Effects of growth temperature on the response of lupin stomata to drought and abscisic acid

Abstract

To assess how growth temperature affects stomatal responses to xylem-transported abscisic acid (ABA), leaf conductance (g), the concentrations of ABA and calcium ions, and the pH of the xylem sap were measured in well-watered and water-stressed Lupinus albus L. plants grown under two thermal regimes: 10/15°C and 20/25°C, night/day temperature. Moderate water deficit was imposed, at the same thermal time, and induced a significant reduction in g regardless of temperature. In the morning, g was higher in plants grown at 20/25°C than in cooler conditions, and these differences could not be explained by dissimilarities in shoot water status or xylem ABA concentration. At midday, the apparent stomatal sensitivity to xylem-carried ABA was increased and the effect of temperature on the relationship between g and xylem ABA was no longer observed. A positive effect of higher temperature on stomatal aperture was also evident when artificial sap containing ABA was fed to leaves of well-watered plants. In response to exogenous ABA, stomata closed to the same extent as observed in the morning in water-stressed plants. However, exogenous ABA feeding could not mimic the relationship between g and xylem ABA determined at midday in intact plants. The pH and the concentration of calcium in xylem were not affected by temperature. At midday, however, the calcium concentrations were higher in water-stressed than in well-watered plants. These changes in the concentrations of calcium or other xylem components, such as ABA conjugates, together with possible changes in the ability of the leaves to degrade and/or to compartmentalise ABA, may partly explain the midday increase in the apparent stomatal sensitivity to xylem ABA.