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

Mesophyll conductance ( g m ) is a key component of internal CO 2 diffusion and can limit photosynthesis in C 3 leaves. Temperature responses of g m 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 g m 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 ( A n ), g m and photorespiration ( R l ). Growth temperature strongly altered the thermal sensitivity of g m in both species. In M82, g m Q 10 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 g m were small, whereas differences in A n were larger and associated with variation in biochemical capacity. Thermal optima of g m shifted with growth temperature in both species, while R l remained consistently temperature sensitive with limited acclimation. The CO 2 response of g m were conserved across growth temperatures. These results show that the temperature sensitivity of g m is not fixed, but acclimates to growth temperature, with a stronger effect than species identity. We conclude that g m thermal sensitivity acclimates to growth temperature, but differences in photosynthetic performance are driven mainly by biochemical capacity rather than g m .