Induction and Repression of CAM inSedum telephiumL. in Response to Photoperiod and Water Stress

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Abstract
Lee, H. S. J. and Griffiths, H. 1987. Induction and repression of CAM in Sedurn relephluni L. in response to photopcnod and water stress.—J. exp. Bot. 38: 834–841. The introduction and repression of CAM in Sedurn telephiunm L, a temperate succulent, was investigated in watered, progressively drouglited and rewatered plants in growth chambers. Measurements were made of water vapour and CO2 exchange, titratable acidity (TA) and xylem sap tension. Effects of photoperiod were also studied. CAM was induced by drought under long or short days, although when watered no CAM activity was expressed. C3-CAM intermediate plants were used for the investigation of water supply. Those which had received water and those drought-stressed both displayed a similar nocturnal increase in TA, with a day-night maximum (δH+) of 69 μmol g−1 fr. wt. The watered plants took up CO2 at a maximum rate of 2·2 μmol m−2 s−1 only in the light period, while the droughted plants showed a maximum nocturnal CO2 uptake rate of 0·69 μmol m−2 s−1. Subsequently, as CAM was repressed, the watered S. telephiwn displayed little variation in TA, with constant levels at 42 μmol g−1 fr. wt. (day 10). After 10 d of drought stress, the CAM characteristics of S. telephiurn were aLso affected, with reduced net CO2 uptake and δH+. The transition between C3 and CAM in S. telephium can be described as a progression in terms of the proportion of respiratory CO2 which is recycled and refixed at night as malic acid, in comparison with net CO2 uptake. Recycling increased from 20% (day 1) to 44% (day 10) as a result of the drought stress and was high in both the CAM-C3 stage (no net CO2 uptake at night) and also in the drought-stressed CAM stage (reduced net CO2 uptake at night). The complete C3-CAM transition occurred in less than 8 d, and the stages could be characterized by xylem sap tension measurements: CAM = 0·50 MPa C3-CAM = 0·36 MPa C3 = 0·29 MPa.