Mycorrhization and phosphorus nutrition affect water relations and CAM induction by drought in seedlings of Clusia minor.

BACKGROUND AND AIMS: Crassulacean acid metabolism (CAM) is currently viewed as an adaptation to water deficit. In plants of Clusia minor, which grow mostly on acidic, P-deficient soils, CAM is induced by water deficit. The symbiosis between plants and mycorrhizal fungi alleviates the symptoms of P deficiency and may influence drought resistance. Therefore, the effect of P supply, modified by three different experimental treatments, on the induction of CAM by drought in C. minor was investigated to test the hypothesis that P deficiency will produce greater CAM activity and, in addition, that treatment will modify drought tolerance.
METHODS: Seedlings were grown in forest soil sterilized and inoculated with Scutellospora fulgida (SF treatment), sterilized and supplemented with P (Ph treatment) or non-sterilized and containing native mycorrhizae (Nat treatment). Leaf turgor potential (psi(T)) was determined psychrometrically, and CAM activity as nocturnal acid accumulation (DeltaH(+)) by titration of dawn and dusk leaf sap. KEY
RESULTS: Plant mass and P content were higher in SF and Ph than in Nat seedlings. After 21 d of water deficit, psi(T) increased in SF, decreased in Ph and remained unchanged in Nat, and, after 7 and 14 d of water deficit, DeltaH(+) in Nat was three times higher than at the beginning of drought, whereas in SF and Ph DeltaH(+) was lower than on day 0.
CONCLUSIONS: P deficiency in Nat seedlings was ameliorated by inoculation or P addition. The SF and Nat seedlings showed greater tolerance of drought than Ph. P deficiency promoted the induction of CAM by drought in Nat seedlings, whereas P fertilization and mycorrhization did not. Nocturnal acid accumulation was highly and negatively correlated with plant P and N contents, indicating that P and N deficiencies are promoters of CAM in droughted plants of C. minor.