Water-use efficiency may influence the distribution of <i>Ipomopsis aggregata</i>, <i>I. tenuituba</i>, and their natural hybrids along an environmental gradient

Abstract

In flowering plants, environment-mediated hybrid fitness plays a large role in hybrid zone dynamics. The relationship of physiological traits to hybrid fitness, however, has not been described for many plant systems. In this study, we investigated the effects of soil moisture on water-use efficiency (WUE), above-ground relative growth rate (RGR), and other leaf traits for Ipomopsis aggregata, I. tenuituba, and their natural hybrids from two contact sites in Gunnison County, Colorado. In nature, hybrids occur in extremely dry, hot conditions, while I. tenuituba and I. aggregata occur in relatively dry and moist conditions, respectively. One contact site (Poverty Gulch), furthermore, appears to be generally drier than the other (Deadman’s Gulch). During the study, all plant types increased WUE as soil moisture decreased, but the effect for hybrids was especially strong. WUE, however, only affected RGR for I. tenuituba, where it dramatically reduced above-ground growth. Differences in site of origin alternately constrained plasticity of leaf traits or became less important in determining leaf traits over the study period. Specific leaf area, for example, was consistently higher for plants from Poverty Gulch than plants from Deadman’s Gulch, but initial site differences in chlorophyll a index disappeared by the end of the study. The fact that contact sites differed in mean temperature and relative humidity, and that site of origin affected several plant traits, suggests that these environmental factors drive at least some fitness trait differences in Ipomopsis parents and hybrids. However, because I. tenuituba naturally occurs in dry conditions, where it has the lowest RGR, WUE cannot entirely explain the distribution of Ipomopsis parents and hybrids, if above-ground biomass controls fitness. It is possible, however, that WUE has positive effects on other fitness traits, such as below-ground biomass.

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