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Compettive Gravitropic& Phototropic Stimuli in Coleus

Joseph E. Laferriere

Plants exhibit tropic responses to a ene and gibberellic acid as well as to be oriented straight upward even variety of environmental stimuli, the , and starch-containing statoliths when vegetative stalks are at an angle. Downloaded from http://online.ucpress.edu/abt/article-pdf/55/6/370/46297/4449683.pdf by guest on 01 October 2021 most important being light and gravity. which respond to gravity (Kaufman & These results were obtained with a Phototropism and in Song 1987). single incandescent bulb at a distance are both mediated by growth- The common houseplant, Coleus of 30 cm in an otherwise dark room. inducing hormones, primarily auxin blumei, produces rapid and pro- Results will of course vary depending (IAA, indole-3-acetic acid). Auxin stim- nounced reactions to both these stim- on light intensity, light color, and age ulates cell elongation and division by uli, making it an excellent organism and health of the plants. increasing production of mRNA (mes- for demonstrating both responses. Petioles also exhibit a response senger RNA) and causing changes in The species is frequently used in re- among plants in the same horizontal pH and membrane permeability. This search on the subject of hor- plane as the light source, turning so results in increased uptake of water and monal responses (Baum et al. 1991). It that the leaf blades are oriented per- calcium, synthesis of protein, and elas- also has the advantage of being easy to pendicular to the radiation. Plants ticity of the cell wall (Reinecke & grow from stem cuttings. placed above the source do not show Bandurski 1987; Cleland 1987). Bio- To illustrate hormonal effects to in- this rotation, the result being that chemical changes such as increases in troductory students, subject the plant the leaves are lit from below. The net RNA production and calcium accumu- to both stimuli simultaneously to result in each case is favorable to lation can be manifest within a few show which of the two tendencies is the plant, as both the plant lit from the hours (Hagen 1989; Parker & Briggs stronger. This can be done growing a side and the one lit from below have 1990; Gerhring et al. 1990). Bending of plant in a small pot until it is approx- leaves oriented perpendicular to the isolated stem and sections can imately 15 cm tall, then turning it so incident radiation permitting maxi- be noted within 10 minutes, whereas that its stem is oriented horizontally, mum light interception. visible reactions of intact plants can take and illuminating the plant from below For instructional purposes, the ex- a few days. or from the side. The pots should be periment can be set up as a demon- Plants bend toward the light be- loosely wrapped in plastic so the soil stration or given as a class exercise to cause of different concentrations of does not spill out. Several different be set up one week and the results auxin on sunlit and shaded sides of angles of illumination may be investi- obtained the next. Either way, the the stem (Reinhold 1978; Klambt gated by placing a source of illumina- technique represents a convenient, 1990). The mechanism by which the tion in a dark room and arranging low-cost method for introducing stu- differential concentration arises is un- plants in various positions at a given dents to the effects of plant hormonal clear. The auxin in the stem is made in distance from the light. Plants placed control. the apical meristem and transported in the same horizontal plane with the downward. Some plant physiologists light source may be oriented with Acknowledgments maintain that the auxin migrates from the stem parallel or perpendicular to the sunlit side of the meristem to the the incident radiation. I thank Deborah Ellstrom, Kim shaded side before basipetal transport, Results typically indicate that the Steiner and Robert Mellor for helpful whereas others claim that transport effects of gravitropism predominate suggestions and comments. down the stem is slower on the sunlit over those of phototropism, but not side (Kaufman & Song 1987). Gravit- completely. Plants placed horizontally References ropism, the bending of stems away directly above the light source gener- from gravity and the growth of ally grow upward, indicating a stron- Baum, S.F., Aloni R., & Peterson C.A. toward gravity, is even less under- ger gravitropic response. However, (1991). Role of cytokinin in vessel stood, but apparently involves ethyl- plants placed on the same horizontal regeneration in wounded Coleus in- plane as the light source, with the ternodes. Annals of , 67, 543- stem initially pointing straight at the 548. Joseph E. Laferriere, Ph.D., is a research light, grow at approximately a 30.5 Cleland, R.E. (1987). Auxin and cell associate with Arnold Arboretum, Har- degree angle from the vertical. Hence elongation. In P.J. Davies (Ed.), vard University, 22 Divinity Ave., Cam- the phototropic response is not com- Plant hormones and their role in plant bridge, MA 02138. pletely suppressed even though it is growth and development(pp. 132-148). less important. Flowering stems tend Dordrecht: Martinus Nijhoff.

370 THEAMERICAN TEACHER, VOLUME 55, NO. 6, SEPTEMBER1993 Gehring, C.A., Williams, D.A., Cody, and development (pp. 375-392). Letham,R.B. Goodwin & T.J.V.Hig- S.H. & Parish, R.W. (1990). Photot- Dordrecht:Martinus Nijhoff. gins (Eds.), Phytohormonesand related ropism and geotropism in maize co- Klambt, D. (1990). A view about the compounds:A comprehensivetreatise, leoptiles are spatiallycorrelated with function of auxin-bindingproteins at Vol. II (pp. 251-289). Amsterdam: increases in cytosolic free calcium. plasma membranes. Plant Molecular Elsevier/North Holland Biomedical Nature,345, 528-530. Biology,14, 1045-1050. Press. Hagen, G. (1989). Molecular ap- Parker, K.E. & Briggs, W.R. (1990). Reinecke, D.M. & Bandurski, R.S. proaches to understand auxin ac- Transport of indole-3-acetic acid tion. New Biologist,1, 19-23. during gravitropismin intact maize (1987). Auxin biosysthesis and me- Kaufman, P.B. & Song I. (1987). Hor- . , 94, tabolism. In P.J. Davies (Ed.), Plant mones and the orientation of 1763-1769. hormonesand theirrole in plantgrowth growth. In P.J. Davies (Ed.), Plant Reinhold, L. (1978). Phytohormones and development (pp. 24-42). hormonesand theirrole in plantgrowth and the orientationof growth.In D.S. Dordrecht:Martinus Nijhoff. Downloaded from http://online.ucpress.edu/abt/article-pdf/55/6/370/46297/4449683.pdf by guest on 01 October 2021

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