HORTICULTURE

WHAT PLANTS NEED FOR GROWTH

Plants can and do exist under incredibly adverse environmental con­ditions. They survive in the arctic as well as the desert. They live atop mountains and in the depth of canyons. They survive on land and in fresh and salt water.

The production of ornamental plants requires more than helping plants survive, however. It requires maximizing their rate of growth, their quality, or both. The growth potential of a plant is determined by two things: its genetic heritage and the environment in which it devel­ops. Its genetic heritage is predetermined; there is nothing a grower can do to change it. Its environment, on the other hand, is under the grower’s control to some extent. Environmental factors play a signifi­cant role in how quickly, if ever, the plant reaches its maximum size, when it blooms, how large its leaves become, whether it sets fruit, and many other qualitative factors.

A plant’s environment exists both above and below ground. It con­sists of the soil and the air, and those natural elements common to them both—water and gases.

Soil

The soil is the environment of the root zone. It provides the mineral ele­ments needed by plants for use during photosynthesis. It also collects and supplies water for uptake by the plants. To function most effectively, the soil must be porous, with sufficient space between the soil particles to store air and water. It must be loose enough to allow roots to grow through it, yet supportive enough to stabilize the plant.

Not all the mineral elements in soil are essential for plant growth. Likewise, not all the elements essential for plant growth are present in necessary amounts in all soils Altogether, seventeen elements are essen­tial at some time during each plant’s life for optimum growth. These elements are basically the same for all plant species. The elements are categorized as macronutrients and micronutrients, depending on the quantities needed by the plant. Designation as a macro - or micronutri­ent is not a measure of importance, since all are essential for growth. (Mineral nutrients are discussed in more detail in Chapter 3.)

When soil is less than ideal for optimum plant growth, it is usually the result of:

1. A deficiency of essential elements (especially nitrogen, phospho­rus, and potassium, used in greatest amounts by plants).

2. Poor aeration (too little air space between soil particles).

3. Improper drainage (either too little or too much).

Changing the soil environment is one way to improve plant growth. For example, fertilizer can add missing nutrients, and sand or peat moss can improve aeration or drainage.

Atmosphere

The atmosphere surrounds the above-ground portion of the plant. It supplies the carbon dioxide needed for photosynthesis and the oxygen needed for respiration. In closed environments such as greenhouses, the addition of carbon dioxide to the air has the effect of increasing the rate of photosynthesis in certain ornamental plants such as roses and carnations. The addition of gaseous pollutants to the atmosphere can have a harmful effect on plants. It may harm their physical appearance and may reduce their rate of photosynthesis.

Water

Water is essential to all life. For plants to grow, they need water for pho­tosynthesis. Water also carries various essential elements within the plant.

Water in liquid form enters the plant through the roots. Water as air vapor also plays a role since the humidity of the air around the plant influences the rate of transpiration. The greater the humidity, the more slowly will the plant transpire.

The presence or absence of water and the natural high or low humidity of a region influence not only the rate but also the nature of plant growth (Figures 1-13 and 1-14). The leafy, lush growth of a

figure 1-13. Abundant rainfall and constant high humidity result in lush plant growth, as in this rain forest. (© Stillfx, 2009. Used under license from Shutterstock. com)

rain forest contrasts sharply with the sparse, stark plants of a desert. Epidermal thickness, type of root system, and population numbers are all influenced by the presence or absence of water.

Light

Light is the energy that permits the existence of life on earth. As previ­ously explained, it is through plant photosynthesis that the light energy of the sun is captured and transformed by plants into a form of chemical energy that is usable initially by plants for their own growth, and later for growth of animals that eat the plants.

Plants respond to light whether it is natural or artificially produced. However, there is a certain minimal amount of light that must be pres­ent to initiate the chemical reactions of photosynthesis. It is termed the threshold value and can be provided by either a natural or artificial light source. If the quantity of light is adequate for growth and distributed evenly as it falls across the plants, growth will be generally straight and uniform. However, if the location of the light source provides more light to one side of a plant than to the other side, the plant will likely respond by bending and growing toward the light source. The general term for the growth of a plant in response to a stimulus is tropism. When light intensity is the stimulus, the applicable term to describe the bending growth is phototropism.

