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PLANT HEAL ANT HEALTH CARE Hormonal Too "H Trees Get Understanding plant hormones and their impact on plant growth By Br andon Gallagher Watson Cytokinin ey,what are the five major groups of plant hormones?"How many times have you been at a cocktail party and this comes up? All the time, right? Well, you no longer have to live with the anxiety of friends and loved ones unexpectedly bringing up phytohormonal inquires and not being prepared.A little knowledge of plant hormones and what they do can give you a better understanding how trees grow, and, perhaps even open up your business to new services. Plant growth is affected by many environmental factors such as light, temperature,gravity,nutrients and,of course,water.Growth is also affect- ed by internal factors such as genetics, vitamins, enzymes and plant hormones.It is generally agreed that there are five major plant hormone groups, all discovered at different times in history so their names and functions are confusingly named, but they all have a few characteristics in common.One is that hormones are effective in small amounts,and are usually produced in one part of the plant then translocat- ed to another part where they do their thing.All plant hormones are responsible for some certain function within plants;in fact,the word"hormone"is derived from the Greek word for "set in motion,"but the function varies widely from hormone to hormone. Auxin Auxins were the first group of compounds found within plants that were considered hor- mones and were shown to affect cell elongation. Research into the nature of auxins began in the 19th Century with scientists such as Charles Darwin among the first chief investigators. Early experiments with auxins showed they involved phototrophism — the tendency for plants to bend toward the light. It turns out that auxins are light sensitive and work better in the dark.This means the shaded side of a shoot will elongate at a faster rate than the sunny side,causing the plant to appear to be bending toward the light.Auxins directly affect or participate in many different functions in plants,and are also responsible for the reg- ulation of other plant hormones. 12 Arbor Age / July/August 2012 Cytokinins are a group of plant hormones so named because of their effect on cytokinesis — a fancy term for cell division.With more than 200 distinct compounds considered cytokinins,they are found in meristems, roots, seeds,cambium and any other location of rapid growth within a plant. Cytokinins and auxins often work in concert with each other in plant development, the former promoting cell division, and the latter helping make those cells larger.These two also play a role in determining apical dominance in plants.Auxin is produced in the apical bud,which helps regulate the cytokinin produced in the lateral buds.As soon as the apical bud is removed, the cytokinins in the lateral buds are free to do their cell dividing,and we get the sucker growth we see after pruning. Ethylene Ethylene is a unique plant hormone as it is the only one that is a gas. Ethylene helps ripen fruit. Ethylene is primarily known for its role in fruit ripening,and,even if you were not aware of its function as a hormone,most people are familiar with ethylene's effects.Ever hear that one bad apple can spoil the bunch? That's true, as the one bad will release large amounts of ethylene, causing the other apples to ripen too quickly and rot.If you have ever put avocados in a paper bag to get them to ripen faster you are using ethylene.Commercially,many fruits are picked green,shipped long distances,then gassed with ethylene to ripen them to the correct color before going on the shelves.Ethylene has also been shown to be responsible for leaf and fruit abscission,which also has commercial applications. Certain types of fruits are harvested by gassing the tree with ethylene, causing the leaves and fruit to fall easily.The fruits are then harvested by shaking the tree vigorously. Abscisic acid The hormone that used to be thought responsible for abscission was aptly named "abscisic acid."Although it is now known that ethylene is the main player in that function,abscisic acid plays some other interesting roles in plants.Commonly known as "the stress hormone, "abscisic acid www.arborage.com