Interactive effects of drought and shade on three arid zone chenopod shrubs with contrasting distributions in relation to tree canopies

1. Plants that grow beneath trees in arid systems may frequently experience both water and light limitation, although protection from high radiation loads during drought may compensate for a loss of productivity due to reduced light availability when water is plentiful. 2. We examined the effects of shading, during an imposed water deficit, on the carbon gain, stomatal conductance (g(s)) and shoot water potential (Psi(s)) of seedlings of three shrubs: Atriplex vesicaria (Heward ex Benth.), a C-4 species, and Enchylaena tomentosa (R. Br.) and Rhagodia spinescens (R. Br.), which are restricted to shaded sites beneath trees. 3. Under conditions of limiting water, photosynthetic rates measured at saturating light (A(max)) were negative in high-light grown Enchylaena plants but remained positive in shade-grown plants. When water was not limiting, A(max) was reduced in shade-grown Atriplex but shade did not affect carbon gain in the other two species. 4. AtriplexPsi(s) was higher in shaded than in unshaded plants, but in unshaded plants positive carbon gain was maintained at Psi(s) below -10 MPa. Stomatal conductance and A(max) decreased more slowly with increasing water deficit in shaded conditions in all species. 5. Atriplex was tolerant of a broader range of light and soil moisture conditions than Enchylaena, with Rhagodia intermediate between these two species. The interactive effect between drought and shade and the ecophysiological tolerances of these three species have consequences for their field distributions.