Two slow-growing plant species (Chamaerops humilis, L. and Cycas revoluta Thunb.) were exposed to elevated CO 2 conditions over a 20-month period in order to study the CO 2 effect on growth, photosynthetic capacity and leaf carbon (C) management. The ambient isotopic 13 C/ 12 C composition (d 13 C) of the greenhouse module corresponding to elevated CO 2 (800 lmol mol À1 CO 2 ) conditions was changed from d 13 C ca. À12.8 AE 0.3% to ca. À19.2 AE 0.2%. Exposure of these plants to elevated CO 2 enhanced dry mass (DM) by 82% and 152% in Chamerops and Cycas, respectively, mainly as a consequence of increases in plant level photosynthetic rates. However, analyses of A-C i curve parameters revealed that elevated CO 2 diminished leaf photosynthetic rates of Chamaerops whereas in Cycas, no photosynthetic acclimation was detected. The fact that Chamaerops plants had a lower DM increase, together with a longer leaf C residence time and a diminished capacity to respire recently fixed C, suggests that this species was unable to increase C sink strength. Furthermore, the consequent C source/sink imbalance in Chamaerops might have induced the downregulation of Rubisco. Cycas plants were capable of avoiding photosynthetic downregulation due to a greater ability to increase C sink strength, as was confirmed by DM values, and 12 C-enriched CO 2 labeling data. Cycas developed the ability to respire a larger proportion of recently fixed C and to reallocate the recently fixed C away from leaves to other plant tissues. These findings suggest that leaf C management is a key factor in the responsiveness of slow-growing plants to future CO 2 scenarios.

New methodologies to assess nitrogen use efficiency (NUE) in field crops could help in the characterisation of large numbers of genotypes and growing conditions. The effects of chemical nitrogen fertilisation on yield: nitrogen taken up by the crop, NUE and its components utilisation efficiency (UTE) and uptake efficiency (UPE) and the stable carbon (d 13 C) and nitrogen (d 15 N) composition of mature grains and straw were evaluated. A set of 24 wheat genotypes generated over the past four decades by International Maize and Wheat Improvement Center and the Mexican Institute of Forestry, Agriculture and Livestock Research were studied under well-irrigated field conditions. Five concentrations of urea were applied as a source of N fertilisation. Fertilisation significantly decreased d 15 N and increased d 13 C, but d 15 N was the isotopic trait most strongly correlated with absolute changes in yield and crop nitrogen (N) accumulation caused by different levels of N fertilisation. Both grain d 15 N and grain d 13 C correlated positively with NUE and UTE. Differences across genotypes in d 13 C correlated positively with UTE and negatively with grain N content within each of the N levels assayed. Genotypic differences in d 15 N correlated negatively with total grain N accumulated but only at the intermediate levels of N fertilisation. We conclude that under our experimental conditions, natural abundances of 15 N and 13 C may provide complementary information on how nitrogen fertiliser is used by the plant. However, whereas grain yield and biomass, as well as total N accumulated and NUE, increased in the most recent genotypes, only a tendency for higher d 15 N was observed and there was no clear trend for d 13 C. Changes in NUE were paralleled by changes in UPE rather than UTE.

Very little is known about the primary carbon metabolism of the high mountain plant Ranunculus glacialis. It is a species with C3 photosynthesis, but with exceptionally high malate content in its leaves, the biological significance of which remains unclear. 13C/12C-isotope ratio mass spectrometry (IRMS) and 13C-nuclear magnetic resonance (NMR) labelling were used to study the carbon metabolism of R. glacialis, paying special attention to respiration. Although leaf dark respiration was high, the temperature response had a Q10 of 2, and the respiratory quotient (CO2 produced divided by O2 consumed) was nearly 1, indicating that the respiratory pool is comprised of carbohydrates. Malate, which may be a large carbon substrate, was not respired. However, when CO2 fixed by photosynthesis was labelled, little labelling of the CO2 subsequently respired in the dark was detected, indicating that: (i) most of the carbon recently assimilated during photosynthesis is not respired in the dark; and (ii) the carbon used for respiration originates from (unlabelled) reserves. This is the first demonstration of such a low metabolic coupling of assimilated and respired carbon in leaves. The biological significance of the uncoupling between assimilation and respiration is discussed.

Most of the literature focused on internal CO(2) (Ci) determinations in plants has used indirect methods based on gas-exchange estimations. We have developed a new method based on the capture of internal air gas samples and their analysis by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). This method provided a direct measure of intercellular CO(2) concentrations combined with stable carbon isotopic composition in O. ficus-indica plants. Plants were grown at both ambient and elevated CO(2) concentration. During the day period, when the stomata are closed, the Ci was high and was very (13)C-enriched in both ambient and elevated CO(2)-grown plants, reflecting Rubisco's fractionation (this plant enzyme has been shown to discriminate by 29 per thousand, in vitro, against (13)CO(2)). Other enzyme fractionations involved in C metabolism in plants, such as carbonic anhydrase, could also be playing an important role in the diurnal delta(13)C enrichment of the Ci. During the night, when stomata are open, Ci concentrations were higher in elevated (and the corresponding delta(13)C values were more (13)C-depleted) than in ambient CO(2)-grown plants.