Valladares F, Pearcy R W
Centro de Ciencias Medioambientales, CSIC, Serrano 115 dpdo. E-28006 Madrid, Spain e-mail:
Division of Biological Sciences, Section of Ecology and Evolution, University of California at Davis, CA 95616, USA, , , , , , US.
Oecologia. 1999 Nov;121(2):171-182. doi: 10.1007/s004420050919.
The influence of leaf orientation and position within shoots on individual leaf light environments, carbon gain, and susceptibility to photoinhibition was studied in the California chaparral shrub Heteromeles arbutifolia with measurements of gas exchange and chlorophyll fluorescence, and by application of a three-dimensional canopy architecture model. Simulations of light absorption and photosynthesis revealed a complex pattern of leaf light environments and resulting leaf carbon gain within the shoots. Upper, south-facing leaves were potentially the most productive because they intercepted greater daily photon flux density (PFD) than leaves of any other orientation. North-facing leaves intercepted less PFD but of this, more was received on the abaxial surface because of the steep leaf angles. Leaves differed in their response to abaxial versus adaxial illumination depending on their orientation. While most had lower photosynthetic rates when illuminated on their abaxial as compared to adaxial surface, the photosynthetic rates of north-facing leaves were independent of the surface of illumination. Because of the increasing self-shading, there were strong decreases in absorbed PFD and daily carbon gain in the basipetal direction. Leaf nitrogen per unit mass also decreased in the basipetal direction but on a per unit area basis was nearly constant along the shoot. The decrease in leaf N per unit mass was accounted for by an increase in leaf mass per unit area (LMA) rather than by movement of N from older to younger leaves during shoot growth. The increased LMA of older lower leaves may have contributed directly to their lower photosynthetic capacities by increasing the limitations to diffusion of CO within the leaf to the sites of carboxylation. There was no evidence for sun/shade acclimation along the shoot. Upper leaves and especially south-facing upper leaves had a potential risk for photoinhibition as demonstrated by the high PFDs received and the diurnal decreases in the fluorescence ratio F /F . Predawn F /F ratios remained high (>0.8) indicating that when in their normal orientations leaves sustained no photoinhibition. Reorientation of the leaves to horizontal induced a strong sustained decrease in F /F and CO exchange that slowly recovered over the next 10-15 days. If leaves were also inverted so that the abaxial surface received the increased PFDs, then the reduction in F /F and CO assimilation was much greater with no evidence for recovery. The heterogeneity of responses was due to a combination of differences between leaves of different orientation, differences between responses on their abaxial versus adaxial surfaces, and differences along the shoot due to leaf age and self-shading effects.
通过气体交换和叶绿素荧光测量，并应用三维冠层结构模型，研究了加州丛林灌木光叶石楠（Heteromeles arbutifolia）中叶片取向和在枝条内的位置对单个叶片光环境、碳增益以及光抑制敏感性的影响。光吸收和光合作用的模拟揭示了枝条内叶片光环境和由此产生的叶片碳增益的复杂模式。上部朝南的叶片可能是最具生产力的，因为它们截获的每日光子通量密度（PFD）比任何其他取向的叶片都要大。朝北的叶片截获的PFD较少，但由于叶片角度陡峭，其中更多的是在叶背面接收的。叶片对叶背与叶面光照的反应因其取向而异。虽然大多数叶片在叶背受光时的光合速率低于叶面受光时，但朝北叶片的光合速率与受光面无关。由于自遮荫增加，基部方向的吸收PFD和每日碳增益大幅下降。单位质量的叶片氮含量在基部方向也下降，但单位面积的叶片氮含量沿枝条几乎保持恒定。单位质量叶片氮含量的下降是由于单位面积叶片质量（LMA）的增加，而不是由于枝条生长过程中氮从老叶向新叶的转移。下部老叶LMA的增加可能通过增加叶片内CO2向羧化位点扩散的限制，直接导致其光合能力较低。没有证据表明枝条上存在阳生/阴生适应。上部叶片，尤其是朝南的上部叶片，存在光抑制的潜在风险，这表现为所接收的高PFD以及荧光比率Fv/Fm的昼夜下降。黎明前的Fv/Fm比率仍然很高（>0.8），表明叶片在正常取向时没有受到光抑制。将叶片重新定向为水平会导致Fv/Fm和CO2交换强烈持续下降，并在接下来的10 - 15天内缓慢恢复。如果叶片也倒置，使叶背面接收增加的PFD，那么Fv/Fm和CO2同化的降低会更大，且没有恢复的迹象。反应的异质性是由于不同取向叶片之间的差异、叶背与叶面反应之间的差异以及由于叶龄和自遮荫效应导致的枝条上的差异共同作用的结果。
