Gas Exchange Responses to Variable Sunlight in Two Sonoran Desert Herbs: Comparison with Subalpine Species

Responses in net photosynthesis (A) and stomatal conductance to water vapor (g) in two Sonoran desert herbaceous perennials exposed to atypical conditions of variable sunlight were compared to previously documented responses in subalpine herbs that grow in an environment of extreme sunlight variability. In the desert herb Penstemon parryi, little variation in g (< 80 mmol m-2 s-1) or xylem pressure potential (ψ) was observed during periods of sunlight variability, although A varied substantially. This response was consistent with those subalpine plants that also maintained relatively constant ψ during sun/shade periods. ψ differed significantly between sun and shade periods in the desert herb Ambrosia ambrosioides (-1.65 vs. -1.36 MPa, respectively), and reductions in g during shade were as rapid (24.5 mmol m-2 s-1min-1) as observed in subalpine species with similarly variable ψ. However, recovery in g during sun periods occurred at a rate well below that measured in subalpine herbs. Thus, alterations in g did not closely correspond to variations in A and sunlight in this desert herb. This response pattern was unlike any measured in 22 subalpine species previously studied. Rapid recovery in g during sun periods that follow shade has likely been selected for in subalpine herbs that are exposed daily to sunlight variability. Such a response can substantially increase carbon gain. We suggest that the benefits of a strong tracking response in g to variations in A and sunlight are not realized in desert environments with relatively constant solar regimes. However, rapid stomatal closure during shade periods does insure higher water use efficiency and the potential for recovery in ψ in this desert herb.