Effect of elevated temperature and water availability on CO2 exchange and nitrogen fixation of nodulated alfalfa plants

The aim of this study was to investigate the effects of predicted temperature increases and drought conditions of Mediterranean environments on N2-fixing alfalfa plants. One-month-old plants inoculated with Sinorhizobium meliloti strain 102F78 were grown in growth chambers under different temperature (25/15 or 28/18 °C, day/night) and water availability (control or drought) regimes. Elevated temperature and drought reduced plant dry mass and leaf area, especially when both stresses were combined. The inhibitory effect of elevated temperature on plant growth was a consequence of decreased CO2 and N2 fixation rates. A photosynthetic decrease resulted from the inhibition of rubisco activity, probably associated with a lower activation state. An absence of differences in photosynthesis in relation to water availability suggests that drought decreased plant growth due to its negative effect on leaf area. Rising temperature and drought affected the nitrogen content negatively, although effects differed. Elevated temperature inhibited nodule activity drastically, whereas the inhibitory effect resulting from drought centred on nodule dry mass (DM) production. Plants exposed to a combination of elevated temperature and drought were the most negatively affected.