Poulson Mary E, DeLucia Evan H
Department of Plant Biology, University of Illinois, 265 Morrill Hall, 505 South Goodwin Avenue, 61801, Urbana, IL, USA.
Oecologia. 1993 Sep;95(3):393-400. doi: 10.1007/BF00320994.
The azimuth of vertical leaves of Silphium terebinthinaceum profoundly influenced total daily irradiance as well as the proportion of direct versus diffuse light incident on the adaxial and abaxial leaf surface. These differences caused structural and physiological adjustments in leaves that affected photosynthetic performance. Leaves with the adaxial surface facing East received equal daily integrated irradiance on each surface, and these leaves had similar photosynthetic rates when irradiated on either the adaxial or abaxial surface. The adaxial surface of East-facing leaves was also the only surface to receive more direct than diffuse irradiance and this was the only leaf side which had a clearly defined columnar palisade layer. A potential cost of constructing East-facing leaves with symmetrical photosynthetic capcity was a 25% higher specific leaf mass and increased leaf thickness in comparison to asymmetrical South-facing leaves. The adaxial surface of South-facing leaves received approximately three times more daily integrated irradiance than the abaxial surface. When measured at saturating CO and irradiance, these leaves had 42% higher photosynthetic rates when irradiated on the adaxial surface than when irradiated on the abaxial surface. However, there was no difference in photosynthesis for these leaves when irradiated on either surface when measurements were made at ambient CO. Stomatal distribution (mean adaxial/abaxial stomatal density = 0.61) was unaffected by leaf orientation. Thus, the potential for high photosynthetic rates of adaxial palisade cells in South-facing leaves at ambient CO concentrations may have been constrained by stomatal limitations to gas exchange. The distribution of soluble protein and chlorophyll within leaves suggests that palisade and spongy mesophyll cells acclimated to their local light environment. The protein/chlorophyll ratio was high in the palisade layers and decreased in the spongy mesophyll cells, presumably corresponding to the attentuation of light as it penetrates leaves. Unlike some species, the chlorophyll a/b ratio and the degree of thylakoid stacking was uniform throughout the thickness of the leaf. It appears that sun-shade acclimation among cell layers of Silphium terebinthinaceum leaves is accomplished without adjustment to the chlorophyll a/b ratio or to thylakoid membrane structure.
松香草垂直叶片的方位对每日总辐照度以及入射到叶片近轴面和远轴面的直射光与漫射光比例有深远影响。这些差异导致叶片结构和生理调节，进而影响光合性能。近轴面朝东的叶片在每个表面接受的日积分辐照度相等，并且当在近轴面或远轴面照射时，这些叶片具有相似的光合速率。朝东叶片的近轴面也是唯一接受直射辐照度多于漫射辐照度的表面，并且这是唯一具有明确柱状栅栏层的叶片面。与不对称的朝南叶片相比，构建具有对称光合能力的朝东叶片的潜在代价是比叶质量高25%且叶片厚度增加。朝南叶片的近轴面接受的日积分辐照度比远轴面大约多三倍。在饱和CO₂和辐照度下测量时，这些叶片在近轴面照射时的光合速率比在远轴面照射时高42%。然而，在环境CO₂浓度下测量时，当在任一表面照射时，这些叶片的光合作用没有差异。气孔分布（平均近轴/远轴气孔密度 = 0.61）不受叶片方位影响。因此，在环境CO₂浓度下，朝南叶片近轴栅栏细胞的高光合速率潜力可能受到气孔对气体交换限制的制约。叶片内可溶性蛋白和叶绿素的分布表明，栅栏和海绵叶肉细胞适应了它们局部的光环境。蛋白/叶绿素比值在栅栏层中较高，在海绵叶肉细胞中降低，大概与光穿透叶片时的衰减相对应。与一些物种不同，叶绿素a/b比值和类囊体堆叠程度在叶片整个厚度上是均匀的。看来，松香草叶片细胞层之间的阳生-阴生适应是在不调整叶绿素a/b比值或类囊体膜结构的情况下完成的。
