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

Mesophyll conductance (g_m) is a key component of internal CO₂ diffusion and can limit photosynthesis in C₃ 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 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 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 CO₂ 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.