EFFECTS OF TEMPORARY DROUGHT ON NITRATE-FED AND NITROGEN-FIXING ALFALFA PLANTS

The effect of temporary drought on growth, carbon exchange and solute accumulation has been examined in alfalfa plants dependent on either Nz or nitrate. Plants were subjected to cyclical moderate or severe drought (drought/recovery). Growth parameters, photosynthetic rate (P-n), leaf conductance to water vapour (g(w)), chlorophyll content, and solute accumulation were determined. Growth decreased markedly under water deficit, but no significant differences between either groups of plants were found. Nitrogen-fixing plants developed higher root/shoot ratios maintaining larger leaves with increased specific leaf area and greater chlorophyll content than nitrate-fed ones. Leaf conductance and net photosynthetic rate declined simultaneously with the drought treatments in both groups of plants; however, N-2 fixing plants retained higher P-n and g(w) values than nitrate-fed plants at lower RWC. Upon rewatering, a considerable stomatal closure remained in nitrate-fed plants. Drought treatment induced an increase in solute concentrations, mainly potassium, especially important in nitrate-fed plants. The interactions between the type of N nutrition and drought tolerance in alfalfa plants during temporary drought are discussed.