Day-Night Variations in Malate Concentration, Osmotic Pressure, and Hydrostatic Pressure in Cereus validus

Abstract

Malate concentration and stem osmotic pressure concomitantly increased during nighttime CO2 fixation and then decreased during the daytime. Changes in malate osmotic pressure matched changes in stem osmotic pressure, indicating that changes in malate level affected the water relations of the succulent stems. In contrast to stem osmotic pressure, stem water potential showed little day-night changes, suggesting that changes in cellular hydrostatic pressure occurred. This was corroborated by direct measurements of hydrostatic pressure which indicated a 4-fold increase in hydrostatic pressure from dusk to dawn. A transient increase of hydrostatic pressure at the beginning of the dark period was correlated with a short period of stomatal closing between afternoon and nighttime CO2 fixation, suggesting that the hydrostatic pressure patterns could be explained by an interplay between the effects of transpiration and malate levels. A second crassulacean acid metabolism (CAM) plant, Agave desertii, showed similar day-night changes in hydrostatic pressure in its succulent leaves. It is concluded that, in addition to the inverted stomatal rhythm, the oscillations of malate markedly affect osmotic pressures and hence water relations of CAM plants