SHORT-TERM CHANGES IN CARBON-ISOTOPE DISCRIMINATION IN THE C3-CAM INTERMEDIATE CLUSIA-MINOR L GROWING IN TRINIDAD

On-line instantaneous carbon isotope discrimination was measured in co njunction with net uptake of CO2 in leaves of exposed and shaded plant s of the C3-CAM intermediate Clusia minor growing under natural condit ions in Trinidad. At the end of the rainy season (late January-early F ebruary, 1992) C3 photosynthesis predominated although exposed leaves recaptured a small proportion of respiratory CO2 at night for the synt hesis of malic acid. Citric acid was the major organic acid accumulate d by exposed leaves at this time with a citric:malic acid ratio of 11: 1. Values of instantaneous discrimination (DELTA) in exposed leaves du ring the wet season rose from 17.1 parts-per-thousand shortly after da wn to 22.7 parts-per-thousand around mid-day just before stomata close d, suggesting that most CO2 was fixed by Rubisco at this time. During the late afternoon, instantaneous DELTA declined from 22.2 parts-per-t housand to 17 parts-per-thousand, probably reflecting the limited cont ribution from PEPc activity and an increase in diffusional resistance to CO2 in exposed leaves. Shaded leaves showed no CAM activity and CO2 uptake proceeded throughout the day in the wet season. The decrease i n instantaneous DELTA from 27 parts-per-thousand in the morning to 19. 2 parts-per-thousand in the late afternoon was therefore entirely due to diffusional limitation. Leaves sampled in the dry season (mid-March , 1992) had by now induced full CAM activity with both malic and citri c acids accumulated overnight and stomata closed for 4-5 h over the mi ddle of the day. Values of instantaneous DELTA measured over the first 3 h after dawn (6.4-9.1 parts-per-thousand) indicated that C4 carboxy lation dominated CO2 uptake for most of the morning when rates of phot osynthesis were maximal, implying that under natural conditions, the d own regulation of PEPc in phase II occurs much more slowly than labora tory-based studies have suggested. The contribution from C3 carboxylat ion to CO2 uptake during phase II was most marked in leaves which accu mulated lower quantities of organic acids overnight. In exposed leaves , measurements of instantaneous DELTA during the late afternoon illust rated the transition from C3 to C4 carboxylation with stomata remainin g open during the transition from dusk into the dark period. Uptake of CO2 by shaded leaves during the late afternoon however appeared to be predominantly limited by decreased stomatal conductance. The short-te rm measurements of instantaneous DELTA were subsequently integrated ov er 24 h in order to predict the leaf carbon isotope ratios (delta(p)) and to compare this with the delta(p) measured for leaf organic materi al. Whilst there was close agreement between predicted and measured de lta(p) for plants sampled in the wet season, during the dry season the predicted carbon isotope ratios were 5-9 parts-per-thousand higher th an the measured isotope ratios. During the annual cycle of leaf growth most carbon was fixed via the C3 pathway although CAM clearly plays a n important role in maintaining photochemical integrity in the dry sea son.