INTERACTIONS BETWEEN DROUGHT AND ELEVATED CO2 ON ALFALFA PLANTS

Alfalfa (Medicago sativa L.) plants were grown in open top chambers at ambient (340 ppm) and high (600 ppm) CO2 concentrations. Twenty-five days after the first cutting one set of both plants was subjected to w ater deficit conditions by withholding water for 5 days. A chamber eff ect on proteolytic activity, monogalactosyl diacylglycerol to digalact osyl diacylglycerol molar ratio, total non-structural carbohydrates an d soluble protein contents occurred. In contrast, no change in leaf wa ter potential was observed between plants grown outdoors and inside th e chambers. Plants grown at high CO2 concentration showed a lower decr ease in leaf water potential in comparison with plants grown at atmosp heric CO2 when subjected to water stress. Under high CO2 concentration leaf nitrogen content decreased whereas starch accumulation and a hig her proteolytic activity were recorded. Following water depletion, CO2 -enriched plants showed a decrease in total non-structural carbhydrate s and soluble proteins. In thylakoid membranes high CO2 caused an incr ease in chlorophyll and lipid contents and a degradation of monogalact osyl diacylglycerol. A higher degree of unsaturation in the main thyla koid lipids was also observed. CO2-enriched plants were less affected by water stress as shown by reduced chlorophyll degradation and a high er membrane stability.