2Interest ApproachLet’s talk for a moment about yourself. What are some differences between your present body and the one you had as a child?What kinds of chemicals were involved in those changes?These chemicals are called hormones; for example, estrogen, testosterone, adrenaline, etc.
3ContinuedPlants also experience growth changes with the help of hormonesSometimes these changes are more dramatic than what you experienced!Take a look at this plant to the right which fell over the night before. What happened to the plant?
5Student Objectives 1. Describe the work of plant growth regulators 2. Explain the functions of several plant hormones3. List several commercial uses for plant growth regulators
6What Do Plant Growth Regulators Do? Plant growth regulators are chemicals which affect the plant in many complex waysThey can control such activities as cell division and differentiation, root and shoot growth, flowering and ripeningPlant growth regulators which are made by the plant are called plant hormonesThey are moved around the plant in very low concentrations
7Some growth regulators are man-made, or synthetic They can be applied to plants to obtain a wide variety of changesThese changes often make the plant better or more saleable!Which grapes would you probably buy?The ones on the right were treated with gibberellic acid to make them larger.
8What Are the Functions of Several Plant Hormones? Several hormones are made by tissues of the plantEach hormone has many different jobs, thus they are very much like human hormonesThere are five groups of plant regulators: auxins, cytokinins, ethylene, gibberellins and abscisic acid
9AuxinsThis growth hormone is produced by the tip, or apical meristem, of the stemIt causes the stem cells to elongate and divideThey also flow down from the tip of the stem, preventing lateral buds from sprouting until neededThere are three major affects caused by auxins on the plant
10Effects of Auxin on Plants This trunk should be a single stem; however, the terminal bud was removed forcing it to branch.1. Auxin causes the tip of the middle stem to grow at a faster rateThis is known as apical dominanceApical dominance is why many conifers have a pyramid shapeIt can be overcome by cutting off the dominant or terminal stem, losing the source of auxin
112. Auxins are responsible for allowing a plant stem to grow toward the sun This is known as phototropismSunlight slowly breaks down auxin; When the side not exposed to the sun grows faster, the stem bends towards the lightThe left side of the plant was exposed to the sun. The right side had shade and grew faster.
123. Auxins allow a plant to respond to the touch of a person or other object This is known as thigmotropismThe repeated touch of an object causes less auxin to remain on that side of the stemWhen the auxin side starts to grow faster, the plant grows towards the object and ultimately wraps around it
13CytokininsThese are hormones that are mostly responsible for cell division and differentiationThey are produced in the root tips in seedsThey tend to travel up the stemIn tissue culture, cell division or root growth can be encouraged by adjusting hormones in the agar. If given higher levels of auxins in the agar, roots are produced. If given higher levels of cytokinins, shoots multiply.Courtesy of McGraw Hill Publishers
14Ethylene This is a gas that affects the plant like a hormone It is produced in ripening fruit and dying plant materialIt stimulates flowering in some plants and causes other fruits and flowers to ripen more quickly and evenly
15Effects of EthyleneEthylene gas is why fruit will ripen faster in a paper bag, than on the counterThe bag helps to concentrate the gas in a specific areaEthylene has a negative effect on cut flowers & foliagesIt causes them to age more quickly, reducing their useful lifeIn this experiment, two holly twigs were placed under separate jars for a week. At the same time, an apple was placed in the second jar. The ethylene given off by the apple caused the holly to lose its leaves and die.
16GibberellinsThese hormones cause the internode of a stem to elongate and cell division to occurThey are produced in the stems, roots and young leavesGibberellins are commonly used on commercially grown dessert grapes to spread the fruits out and cause them to be biggerFlowering of plants and the breaking off of seed dormancy can also be achieved by adding gibberellins
17Effects of Gibberellins The cabbage plants on the left were grown on their ownThe plants on the right were given gibberellic acid once a week for eight weeksNotice the long stems and flowers at the top of the plants on the rightThey are a result of this hormone
18Abscisic Acid This hormone inhibits (prevents) growth It is found in seeds which are dormant and in dying leavesIt also appears to help a plant prepare its buds for winterThe dormant bud on the left has a high concentration of abscisic acid. As spring nears, the acid level drops and the bud begins to develop a new shoot.
19What are Several Commercial Uses for Plant Growth Regulators? Synthetic growth regulators are very useful for commercial plant cropsThey can save money, time and can lead to a more sellable cropThere are at least three commercial uses of regulators:1. Growth regulators are routinely sprayed on crops such as poinsettias, Easter lilies and mums to reduce size and make a shorter, bushier and more attractive plantProducts such as A-rest, B-nine, Cycocel and Florel are commonly usedPrevents greenhouse plants from getting ‘leggy’
202. Growth regulators are commonly used to help plants root more completely These are often sold as a powder under the names Rootone and Hormodin3. Ethylene gas is used commercially to ripen bananas, tomatoes once they get to market; and to induce flowering in pineapple crops
21Summary What is a plant growth regulator called and what does it do? Where in a plant is auxin produced?Explain how apical dominance works in a plant.How is phototropism different from thigmotropism?
22Summary continued What do cytokinins control? How does ethylene effect plants?What does gibberellin cause in a plant?What does abscisic acid control?Name one way that hormones are used commercially to control plants.