Rubisco activity in Mediterranean species is regulated by the chloroplastic CO2 concentration under water stress

Water stress decreases the availability of the gaseous substrate for ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) by decreasing leaf conductance to CO2. In spite of limiting photosynthetic carbon assimilation, especially in those environments where drought is the predominant factor affecting plant growth and yield, the effects of water deprivation on the mechanisms that control Rubisco activity are unclear. In the present study, 11 Mediterranean species, representing different growth forms, were subject to increasing levels of drought stress, the most severe one followed by rewatering. The results confirmed species-specific patterns in the decrease in the initial activity and activation state of Rubisco as drought stress and leaf dehydration intensified. Nevertheless, all species followed roughly the same trend when Rubisco activity was related to stomatal conductance (g(s)) and chloroplastic CO2 concentration (C-c), suggesting that deactivation of Rubisco sites could be induced by low C-c, as a result of water stress. The threshold level of C-c that triggered Rubisco deactivation was dependent on leaf characteristics and was related to the maximum attained for each species under non-stressing conditions. Those species adapted to low C-c were more capable of maintaining active Rubisco as drought stress intensified.