Photosynthetic carbon uptake by tropical forests is of critical importance in regulating the earth's climate, but rising temperatures threaten this stabilizing influence of tropical forests. Most research on how temperature affects photosynthesis focuses on fully sun-exposed leaves, and little is known about shade leaves, even though shade leaves greatly outnumber sun leaves in lowland tropical forests. We measured temperature responses of light-saturated photosynthesis, stomatal conductance, and the biochemical parameters VCMax (maximum rate of RuBP carboxylation) and JMax (maximum rate of RuBP regeneration, or electron transport) on sun and shade leaves of mature tropical trees of three species in Panama. As expected, biochemical capacities and stomatal conductance were much lower in shade than in sun leaves, leading to lower net photosynthesis rates. However, the key temperature response traits of these parameters-the optimum temperature (TOpt) and the activation energy-did not differ systematically between sun and shade leaves. Consistency in the JMax to VCMax ratio further suggested that shade leaves are not acclimated to lower temperatures. For both sun and shade leaves, stomatal conductance had the lowest temperature optimum (~25 °C), followed by net photosynthesis (~30 °C), JMax (~34 °C) and VCMax (~38 °C). Stomatal conductance of sun leaves decreased more strongly with increasing vapor pressure deficit than that of shade leaves. Consistent with this, modeled stomatal limitation of photosynthesis increased with increasing temperature in sun but not shade leaves. Collectively, these results suggest that modeling photosynthetic carbon uptake in multi-layered canopies does not require independent parameterization of the temperature responses of the biochemical controls over photosynthesis of sun and shade leaves. Nonetheless, to improve the representation of the shade fraction of carbon uptake dynamics in tropical forests, better understanding of stomatal sensitivity of shade leaves to temperature and vapor pressure deficit will be required.

中文翻译：

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三种热带树种的日光和阴凉叶光合作用参数的温度依赖性相似。

热带森林吸收光合作用的碳对于调节地球的气候至关重要，但是温度升高威胁着热带森林的这种稳定影响。关于温度如何影响光合作用的大多数研究都集中在完全暴露在阳光下的叶子上，而对遮荫的叶子知之甚少，即使在低地热带森林中遮荫的叶子大大超过了太阳的叶子。我们测量了三种成熟热带树木的阳光和阴凉处的光饱和光合作用，气孔导度以及生化参数VCMax（RuBP羧化的最大速率）和JMax（RuBP再生的最大速率或电子传递的最大速率）的温度响应。在巴拿马。不出所料，阴凉处的生化能力和气孔导度比太阳叶低得多，导致净光合作用率降低。但是，这些参数的关键温度响应特性-最佳温度（TOpt）和活化能-在太阳和阴暗叶子之间没有系统地不同。JMax与VCMax比率的一致性进一步表明，遮阴叶无法适应较低的温度。对于日光和阴暗叶，气孔导度具有最低的最佳温度（〜25°C），其次是净光合作用（〜30°C），JMax（〜34°C）和VCMax（〜38°C）。与阴叶相比，随着蒸气压亏缺的增加，太阳叶的气孔导度下降的幅度更大。与此相一致的是，模拟的光合作用的气孔限制随着太阳温度的升高而增加，但不包括遮荫的叶子。总的来说，这些结果表明，对多层冠层的光合碳吸收进行建模不需要对日光和阴叶的光合作用的生化控制的温度响应进行独立的参数化。然而，为了改善热带森林中碳吸收动态的阴影部分的表示，将需要更好地了解阴影叶片对温度和蒸气压赤字的气孔敏感性。