Coordinated stomatal, mesophyll, and biochemical functions in photosynthetic responses to heat and dryness

Abstract

The intrinsic link between temperature and leaf-to-air vapor pressure difference (Δ e ) complicates isolation of their individual effects on photosynthesis. Consequently, how CO 2 diffusion changes under heat and high evaporative demand, particularly through mesophyll conductance ( g m ) responses, remains poorly understood. The conditions under which biochemical colimitation occurs, meaning Rubisco carboxylation and RuBP regeneration capacities match, are also unclear. To advance understanding of plant responses to climate change, we separated temperature and Δ e effects by holding Δ e at 1 and 2 kPa while varying leaf temperature ( T leaf ) from 20 to 40 °C across five CO 2 levels (150 to 800 μmol mol −1 ) in cotton, sunflower, and dwarf bean. Gas exchange and chlorophyll fluorescence measurements showed that g m responses partly counteract increases in stomatal conductance to CO 2 ( g sc ) at high temperatures and declines in g sc at elevated Δ e . Coordination between g sc and g m buffers effects of heat and dryness on CO 2 diffusion and stabilizes chloroplast-to-ambient CO 2 ratio ( C c / C a ) across measured T leaf and Δ e ranges. C c / C a is more conservative with increasing T leaf at C a ≤ 400 μmol mol −1 than at elevated C a . Across tested T leaf and Δ e conditions, the transition from Rubisco carboxylation to RuBP regeneration limitation remains near C a of 400 μmol mol −1 , indicating that biochemical colimitation occurs near current atmospheric CO 2 levels. Our findings reveal that plants alleviate diffusional limitations under heat and dryness through coordinated responses of g sc and g m , and maintain biochemical colimitation over broad T leaf and Δ e conditions to efficiently utilize Rubisco carboxylation and RuBP regeneration capacities at near-atmospheric CO 2 levels.