Photosynthetic gas exchange measurements were combined with measurements of the carbon and oxygen stable isotopic composition of CO2 after it passed over a leaf of Phaseolus vulgaris or Senecio spp. plants held in a controlled environment chamber. Calculations were then made of discrimination by the leaf against 13CO2 and C18O16O. Leaves were maintained at different vapour pressure gradients in order to generate a range of leaf water 18CO/16CO ratios. The 18CO content of leaf water increased when plants were exposed to higher vapour pressure deficits. The observed C18O16O discrimination values also increased with an increase in the leaf-air vapour pressure gradient and the associated change in leaf water 18/CO16CO values. In addition, the observed C18O16O discrimination values were strongly correlated with values predicted by a mechanistic model of isotopic fractionation.

The ability of two solar-tracking desert winter annuals, Lupinus arizonicus Watson and Malvastrum rotundifolium Gray, to acclimate their photosynthetic characteristics to different growth temperatures was examined. Plants were grown in growth chambers under 25/15°C and 40/30°C thermal regimes. The plants grown at 40/30°C were also subjected to a drying cycle in order to assess whether or not drought affected the photosynthetic acclimation to high temperatures. Both species exhibited shifts in their temperature optima and thermal death points for photosynthesis. The droughted plants showed no further acclimation than did the watered plants grown at 40/30°C. Using representative meteorological conditions measured in Death Valley, California, it was predicted that photosynthetic acclimation plays only a small role in enhancing daily carbon-gaining ability for late-season conditions. However, under conditions of drought stress and high temperatures, the acclimation may prevent thermal death and extend plant life.

Summary — Aspects of the water relations of three oak species (Quercus gambelii, Q turbinella and Q macrocarpa) and their hybrids (Q gambelii x turbinella, Q gambelii x macrocarpa) were observed under common garden conditions in northern Utah, USA. In the absence of summer moisture inputs, Q macrocarpa and Q turbinella were unable to maintain active gas exchange through the day; following an early morning peak, leaf conductances to water vapor remained very low through the remainder of the day. In contrast, Q gambelii and the hybrids were able to maintain high leaf conductances throughout this period. Consistent with these observations, Q gambelii is thought to have a root system penetrating to the deeper, winter-recharged layers, a feature apparently absent in both Q macrocarpa or Q turbinella. Based on current hybrid distributions, both Q turbinella and Q macrocarpa once extended into drier more northerly regions than they occupy at present. When these parents retreated, they left behind hybrids with Q gambelii, which do not depend on monsoonal moisture input. Leaf size, leaf longevity, carbon isotope ratio, and minimum winter temperatures appear not to be correlated with the absence of Q macrocarpa and Q turbinella from summer-dry habitats. Instead it appears that reliance on summer monsoon events is one of the critical factors influencing loss of these oaks from summer-dry sites in the intermountain west.