School of Biological Sciences, Molecular Plant Sciences, Washington State University, Pullman, Washington, USA.
J Exp Bot. 2018 Feb 23;69(5):1159-1170. doi: 10.1093/jxb/erx464.
Mesophyll conductance (gm) limits rates of C3 photosynthesis but little is known about its role in C4 photosynthesis. If gm were to limit C4 photosynthesis, it would likely be at low CO2 concentrations (pCO2). However, data on C4-gm across ranges of pCO2 are scarce. We describe the response of C4-gm to short-term variation in pCO2, at three temperatures in Setaria viridis, and at 25 °C in Zea mays. Additionally, we quantified the effect of finite gm calculations of leakiness (ϕ) and the potential limitations to photosynthesis imposed by stomata, mesophyll, and carbonic anhydrase (CA) across pCO2. In both species, gm increased with decreasing pCO2. Including a finite gm resulted in either no change or increased ϕ compared with values calculated with infinite gm depending on whether the observed 13C discrimination was high (Setaria) or low (Zea). Post-transitional regulation of the maximal PEP carboxylation rate and PEP regeneration limitation could influence estimates of gm and ϕ. At pCO2 below ambient, the photosynthetic rate was limited by CO2 availability. In this case, the limitation imposed by the mesophyll was similar or slightly lower than stomata limitation. At very low pCO2, CA further constrained photosynthesis. High gm could increase CO2 assimilation at low pCO2 and improve photosynthetic efficiency under situations when CO2 is limited, such as drought.
叶肉导度（gm）限制了 C3 光合作用的速率，但人们对其在 C4 光合作用中的作用知之甚少。如果 gm 限制了 C4 光合作用，那么它可能在低 CO2 浓度（pCO2）下。然而，关于 C4-gm 在不同 pCO2 范围内的数据却很少。我们描述了在三个温度下，绿色狗尾草（Setaria viridis）和 25°C 下，玉米（Zea mays）中 C4-gm 对 pCO2 短期变化的响应。此外，我们量化了有限 gm 计算泄漏（φ）和气孔、叶肉和碳酸酐酶（CA）对光合作用的潜在限制对 pCO2 的影响。在这两个物种中，gm 随 pCO2 的降低而增加。与无限 gm 计算值相比，包括有限 gm 会导致 φ 不变或增加，这取决于观察到的 13C 歧视是高（绿色狗尾草）还是低（玉米）。PEP 羧化酶最大速率的后转录调节和 PEP 再生限制可能会影响 gm 和 φ 的估计。在低于环境的 pCO2 下，光合作用速率受到 CO2 供应的限制。在这种情况下，叶肉的限制与气孔限制相似或略低。在非常低的 pCO2 下，CA 进一步限制了光合作用。高 gm 可以在低 pCO2 下增加 CO2 同化，并在 CO2 受到限制的情况下（例如干旱）提高光合作用效率。
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