Limitations of evaluating the growth-differentiation balance hypothesis with only two levels of light and water

Abstract

:The growth-differentiation balance hypothesis provides a framework for predicting patterns of allocation by plants to defenses and growth in different resource environments. This hypothesis is unique in that it provides a set of predictions about how plants should allocate to defenses and growth under differing water regimes, in addition to other resources such as light. As predicted by the GDB hypothesis, concentrations of rutin and chlorogenic acid were higher at the high light level. However, in conflict with the GDB hypothesis, there were no differences between phenolic concentrations in low versus high water concentrations. Furthermore, there was a significant positive correlation between plant mass and phenolic concentration. However, it is impossible to either forcefully reject or defend the GDB hypothesis with this data set, because the GDB hypothesis makes predictions about resource allocation by plants across a gradient, and I employed only two levels of water for the test. A full test of the growth-differentiation balance hypothesis requires many more resource levels. Although there was a significant increase in leaf concentration of rutin, an allometric relationship between leaves and plant support structures leads to a dilution of the phenolics at the whole-plant level. This result indicates that multiple plant parts must be examined for defense products and mass contribution to the whole plant to test patterns of plant allocation as it relates to the GDB hypothesis. The within-plant variation in phenolic and nitrogen concentrations that I found may result in tradeoffs for herbivores faced with choosing between new and mature leaves. Furthermore, the choice for an insect herbivore between new and mature leaves in a population of plants may depend on the light environment of the plants.