REGULATION OF PLANT GROWTH

Plant growth is influenced by a number of external and internal factors. The external factors affecting plant growth are:  light  temperature  humidity  oxygen  carbon dioxide  soil water and soil nutrients  pressure (altitude)  gravity

Plant Hormones Most plant hormones are made in minute quantities by actively dividing tissues at the tips of roots and stems. Once produced, they are transported to various parts of the plant. The most important plant hormones are; Auxins Giberrelins Cytokinins Ethylene

AUXINS Hormones that affect the plant growth are called auxins. They may stimulate or slow growth, depending on the type of the tissue and the amount of hormone. Auxins are synthesized mainly in shoot meristem. These hormones increase plant growth by stimulating cells to lengthen. In addition they cause cell to differentiate. Auxins also affect the process of abscission – the dropping off of leaves, flowers, or fruits from a plant.

AUXINS

GIBBERELLINS Hormones that affect the plant growth and development of fruits and seeds are called gibberellins. Unlike auxins, they are distributed evenly throughout the plants tissues. They have important effects on stem growth. Commercially they are used to stimulate flowering and to increase fruit size.

CYTOKININS Cytokinins stimulate cell division and growth during seed germination. They are thought to work together with auxins in stimulating cell differentiation.

ETHYLENE Ethylene, is a gas that plays role, along with auxins, in abscission. It also stimulates the ripening of many fruits and controls aging of the plant.

AUXINS and TROPISM The growth of a plant in a specific direction in response to a stimulus is called a tropism. Plant growth or movement toward a stimulus is called positive tropism, while movement away from a stimulus is called negative tropism.

AUXINS and TROPISM The stem of a plant that is growing toward the light is an example of positive phototropism. Roots, show negative phototropism.

Phototropism

AUXINS and TROPISM Roots generally show positive geotropism- they grown down into the ground in the direction of the force of gravity. Stems show negative geotropism.

Geotropism

Corn geotropism

Tropism, negative response to gravity Onion (Allium cepa)

AUXINS and TROPISM When the tendrils of a grapevine wind themselves around the stem of another plant, they are showing thigmotropism- growth in response to touch.

AUXINS and TROPISM Hydrotropism is observed in plants whose roots grow toward water.

AUXINS and TROPISM The growth responses seen in tropism are thought to be caused by uneven distribution of auxins in the affected plant parts.

In the positive photoperiodism of stems, for example, the concentration of auxins becomes higher on the shaded side of the stem than on the lighted side. Thus, the cells on the shaded side grow faster than the cells on the lighted side. The uneven rates of growth on opposite sides of the stem result in bending toward the side less rapid growth. In this case the stem bends toward the light.

NASTIC MOVEMENTS A plant movement that is in response to a stimulus but independent of the direction of the stimulus is called nastic movement. Most nastic movements involve changes in the internal pressure or turgor pressure of specific cells.

Rapid movements such as closing leaves involve changes in the turgor pressure in cells

Venus flytrap

PHOTOPERIODISM The flowering of many plants is in response to changes in the length of day over the course of the year. The response of a plant to changes in the length of day or night is called photoperiodism. (Photoperiodism is the non-directional developmental responses to non-directional but periodic light stimuli.) In many types of plants, flowering and other processes, such as leaf abscission, are controlled photoperiodically.

Short-day plants Long-day plants  Plants that flower during short days were called (or short-night plants).  They flower when there are short periods of darkness.  These plants usually bloom in the summer.  ex: clover, potato, beet, poppy and gladiolus. Plants that flower during short days (or long-night plants). They require long periods of darkness in order to flower. These plants flower in the early spring. ex: Morning glory, tulip, chrysanthemum and aster

Short-day plants

Long-day plants

Day-neutral plants Plants whose flowering is unaffected by the lengths of light and dark. These plants have long flowering season. ex: tomato, cucumber, dandelion, strawberry, string bean and corn.