Ecophysiology of two solar tracking desert winter annuals : III. Gas exchange responses to light, CO2 and VPD in relation to long-term drought.

The gas exchange responses of potted, outdoor and greenhouse grown plants of the Sonoran Desert annuals Lupinus arizonicus (Wats.) and Malvastrum rotundifolium (Gray) were examined. Light saturation of leaf photosynthetic rates did not occur in either species at quantum flux densities exceeding 2.0 mmol m-2 s-1. Decreasing water potentials due to long-term drought did not alter this pattern of light response, though it did lower both photosynthetic rates and leaf conductances. Absolute maximum net photosynthetic rates exceeded 60 μmol m-2 s-1 and 50 μmol m-2 s-1 for M. rotundifolium and L. arizonicus, respectively. Both species showed a two level control of leaf conductance, responding to bulk leaf water potential and vapor pressure deficit. There were non-stomatal effects of drought upon photosynthesis in each species. Leaves of M. rotundifolium exhibited a constant stomatal, inhibition of approximately 19%, while in leaves of L. arizonicus stomatal inhibition ranged from 12-40% with decreasing leaf water potentials. These physiological data lend support to previous reports on the divergent water use patterns of these co-occurring species.