In species of Clusia , switching from C 3 -photosynthesis (C 3 -PS) to crassulacean acid metabolism (CAM) may be a means of optimizing water use, plant carbon balance and photon utilization during periods of stress. We ask whether, in perennial species of Clusia , the switch from CAM back to C 3 -PS is also of ecophysiological significance. Our objective was to investigate the performance of C. minor L. during a short-term shift from CAM to C 3 -PS. During the transition from CAM to C 3 -PS, nocturnal malate and citrate accumulation decreased whereas CO 2 uptake increased during the daytime. However, after 7 d, marked nocturnal accumulation of citrate and 24 h CO 2 uptake occurred. In contrast to C 3 -like photosynthesis, a pronounced reduction in the effective quantum yield of photosystem II, ▵F/F′ m , together with a sharp increase in non-photochemical quenching were observed during CAM at the beginning and end of the day. After 7 d, integrated CO 2 uptake over 24 h approximately doubled; however, water use efficiency was reduced three-fold due to increased rates of daytime transpiration. Despite very similar maximum ▵F/F′ m between CAM and C 3 -PS, the results suggest that daily photon utilization increases when there is unrestricted CO 2 uptake directly from the atmosphere. Thus, under well-watered conditions and with increased rates of transpiration, C. minor performing C 3 -like photosynthesis may overcome the limitations of the storage capacity of the vacuole for overnight organic acid accumulation, improving its daily carbon balance. Copyright 2001 Annals of Botany Company

Received: 31 January 2001 ; Returned for revision: 28 February 2001 . Accepted: 21 May 2001


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Present address: Departamento de Ecologia, IB, CCS, Universidade Federal do Rio de Janeiro, C.P. 68020, Rio de Janeiro, RJ, 21941 - 970, Brazil.