Resumen: Drought is recognised as the major environmental factor that constrains productivity and stability of plants. Crop yield under future climatic conditions has increased the interest in ¿water stress physiology¿. Plant development under limited water availability together with increasing atmospheric CO2 concentration is of primary interest to ensure crop production under the projected climate scenarios. The expected reduction in precipitation and rising evapotranspiration rates will limit plant growth either by restricting stomatal conductance and photosynthesis or by restricting leaf expansion. Furthermore, alfalfa is a legume that establishes a symbiotic relationship with N2-fixing bacteria and hence drought may indirectly compromise plant production via alterations in nodule performance. The effects of water stress on nodules include not only reduction in nodule mass but decreases in nodule functioning. Furthermore, previous studies have confirmed that the performance of nodules is conditioned by their active interaction with other organs like leaves and roots. After long-term exposure to elevated CO2, photosynthetic downregulation may limit leaf N demand and hence, nodule activity. Moreover, as observed for leaves, nodule responses to water deficit may be altered by the way that drought limitation is imposed. When water shortage is imposed by controlling irrigation levels, plants acclimatise their water status and growth and therefore nodule activity is usually unaffected. In contrast, after progressive drought treatment by withholding water, nodules show significant decreases in nitrogenase activity.