While it is natural to think that a plant stretching toward the light favors the light, the explanation is chemical, not a case of partiality. Plants produce a growth stimulant known as an auxin that is located in the tips of shoots and is responsible for cell elongation. Auxins will build up in a greater amount on the side of a shoot that is away from the light source, creating a greater stimulant for the elongation of cells on that side of the shoot. That enables the shoot to grow faster on that side,

figure 1-14. Typical plant growth of the American Southwest, a region of limited rainfall and high temperature (© Anton Foltin, 2009. Used under license from Shutterstock. com)

thereby causing it to appear to be leaning or growing toward the light (Figure 1-15). When most of the shoots on a plant display the response, it creates a rather dramatic effect, often making the plant look lopsided or windswept. Once a shoot becomes woody, it does not respond to phototropism except at its growing tip.

Another influence of light is photoperiodism. It is the response of plants to

• The number of hours of daylight each day

• The number of hours of darkness each day

• Interruptions of the light period

figure 1-15. An explanation of phototropism (Delmar/Cengage Learning)

Responses by plants to the proportionate number of hours of daylight and darkness are diverse. They include flowering, stem length, the formation of storage organs such as bulbs and tubers, leaf color change, and leaf drop (abscission).

In many plants, light has a dramatic ability to initiate or delay the blooming. Some plants, such as the rose-of-Sharon (Hibiscus syriacus) will flower only when day length is sufficient to reach or exceed a critical day length. Such species are termed long-day plants. The actual number of hours of light per day varies among species, but without the requisite number of long days, the plants will remain in a vegetative state and not initiate flower formation. Conversely, certain species known as short - day plants, will flower only after exposure to day lengths that are shorter (or no longer) than their critical day length. Chrysanthemums and poin - settias exemplify short-day plants. A third group of plants known as the day neutral plants do not have their ability to initiate flowers linked to a critical day length. While some species may produce more or fewer flowers in response to long or short photoperiods, the plant will flower regardless of the length of illumination.

In the natural world, the length of daylight illumination is controlled by the seasons. Leaves don’t change color in early summer when days are long. Neither do leaves fall from the trees in the lengthening days of springtime. When nature begins to lengthen the night hours, the trees respond with color changes and eventually falling leaves. Garden mums grow lush and green during the long days of summer and only start to display their blossoms when the shorter days of autumn begin.

In commercial plant production, knowledge of the critical day length of the ornamentals being grown permits the grower to promote or delay flower production of crops by manipulating the environment of the greenhouse or other production facility.

Temperature

The temperature of the plant’s growing environment, both in the soil and in the air around it, controls key reactions responsible for seed germina­tion, respiration, transpiration, flowering, leaf abscission, color change, dormancy, shoot and root growth, and ultimately the survival of the plant. Temperatures explain in large part why palm trees won’t survive outside in Alaska and why apple trees are not planted in Florida. Because the growth processes of plants are the result of chemical reactions, tempera­ture can accelerate, slow, or render impossible those reactions. When the presence or absence of light is factored into the reaction, many plants grow better in alternating day and night temperatures than they do in a constant temperature setting. Thermoperiodicity is the term applied to this positive reaction by plants to differences in the day and night tem­peratures of the production environment.

Many plants commonly grown as commercial greenhouse crops respond to differential production temperatures. Carnations grow best at a day temperature of 65°F and a night temperature of 50° to 55°F. In contrast, chrysanthemums produced in a greenhouse prefer a warmer growing environment both day and night. The positive response to spe­cific temperature ranges has created a categorization of many plants as cool season crops or warm season crops. Even turf grass species vary throughout the country, with the umbrella-terms of cool season grasses and warm season grasses used to describe when they will be most effec­tive as landscape components.

