Plant Hormones

AUXIN

Auxin is the plant hormone responsible for stimulating and controlling plant growth. Auxin is made in actively growing tissue including young leaves, fruits, the shoot apex, and the root. In phototropism, the shaded side of the shoot in a plant will contain more auxin, thus compelling the plant to grow away from the shaded side and towards the light. While in a root cell, though the shaded side of the root will contain more auxin, the shaded side will grow less than the lit side and cause the root to grow away from the light. Auxin is also involved in gravitropism. If a shoot is placed horizontally, the bottom side will contain more auxin than the top side, compelling the bottom side to grow more than the top side. This causes the shoot to bend and grow against the force of gravity and in the correct direction. If a root is placed horizontally, the bottom side will contain more auxin than the top side, causing the bottom side to grow less than the top side. This allows the root to bend in the direction of the force of gravity and grow in the correct direction. The growing pattern and the concentrations of auxin in the plant can be described in the five models for auxin transport. On a cellular level, auxin is essential for cell growth, promoting cellular division and cellular expansion. Auxin contributes to cell differentiation and specification. Depending on the type of tissue, auxin may compel axial elongation (shoots), lateral expansion (roots), or isodiametric expansion (fruits). 

http://upload.wikimedia.org/wikipedia/commons/thumb/9/97/Model_for_auxin_transport.png/330px-Model_for_auxin_transport.png

http://www.plant-hormones.info/iaa.gif

http://www.plant-hormones.info/went1.gif

ABSCISIC ACID

Abscisic acid (ABA) is a plant hormone also known as abscisin II and dormin. It functions in many plant development processes. Abscisic acid stimulates the closure of the stomata, inhibits shoot growth, induces seeds to synthesize storage proteins, inhibits the affect of gibberellins on stimulating de novo synthesis of a-amylase, effects induction and maintenance of dormancy,  and induces gene transcription. Abscisic acid is also the plant hormone that responds to weather stresses such as cold and drought. ABA maintains the dormancy in seed germination, ensuring its growth in the most advantageous environment. 

http://www.plant-hormones.info/aba.gif

ETHYLENE

Ethylene is commonly used in the agricultural industry. Commercial fruit farmers control the timing of the fruit ripening with the application of ethylene gas. Horticulturalists inhibit leaf dropping in ornamental plants by removing ethylene from green houses using fans and ventilation. Ethylene is a hormone that stimulates fruit ripening, flower wilting, and leaf fall. Aging tissues and nodes of stems also produce ethylene. The most well-known effect of this hormone is the compulsion of fruit ripening. It stimulates the conversion of starch and acids to sugars. One trick people use to accelerate the ripening of fruit is to seal unripe food in a paper bag and let the gas released by the first fruit to mature trigger the ripening of the remaining fruit. Ethylene also plays a role in fruit abscission and flower fading/dropping. 

http://www.chinadaily.com.cn/lifestyle/2006-06/01/xin_050603011349097195622.jpg