An Analysis of the Temperature Dependence of Photosynthesis Considering the Kinetics of RuP₂ Carboxylase and the Pool of RuP₂ in Intact Leaves

Abstract

Sunflower and cotton plants were grown in growth boxes at 460 µmol m^{-2 s-1}. The mesophyll conductance in N₂, the assimilatory charge (post-illumination CO₂ uptake) and the CO₂ capacity (the solubility of CO₂ + HCO₃^- + CO₃^2-) were measured at different temperatures. The mesophyll conductance had its maximum at 29-30°C in sunflower leaves and rapidly declined at higher and lower temperatures. In cotton, the maximum occurred at a somewhat higher temperature. The assimilatory charge changed in parallel with the mesophyll conductance. When the assimilatory charge was measured after a short exposure to CO₂-free N₂, it remained constant at lower temperatures and declined only at superoptimal temperatures. As the assimilatory charge reflects the RuP₂ pool in the leaf, the temperature dependence of the mesophyll conductance at a constant assimilatory charge reveals the actual activation energy of the CO₂ binding reaction of carboxylase (together with the CO₂ transport conductance in the liquid phase of mesophyll cells) which was 29 kJ mol^-¹ for both species.

At superoptimal temperatures, the primary cause for the reversible decrease of photosynthesis was a decrease in the assimilatory charge (RuP₂ pool). The decrease cannot be caused by an inadequate rate of RuP₂ resynthesis but is, presumably, the result of either too rapid drainage of triosephosphates to the cytosol from chloroplasts or de-energisation of thylakoids at high temperatures.