Department of Forest Sciences, PO Box 27, FI-00014 University of Helsinki, Finland.
Tree Physiol. 2012 May;32(5):510-9. doi: 10.1093/treephys/tps023. Epub 2012 Apr 5.
Leaf properties vary significantly within plant canopies, due to the strong gradient in light availability through the canopy, and the need for plants to use resources efficiently. At high light, photosynthesis is maximized when leaves have a high nitrogen content and water supply, whereas at low light leaves have a lower requirement for both nitrogen and water. Studies of the distribution of leaf nitrogen (N) within canopies have shown that, if water supply is ignored, the optimal distribution is that where N is proportional to light, but that the gradient of N in real canopies is shallower than the optimal distribution. We extend this work by considering the optimal co-allocation of nitrogen and water supply within plant canopies. We developed a simple 'toy' two-leaf canopy model and optimized the distribution of N and hydraulic conductance (K) between the two leaves. We asked whether hydraulic constraints to water supply can explain shallow N gradients in canopies. We found that the optimal N distribution within plant canopies is proportional to the light distribution only if hydraulic conductance, K, is also optimally distributed. The optimal distribution of K is that where K and N are both proportional to incident light, such that optimal K is highest to the upper canopy. If the plant is constrained in its ability to construct higher K to sun-exposed leaves, the optimal N distribution does not follow the gradient in light within canopies, but instead follows a shallower gradient. We therefore hypothesize that measured deviations from the predicted optimal distribution of N could be explained by constraints on the distribution of K within canopies. Further empirical research is required on the extent to which plants can construct optimal K distributions, and whether shallow within-canopy N distributions can be explained by sub-optimal K distributions.
叶片特性在植物冠层内有很大的差异，这是由于冠层内光可用性的强烈梯度以及植物需要有效地利用资源。在高光下，当叶片具有高氮含量和供水时，光合作用最大化，而在低光下，叶片对氮和水的需求都较低。对冠层内叶片氮（N）分布的研究表明，如果忽略供水，最佳分布是 N 与光成正比，但实际冠层中的 N 梯度比最佳分布浅。我们通过考虑植物冠层内氮和供水的最佳共分配来扩展这项工作。我们开发了一个简单的“玩具”双叶冠层模型，并优化了两个叶片之间的 N 和水力传导率（K）的分配。我们询问了供水的水力限制是否可以解释冠层中 N 梯度较浅的原因。我们发现，只有当水力传导率 K 也得到最佳分配时，植物冠层内的 N 最佳分布才与光分布成正比。K 的最佳分布是 K 和 N 都与入射光成正比的分布，因此最优 K 位于冠层上部最高。如果植物在构建更高 K 以暴露于阳光下的叶片的能力受到限制，那么最优 N 分布就不会遵循冠层内的光梯度，而是遵循较浅的梯度。因此，我们假设可以通过对冠层内 K 分布的限制来解释测量到的与最优 N 分布的偏差。需要进一步进行关于植物在多大程度上可以构建最优 K 分布的实证研究，以及是否可以通过次优 K 分布来解释较浅的冠层内 N 分布。
