Diversity in the Rubisco temperature response kinetics and photosynthesis among four linted cotton (Gossypium) species.

There is increasing urgency for more productive and resource-use efficient crops to cope with changing climates. One possibility to improve crop productivity is to enhance photosynthesis. Here, we studied four linted cotton species, (Gossypium hirsutum, G. barbadense, G. arboreum and G. herbaceum), for diversity in the temperature response of Rubisco catalysis and impact on carbon assimilation. Compared with the model C3-plant tobacco, each cotton Rubisco had a slower carboxylation rate (kcatc) and higher CO2-affinity (lower Kc21%O2) in response to temperature, with the specificity for CO2 over O2 (Sc/o) of G. hirsutum Rubisco being thermally advantaged above 20°C relative to tobacco. Consistent with high homology between the cotton species Rubisco large subunits, there was little difference in the response of kcatc or Kc21%O2 to temperature. However, at temperatures above 15°C the carboxylation efficiency (kcatc/ Kc21%O2) of G. herbaceum Rubisco significantly exceeded G. hirsutum Rubisco by 23-29%, with the G. herbaceum enzyme predicted to support 20% higher rates of photosynthesis than tobacco Rubisco at 35°C. Leaf-level photosynthetic measurements at 28°C however showed no significant variation in the net photosynthetic CO2 assimilation rates, stomatal conductance, transpiration rate, intrinsic water-use efficiency, or photosynthetic electron transport rates between each cotton species. Using the 'OptiFitACi' A/Ci model parameterised with cotton Rubisco and mesophyll conductance (gm) values, the maximum Rubisco carboxylase activity (Vcmax) was 20% lower in G. herbaceum leaves compared to G. hirsutum. However, the greater Rubisco kinetics of G. herbaceum did not confer higher leaf photosynthesis.