A_Test_of_the_Reserve_Meristem_Hypothesi.pdf - American...

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1789 American Journal of Botany 87(12): 1789–1792. 2000. A TEST OF THE RESERVE MERISTEM HYPOTHESIS USING V ERBASCUM THAPSUS (S CROPHULARIACEAE ) 1 C HRISTOPHER J. L ORTIE 2 AND L ONNIE W. A ARSSEN 3 Queen’s University, Department of Biology, Kingston, Ontario, Canada K7L 3N6 The reserve meristem hypothesis predicts that latent meristems may act as a bet-hedging strategy given high-cost, predictable herbivory. Under this hypothesis, damage to a plant should elicit greater branching. This prediction was tested in Verbascum thapsus with three experiments manipulating the intensity and type of damage to reproductive tissue. In the first experiment, seed set was prevented in the treatment group by stigma excision and lanolin application to 80% of the flowers of each plant. In the second experiment, a minimum of two mating pairs of weevils were added to treated plants prior to the onset of flowering. In the third experiment, all fruits were sliced lengthwise twice. All three treatments significantly reduced seed set. In the first two experiments, treated plants significantly increased degree of branching (branch number and total branch length). This supports the reserve meristem hypothesis as an explanation for greater branching in larger plants of V. thapsus. Interestingly, the fruit destruction experiment failed to elicit a branching response, which suggests that the timing of damage is important. Key words: apical dominance; branching; compensation; herbivory; reproductive tissue; reserve meristem; Scrophulariaceae; Verbascum thapsus. Until recently, an ecological context has been lacking to address what is perhaps the most important aspect of growth form in plants, apical dominance. We performed three exper- iments to test the reserve meristem hypothesis for the occur- rence of branching in large inflorescences of Verbascum thap- sus. The reserve meristem hypothesis proposes that apical dominance is favored by selection because dormant meristems are a better fitness investment than active bud growth given predictable damage followed by a period of lower risk (Aars- sen, 1995). Thus, apical dominance through delayed branching may act as a ‘‘bet-hedging’’ strategy or escape in time (Craw- ley, 1987; van der Meijden, 1990; Whitham et al., 1991; Vail, 1992; Tuomi, Nilsson, and Astrom, 1994; Nilsson, Tuomi, and Astrom, 1996). The reserve meristem hypothesis therefore ex- plicitly predicts that damage induces branching through the release of apical dominance. In the present study, the primary focus was to test for a branching response to explain the nat- ural growth pattern of Verbascum thapsus L. Typically, studies of plant responses to damage (compen- sation) focus almost exclusively on ungulates and either use grazing pressure as an independent variable or simulate this damage by removing the shoot apex (i.e., McNaughton, 1979, 1983; Inouye, 1982; Argall and Stewart, 1984; Aarssen and Turkington, 1987 ; Paige and Whitham, 1987). Unfortunately, field experiments rarely varied either the type of damage (re- moval of shoot apex) or the measure used to estimate the plant response (usually biomass). In this study, we extend the com-

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