Is photosynthesis limited by decreased Rubisco activity and RuBP content under progressive water stress?

Whether decreases in Rubisco activity and the availability of ribulose-1,5-bisphosphate (RuBP) regeneration are responsible for drought-induced depression of photosynthesis is under debate. Here, leaf water potential and relative water content, gas exchange, chlorophyll fluorescence, initial and total Rubisco activity and RuBP content were determined during the time course of drought development in five C, species: Rhamnus alaternus, Rhamnus ludovici-salvatoris, Nicotiana sylvestris, Phaseolus vulgaris and Vitis vinifera. Water was withheld until photosynthesis approached zero (between 6 and 12 d depending on the species). Relative water content and water potential progressively dropped with drought in Rhamnus and Vitis, but not in the other two species. While RuBP content and Rubisco activity remained constant, declining eventually only in the more severe drought situations, light-saturated stomatal conductance (g) and photosynthesis (AN) decreased progressively during drought in all species. This strongly suggests a dominant role of decreased g, in photosynthesis downregulation during drought in these species, which is supported by increased electron transport to AN ratio. It is concluded that impairment of Rubisco activity and RuBP content do not limit photosynthesis until drought is very severe. Moreover, the relative water content at which these mechanisms are impaired is strongly species-dependent.