ORIGINAL PAPER Nocturnal sap flow in the C 3 -CAM species, Clusia minor Ana Herrera Æ Caı ´n Ballestrini Æ Wilmer Tezara Received: 29 November 2007 / Revised: 9 January 2008 / Accepted: 21 January 2008 / Published online: 14 February 2008 Ó Springer-Verlag 2008 Abstract Clusia minor L. is a C 3 -CAM species in which Crassulacean acid Metabolism (CAM) is induced, among other factors, by water deficit. We propose that CAM induction by natural drought in C. minor shifts the sap flow pattern from daytime to a night-time one, and that the decreased osmotic potential due to increased malate con- tent in droughted plants aids in the increase in nocturnal sap flow. In order to test these hypotheses, we followed for 2 years the seasonal changes in parameters of water rela- tionships and sap flow velocity in one single, freestanding tree growing in Caracas. Leaf water and osmotic potential were measured psychrometrically, nocturnal proton accu- mulation by titration of aqueous leaf extracts and sap flow density with thermal dissipation probes. Leaf water, osmotic and turgor potential remained relatively high throughout the seasons. Nocturnal proton accumulation was nil under extreme drought or after frequent and heavy rains, and high after moderate rainfall. Estimated malate and citrate concentrations contributed up to 80 and 60%, respectively, of the value of osmotic potential. The shape of the daily courses of sap flow velocity varied seasonally, from mostly diurnal during the dry season to mostly noc- turnal after a short dry spell during the rainy season, when nocturnal acid accumulation attained high values. There was a strong positive relationship between the proportion of the integrated sap flow courses corresponding to the night and dawn [H + ](r 2 = 0.88). Increased nocturnal sap flow in the CAM stage of the tree of C. minor may be explained by a lower osmotic potential due to an increased acid concentration, together with increased stomatal aperture, as suggested by increased nocturnal acid accumulation probably due to nocturnal CO 2 fixation. Keywords C 3 -CAM Á Clusia minor Á Crassulacean acid metabolism Á Nocturnal sap flow Á Osmotic potential Á Water potential Introduction The occurrence of Crassulacean acid Metabolism (CAM) in the genus Clusia (Clusiaceae) is well documented; Clusia is the only dicotyledonous tree genus known to possess CAM and C 3 -CAM species (Lu ¨ttge 2006). Plants of C. minor are evergreen small freestanding trees, hemi-epiphytes or strangler holo-epiphytes; they live in a broad range of environments, from forests to savannas, and from the forest floor to tree branches (Lu ¨ttge 2006). In C. minor CO 2 fixation is best characterised as C 3 -CAM; watered plants of C. minor show nil nocturnal acid accu- mulation (DH + ), while a short period of drought is capable of inducing the appearance of nocturnal CO 2 fixation with concomitantly high values of DH + (Schmitt et al. 1988). CAM is extremely plastic in this species; in one single branch, leaves subjected to drier air performed CAM, whereas the opposite leaves, under humid air, did not (Schmitt et al. 1988). The shift between C 3 photosynthesis and CAM is rapid and plastic under both field and con- trolled conditions (Borland et al. 1996; Haag-Kerwer et al. 1996; Schmitt et al. 1988; Mattos et al. 1999; Mattos and Lu ¨ttge 2001). In plants of the C 3 -CAM hemi-epiphyte C. uvitana growing on a Ceiba tree, the seasonal variation in leaf Communicated by U. Lu ¨ttge. A. Herrera (&) Á C. Ballestrini Á W. Tezara Instituto de Biologı ´a Experimental, Universidad Central de Venezuela, Apartado 47577, Caracas, Venezuela e-mail: aherrera@ciens.ucv.ve 123 Trees (2008) 22:491–497 DOI 10.1007/s00468-008-0209-8