Leaf Ultrastructure and δ¹³C Values of Three Seagrasses from the Great Barrier Reef

Abstract

Leaf anatomy, ultrastructure and ¹³C/¹²C ratios were studied in three species of seagrasses collected on the Great Barrier Reef: Cymodocea rotundata Ehrenb. & Hempr., C. serrulata (R. Br.) Aschers. & Magnus, and Thalassia hemprichii (Ehrenb.) Aschers. Although they belong to two different mono- cotyledonous families, the three species are quite similar in the characteristics studied. Cells of the epidermal layer of the leaves are extremely thick-walled and have abundant cytoplasm with large chloroplasts and numerous mitochondria. The chloroplast-microbody profile ratio is c. 4-5 : 1 and the mitochondrion-microbody ratio 10-15 : 1. The epidermal cells resemble transfer cells in having a pronounced development of ingrowths on the radial walls. The mesophyll cells have thin walls, a large central vacuole and a thin layer of cytoplasm with relatively few organelles. There is no specialization of mesophyll cells around the vascular bundles. The δ¹³C values for the three sea- grasses range from -6.90, to - 12.40, and thus are characteristic of C₄ land plants, although the seagrasses do not conform to the C₄ syndrome in leaf anatomy or ultrastructure. It is not possible to place the seagrasses in either the C₃, C₄ or crassulacean acid metabolism category of land plants, but whether they constitute yet a fourth group with respect to characteristics related to CO₂ assimilation is not clear.