Changes in leaf water potential and CAM inSempervivum montanum andSedum album in response to water availability in the field.

The short term effects of irrigation on diurnal changes in Ψleaf and titratable acidity were examined both inSempervivum montanum and inSedum album, a facultative CAM plant, in the Spanish Pyrenees. InSemperivivum, Ψleaf responded rapidly to irrigation and, in both the control and irrigated plants, increased during the day and decreased during the night and early morning. By contrast, Ψleaf inSedum responded more slowly to irrigation and showed a decrease during the day and an increase in the period between evening and early morning. Under the conditions of the short-term experiments, changes in acid metabolism were not observed in either species following irrigation. The results suggest that transpirational water loss together with redistribution of water within the plant are more important than the osmotic concentration of malic acid in determining Ψleaf in both species and that daytime water loss is greater inSedum than inSempervivum.The effect of long-term water stress on Ψleaf and acid levels was also assessed in both species over a 3-week period. Both Ψleaf and acidification inSempervivum decreased over this time period but could, at least partially, be reversed by irrigation. InSedum, Ψleaf also declined but a more gradual reduction in acidification occurred than inSempervivum. Irrigation inSedum at least partially reversed the decline in Ψleaf but produced a complex pattern of acid metabolism. Nocturnal acidification in the irrigated plants was lower than in the non-irrigated control when preceded by a cool day but showed complete recovery following a hot day. It is suggested inSedum album that C3 photosynthesis during the preceding light period, as determined by light intensity and leaf temperature, may be important in determining the extent of nocturnal acidification under field conditions.