Photosynthetic characteristics of Sonoran Desert winter annuals.

Photosynthesis in Sonoran Desert winter annuals appeared to be similar to those observed in other C3 photosynthetic pathway herbs, although photosynthetic capacities ranged from 18 to 65 μmol CO1 m(-2) s(-1) under natural conditions. The higher photosynthetic capacities were associated with high leaf conductances to water vapor (up to 39 mm s(-1)). Leaf Kjeldahl nitrogen contents were high, ranging up to 44.9 mg g(-1). We suggest that the high photosynthetic capacities in several species may be related to resource availability and enable successful exploitation of the short, unpredictable growth periods experienced by these annuals. Although photosynthetic rates in desert winter annuals spanned a wide range, the relationship between leaf conductance and maximum photosynthesis appeared simiar to that of other C3 vascular plants. It is possible that the resulting constant intercellular, CO2 concentrations were related to minimizing excessive water loss, while not severely imposing limitations to photosynthetic gains.