The decrease in the rate of photosynthesis may be due to the chronic water stress effect of metabolic inhibition, or the down-regulation of photosynthesis as described by Chaves et al. (2003) and Cornic (2000). Distinguishing between these alternatives is difficult (Flexas et al., 2004). Acclimation to water stress requires responses that enable essential reactions of primary metabolism to continue for the plant to tolerate water deficit (Foyer et al., 1998).