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 (QYmax), and 13C discrimination of leaf dry mass (ΔDM) was measured to estimate bundle-sheath leakiness (BS-leakiness). Averaged over VPD levels, net CO2 assimilation rate at saturating light (Asat) decreased in S. viridis and S. sudanense but not Z. mays under low irradiance. High VPD decreased Asat of S. viridis but increased Asat of Z. mays. QYmax 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 QYmax and BS-leakiness indicates that these C4 species could maintain photosynthetic efficiency under these unfavourable conditions.
Keywords: Bundle-sheath leakiness; C(4) photosynthesis; Irradiance; Light use efficiency; NADP-ME species; Vapour pressure deficit.
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