1. Plant hormones

2. Hormones from Gr. to excite
natural chemical messengers transported to target cells
minute concentrations = substantial change
growth regulators
Chemical messengers produced by one plant organ affect cells in another location within the plant
Very small concentrations can cause substantial changes in the plant
Most often used to regulate the growth and maturity of the plant

3. Plants respond to stimuli
Picture on the left shows a potato grown in the dark. Because of the absence of light, an imbalance of growth hormones causes many sprouts to grow out of the potato in many different directions. This causes the potato to rapidly “eat up” the starch remaining in the tuber.
Picture on the right is a dramatic representation of a plant shoot growing towards the light (candle). You may have seen this in plants that grow in shaded areas on potted plants in window sills.

4. Plant hormones coordinate growth, development and responses to stimuli
Tropisms
growth responses
organs curve towards (+) or away from (-) a stimulus
ex. phototropism, thigmotropism, gravitropism
One important function of plant hormones is to coordinate growth responses called “tropisms”
Plant organs respond to stimuli by growing towards them (positive tropism) or way from them (negative tropism)
Phototropism is response toward light: shoot tips of plants are + phototropic while the root tips are – phototropic
Thigmotropism is response to mechanical pressure or “touch,” some plant organs like tendrils (modified stems) are + thigmotropic, wrapping around other plants or trellises
Gravitropism/geotropism is response to the force of gravity. Shoot tips are – gravitropic, root tips are + gravitropic. In the picture on the lower right, no matter how you orient the seed when planting it, the roots will always grow downward.

5. Studies on phototropism
towards or away from light
differential growth of cells on opposite sides of a shoot or coleoptile
cells on darker side elongate faster than those on the light side
experiments by Darwin and son and FW Went
due to auxin distributions
Phototropism has been much studied since the 1800’s.
How do plants curve towards the light? This involves a difference in growth of the cells on both sides of a plant shoot. (A coleoptile is a special name for the covering of a grass shoot). The cells on the side of the plant that faces away from the source of light should elongate more that the cells on the side of the plant that faces the light source. This causes the plant to appear to “bend” towards a given source.
Experiments by Charles and Francis Darwin as well as FW Went showed that this happens because of the distribution of a plant hormone called auxin.

6. The Darwins discovered that the substance that caused the bending was produced by the plant’s shoot tip even if the bending itself occurred on the shoot, a short distance away from the tip.
Went discovered that the substance that caused the bending was a chemical called “auxin,” (from the Gr. word “to grow/increase”). This auxin was produced by the shoot tip of plants in response to light. When a shoot tip was cut and placed on a porous material, like a block of agar, auxin would diffuse into the agar block.
Went wanted to demonstrate that it was the concentration of auxin that caused the difference in growth so he took the agar block with the auxin and placed it on various positions on top of shoots that had their tips cut off.
In the drawing above, the first two shoots were the – controls. They showed no growth differences. The third had the auxin-agar block placed evenly on top. It too showed no growth difference. This is because the auxin was distributed evenly on both sides of the plant shoot. The fourth and fifth shoots had the auxin-agar block placed a little bit off-center, the fourth had it placed on the right, the last had it placed on the left side. This caused the shoots to curve as auxin caused the cells on the side it was on to elongate more.
Went experiment

7. There are five major classes of plant hormones (and many more minor classes). This table summarizes their origins, locations, and major functions. All of these hormones work together to ensure proper growth and development and proper response to stimuli.
Familiarize yourself with this table.