Plant physiology is a discipline of botany which deals with the functioning and physiology of plants. Fundamental processes of plants like photosynthesis, plant nutrition, respiration, function of plant hormones, tropism, nastic movements, photoperiodism, photomorphogenesis, circadian rhythms, environmental stress, seed germination, transpiration and plant water relations are studied under plant physiology.
Environmental physiology is a field of study in plant ecology. It deals with the way plants respond to their environment. Plant response to physical factors like radiation, temperature, fire, wind, humidity, pH, and soil. Ecophysiology of plants is concerned with mechanisms and integration to variable conditions. Mechanisms of plants like the responses and sensation of plants to environmental changes and their responses to highly variable conditions like high sunlight and shade within tree canopies.
Unlike animals, plants are not able to escape factors like heat, cold, floods and droughts. Hence, plants have to survive through adverse conditions or be destroyed. Phenotypically plants are flexible and they have a wide genetic variation and it aids to adapt alternating conditions.
Plants sense direction, wavelength, intensity and periodicity of light. Photomorphogenesis is induced by light and many other processes like phototropism, chloroplast differentiation, flowering and germination. With respect to developmental biology, photomorphogenesis is a light mediated development.
In plants, photomorphogenesis is often studied by using frequency controlled light sources to grow the plants. Light has pronounced effect on plant development. The most noticeable effect is observed during seed germination when the seed emerges from the soil and is exposed light for the first time.
Photoperiodism is the reaction of plants to the length of day or night. It can also be defined as the developmental responses of plants to the relative lengths of light and dark periods. In most flowering plants, the pigment phytochrome is used to sense seasonal changes in the length of the day. This sensitivity to day length, is sent as signals by the phytochromes to the flower and is known as photoperiodism.
Plants can be classified based on their responses to the changes in the day length. They are classified into long day plants, short day plants, or day neutral plants. Plants use their phytochrome system to sense the day length or photoperiod.
Plant hormones are also known as phytohomones, they are chemical messengers that regulate plant growth and are termed as plant growth substances. They are also known as plant growth regulators (PGR's). Plants lack specific organs or tissues to produce hormones and they are not transported to other plant parts and the production is not limited to specific locations.
Plant hormones are chemical messengers which are secreted in small amounts, they promote and influence many physiological process like seed development, dormancy and seed germination. They regulate direction of tissue growth, leaf formation, stem growth, development of fruit, ripening and leaf abscission and even death.
Phytochemistry is the study of phyto chemicals, which are derived from plants. It is often used to describe a large number of secondary metabolic compounds found in plants. These chemical compounds are known to provide protection against attacks of insects and plant diseases. These chemicals also exhibit a number of protective functions in humans also. Phytochemistry is used in the field of herbal medicine.
Plant nutrition is the study of chemical elements and compounds that are required for plant growth, and also the supply to their external environment and internal metabolism of these substances. According to Epstein there are two different criteria for an essential nutrient from for plant growth. They are:
- The absence of essential nutrient in the plant is unable to complete a normal life cycle or
- The essential element is a part of some essential plant metabolite or constituent.
For a plant, according of Leibig's law of minimum, there are about 14 essential plant nutrients. Elements like carbon and oxygen are absorbed from air and the other nutrients are obtained from the soil.
The following nutrients are obtained by a plant from growing media:
- Elements like nitrogen, phosphorus, potassium are primary macronutrients.
- The secondary macronutrients include calcium, sulphur, magnesium.
- The micronutrients or trace minerals are boron, chlorine, manganese, copper, zinc, iron, nickel, molybdenum.
Plants respond to stimulus may be directional or non-directional. Response of plants to directional stimulus like gravity or sunlight is known as tropism. Tropisms are associated with plants where it is capable of directed physical motility, activity in response to a specific stimulus. Tropism in plants results in differential growth where the cells on one side of the plant elongates more than other side resulting in it to bend to a side and lesser growth.
Plants respond to directional and non-directional stimuli. Plants response to nondirectional stimuli like temperature, humidity is known as nastic movement. Nastic movements are a result of differential cell gowth like epinasty or hiponasty, or it may also result from changes in the turgor pressure within plant tissues which may occur rapidly.
Nastic movements differ from tropic movements as the direction of nastic movements is independent of the position of the stimulus. The frequency or the rate of responses increase with the increase in the intensity of the stimulus.
Pigments are important molecules involved in plant functions. There are variety of plant pigments which include molecules like porphyrines, anthocyanins, and carotenoids. These biological pigments absorb certain light and reflect others. The light that is absorbed by these pigments are used by the plants to power chemical reactions while the reflected light determines the color pigment that appears to the eye.
- The primary pigment in plants is chlorophyll which is a porphyrin. It absorbs light to power the process of photosynthesis.
- Carotenoids are accessory pigments in plants which help in the process of photosynthesis by gathering wavelengths that is not absorbed by chlorophyll.
- Anthocyanins are flavonoid pigments which are water-soluble. These pigments occur in almost all tissues of higher plants, they are most visible in petals of flowers.
The study of diseases in plants and the resistance of plants to certain diseases and infection is known as plant pathology. Plants are susceptible to diseases which are caused by bacteria, viruses, and fungi and also due to physical invasion of roundworms and insects. The symptoms and responses of plants is different to animals.
Plants may respond by just shedding leaves or flowers to prevent the spread of disease. The plant pathogens are spread via animal vectors or spores. Plant disease control is a crucial part as it is reliable for food production and it also proves useful as there is reduction in agricultural use of land, water and other inputs.