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 water deficit conditions by withholding water for 5 days. A chamber effect on proteolytic activity, monogalactosyl diacylglycerol to digalactosyl diacylglycerol molar ratio, total non-structural carbohydrates and soluble protein contents occurred. In contrast, no change in leaf water potential was observed between plants grown outdoors and inside the chambers. Plants grown at high CO2 concentration showed a lower decrease in leaf water potential in comparison with plants grown at atmospheric CO2 when subjected to water stress. Under high CO2 concentration leaf nitrogen content decreased whereas starch accumulation and a higher proteolytic activity were recorded. Following water depletion, CO2-enriched plants showed a decrease in total non-structural carbhydrates and soluble proteins. In thylakoid membranes high CO2 caused an increase in chlorophyll and lipid contents and a degradation of monogalactosyl diacylglycerol. A higher degree of unsaturation in the main thylakoid lipids was also observed. CO2-enriched plants were less affected by water stress as shown by reduced chlorophyll degradation and a higher membrane stability.