Stereology of the Internal Structures of Leaves in Peperomia obtusifolia, P. camptotricha, and P. scandens

Abstract

The internal structure of leaves from Peperomia obtusifolia, P. camptotricha, and P. scandens were analyzed with stereology to detect relationships between the internal leaf structure of each species with its diurnal cycles of gas exchange and organic acid fluctuation. In these succulent leaves, the relative volume of leaf occupied by each of three tissues (multiple epidermis and median and spongy mesophyll) varied according to the species and leaf development. In P. obtusifolia, which exhibits light-period gas exchange and little acid fluctuation, 75% of the leaf volume was multiple epidermis. In P. scandens, which exhibits dark-period gas exchange and large acid fluctuation, 77% of the leaf volume was spongy mesophyll. In P. camptotricha, which exhibits increasing dark-period gas exchange and acid fluctuation during leaf development, the multiple epidermis and spongy mesophyll were each about 48% of the leaf volume. In each species the median mesophyll was about 4% of the leaf volume. The area of cell wall exposed to the internal air space of the leaf was dominated by the spongy mesophyll in each species; the area was least in P. obtusifolia, greater in thicker leaves of P. camptotricha, and greatest in P. scandens. The relative volumes of cellular components in P. camptotricha were determined for leaves of different thickness. A high density of chloroplasts in the median mesophyll gave a low ratio of vacuole to cytoplasm, which was similar to the ratio in bundle sheath cells of many plants with C4 metabolism. The vacuole predominated in cells of the multiple epidermis and spongy mesophyll, and the cells were much larger than those in the median mesophyll. However, only the spongy mesophyll contained chloroplasts with the stroma and thylakoid volume more typical of chlorenchyma in plants with Crassulacean acid metabolism. Tissue dimorphism in cell size, chloroplast content, vacuole volume, and cell wall exposed to air space were the structural features most easily associated with the known physiological data of gas exchange and organic acid fluctuation patterns in these species of Peperomia.