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

Abstract

On-line instantaneous carbon isotope discrimination was measured in conjunction with net uptake of CO in leaves of exposed and shaded plants of the C-CAM intermediate Clusia minor growing under natural conditions in Trinidad. At the end of the rainy season (late January-early February, 1992) C photosynthesis predominated although exposed leaves recaptured a small proportion of respiratory CO 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 CO 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 CO in exposed leaves. Shaded leaves showed no CAM activity and CO 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 C carboxylation dominated CO 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 C carboxylation to CO 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 C to C carboxylation with stomata remaining open during the transition from dusk into the dark period. Uptake of CO 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 (δ) and to compare this with the δ measured for leaf organic material. Whilst there was close agreement between predicted and measured δ 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 C pathway although CAM clearly plays an important role in maintaining photochemical integrity in the dry season.

Carbon-Isotope Composition of Biochemical Fractions and the Regulation of Carbon Balance in Leaves of the C3-Crassulacean Acid Metabolism Intermediate Clusia minor L. Growing in Trinidad

“…Despite the considerable capacity for CAM in C. minor (Borland et al, 1992) and the observation that C4 carboxylation can account for up to 30% of C 0 2 fixed during the day in the dry season (Borland et al, 1993), 613Cs of leaf organic material reveal that during the annual cycle of leaf growth most carbon is fixed via the C3 pathway (Borland et al, 1992). However, given the large diel changes in the size of organic acid and carbohydrate pools in CAM plants, measurements of 613C of different biochemical fractions at dawn and dusk could provide a means of reconciling hourly changes in instantaneous A with seasonal changes in 613C.…”

“…Using 613C analyses and mass balance interconversions of organic acid and carbohydrate fractions associated with CAM, one can now calculate the carbon-isotope discrimination expressed during the dark period. Leaves were sampled during the wet and dry seasons in Trinidad at dawn and dusk on the days when on-line measurements of instantaneous discrimination had been made (Borland et al, 1993). Analyses of the 6I3C composition of individual metabolic components during the day-night cycle were made to illustrate how products of C3 and C4 carboxylation regulate plant carbon balance during daily and seasonal time scales.…”

“…Under natural conditions, short-term changes in on-line, instantaneous A have illustrated how shifts in C3 and C4 carboxylation occur on a daily basis in C. minor, reflecting changes in environmental parameters (Borland et al, 1993). These variations in carboxylation pathway may in the first instance…”

“…Estimates of carbon-isotope discrimination expressed during CAM, derived from the trafficking among inorganic carbon, organic acids, and carbohydrate pools overnight, ranged from 0.9 to 3.1%0. The 613C of structural material did not change significantly between wet and dry seasons, indicating that most of the carbon used in growth was derived from C3 carboxylation.Clusia minor L. is a tropical dicotyledonous tree that shows extreme flexibility in regulating the relative amounts of C 0 2 fixed during the day and night in response to changes in the environment (Franco et al, 1990Borland et al, 1992Borland et al, , 1993Winter et al, 1992;Zotz and Winter, 1993). Under natural conditions, short-term changes in on-line, instantaneous A have illustrated how shifts in C3 and C4 carboxylation occur on a daily basis in C. minor, reflecting changes in environmental parameters (Borland et al, 1993).…”

The effects of salinity, crassulacean acid metabolism and plant age on the carbon isotope composition of Mesembryanthemum crystallinum L., a halophytic C3-CAM species

“…Stomatal conductance and thus net CO 2 uptake during the light may be reduced, thereby increasing the relative contribution of dark CO 2 fixation to carbon gain (Holtum and Winter 1999), which should give rise to less negative d 13 C values. In some highly flexible CAM species, such as members of the genus Clusia, water deficit may stimulate dark CO 2 fixation in a reversible fashion Borland et al 1993;Lü ttge 1996). Independent of such stress-induced alterations of the proportion of CO 2 assimilated during the dark and light, d 13 C value can also be affected by stress-related effects on the diffusional limitation of CO 2 uptake.…”