Heterogeneity of anatomical structure in giant leaves of <italic>Musa balbisiana</italic>

Abstract

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.