Lecture Notes for Monday, October 19; BTNY 1210, Fall 1998

New Handout today - Plant Structure Lab Exercise for this week.

Outline

SECONDRY PLANT GROWTH (continued)

CONTROL OF PLANT DEVELOPMENT

SECONDARY PLANT GROWTH (continued)

READ and STUDY:

External features of twigs (page 657). Where do new leaves come from?

Commercial cork (pages 658-9). Where does it come from?

Knots in wood (page 668). What do they represent? What's the difference between a tight knot and a loose knot?

Climate is recorded in the width of annual rings (pages 664-5). Explain. Examine Figure 27-28 an old bristle cone pine tree and a section of the very narrow rings in its wood. Be reminded that plants have to continue growing throughout their lives, even if it is only a tiny amount of growth as in these trees.

CONTROL OF PLANT DEVELOPMENT

Plants cannot move as animals do, but they can respond to their environment through growth and development. This is another fundamental difference between plants and animals (see LS #2). Examples of plant growth responses which substitute for animal movement include: flowers (reproduction); seeds (survival and dispersal); fruits (aid seed dispersal); phototropism (shoots grow toward the light) and gravitropism (shoots grow up and roots grow down).

These growth and developmental processes are controlled largely by hormones; we understand some and not others. The textbook lists several types of plant hormones. Realize that many hormone responses are controlled by a balance between the concentrations of at least two hormones -- one hormone often stimulates a particular process while a second hormone often inhibits the same process. Not unlike the gas and brake pedals in a car, a two-component control system like this allows for more sensitive control of the process than simply having one stimulating hormone (e.g. you could drive a car with only a gas pedal, but it wouldn't be very safe).

PHOTOTROPISM is defined as the growth of a shoot toward the light. The hormone involved is auxin (also called IAA which stands for its chemical name). I use phototropism as a well-studied example of a hormone-mediated response in plants. Thoroughly understand phototropism (pages 675-676; 702-704), its significance to plants (why is this a good thing for plants to do?), the role of auxin in phototropism and Figures 28-1, 29-1 and 29-2. The coleoptile is a sheath of tissue that surrounds and protects the embryonic shoot in grasses until it above ground; it is especially sensitive to the direction from which light is coming and therefore is a good experimental tissue to use in the study of phototropism. Auxin is produced in stem tips (including the tip of the coleoptile) and always moves down the stem. Examine these three figures, read the legends and understand what is being demonstrated about the action of auxin in phototropism.

Figure 28-1 (page 675) illustrates the first known experiments on phototropism carried out on the coleoptile of oat seedlings by Charles Darwin and his son Francis more than 100 years ago. They were trying to figure out what part of the coleoptile "sees" the light. In (a) the control coleoptile is shown undergoing the events described as phototropism - the coleoptile on this oat seedling is bending so as to grow toward the source of the light coming from the right side. Notice that bending occurs below the tip, not right at the tip. In (c) they covered up the part of the coleoptile that exhibited the bending, but bending still occurred. In (b) the tip is covered up and this treatment stopped the bending response - the tip must be "seeing" the light. They concluded that "some influence is transmitted from the upper part to the lower part, causing the latter to bend."

Figure 29-1 (page 702) illustrates some of the experiments done by Fritz Went in 1926 who showed that the "influence" is a chemical. He named it auxin but it took a number of years before someone identified the chemical structure of auxin. The experiment shows that auxin from the coleoptile tip will diffuse into agar as shown by the blue color. When one of these blocks is placed unevenly on a cut coleoptile stump, one side gets more auxin (as shown by the blue color). More auxin on one side of the stem results causes the uneven growth which results in bending in the direction shown. What would happen if the block had been placed on the other side of the cut stump? These and other experiments show that auxin stimulates cell elongation and that a higher concentration of auxin (and thus more cell elongation) on the shady side of the stem is responsible for the bending.

Figure 29-2 (page 703) illustrates experiments testing the following hypotheses regarding how a higher concentration of auxin develops on the shady side of the stem. 1) Differential synthesis - there is more synthesis of auxin on the shady side of the apical meristem than on the sunny side. 2) Differential destruction - there is equal synthesis of auxin across the apical meristem, but there is more destruction of auxin on the sunny side than on the shady side. 3) Lateral migration - there is equal synthesis of auxin across the apical meristem, but there is lateral migration of auxin from the sunny side to the shady side of the stem. Figs. (a) and (b) show that there is equal synthesis of auxin in the light and dark. Figs. (c) and (d) are controls for (e) and (f); they show that dividing the coleoptile with a thin piece of glass has no effect on auxin synthesis. Fig. (e) shows that if the coleoptile is divided completely, both sides produce the same amount of auxin. Fig. (f) shows that partial division of the coleoptile allows for lateral migration (away from the direction of the light) and an accumulation of auxin on the shady side of the stem. Thus hypothesis 3 is supported and the other two hypotheses are rejected.

The bottom line on phototropism is as follows: under the influence of unilateral light (light coming from the side), auxin, which is produced in the coleoptile tip, migrates to the shady side of the coleoptile tip and then migrates down the coleoptile. Auxin promotes cell elongation. The higher auxin concentration on the shady side promotes more cell elongation and the coleoptile bends (grows) toward the light. The "how?" question (how does auxin get moved across the stem) is not yet answered. The "why?" question should be obvious - phototropism is an adaptive response on the part of the plant to acquire more light, a necessary resource.

READ and STUDY about these additional examples of hormone-mediated responses in plants.

Auxin and Cuttings (Fig. 28-6). It is possible to make cuttings of many (but not all) plants by placing a cut stem with a few leaves into moist soil and waiting for adventitious roots to grow from the stem. Auxin stimulates the development of root growth under these conditions. Thus, dusting the stem with an auxin-containing powder (obtained from your local nursery) greatly increases the chances of successfully starting the cutting. Commercial growers make extensive use of this phenomenon.

Apical Dominance (Fig. 29-5) Auxin (produced by the growing terminal bud) inhibits lateral bud development whereas cytokinin (produced by the roots) is the antagonistic hormone which counteracts the effect of auxin by promoting lateral bud development. The benefit to a plant of being able to suppress the development of lateral buds is that all the resources for growth can be channeled to the terminal bud (apical meristem) so the plant can grow tall and (maybe) outgrow its competitors. If too many lateral buds developed, the plant would be very bushy and very short and the leaves would tend to shade each other. You have probably noticed this phenomenon of apical dominance in your yard -- when you prune bushes you promote the growth of lateral buds because you have removed the dominant stem with its supply of auxin. This phenomenon illustrates the developmental flexibility characteristic of plants (see LS #2).

Ethlylene and Ripening (see page 567) Ethylene, a simple 2-carbon gas, is generally involved in promoting aging in plants. Ethylene also stimulates fruit ripening; you should be aware of the commercial applications. For example, ethylene is added to promote ripening and removed to retard ripening.

Plant Movements (more growth responses)

Phototropism has already been discussed. Gravitropism (see Fig. 29-3) is the plant response to gravity - shoots grow up and roots grow down. What would happen if a light were placed underneath the tomato plant shown in Fig. 29-3b. Thigmotropism (see Fig. 29-6) is a plant's response to touch, most easily seen in tendrils which wrap around structures.

Photoperiodism (pages 709-711)

Understand that it is important for plants and animals to be able to predict the changing seasons so as to reproduce (or carry out other activities) at the most appropriate time of year. In the absence of calendar-keeping ability, many plants and animals predict the seasons by measuring the length of the day and night. If the days are getting shorter, a cooler season lies ahead. If the days are getting longer, a warmer season lies ahead.