Effect of Competition on Plant Allometry and Defense

By Stamp, N.; Bradfield, M. et al. | The American Midland Naturalist, January 2004 | Go to article overview

Effect of Competition on Plant Allometry and Defense


Stamp, N., Bradfield, M., Li, S., Alexander, B., The American Midland Naturalist


ABSTRACT.-A series of experiments was conducted to examine the effects of competition on allometry and defense using tomato plants (Lycopersicon esculentum). No mortality occurred over the 4-wk test period, even with a plant density of 30 plants per 10-cm^sup 2^. However, allometric patterns for log canopy diameter vs. log height indicated that competition occurred as early as 8 d after transplant and for pots with as few as two young plants (density of about 2 plants per 10-cm ). There was a change towards less root mass for greater height as competition increased. Leaf concentration of the allelochemicals chlorogenic acid, rutin and tomatine increased with decreasing competition, with the sum of these exhibiting a sigmoid-like pattern for allelochemical concentration. But competition did not affect leaf proteinase inhibitor activity, or petiole glandular trichomes or total trichomes. While the results are compatible with the growth-differentiation balance (GDB) hypothesis, they also indicate some of the difficulties that will be encountered in testing the GDB hypothesis rigorously.

INTRODUCTION

Although there is a negative correlation between plant density and herbivore abundance (Pimentel, 1961; Kareiva, 1983), suggesting that competition among plants can affect defense by plants against herbivores, few studies have examined the effect of competition on plant resistance to herbivores (but see Karban, 1993). The most relevant study showed that cotton seedlings grown at extreme low density (one plant per pot) exhibited high numbers of herbivorous mites per unit of leaf mass, whereas plants grown at higher densities had lower numbers of mites, implying greater defensive chemical production by the latter (Karban et al., 1989).

The degree to which plant defenses change with different levels of competition and whether they do so in a linear or curvilinear pattern remains an open question (Stamp, 2003). The pattern of change is of interest because it is expected to influence herbivore attack.

The growth-differentiation balance (GDB) hypothesis predicts how plants balance allocation between differentiation-related processes and growth-related processes over a range of environmental conditions (Loomis, 1932, 1953). Specifically, it predicts that the pattern of the sum of defenses (e.g., secondary metabolites) should be curvilinear across a resource gradient of soil nutrients, with a peak of defense at intermediate resource levels (Herms and Mattson, 1992).

For plants experiencing competition, i.e., gradients of both soil nutrients and light, the GDB hypothesis would predict a curve of defense concentration that results from a combination of a bell-shaped curve, in response to a soil nutrient gradient, and a positive linear pattern, in response to a light gradient. The combination of a bell-shaped curve and a positive-sloped line would yield a sigmoid-like curve of defense concentration. The exact shape of the curve would depend on the relative effects of the soil nutrients and light.

One of the major problems in ascertaining the pattern of plant defense at different levels of competition (or over a resource gradient) is that not enough of the competitive or resource continuum, in particular the ends of the continuum, is examined (Wilkens, 1997). For example, testing the GDB hypothesis requires some sampling near the endpoints of the continuum. To examine the effects of competition, one plant per experimental unit serves as the extreme high end of resource availability, while a high density of plants close to the point of "self-thinning" serves as the low end of resource availability. "Self-thinning" of a plant population refers to a decrease in plant numbers due to mortality that preferentially strikes plants that have experienced reduced growth as a consequence of competition (Adler, 1996).

In this study, some assumptions and sub-hypotheses of the GDB hypothesis were examined. …

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