Diversity in the Rubisco temperature response kinetics and photosynthesis among four linted cotton 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 (k_cat^c) and higher CO₂ affinity (lower K_C^21%O2) in response to temperature, with the specificity for CO₂ over O₂ (S_c/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 k_cat^c or K_C^21%O2 to temperature. However, at temperatures above 15 °C, the carboxylation efficiency (k_cat^c/K_C^21%O2) of G. herbaceum Rubisco significantly exceeded that of 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/C_i model parameterized with cotton Rubisco and mesophyll conductance (g_m) values, the maximum Rubisco carboxylase activity (V_cmax) was 20% lower in G. herbaceum leaves compared with G. hirsutum. However, the greater Rubisco kinetics of G. herbaceum did not confer higher leaf photosynthesis.