Plant HormonesHow do we know they exist?Why do the shoots grow up,but the roots grow down?

Plant HormonesPhototropism experiments by Charles and Francis Darwin:

Plant HormonesBoysen-Jensens experiments:Mechanisms of bending:

Plant HormonesFrits Went discovered the hormone Auxin. Concentration of auxins in different parts of roots and shoots determines growth rates of the cells Since distribution of auxins is affected byboth light and gravity, auxins are involved in plants phototropic and gravotropic growthpatterns.

Plant HormonesMajor plant hormones and their action:

Abscisic acidclosing of stomata; seed dormancy

Auxinselongation of shoots and roots, gravotropism,phototropism

Cytokininspromotion of sprouting of lateral buds

Ethyleneripening of fruit

Gibberellinsgermination of seeds and sprouting of buds;elongation of stems; stimulation of flowering;development of fruit

Plant HormonesHormones can work with each other to regulate variousaspects of plant biology.

Here, auxin and cytokinin worktogether in regulating lateral bud formation the sproutingof lateral buds depends on the ratio of cytokinin to auxin

Plant HormonesPlants use starch-filled organelles called statoliths to detect gravity this is very similarto how jellyfish detect gravity, and in some ways, it is even similar to how humans keep themselves balanced

Plant HormonesA hormone called florigen both stimulates and inhibits flowering. Since flowering is sometimes a function of day length, pigments called phytochromes are involved in measuring day length. The ratio of different forms of this pigments change as a functionsunlight exposure and can thus be used by the plants to set their internal biological clocks.

Chemical Communication in Plants

Plant CommunicationThe Venus flytrap catches flies as a source of nitrogen.Insects are attracted to the nectar in the trap, but when they trigger sensory hariswithin the trap, they are capturedWhat is the mechanism?

Plant CommunicationThe mechanism is based on irreversible, differential growth.Triggering of the hairs creates an electrical potential that pumps H+ into outer cell walls.The acidic condition created by the H+ induces enzymes to loosen the cellulose fibers ofthe walls, causing water to rush in the growing or swelling of the outer cells then causes the rapid closing of the trap