Physiology is the study of the various functions performed in and by living organisms. Physiological processes of plants include the flow of energy, movement of solutes, and control by hormones. Chemical reactions are mediated (their rate is controlled) by enzymes.
For example, those studying plant physiology are concerned with the way that plants trap light energy as light is absorbed by chlorophyll. As a result of a series of chemical reactions, glucose, a six-carbon carbohydrate, is formed (preceding glucose are molecules of PGAL, a three-carbon sugar, which pair to form glucose). The glucose may be oxidized within the same cell or within another cell of the same plant (or utilized by an animal). This oxidation process produces adenosine triphosphate (ATP). As this compound is converted to adenosine diphosphate (ADP), energy is released, allowing organisms to perform other essential energy-requiring life activities.
Plant physiologist would also be concerned with the transport of nutrients and water throughout a plant. By means of their roots, plants absorb water and dissolved materials from the soil, after which they are conducted upward, by means of xylem tissue, to all parts of the plant. This upward movement is called transpiration. The glucose formed in leaves is dissolved in water and transported, by means of phloem tissue, to all parts of the plant. This movement, called translocation, is commonly downward, but also may be upward. By these processes, water, minerals, and sugars are transported to all parts of a plant.
Another area of concern for plant physiologists is the function of plant hormones (phytohormones), in fact, those specializing in this area have their own discipline, endocrinology. Phytohormones are compounds produced within a plant. They are transported to other parts of plant, where they regulate growth and development. Early in the twentieth century, auxin was the first phytohormone to be discovered. It promotes growth by causing cells to elongate but was found also to inhibit growth of lateral buds. Gibberellins, a second group of hormones, also stimulate growth by causing cell elongation. Among their activities is the promotion of seed germination. Cytokinins are abundant in dividing tissues, where they stimulate cell division. Abscisic acid is a growth inhibiting hormone that maintains dormancy in buds and fruits and also is associated with the falling of leaves in autumn. Ethylene causes fruits to ripen. Several hormones are used in agriculture for increasing growth rates of crops.
See also: Cytology, Anatomy
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