Plant Physiology

Plant physiology is a subdiscipline of

botany

concerned with the functioning, or physiology, of

plant

s. Closely related fields include

plant morphology

(structure of plants), plant ecology (interactions with the environment), phytochemistry (

biochemistry

of plants), cell biology, genetics, biophysics and molecular biology.

Fundamental processes such as

photosynthesis

,

respiration

, plant nutrition, plant hormone functions, tropisms, nastic movements, photoperiodism, photomorphogenesis, circadian rhythms, environmental stress physiology, seed

germination

, dormancy and

stoma

ta function and

transpiration

, both parts of plant water relations, are studied by plant physiologists.

Aims

The field of plant physiology includes the study of all the internal activities of plants—those chemical and physical processes associated with

life

as they occur in plants. This includes study at many levels of scale of size and time. At the smallest scale are

molecular

interactions of

photosynthesis

and internal

diffusion

of water, minerals, and nutrients. At the largest scale are the processes of plant development,

season

ality, dormancy, and reproductive control. Major subdisciplines of plant physiology include phytochemistry (the study of the

biochemistry

of plants) and phytopathology (the study of disease in plants). The scope of plant physiology as a discipline may be divided into several major areas of research.

First, the study of phytochemistry (plant chemistry) is included within the domain of plant physiology. To function and survive, plants produce a wide array of chemical compounds not found in other organisms.

Photosynthesis

requires a large array of pigments,

enzyme

s, and other compounds to function. Because they cannot move, plants must also defend themselves chemically from herbivores, pathogens and competition from other plants. They do this by producing toxins and foul-tasting or smelling chemicals. Other compounds defend plants against disease, permit survival during drought, and prepare plants for dormancy, while other compounds are used to attract

pollinator

s or herbivores to spread ripe seeds.

Secondly, plant physiology includes the study of biological and chemical processes of individual plant cells. Plant cells have a number of features that distinguish them from cells of

animal

s, and which lead to major differences in the way that plant life behaves and responds differently from animal life. For example, plant cells have a cell wall which restricts the shape of plant cells and thereby limits the flexibility and mobility of plants. Plant cells also contain

chlorophyll

, a chemical compound that interacts with

light

in a way that enables plants to manufacture their own nutrients rather than consuming other living things as animals do.

Thirdly, plant physiology deals with interactions between cells, tissues, and organs within a plant. Different cells and tissues are physically and chemically specialized to perform different functions.

Root

s and rhizoids function to anchor the plant and acquire minerals in the soil.

Leaves

catch light in order to manufacture nutrients. For both of these organs to remain living, minerals that the roots acquire must be transported to the leaves, and the nutrients manufactured in the leaves must be transported to the roots. Plants have developed a number of ways to achieve this transport, such as vascular tissue, and the functioning of the various modes of transport is studied by plant physiologists.

Fourthly, plant physiologists study the ways that plants control or regulate internal functions. Like animals, plants produce chemicals called hormones which are produced in one part of the plant to signal cells in another part of the plant to respond. Many

flowering plant

s bloom at the appropriate time because of light-sensitive compounds that respond to the length of the night, a phenomenon known as photoperiodism. The ripening of

fruit

and loss of leaves in the winter are controlled in part by the production of the gas ethylene by the plant.

Finally, plant physiology includes the study of plant response to environmental conditions and their variation, a field known as environmental physiology. Stress from water loss, changes in air chemistry, or crowding by other plants can lead to changes in the way a plant functions. These changes may be affected by genetic, chemical, and physical factors.

Biochemistry of plants

The chemical elements of which plants are constructed—principally

carbon

,

oxygen

,

hydrogen

,

nitrogen

,

phosphorus

,

sulfur

, etc.—are the same as for all other life forms: animals, fungi,

bacteria

and even

virus

es. Only the details of their individual molecular structures vary.

Despite this underlying similarity, plants produce a vast array of chemical compounds with unique properties which they use to cope with their environment. Pigments are used by plants to absorb or detect light, and are extracted by humans for use in dyes. Other plant products may be used for the manufacture of commercially important

rubber

or

biofuel

. Perhaps the most celebrated compounds from plants are those with pharmacological activity, such as

salicylic acid

from which

aspirin

is made,

morphine

, and

digoxin

. Drug companies spend billions of dollars each year researching plant compounds for potential medicinal benefits.

Constituent elements

Plants require some nutrients, such as

carbon

and

nitrogen

, in large quantities to survive. Some nutrients are termed macronutrients, where the prefix ''macro-'' (large) refers to the quantity needed, not the size of the nutrient particles themselves. Other nutrients, called micronutrients, are required only in trace amounts for plants to remain healthy. Such micronutrients are usually absorbed as ions dissolved in water taken from the soil, though

carnivorous plant

s acquire some of their micronutrients from captured prey.

The following tables list element nutrients essential to plants. Uses within plants are generalized.

Pigments

Among the most important molecules for plant function are the pigments. Plant pigments include a variety of different kinds of molecules, including

porphyrin

s, carotenoids, and

anthocyanin

s. All biological pigments selectively absorb certain

wavelength

s of

light

while reflecting others. The light that is absorbed may be used by the plant to power

chemical reaction

s, while the reflected wavelengths of light determine the

color

the pigment appears to the eye.

Chlorophyll

is the primary pigment in plants; it is a

porphyrin

that absorbs red and blue wavelengths of light while reflecting

green

. It is the presence and relative abundance of chlorophyll that gives plants their green color. All land plants and green algae possess two forms of this pigment: chlorophyll ''a'' and chlorophyll ''b''.

Kelp

s,

diatom

s, and other photosynthetic heterokonts contain chlorophyll ''c'' instead of ''b'',

red algae

possess chlorophyll ''a''. All chlorophylls serve as the primary means plants use to intercept light to fuel

photosynthesis

.

Carotenoids are red, orange, or yellow tetraterpenoids. They function as accessory pigments in plants, helping to fuel

photosynthesis

by gathering wavelengths of light not readily absorbed by chlorophyll. The most familiar carotenoids are

carotene

(an orange pigment found in

carrot

s),

lutein

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