The size and density of stomatal pores limit the maximum rate of leaf carbon gain and water loss (g_max) in land plants. The limits of g_max due to anatomy, and its constraint by the negative correlation of stomatal size and density at broad phylogenetic scales, has been unclear and controversial. The prevailing hypothesis posits that adaptation to higher g_max is typically constrained by geometry and/or an economic need to reduce the allocation of epidermal area to stomata (stomatal-area minimization), and this would require the evolution of greater numbers of smaller stomata. Another view, supported by the data, is that across plant diversity, epidermal area allocated to guard cells versus other cells can be optimized without major trade-offs, and higher g_max would typically be achieved with a higher allocation of epidermal area to stomata (stomatal-area increase). We tested these hypotheses by comparing their predictions for the structure of the covariance of stomatal size and density across species, applying macroevolutionary models and phylogenetic regression to data for 2408 species of angiosperms, gymnosperms, and ferns from forests worldwide. The observed stomatal size-density scaling and covariance supported the stomatal-area increase hypothesis for high g_max. A higher g_max involves construction costs and maintenance costs that should be considered in models assessing optimal stomatal conductance for predictions of water use, photosynthesis, and water-use efficiency as influences on crop productivity or in Earth System models.

中文翻译：

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森林植物气孔大小和密度之间的比例

气孔的大小和密度限制了陆地植物叶片碳增失的最大速率 ( g _max )。由于解剖学引起的g _max的限制，以及它在广泛的系统发育尺度上气孔大小和密度的负相关性的约束，一直不清楚和有争议。普遍的假设假定适应更高的g _max通常受几何形状和/或经济需求的限制，以减少表皮面积分配给气孔（气孔面积最小化），这将需要更多数量的较小气孔的进化。数据支持的另一种观点是，在植物多样性中，分配给保卫细胞与其他细胞的表皮区域可以在没有重大权衡的情况下进行优化，以及更高的g _max通常通过将表皮面积分配给气孔（气孔面积增加）来实现。我们通过比较它们对跨物种气孔大小和密度的协方差结构的预测，将宏观进化模型和系统发育回归应用于来自全球森林的 2408 种被子植物、裸子植物和蕨类植物的数据来检验这些假设。观察到的气孔大小密度标度和协方差支持高g _max的气孔面积增加假设。更高的g _max 涉及建设成本和维护成本，在评估最佳气孔导度以预测用水、光合作用和用水效率作为对作物生产力的影响的模型或地球系统模型中应考虑这些成本。