Plant Physiology is the science which deals with the life processes of plants, or Plant Physiology is the science which deals with the functions of cells, tissues, organs or the plant as a whole.
Life process or Physiological process may be defined as any chemical or physical change occuring within a cell or organism and any exchange of substances between the cell or organism and its environment. According to this definition of life process or Physiological process, the physical process of diffusion (including osmosis, gas diffusion, transpiration), water conduction, solute absorption, absorption or loss or transport of radiant energy and other internal physical changes or exchanges with the environment would be considered as physiological process, similarly all metabolic changes like those concerned in respiration, photosynthesis, synthesis or degradation of lipids, amino acids, proteins etc. , when they occur within the cell or organism would be considered as physiological process. Many physiological processes such as growth and photosynthesis involve both physical and chemical changes.
Although there is a tendency to discuss the physiological processes one at a time but we should not lose sight of the fact that in the plant most of the processes are taking place simultaneously and each one may influence and be influenced by the others.
We know that a tree is just not a large seed.Actually growth took place in the seed and this is accompanied by differentiation of roots,shoots,leaves,flowers and fruits thus taking it a complete plant. A large number of diverse and complex processes takes place during various stages of developments. First of all the seeds imbibe water and swell up resulting in bursting of seed coats. Soon after radicle and plumule emerge from the seed,the food reserves of the seed are mobilized to meet the requirements of developing radicle and plumule. The radicle penetrates into the soil and develops root system, the plumule goes into the air and produces branches, leaves, flowers and fruits.
Through root, water and nutrient-salts pass into the vascular tissue and are carried through the stem to the leaves. Carbon dioxide enters through the stomata of the leaves and with the help of sunlight, carbohydrates are synthesized from carbon dixoide and water, in the chlorophyll bearing cells. From the carbohydrates and salts the plant later synthesizes other compounds such as amino acids, fatty acids and vitamins. These manufactured foodstuffs are then distributed in a solution to cells in all parts of the plant and the excess may be stored in the seed, fruit, root and stem. Although photosynthesis occurs only in some special cells of the seed plant, all cells of the plant body must be supplied with foods and they must be freed of cell wastes.
Plants grow using part of the food manufactured and after reaching maturity they reproduce. Growing plants respond to gravity, to light and to chemical and other factors in their environment. Thus the root grows downward in response to gravity, whereas the stem grows upward, the root grows away from the light while the stem grows towards it. Movements of plants are mainly growth movements and although they are too slow for us to be seen they can be readily demonstrated in time lapse photography. The more rapid turgor movements can be seen in the sensitive plant Mimosa.
To carry out all the physiological processes listed above energy is required. A given weight of an organic compound (carbohydrates, proteins, fats) contains a fixed amount of potential energy locked up in the bonds between the carbon, hydrogen and oxygen atoms in the sugar-glucose. This energy is released step wise by oxidation of different chemicals for carrying out all the functions of life. The agents for this controlled oxidation are the enzymes-the organic catalysts.
The cell employs dozens of oxidative enzymes, each specialized to catalyze one reaction in the series that ultimately convert the foods into carbon-dioxide and water. But the biologically significant product of the whole chain of transaction is energy and not water and carbon dioxide which are mere waste products. As the energy is liberated in the breakdown and oxidation reactions it is captured in the chemical bonds of a special energy storing molecule (ATP) and delivered thereby to the energy-consuming activities of the cell.
The common denominator required for the performance of all physiological processes of a plant is the cell. It is through the cell of the plant that all the flow of matter and flow of energy occurs. The cell is a fundamental unit, not only of structure but also of function of all living organisms.