TRANSPIRATION EFFICIENCY OF 3 MEDITERRANEAN ANNUAL PASTURE SPECIES AND WHEAT

Attempts to improve water use efficiency in regions with Mediterranean climates generally focus on increasing plant transpiration relative t o evaporation from the soil and increasing transpiration efficiency. O ur aim was to determine if transpiration efficiency differs among key species occurring in annual pastures in southern Australia. Two glassh ouse experiments were conducted with three key pasture species, subter ranean clover (Trifolium subterraneum L.), capeweed [Arctotheca calend ula (L.) Levyns] and annual ryegrass (Lolium rigidum Gaudin), and whea t (Triticum aestivum L.). Transpiration efficiency was assessed at the levels of whole-plant biomass and water use (W), leaf gas exchange me asurements of the ratio of CO? assimilation to leaf conductance to wat er vapour (A/g), and carbon isotope discrimination (Delta) in leaf tis sue. In addition, Delta was measured on shoots of the three pasture sp ecies growing together in the field. In the glasshouse studies, annual ryegrass had a consistently higher transpiration efficiency than subt erranean clover or capeweed by all methods of measurement. Subterranea n clover and capeweed had similar transpiration efficiencies by all th ree methods of measurement. Wheat had W values similar to ryegrass but Alg and Delta values similar to subterranean clover or capeweed. The high W of annual ryegrass seems to be related to a conservative leaf g as exchange behaviour, with lower assimilation and conductance but hig her.A/g than for the other species. In contrast to the glasshouse resu lts, the three pasture species had similar Delta values when growing t ogether in mixed-species swards in the field. Reasons for these differ ing responses between glasshouse and field-grown plants are discussed in terms of the implications for improving the transpiration efficienc y of mixed-species annual pasture communities in the field.