Department of Plant Sciences, University of California, Davis, One Shields Ave, Davis, CA, 95616, USA.
New Phytol. 2021 Jun;230(6):2246-2260. doi: 10.1111/nph.17199. Epub 2021 Mar 8.
Photosynthetic capacity per unit irradiance is greater, and the marginal carbon revenue of water (∂A/∂E) is smaller, in shaded leaves than sunlit leaves, apparently contradicting optimization theory. I tested the hypothesis that these patterns arise from optimal carbon partitioning subject to biophysical constraints on leaf water potential. In a whole plant model with two canopy modules, I adjusted carbon partitioning, nitrogen partitioning and leaf water potential to maximize carbon profit or canopy photosynthesis, and recorded how gas exchange parameters compared between shaded and sunlit modules in the optimum. The model predicted that photosynthetic capacity per unit irradiance should be larger, and ∂A/∂E smaller, in shaded modules compared to sunlit modules. This was attributable partly to radiation-driven differences in evaporative demand, and partly to differences in hydraulic conductance arising from the need to balance marginal returns on stem carbon investment between modules. The model verified, however, that invariance in the marginal carbon revenue of N (∂A/∂N) is in fact optimal. The Cowan-Farquhar optimality solution (invariance of ∂A/∂E) does not apply to spatial variation within a canopy. The resulting variation in carbon-water economy explains differences in capacity per unit irradiance, reconciling optimization theory with observations.
单位辐照度下的光合能力较大，水分的边际碳收益（∂A/∂E）在荫蔽叶片中比在光照叶片中较小，这显然与最优化理论相矛盾。我检验了这样一个假设，即这些模式是由受到叶片水势的生物物理限制的最优碳分配引起的。在一个具有两个冠层模块的整体植物模型中，我调整了碳分配、氮分配和叶片水势，以最大化碳收益或冠层光合作用，并记录了在最优条件下，荫蔽和光照模块之间的气体交换参数如何比较。该模型预测，与光照模块相比，单位辐照度下的光合能力应该更大，∂A/∂E 更小。这部分归因于辐射驱动的蒸散需求差异，部分归因于由于需要在模块之间平衡茎碳投资的边际回报，因此水力传导率的差异。然而，该模型验证了实际上最优的是氮的边际碳收益（∂A/∂N）的不变性。Cowan-Farquhar 最优解（∂A/∂E 的不变性）不适用于冠层内的空间变化。由此产生的碳-水经济的变化解释了单位辐照度下的容量差异，使优化理论与观察结果相一致。
