity is also high, which greatly weakens the transpiration of the plants. Long-term humidity has made them adaptable in morphology and structure. Typical hygrophilous flowers have large, smooth and hairless leaves, thin cuticles, no wax layer, and many stomata that are often open. Many species also produce water-secreting tissues (water pores) to promote water metabolism. The absorption and conduction tissues of hygrophilous flowers are usually simplified, with shallow root systems, few lateral roots that do not extend far, underdeveloped central columns, few vessels, and sparse leaf veins. In addition, because they live in a highly humid environment, cells are often in a state of expansion, the function of the mechanism is reduced and simplified, and the ventilation tissue is extremely developed.

   Wet-growing flowers are mainly shade-loving foliage plants. They can be said to be the terrestrial flowers with the least drought resistance. When cultivating and managing them, you should pay attention to the watering principle of "better wet than dry" during the growing period, and water them as soon as the topsoil is dry.

  5. Aquatic flowers

   Aquatic plants grow completely or partially in water or float on the water surface. Common aquatic flowers include Victoria amazonica, lotus, water lily, etc. The surface of aquatic plants has absorption function, so their root system is not well developed, the conduction system is also very weak, and the ventilation tissue is well developed or there are large intercellular spaces in the body.

   For a specific flower, the water requirements are somewhat different at different stages of its growth and development. For example, after sowing, a higher soil humidity is required so that the seeds can easily absorb water, which is conducive to the germination of the radicle and plumule. After the seeds emerge from the soil, the root system is shallow and the seedlings are very thin and weak, so the topsoil should be kept moderately moist. In the future, in order to prevent leggy growth and promote the healthy growth of seedlings, the soil humidity should be reduced. When the growing plants are growing vigorously, more water is needed. Less water is needed during dormancy.

   Water also affects the differentiation of some flower buds. Some flowers can achieve the purpose of controlling vegetative growth and promoting flower bud differentiation by controlling water supply, such as bougainvillea and four-season orange.

  (IV) Requirements of flowers for air humidity

   The relative humidity is usually expressed as a percentage of air humidity. During the day, the relative humidity is lowest in the afternoon when the temperature is highest, and highest in the early morning. However, on mountain tops or in coastal areas, the two are often consistent or vary little. There are also differences in air humidity throughout the year. For example, in inland dry areas, the air humidity is highest in winter and lowest in summer, while the opposite is true in monsoon areas.

   Most flowers require an air humidity of about 65-70%, while flowers native to arid and desert climates require much lower humidity. Usually when the air humidity decreases, the color of the flower becomes darker due to the formation of more pigments.

   For the above-mentioned hygrophilous flowers and epiphytic flowers native to tropical rainforests, such as tropical orchids, the air humidity requirement is higher (more than 80%). If the air is dry, the leaves are prone to become rough, the tips and edges of the leaves are scorched, and even the entire leaves are scorched, which seriously affects their ornamental value. Therefore, in the dry season, they need to frequently use measures such as spraying water and misting to increase the air humidity.

   When the air is saturated (humidity is 100%), the survival rate of softwood cuttings can be increased. When flowers are transplanted or planted, increasing the humidity can reduce the transpiration of the plants and thus reduce mortality.

   However, when the air humidity is high, diseases are often prone to occur, so if the air humidity is too high, you must pay attention to ventilation and disease prevention.

(V) Water quality requirements of flowers

   Urban sewage and water contaminated by factory and mine wastewater cannot be used for flower irrigation.

   The water we usually use for flower irrigation includes groundwater, well water, river water, pond water, rainwater, tap water, etc. These waters contain soluble salts of varying types and quantities. The quality of water mainly refers to its main components and total salinity content.

   Pure water is not conductive. When soluble salts are added to pure water, it can conduct electricity. The higher the concentration of salts, the greater the current that passes through. The reciprocal of the resistance of a 1cm×1cm×1cm aqueous solution is called conductivity, which is represented by EC and the unit is usually millisiemens/cm (ms/cm).

   Natural water almost always contains calcium salts. Although gypsum is slightly soluble in water among natural calcium salts, if water contains CO 2, calcium carbonate can also dissolve into a solution of calcium bicarbonate, and the pH value of the water becomes alkaline. Natural water containing more calcium and magnesium salts is called hard water, and soft water refers to water containing little or no calcium and magnesium. Usually groundwater, well water and tap water (from groundwater) are hard water, while pond water, rainwater, river water and lake water are soft water.

   If hard water is sprayed on plant leaves or in the leaf tubes of pineapples, it will cause serious salt deposition and damage the plants. Although many flowers have a high tolerance to calcium and magnesium in the soil (Ca and Mg are essential elements for flowers), long-term use of hard water to irrigate the soil will not only cause salt accumulation, but also make the soil alkaline, thereby reducing the effectiveness of nutrients such as P, Fe, Mn, and B in the soil, causing nutrient deficiencies.

