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Cell wall properties play a major role in determining photosynthetic carbon uptake and water use through their impact on mesophyll conductance (CO2 diffusion from substomatal cavities into photosynthetic mesophyll cells) and leaf hydraulic conductance (water movement from xylem, through leaf tissue, to stomata). Consequently, modification of cell wall (CW) properties might help improve photosynthesis and crop water use efficiency (WUE). We tested this using 2 independent transgenic rice (Oryza sativa) lines overexpressing the rice OsAT10 gene (encoding a “BAHD” CoA acyltransferase), which alters CW hydroxycinnamic acid content (more para-coumaric acid and less ferulic acid). Plants were grown under high and low water levels, and traits related to leaf anatomy, CW composition, gas exchange, hydraulics, plant biomass, and canopy-level water use were measured. Alteration of hydroxycinnamic acid content led to statistically significant decreases in mesophyll CW thickness (−14%) and increased mesophyll conductance (+120%) and photosynthesis (+22%). However, concomitant increases in stomatal conductance negated the increased photosynthesis, resulting in no change in intrinsic WUE (ratio of photosynthesis to stomatal conductance). Leaf hydraulic conductance was also unchanged; however, transgenic plants showed small but statistically significant increases in aboveground biomass (AGB) (+12.5%) and canopy-level WUE (+8.8%; ratio of AGB to water used) and performed better under low water levels than wild-type plants. Our results demonstrate that changes in CW composition, specifically hydroxycinnamic acid content, can increase mesophyll conductance and photosynthesis in C3 cereal crops such as rice. However, attempts to improve photosynthetic WUE will need to enhance mesophyll conductance and photosynthesis while maintaining or decreasing stomatal conductance.

In habitats with low water availability, a fundamental challenge for plants will be to maximize photosynthetic C‐gain while minimizing transpirational water‐loss. This trade‐off between C‐gain and water‐loss can in part be achieved through the coordination of leaf‐level photosynthetic and hydraulic traits. To test the relationship of photosynthetic C‐gain and transpirational water‐loss, we grew, under common growth conditions, 18 C₄grasses adapted to habitats with different mean annual precipitation (MAP) and measured leaf‐level structural and anatomical traits associated with mesophyll conductance (g_m) and leaf hydraulic conductance (K_leaf). The C₄grasses adapted to lower MAP showed greater mesophyll surface area exposed to intercellular air spaces (S_mes) and adaxial stomatal density (SD_ada) which supported greater g_m. These grasses also showed greater leaf thickness and vein‐to‐epidermis distance, which may lead to lower K_leaf. Additionally, grasses with greater g_mand lower K_leafalso showed greater photosynthetic rates (A_net) and leaf‐level water‐use efficiency (WUE). In summary, we identify a suite of leaf‐level traits that appear important for adaptation of C₄grasses to habitats with low MAP and may be useful to identify C₄species showing greater A_netand WUE in drier conditions.