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.