Optimization of photosynthesis and stomatal conductance in the date palm Phoenix dactylifera during acclimation to heat and drought

We studied acclimation of leaf gas exchange to differing seasonal climate and soil water availability in slow-growing date palm (Phoenix dactylifera) seedlings. We used an extended Arrhenius equation to describe instantaneous temperature responses of leaf net photosynthesis (A) and stomatal conductance (G), and derived physiological parameters suitable for characterization of acclimation (T-opt, A(opt) and T-equ). Optimum temperature of A (T-opt) ranged between 20-33 degrees C in winter and 28-45 degrees C in summer. Growth temperature (T-growth) explained c. 50% of the variation in T-opt, which additionally depended on leaf water status at the time of measurement. During water stress, light-saturated rates of A at T-opt (i.e. A(opt)) were reduced to 30-80% of control levels, albeit not limited by CO2 supply per se. Equilibrium temperature (T-equ), around which A/G and substomatal [CO2] are constant, remained tightly coupled with T-opt. Our results suggest that acclimatory shifts in T-opt and A(opt) reflect a balance between maximization of photosynthesis and minimization of the risk of metabolic perturbations caused by imbalances in cellular [CO2]. This novel perspective on acclimation of leaf gas exchange is compatible with optimization theory, and might help to elucidate other acclimation and growth strategies in species adapted to differing climates.