Potential growth and drought tolerance of eight desert grasses: lack of a trade-off?

Eight perennial C-4 grasses from the Jornada del Muerto Basin in southern N ew Mexico show fivefold differences in relative growth rates under well-wat ered conditions (RGR(max)). In a controlled environment, we tested the hypo thesis that there is an inverse relationship (trade-off) between RGR(max) a nd the capacity of these species to tolerate drought. We examined both phys iological (gas exchange) and morphological (biomass allocation, leaf proper ties) determinants of growth for these eight species under three steady-sta te drought treatments (none=control, moderate, and severe). When well water ed, the grasses exhibited a large interspecific variation in growth, which was reflected in order-of-magnitude biomass differences after 5 weeks. The species had similar gas-exchange characteristics, but differed in all the m easured allocation and morphological characteristics, namely tiller mass an d number, root:shoot ratio, dry-matter content, and specific leaf area (SLA ). Drought affected tillering, morphology, and allocation, and reduced grow th by 50 and 68% (moderate and severe drought, respectively) compared to th e well-watered controls. With the exception of SLA, none of these variables showed a significant species-by-treatment interaction. We calculated three indices of drought tolerance, defined as the ratio in final biomass betwee n all the possible "dry"/"wet" treatment pairs: severe/moderate, moderate/c ontrol. and severe/control. We found no significant correlation between the se drought tolerance indices, on the one hand, and three indices of growth potential (greenhouse RGR(max), final biomass in the control treatment, and final:initial biomass ratio in controls), on the other. Based on these con trolled-environment results, we hypothesize that the commonly reported corr elation between plant growth potential and drought tolerance in the field m ay in some cases be explained by differential effects of plants on soil-wat er content rather than by differences in species responses to drought.