Knowledge of the relationship between the rate of plant growth and the influence of temperature(s) on that rate of growth permits growers to manipulate the time when a crop will be ready for sale. Customers do not buy poinsettias after Christmas or Easter lilies on the day after Easter. Neither will they buy them too far in advance, so the ability to control the flowering response of the plants by raising or lowering day and/or night temperatures can determine if and when a crop is ready for harvest. In other crops, temperature manipulation can also make the difference between a compact, full plant and a spindly, sparse one.

The chief danger of an introductory chapter such as this one is oversimplification. A discussion of the botanical sciences in so few pages may create the impression that the green plant is like a Tinker Toy®, simplistic in form and easy to understand. Nothing could be more incorrect. The green plant is a marvel of biological and evolutionary engineering, about which much is still unknown. Any student of orna­mental horticulture should first be a student of botany.

SUMMARY______________________________________

The survival of the human race and all other animal life depends on the oxygen provided and recycled by the green plants of the earth. Green plants also capture the sun’s energy and convert it into forms usable by members of the animal kingdom. Plants are the only organisms capable of manufacturing their own food. Our dependence on them is total.

The classification of plants is a dynamic process, changing as new knowledge is acquired and new members are identified. The scientific systems of classification are natural ones based on the genetic and evo­lutionary relationships among plants.

Members of the higher plants (the Division Tracheophyta) have roots, stems, and leaves, and may have cones (Class Coniferopsida) or flowers (Class Angiospermopsida). Flowers and cones are the reproduc­tive structures. Roots serve to absorb water and mineral nutrients from the soil, anchor the plant, and store food materials produced in the leaves. Stems are the central axis of plants. Their function is to conduct water and minerals from the roots to the leaves, and food materials from the leaves to the roots and other plant parts. Leaves are the major sites of food manufacturing in the plant.

The formation of roots, stems, leaves, cones, and flowers in the higher plants is attributable to the ability of plant cells to differentiate and assume assorted roles for the growth and development of the plant. Plant cells may be meristematic, parenchyma, collenchyma, or scleren - chyma in type. All contain similar materials but differ in the compara­tive amounts of those materials. They also appear in differing numbers, depending on the tissue or organ they comprise.

It is in the leaves where the important processes of photosynthesis, respiration, and transpiration occur most actively. Photosynthesis is the process by which food in the form of sugar is manufactured from water and carbon dioxide in the presence of the green pigment, chlorophyll, and light. Respiration is the process that permits living cells to trans­form organic material into energy. Oxygen and enzyme catalysts oxidize sugar to carbon dioxide and water, releasing energy simultaneously. Transpiration is the process by which a plant loses water vapor through the stomata in the leaves. Translocation is the process of moving organic solutes from their place of production to their place of use or storage.

Color in plants is caused by the presence of pigments, with the dominant pigment, usually chlorophyll, in a healthy, growing plant, giving the plant its tone. Lacking the dominance of chlorophyll, one or more colors may appear due to the presence of other pigments such as xanthophyll, carotene, or anthocyanins in the tissue.

The growth potential of a plant is determined by its genetic heritage and its environment. The genetic heritage is predetermined, but a grow­er can exercise some control over environmental factors such as the soil, atmosphere, water, light, and temperature. The specific response of the plant to environmental change is sometimes related to whether the plant is in its juvenile or its mature stage.

ACHIEVEMENT REVIEW

A. ESSAY

What relationship exists between the plant king­dom and the animal kingdom? Which is most dependent on the other? What is the basis for the dependency? What responsibility do human beings have for the well-being of the plant kingdom? Write a short essay on the value of green plants that incorporates answers to these questions.

B. MULTLIPLE CHOICE

From the choices given, select the answer that best completes each of the following statements.

