Leaf Anatomy of C3-C4 Species as Related to Evolution of C4 Photosynthesis

Abstract

This study was undertaken to examine the degree of Kranz anatomy development in the species intermediate to C(3) and C(4) types (C(3)-C(4)) in Panicum, Neurachne, Flaveria, and Moricandia. In each genus, C(3) and/or C(4) species were used for comparison. Leaf transections from each species were examined by light and transmission electron microscopy. The percentages of leaf photosynthetic cell profiles partitioned to bundle sheaths were higher in C(4) than in C(3) species, while C(3)-C(4) species tended to be in between. However, percentages for C(3)-C(4) species in Moricandia and some C(3)-C(4)Flaveria species were not greater than C(3). When expressed on a cell profile area basis, C(3)-C(4) species partitioned more photosynthetic tissue to bundle sheaths than C(3) species in Moricandia, but not in Flaveria. Neurachne minor S. T. Blake (C(3)-C(4)) partitioned a very small portion of cell profile area to the inner bundle sheaths (5%) compared to Neurachne munroi F. Muell (C(4)) (21%). The percentage of organelles partitioned to bundle sheaths was much greater in C(3)-C(4) than in C(3) species. The average C(3) percentages for mitochondria plus peroxisomes were 19, 8, and 19.5% for Neurachne, Flaveria, and Moricandia, respectively, compared to 41, 29, and 46.5% for the C(3)-C(4) species. The CO(2) compensation concentration was negatively related to the partitioning of tissue to bundle sheaths and to the percentage of organelles in bundle sheaths. It is concluded that all of the C(3)-C(4) species examined have developed some degree of Kranz anatomy and that this altered anatomy is involved in their reduced apparent photorespiration.