PHOTOSYNTHETIC NUTRIENT USE EFFICIENCY, NODULE ACTIVITY AND SOLUTE ACCUMULATION IN DROUGHT-STRESSED ALFALFA PLANTS

Effect of temporary drought on the photosynthetic characteristics, the photosynthetic nutrient-use efficiency (PNUE, PPUE), the water-use efficiency (WUE), acetylene reduction nodule activity (ARA) and the solute accumulation has been examined in nodulated alfalfa plants. The plants were subjected to moderate or severe cycles of drought (drought-recovery). Growth decreased markedly under the water deficit, but no significant differences were found between either level of drought. As stress progressed, the plants developed higher root/shoot ratios than controls. ARA declined progressively during water deficit treatments and upon rewatering no recovery was observed. PNUE and PPUE markedly decreased, whereas WUE increased in drought treated plants. Proline and inorganic cations (K, Ca, Mg) concentrations were always higher in drought-stressed plants, but sugar (TSS) content increased only in roots and nodules. Hence, under cyclic drought nodulated alfalfa plants adjusted osmotically and achieved an improved WUE, which suggested an enhanced drought tolerance.