Responses of gas exchange, photosynthesis, nocturnal acid accumulation and water relations of Aptenia cordifolia to short-term drought and rewatering

In a controlled environment chamber study, changes of diurnal gas exchange, malic and citric acid accumulation, water relations and chlorophyll fluorescence in response to a short-term drought (10 days) and to rewatering were examined in Aptenia cordifolia. This all-cell leaf succulent Mesembryanthemaceae is native to summer rainfall regions of the eastern parts of the coastal deserts in South Africa and Namibia. When well-watered, A. cordifolia showed all attributes of CAM, i.e. nocturnal accumulation of malate (Delta-malate) and, to a lesser extent, of citrate (Delta-citrate; 24 % that of Delta-malate) and nighttime carbon uptake. During drought, daytime CO2 uptake ceased within 2 days due to stomatal closure. All changes in light period CO2 exchange and transpiration were highly positively correlated. Nocturnal carbon uptake gradually diminished but malate and citrate accumulation was slightly enhanced and the ratio of Delta-citrate to Delta-malate increased to almost 0.5. While water potential was reduced with decreasing water content, bulk leaf pressure potential was constant due to active osmotic adjustment. Changes in tissue osmotic content, which were fully reversible upon rewatering, mainly (approximate to 50 %) resulted from variations in citrate content. However, during prolonged drought even the constancy of bulk leaf pressure potential and CAM could not prevent reduction of maximum photochemical efficiency as estimated from dark-adapted fluorescence parameters (F-V/F-M). This indicated a photoinhibitory disturbance of photosynthesis, which probably not only resulted from CO2 deprivation due to stomatal limitations. Partial non-stomatal restrictions at the biochemical level may be deduced from the close relationship between changes in F-V/F-M and tissue osmolarity. Kinetics of recovery after rewatering showed that daytime and nighttime gas exchange may be regulated differentially. The former was independent of water content and pressure potential as was the diurnal rhythmicity of organic acids.