1. Macroscopic plants are_____ to the

unaided eye.

a. invisible c. green

b. visible d. nongreen

2. Microscopic plants require_____ to be

seen.

a. a microscope c. an unaided eye

b. a magnifying d. an electron

glass microscope

3. The current systems that use genetic

relationships as the basis for classifying plants are systems.

a. artificial c. horticultural

b. Latinized d. natural

4. Flowering plants are in the division____ .

a. Phaeophyta c. Bryophyta

b. Chlorophyta d. Tracheophyta

5. The basic structural unit of plants is the

a. nucleus c. cambium

b. cell d. tissue

6. The major difference between plant cells

and animals cells is the presence of a____

in plants.

a. cell membrane

b. vacuole

c. cell wall

d. nucleus

7. The process of converting energy from

solar to chemical form through the manufacture of sugar is.

a. respiration c. guttation

b. transpiration d. photosynthesis

8. The green pigment plants need to capture

the sun’s energy is_____ .

a. chlorophyll c. photosynthesis

b. protoplasm d. lignin

9. Water, nutrients, and food materials travel

up and down the stem in the____ .

a. phloem c. xylem

b. intercellular d. vascular

spaces bundles

10. Water and minerals are transported in the

a. phloem c. xylem

b. intercellular d. vascular bundles spaces

11. Food materials are transported down the

stem in the____ .

a. phloem c. xylem

b. intercellular d. vascular bundles

spaces

12. Stems increase in diameter due to the

production of vascular tissue by the____ .

a. cell walls c. middle lamella

b. cambium d. epidermis

13. The many pore-like openings in a leaf

through which transpiration releases water vapor and through which gaseous exchange occurs are the___________________________ .

a. hydathodes c. guard cells

b. stomata d. vascular bundles

14. Roots grow in the region called the_____ .

a. root hairs c. endodermis

b. root cap d. apical meristem

15. Photosynthesis uses the raw materials of

water and ____ to produce simple sugar,

water, and____ .

a. carbon dioxide/oxygen

b. oxygen/carbon dioxide

c. glucose/oxygen

d. sunlight/carbon dioxide

16. Respiration uses the raw materials of

glucose and____ in the presence of

enzymes to produce carbon dioxide, water, and.

a. carbon dioxide/oxygen

b. water/oxygen

c. sunlight/simple sugar

d. oxygen/energy

17. A cultivar is a variety that is sustained by

a. nature c. volunteers

b. propagators d. cell differentiation

and growers

18. The simplest embryonic cells are the cells.

a. parenchyma c. meristematic

b. collenchyma d. scleroid

19. Complex____ can result when cells

differentiate, then group together to carry on the same function.

a. cell walls c. tissues

b. pigments d. photosynthesis

C. SHORT ANSWER

Answer each of the following questions as briefly as possible.

1. What are the four major parts of a higher green plant?

2. What are the reproductive structures of Angio spermopsida? Coniferopsida?

3. What are the two forms of root systems?

4. What is the name of the specialized root type formed during vegetative reproduction?

5. Why do trees and shrubs change colors in the autumn in temperate regions of the country?

6. Label the parts in this cross-sectional diagram of a root.

9. What is the term used for a flower that:

a. has all of the floral organs

b. lacks one or more of the floral organs

c. has both pistils and stamens

d. lacks either pistils or stamens

10. What is the term used for a plant that carries:

a. both pistillate and staminate flowers on the same plant

b. pistillate and staminate flowers on separate plants

D. TRUE/FALSE

Indicate if the following statements are true or false.

1. The growth potential of a plant is determined solely by its genetic heritage.

2. Plant size can be modified by a grower.

3. Not all mineral elements in the soil are essential for plant growth.

4. Macronutrients are more important to plant growth than micronutrients.

5. Nitrogen, phosphorus, and potassium are used in the greatest quantities by green plants.

6. Air pollution can affect plant growth.

7. A plant growing in an environment of low humidity can be expected to transpire more than one growing in a highly humid area.

8. Phototropism is the influence of varying durations of light on plant growth and development.

9. Photoperiodism is plant movement in response to light.

10. Auxins promote cell elongation.

11. Long-day plants require shorter night periods than short-day plants to initiate flowers.

12. The terms cool season and warm season crops refer to the temperature conditions most suitable for the plants’ growth.

13. Translocation is a process of moving water and nutrients from the roots to the leaves. A different process explains the movement of food from leaves to points of plant growth.

14. Respiration occurs within the cells’ chloroplasts.