热带巨型叶植物芭蕉叶片内结构异质性

叶片是植物进行光合作用的主要场所,叶片面积是决定叶片光合作用的重要因子之一.以往对于叶片的解剖结构和生理功能的研究中,常常忽略同一叶片不同部位的结构及功能的差异,尤其是对于某些巨大叶片的结构和功能的异质性更是缺乏了解.为什么具有巨大叶片的植物在自然界十分稀少仍然是科学之谜.本研究选取了具有典型巨型叶片的单子叶植物芭蕉(Musa balbisiana Colla)作为实验材料,测定了叶片不同部位的结构和解剖特征.结果发现,沿主脉方向从叶片基部到叶片尖端,主脉导管直径、叶片厚度、保卫细胞长度呈剧烈下降趋势,比叶重在上部约1/2处呈下降趋势,而栅栏组织和海绵组织的比(P/S值)和气孔密度呈增长趋势,叶绿素含量、叶脉密度和气孔面积指数则无明显变化.沿平行脉从叶片中部到叶片两侧边缘,叶片厚度和比叶重呈现剧烈下降趋势,叶绿素含量、气孔密度和气孔面积指数在边缘约1/3范围内剧烈下降,栅栏组织和海绵组织的比和叶脉密度则呈现上升的趋势.从叶基到叶顶端主脉的导管直径急剧减少可能会影响叶片顶端的水分供应,而叶片两侧边缘气孔面积指数的明显减小、再加上大叶片水汽界面层厚会使边缘部位蒸腾散热功能受到抑制,从而抑制该部位的生理功能,这些因素可能导致芭蕉叶片面积不能继续增大.与叶片小一些的海芋大型叶相比,芭蕉叶内结构的异质性更加强烈.

Heterogeneity of anatomical structure in giant leaves of Musa balbisiana

Leaves are the site of photosynthesis in higher plants. Leaf size is one of the factors that determine energy capture and carbon acquisition; however, previous studies have considered leaves to be homogeneous in both structure and physiology. Consequently, little is known about how these parameters may vary in different parts of a leaf, especially in giant leaves. To test the hypothesis that very large leaves have a high level of within-leaf heterogeneity, we examined the anatomical structure of the giant leaves of Musa balbisiana Colla (Musaceae). We also calculated the stomatal pore area index (SPI) based on the anatomical data. We found that, from the leaf base to the tip along the midrib, vessel diameter in the midrib, leaf thickness, and guard cell length decreased sharply, while leaf mass per unit area (LMA) decreased in distal half of the leaf. Conversely, the ratio of palisade tissue to spongy tissue (P/S ratio), stomatal density, and SPI increased along this axis. However, neither chlorophyll content nor minor vein density varied from the basal to apical parts of the leaf. From the midrib to the lateral leaf margins, leaf thickness, LMA, chlorophyll content, guard cell length, stomatal density, and SPI decreased, while the P/S ratio and minor vein density increased. Our results indicated a remarkable level of structural heterogeneity within the giant leaves of M. balbisiana. The apical region of its leaves is likely limited by water supply, while the leaf lateral margins probably suffer mainly from heat stress due to reduced transpirational cooling. These constraints at the edges of the leaves could restrict size in large leaves.