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

To assess how growth temperature affects stomatal responses to xylem-transp orted 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 degrees C and 20/25 degrees C, night/day temperature. Moderate wate r 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 p lants grown at 20/25 degrees C than in cooler conditions, and these differe nces could not be explained by dissimilarities in shoot water status or xyl em ABA concentration. At midday, the apparent stomatal sensitivity to xylem -carried ABA was increased and the effect of temperature on the relationshi p between g and xylem ABA was no longer observed. A positive effect of high er temperature on stomatal aperture was also evident when artificial sap co ntaining ABA was fed to leaves of well-watered plants. In response to exoge nous ABA, stomata closed to the same extent as observed in the morning in w ater-stressed plants. However, exogenous ABA feeding could not mimic the re lationship between g and xylem ABA determined at midday in intact plants. T he pH and the concentration of calcium in xylem were not affected by temper ature. At midday, however, the calcium concentrations were higher in water- stressed than in well-watered plants. These changes in the concentrations o f calcium or other xylem components, such as ABA conjugates, together with possible changes in the ability of the leaves to degrade and/or to compartm entalise ABA, may partly explain the midday increase in the apparent stomat al sensitivity to xylem ABA.