HORTICULTURE

MAJOR PLANT PROCESSES

A green plant is comparable to a machine that operates nonstop. However, unlike a machine that would perform a single function or programmed series of functions, the plant “machine” performs dif­ferent functions simultaneously. At different times and under varying environmental conditions, certain plant processes increase and others decrease.

The physiological (functional) processes of the higher plants are numerous, but three stand out: photosynthesis, respiration, and tran­spiration.

Photosynthesis

Photosynthesis has been referred to earlier. It is a process unique to green plants in which food (sugar) is manufactured from water and car­bon dioxide in the presence of chlorophyll. Light energy, from the sun or other sources, drives the chemical reaction, and oxygen is released in the process. Water, used in the process, is also produced. The chemical equation for photosynthesis is believed to be:

chloroplast

6CO2 + 12H2O------------ > 6H12O6 + 6O2 + 6H2O

light energy

This equation is more of a summary than a specific explanation. Photosynthesis is an assemblage of many reactions that scientists con­tinually strive to understand. Considering that photosynthesis is the source of all fuel and all food on the planet, the need to understand the process is apparent. The reactions occur in the chloroplasts of the cells, where the chlorophyll is contained.

The importance of the green pigment chlorophyll cannot be over­stated. It is the primary means by which the unusable energy of the sun is transformed into usable chemical energy. Chlorophyll is a complex molecule, which is not surprising considering its almost magical accom­plishment. It occurs in several modified forms (chlorophyll a, b, c, d, and e). The modifications are based on which form of the chlorophyll mole­cule absorbs which wavelength of light, since the light coming from the sun contains varied wavelengths (Figure 1-12). Chlorophyll a is found in all photosynthetic plants, absorbing nearly all of its light energy from the violet, blue, red-orange, and red wavelengths. Chlorophyll b absorbs more light energy from the green wavelength. The absorption of the light energy triggers the chemical reaction that is photosynthesis. While

Visible light spectrum
(Wavelength in millimicrons)

figure 1-12. Electromagnetic Spectrum and Special distribution of visible light (Delmar/Cengage Learning)

there are other modified forms of chlorophyll, it is the a and b forms of the pigment that account for most photosynthetic activity within the cellular chloroplasts of living higher plants.

Photosynthesis occurs in two stages, sometimes called the light and dark phases, and sometimes referred to as the light dependent process and the light independent process. It is not the purpose of this brief treatise to explain in detail the physiology of photosynthesis. Suffice it to say that some of the reactions occurring during photosynthesis require light, since it is through those reactions that light energy is captured and water is broken apart into hydrogen and oxygen. The oxygen is released in gaseous form. The hydrogen is then used in the formation of sugars by combination with an acid derived from CO2. This part of the photo­synthetic process does not require light, hence the name: dark phase.

The rate of the process varies with the light intensity, tempera­ture, and concentration of carbon dioxide in the plant’s atmosphere. Also, excessive accumulation of the end product, sugar, can slow the reaction.

Respiration

Respiration is the process that permits living cells to obtain energy from organic material (usually glucose sugar). It is a breaking-down process, unlike photosynthesis, which is a manufacturing process. Respiration uses oxygen and enzyme catalysts to oxidize the sugar to carbon diox­ide and water. In the process, energy is produced. The reaction can be stated as follows:

enzymes

glucose + oxygen------------- > carbon dioxide + water + energy

This process converts the chemical energy captured through photo­synthesis into a form of energy available to the plant for growth, repro­duction, and cell maintenance. The process does not create energy but simply changes its form.

While photosynthesis can only occur during the hours of daylight or under artificial light, and only in cells that contain chlorophyll, respira­tion is ongoing every hour of the day and in the mitochondria of every cell of the plant. Energy is released as adenosine triphosphate, com­monly termed ATP.

Since photosynthesis produces the products needed for respiration, and since photosynthesis occurs for fewer hours each day than does respiration, a balance is needed. That balance is made possible by the greater efficiency of the photosynthetic process. In a healthy plant, the reaction rate of photosynthesis is higher than that of respiration, there­by producing more food during the day than can be transformed by respiration during the dark hours. That permits plants to store food in the form of fruits and seeds. If conditions alter the balance and permit respiration to exceed photosynthesis, the plant will suffer and decline.

Transpiration

Transpiration is the loss of water in vapor form from a plant. Water enters the plant through the roots and saturates the intercellular spaces throughout the plant. Then, because the amount of water vapor in the atmosphere is nearly always less than the amount of water vapor inside the plant tissue, the vapor leaves the plant. Most transpiration occurs

through the stomata, 90 percent of which are located on the lower surface of the leaves. Smaller amounts of water vapor are lost through the cuticle. When water uptake exceeds the rate of transpiration, water passes out of the plant in liquid form through leaf openings called hydathodes. This slow exudation of liquid water is called guttation.

Transpiration rates are accelerated by increased temperatures or light. As the humidity in the air around the plant increases or decreases, the rate of transpiration decreases or increases in response.

Everyone has observed wilted plants. Most have probably also observed that the majority of plants will recover when water is added to the soil. However, sometimes the plant is so extremely dehydrated that the addition of water cannot reverse the wilting. Still other times, the soil is moist yet the plant wilts, only to recover later in the day, when the sun goes down. Each observation exemplifies a different type of wilting, but all are related to the process of transpiration. When the soil begins to dry out, plants may transpire water faster than the roots can absorb the diminishing water resource. The resultant wilting will be temporary as long as water is added before permanent injury occurs. When there is ample water in the soil, yet the plant is still wilted, the condition is incipient, meaning that transpiration is occurring faster than water can be absorbed by the roots. The condition usually results when the environment of the plant is either excessively warm or bright or both. As evening comes and environmental conditions moderate, the wilting subsides as water absorption catches up with transpiration. If there is so little water in the soil that the plant cannot replace the water vapor it has lost even after transpiration slows, then the plant is in a state of permanent wilting and cannot recover even if water is added to the soil.

Translocation

Translocation is the movement of organic materials from one part of the plant to another part. Occurring in the phloem, translocation allows the products of photosynthesis to move to where they are needed for plant growth and reproduction, or to be stored to become fruits and seeds. Depending on the stage of plant development, translocation can occur in any direction within the plant.