CAM INDUCTION IN CLUSIA-MINOR L DURING THE TRANSITION FROM WET TO DRYSEASON IN TRINIDAD - THE ROLE OF ORGANIC-ACID SPECIATION AND DECARBOXYLATION

The interrelationships between the induction of CAM and the turnover o f malate and citrate in the dicotyledenous tree Clusia minor were comp ared with seasonal changes in rainfall, leaf water status, PFD and pho toinhibitory responses during the transition from wet to dry season in Trinidad, Over a period of 8 weeks, as rainfall declined from a maxim um observed around week 3, leaf xylem tensions measured at dusk and da wn reflected the concurrent reduction in day-time carbon gain and an i ncrease in the diel turnover of malate (exposed leaves) and citrate (s haded leaves), Clear seasonal trends were observed in the turnover of malate and citrate during the transition from wet to dry season, In co ntrast to the declining background concentrations of citrate during th e wet-dry season transition, malate accumulation was markedly enhanced and the ratio of malate:citrate accumulated overnight increased as th e dry season advanced, Photoinhibitory responses, assessed by chloroph yll fluorescence, indicated that photochemistry was largely determined by the diurnal course of PFD incident on leaves, regardless of the ma gnitude of internal CO2 release from malate and citrate decarboxylatio n, In the long term, photochemical efficiency in both shaded and expos ed leaves appeared to decline as the dry season progressed, Although t here was a clear linear relationship between integrated PFD and overni ght accumulation of malate, no such correlation was found for citrate, However, citrate breakdown during the day showed a much closer correl ation with PFD as compared to malate, with levels of citrate measured at dusk declining in response to higher daily light intensities, Moreo ver, enhanced citrate decarboxylation during the day was strongly corr elated with increased CAM and overnight accumulation of both malate an d citrate.