Growth Temperature Alters the Thermal Sensitivity of Mesophyll Conductance in Wild and Cultivated Tomato.

Mesophyll conductance (gm) is a key component of internal CO2 diffusion and can limit photosynthesis in C3 leaves. Temperature responses of gm are commonly treated as intrinsic properties of the leaf, yet it remains unclear whether they acclimate to growth conditions. Here, we tested whether growth temperature alters the thermal sensitivity of gm and whether this differs between wild and cultivated tomato. We compared the wild species Solanum pennellii and the cultivated Solanum lycopersicum cv. M82 grown under four temperature regimes (16°C/10°C to 34°C/28°C, day/night). Using combined gas exchange and chlorophyll fluorescence, we quantified net photosynthesis (An), gm and photorespiration (Rl). Growth temperature strongly altered the thermal sensitivity of gm in both species. In M82, gm Q10 increased from 0.92 to 1.29 across growth temperatures, and in S. pennellii from 1.03 to 1.44. Activation energy showed a similar shift, from values near zero under cold growth to 19.6 and 27.8 kJ mol-1 under warm growth. In contrast, species effects on gm were small, whereas differences in An were larger and associated with variation in biochemical capacity. Thermal optima of gm shifted with growth temperature in both species, while Rl remained consistently temperature sensitive with limited acclimation. The CO2 response of gm were conserved across growth temperatures. These results show that the temperature sensitivity of gm is not fixed, but acclimates to growth temperature, with a stronger effect than species identity. We conclude that gm thermal sensitivity acclimates to growth temperature, but differences in photosynthetic performance are driven mainly by biochemical capacity rather than gm.