Photosynthetic efficiency of three C4 species is maintained under low light and high vapour pressure deficit

The photosynthetic efficiency of C₄ plants could be impaired in environments with low light and high vapour pressure deficit (VPD). However, the interactive effect of low light and high VPD on C₄ photosynthetic efficiency remains unexplored. We grew three C₄ species, Setaria viridis L., Sorghum sudanense (Piper) Stapf., and Zea mays L., under controlled high and low irradiance and VPD conditions in growth chambers. Gas exchange and chlorophyll fluorescence parameters were measured to estimate apparent maximum quantum yield (QY_max), and ¹³C discrimination of leaf dry mass (Δ_DM) was measured to estimate bundle-sheath leakiness (BS-leakiness). Averaged over VPD levels, net CO₂ assimilation rate at saturating light (A_sat) decreased in S. viridis and S. sudanense but not Z. mays under low irradiance. High VPD decreased A_sat of S. viridis but increased A_sat of Z. mays. QY_max was not significantly affected by low light, high VPD or their interaction, which was associated with the unchanged photochemical efficiency of photosystem II. Low light did not significantly influence BS-leakiness, while high VPD only increased BS-leakiness of Z. mays by 0.08 compared with low VPD at low irradiance. The insignificant interaction of low light and high VPD on QY_max and BS-leakiness indicates that these C₄ species could maintain photosynthetic efficiency under these unfavourable conditions.