C3 photosynthesis in the desert plant Rhazya stricta is fully functional at high temperatures and light intensities

The C-3 plant Rhazya stricta is native to arid desert environment zones, where it experiences daily extremes of heat, light intensity (PAR) and high vapour pressure deficit (VPD). We measured the photosynthetic parameters in R. stricta in its native environment to assess the mechanisms that permit it to survive in these extreme conditions. Infrared gas exchange analysis examined diel changes in assimilation (A), stomatal conductance (g(s)) and transpiration (E) on mature leaves of R. stricta. A/c(i) analysis was used to determine the effect of temperature on carboxylation capacity (V-c,V-max) and the light- and CO2-saturated rate of photosynthesis (A(max)). Combined chlorophyll fluorescence and gas exchange light response curve analysis at ambient and low oxygen showed that both carboxylation and oxygenation of Rubisco acted as the major sinks for the end products of electron transport. Physiological analysis in conjunction with gene expression analysis suggested that there are two isoforms of Rubisco activase which may provide an explanation for the ability of R. stricta to maintain Rubisco function at high temperatures. The potential to exploit this ability to cope with extreme temperatures is discussed in the context of future crop improvement.