Niinemets Ü, Kull O, Tenhunen JD
Int J Plant Sci. 1999 Sep;160(5):837-848. doi: 10.1086/314180.
Morphology, chemical composition, and photosynthetic capacity of leaf laminas were investigated in Populus tremula L. and Tilia cordata Mill. along a canopy light gradient. Variables determining the thickness of boundary layer for heat and water exchange at a given wind speed-effective leaf width (Ww) and length (Wd)-scaled positively with daily integrated quantum flux density averaged over the season (Qint, mol m-2 d-1) in T. cordata, but Wd decreased and Ww was constant with increasing Qint in P. tremula, bringing about a moderately improved capacity for convective cooling at greater irradiances in the latter species. Foliar stable carbon isotope discrimination (Delta) decreased with increasing Qint, demonstrating that, possibly because of more severe foliar water stress, leaves operated at a lower intercellular CO2 concentration in the upper canopy. Further analysis of foliar characteristics provided additional evidence of the interaction between water stress and Qint. Leaf dry matter content and both components of lamina dry mass per area (MA)-lamina thickness and density (dry mass per unit volume, rhoB)-increased with increasing Qint in both species. The rhoB and lamina dry matter content were also positively related to lamina carbon concentration, variability in which along the canopy was related to changes in carbon-rich lignin concentration. Since both increases in lamina density and lignin concentration improve leaf tolerance of low-water potentials, these foliar modifications were interpreted as indicative of acclimation to enhanced water limitations in high light. For the whole material, foliar nitrogen concentrations decreased with increasing rhoB, suggesting that an improvement of foliar mechanical strength may result in declining foliar assimilative potential. However, foliar photosynthetic electron transport capacity per unit area increased with increasing rhoB, possibly because increases in rhoB with light are not only attributable to greater cell wall lignification but also to denser packing of leaf cells, in particular, in fractional increases in palisade tissues with Qint. Because of a positive scaling of leaf thickness and density with total tree height, MA was greater in taller trees of T. cordata, foliage of which also had lower Delta and was likely to function with less open stomata. In summary, we conclude that leaf water stress, which scales with both Qint and total tree height, is a major factor altering foliage structure and assimilative capacity.
沿着树冠光照梯度，对欧洲山杨（Populus tremula L.）和心叶椴（Tilia cordata Mill.）叶片的形态、化学成分及光合能力进行了研究。在给定风速下，决定热量和水分交换边界层厚度的变量——有效叶宽（Ww）和长度（Wd），在心叶椴中与季节平均日积分量子通量密度（Qint，mol m-2 d-1）呈正相关，但在欧洲山杨中，随着Qint增加，Wd减小，Ww保持不变，这使得后者在更高辐照度下对流冷却能力适度提高。叶片稳定碳同位素判别值（Delta）随Qint增加而降低，这表明，可能由于叶片水分胁迫更严重，树冠上部叶片在较低的胞间CO2浓度下运行。对叶片特征的进一步分析提供了水分胁迫与Qint之间相互作用的额外证据。两种植物的叶片干物质含量以及单位面积叶片干质量（MA）的两个组成部分——叶片厚度和密度（单位体积干质量，rhoB）均随Qint增加而增加。rhoB和叶片干物质含量也与叶片碳浓度呈正相关，其沿树冠的变化与富含碳的木质素浓度变化有关。由于叶片密度和木质素浓度的增加均提高了叶片对低水势的耐受性，这些叶片变化被解释为适应高光下增强的水分限制的表现。对于整个材料，叶片氮浓度随rhoB增加而降低，这表明叶片机械强度的提高可能导致叶片同化潜力下降。然而，单位面积叶片光合电子传递能力随rhoB增加而增加，这可能是因为rhoB随光照增加不仅归因于细胞壁木质化程度提高，还归因于叶细胞更紧密的排列，特别是栅栏组织随Qint呈分数增加。由于叶片厚度和密度与树高呈正相关，心叶椴较高树木的MA更大，其叶片的Delta也较低，且可能以较少开放的气孔发挥功能。总之，我们得出结论，与Qint和树高均相关的叶片水分胁迫是改变叶片结构和同化能力的主要因素。
