Short-term changes in carbon-isotope discrimination in the C3-CAM intermediate Clusia minor L. growing in Trinidad

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Abstract
On-line instantaneous carbon isotope discrimination was measured in conjunction with net uptake of CO2 in leaves of exposed and shaded plants of the C3-CAM intermediate Clusia minor growing under natural conditions in Trinidad. At the end of the rainy season (late January-early February, 1992) C3 photosynthesis predominated although exposed leaves recaptured a small proportion of respiratory CO2 at night for the synthesis of malic acid. Citric acid was the major organic acid accumulated by exposed leaves at this time with a citric: malic acid ratio of 11:1. Values of instantaneous discrimination (Δ) in exposed leaves during the wet season rose from 17.1‰ shortly after dawn to 22.7‰ around mid-day just before stomata closed, suggesting that most CO2 was fixed by Rubisco at this time. During the late afternoon, instantaneous Δ declined from 22.2‰ to 17‰, probably reflecting the limited contribution 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 in instantaneous Δ from 27‰ in the morning to 19.2‰ 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 citric acids accumulated overnight and stomata closed for 4–5 h over the middle of the day. Values of instantaneous Δ measured over the first 3 h after dawn (6.4–9.1‰) indicated that C4 carboxylation dominated CO2 uptake for most of the morning when rates of photosynthesis were maximal, implying that under natural conditions, the down regulation of PEPc in phase II occurs much more slowly than laboratory-based studies have suggested. The contribution from C3 carboxylation to CO2 uptake during phase II was most marked in leaves which accumulated lower quantities of organic acids overnight. In exposed leaves, measurements of instantaneous Δ during the late afternoon illustrated the transition from C3 to C4 carboxylation with stomata remaining 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-term measurements of instantaneous Δ were subsequently integrated over 24 h in order to predict the leaf carbon isotope ratios (δp) and to compare this with the δp measured for leaf organic material. Whilst there was close agreement between predicted and measured δp for plants sampled in the wet season, during the dry season the predicted carbon isotope ratios were 5–9‰ higher than the measured isotope ratios. During the annual cycle of leaf growth most carbon was fixed via the C3 pathway although CAM clearly plays an important role in maintaining photochemical integrity in the dry season.

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