Effects of Drought on CAM and Water Relations in Plants of Peperomia carnevalii

Abstract

The mechanisms underlying the drought tolerance of Peperomia carnevalii Steyermark (Piperaceae), a succulent herb growing in the understorey of seasonally dry forests, were examined. Crassulacean acid metabolism (CAM) was studied in the field and laboratory, and measurements of water status were made in plants subjected to drought in the greenhouse. Nocturnal acid accumulation and day and night-time CO2assimilation rates were greatest in watered plants and decreased in drought. The proportion of CO2recycled through CAM in droughted plants, with nocturnal CO2uptake close to zero, was higher than in watered plants. Maximum quantum yield of chlorophyll fluorescence remained unchanged during drought, but the PSII quantum yield at the photosynthetic photon flux density at which the plants were grown was significantly decreased. Leaf anatomy consists of a chlorophyll-less hydrenchyma located beneath the upper epidermis, and a two-layered mesophyll. Leaves nearer to the apex are thinner than those nearer to the base of the shoot. Drought caused a reduction in leaf thickness due to shrinkage of the hydrenchyma, but not of the mesophyll. This was associated with the occurrence of a gradient of osmotic potential between these tissues. Comparison of water loss from thin leaves of watered and droughted plants, either partly defoliated at the lower nodes or intact, suggested that water moved from the thick to the thin leaves. This process was related to the occurrence of a gradient of water potential between the thick and the thin leaves. Drought tolerance in P. carnevalii is achieved by the operation of CAM and the occurrence of water movement within and between leaves.