Plant Hormones http://plantsinmotion.bio.indiana.edu/plantmotion/starthere.html

5 Main Plant Hormones Abscisic acid Auxin Cytokinins Ethylene Gibberellins

Abscisic acid Regulator and “babysitter” Prevents seeds from germinating Inhibits cell growth Closes stomata when water is low

Auxin (IAA) Performs many functions Elongation of stems Phototropism and geotropism

Phototropism http://plantsinmotion. bio. indiana

Phototropism Auxins secreted from shoot tips again diffuse downwards from cells to cells Then the auxins will migrate away from the illuminated side of the shoot towards the side with less exposure to light. The accumulation of auxins on the side with less exposure to light leads to its faster growth than the other side, and hence the shoot will bend towards the light = WHY?

Geotropism

Geotropism - roots auxins are produced in the root cap and then they migrate towards the center of the plant imagine that the root is oriented horizontally, auxins will then accumulate at the lower side of the root (gravity) auxins inhibits the growth of the root, the lower side of the root accumulated with auxins will grow much slower than the upper side

Geotropism - stems when the shoot is oriented horizontally, auxins secreted from the shoot tips accumulate on the lower side of the shoot high concentration of auxins can promote the growth of the shoot the high concentration of auxins causes the lower side of the shoot to grow much faster than the upper side

Cytokinins Works with auxin to promote cell division Slows down growth in aging plants to help maintain their health Promotes growth of lateral buds (leaves)

Ethylene Gaseous hormone Initiates fruit ripening Causes flowers and leaves to drop from trees Fruit in brown paper bag = ripened

Gibberellins (GA) Stimulate stem elongation by stimulating cell division and elongation. Stimulates bolting/flowering in response to long days. Breaks seed dormancy in some plants which require stratification or light to induce germination. Stimulates enzyme production (a-amylase) in germinating cereal grains for mobilization of seed reserves.

As the seed imbibes water the embryo produces GA As the seed imbibes water the embryo produces GA. This induces synthesis of amylase which is secreted into the endosperm. Amylase breaks down starch to glucose which diffuses to the embryo and is used for the early stages of plant growth.

Photoperiodism Biological clock for plants The response by a plant to the change in the length of days Flowering plants – florigen accumulates based on length of day as produced by phytochrome, florigen will cause flowering Short-day plants vs. long-day plants Poinsettias – flower w/ long nights (fall – winter) Spinach – flower w/short nights (spring – summer)