PHOTOSYNTHESIS, WATER RELATIONS, AND LEAF MORPHOLOGY OF XERIC VERSUS MESIC QUERCUS-RUBRA ECOTYPES IN CENTRAL PENNSYLVANIA IN RELATION TO MOISTURE STRESS

Differences in photosynthesis and water relations during drought, and in leaf morphology, were evaluated in 1-year-old greenhouse-grown Quercus rubra L. seedlings from neighboring mesic and xeric sites in central Pennsylvania, U.S.A. Xeric seedlings had higher photosynthetic rates and stomatal conductance to water vapor than mesic seedlings during well-watered conditions. During a drought cycle, xeric seedlings maintained positive net photosynthesis at significantly lower predawn leaf water potential and relative water content. Mesic seedlings exhibited greater intrasource variation in gas exchange during drought, which may be related to reduced selection pressure for drought tolerance characteristics on the mesic site or to gene flow from the adjacent, upslope xeric site. Nonstomatal limitations to photosynthesis increased during drought and accounted for 98% of the reduction in photosynthesis as both sources approached incipient wilting. Xeric seedlings exhibited less nonstomatal inhibition of photosynthesis during early and midstages of drought, suggesting a greater tolerance of the photosynthetic apparatus for tissue water deficit. Xeric seedlings also exhibited the expected leaf morphological characteristics, which included smaller leaf area and greater leaf thickness and specific leaf mass. These results indicate that Q. rubra growing on adjacent sites in central Pennsylvania exhibit ecotypic differences in ecophysiology that are consistent with site moisture conditions.