   Some water contains too much sodium ions. Long-term use will cause poisoning (excessive absorption) and accumulation in the soil, affecting the water and fertilizer absorption of plants. The water quality in the south may also have problems with high iron and manganese content.

   The total content of calcium and magnesium in hard water is called the total hardness of the water. Removing calcium and magnesium is called water softening, which is usually done by precipitation and ion exchange. The boiling water method in the precipitation method, that is, boiling the water, can turn the calcium bicarbonate and magnesium bicarbonate in the water into calcium carbonate and magnesium carbonate precipitation. The main problem with using hard water to irrigate the soil is that it is alkaline, so the pH value of the water can be lowered by adding organic acids such as citric acid, acetic acid (edible vinegar), etc., or acidic chemicals such as ferrous sulfate. Strong acids such as sulfuric acid are generally not used to avoid burning the roots and causing soil compaction.

   Bleach or liquid chlorine is often added to tap water. Chlorine is also one of the essential elements for plants, but too much chlorine is harmful to flowers. Therefore, tap water can be left for one or two days to allow the chlorine to dissipate before use. However, some people think that the chlorine content in tap water is very low and is not a problem. It is precisely because there is no chlorine in the water (not only tap water, but also rainwater and clean well water), which can meet the needs of plants, that in soilless cultivation, there is no need to add chlorine to the nutrient solution.

  6. Drought and waterlogging

   The damage to flowers due to lack of water is called drought damage. On the one hand, flower plants absorb a large amount of water from the soil, and on the other hand, the leaves lose most of it through transpiration. Only when the absorption and consumption of water reach a balance can the plants grow and develop normally. If the absorption is less than the consumption, the absorption of nutrients will also decrease, and the metabolic processes of the plants, such as photosynthesis, will become slow. Plants are often in a state of water deficit, and the depth of the water will increase; the leaves are few and thin, the leaf body and leaf area become smaller; there are few branches, the new shoots are weakened, and the fullness and the contained water are insufficient; the color of the stems and leaves turns darker, sometimes red; the tip of the leaf, the edge of the leaf or the tissue between the veins is yellowed, and this phenomenon often develops from the base leaves to the top, causing early leaf fall, flower fall, fruit fall, and reduced flower bud differentiation. Insufficient water can also easily increase the concentration of deep soil liquid and cause salt damage, which is more likely to occur in grass flowers, so more attention should be paid when applying more fertilizers.

   If there is a serious lack of water at one time, the plant will wilt, the tender branches and leaves will droop, and the leaves will curl up or close. If the stems and leaves will become upright again after watering or rainfall after wilting occurs, this kind of wilting is called temporary wilting. In the hot summer noon, due to the strong transpiration, the plant may also wilt temporarily. If watering or rainfall still cannot make the plant stand upright again after wilting occurs, this kind of wilting is called permanent wilting. If flowers generally wilt permanently, it means the death of the plant. But for some flowers, such as many lawn grasses, the above-ground parts will also die due to long-term drought. The underground parts will enter dormancy, and the buds on the root neck, rhizomes and runners will still survive. Once there is water, new top growth will begin.

   The above is about the result of lack of water in the soil. There is also a situation where the soil contains enough water, but it cannot be absorbed and used by the plants due to other reasons, such as low soil temperature, poor ventilation, high salt content, etc., which also causes the same damage to the plants.

   If the soil moisture is often in a state of excessive moisture, it will be detrimental to the elongation growth of the roots and the drought resistance will be reduced; the plants will grow weak and the cold resistance will be reduced; and diseases will be more likely to occur.

   If the soil is poorly drained and water accumulates, or if heavy rain and floods cause part of the plant to be submerged and cause damage to the plant, it is called waterlogging. Waterlogging causes the roots of the plant to lack oxygen and can only perform anaerobic respiration, so the absorption of water and nutrients is affected, causing physiological soil drought and nutrient deficiency. In addition, waterlogging causes the anaerobic bacteria in the soil to become active, causing the accumulation of organic and inorganic acids in the soil, increasing the concentration of the soil solution, affecting the plant's absorption of nutrients; at the same time, some toxic substances such as H2S and NH3 are produced, causing root poisoning. Third, part of the aboveground part of the waterlogged plant is immersed in water, affecting photosynthesis and respiration. Waterlogging will cause yellow leaves, lighter flower color, reduced fragrance of flowers, falling leaves, flowers, and fruits on the appearance of the plant. In severe cases, the root cells are in a state of suffocation and die, the root system rots, and even the whole plant dies. If the soil is watered too frequently and is in a wet state, some flowers may also experience some phenomena similar to waterlogging.