Carbon isotopic ratios of atmospheric CO(2) affect the delta(13)C values of heterotrophic growth in Nicotiana tabacum

Needle-age related variability in nitrogen, mobile carbohydrates, and δ13C within Pinus koraiensis tree crowns

For both ecologists and physiologists, foliar physioecology as a function of spatially and temporally variable environmental factors such as sunlight exposure within a tree crown is important for understanding whole tree physiology and for predicting ecosystem carbon balance and productivity. Hence, we studied concentrations of nitrogen (N), non-structural carbohydrates (NSC = soluble sugars + starch), and δ(13)C in different-aged needles within Pinus koraiensis tree crowns, to understand the needle age- and crown position-related physiology, in order to test the hypothesis that concentrations of N, NSC, and δ(13)C are needle-age and crown position dependent (more light, more photosynthesis affecting N, NSC, and δ(13)C), and to develop an accurate sampling strategy. The present study indicated that the 1-yr-old needles had significantly higher concentration levels of mobile carbohydrates (both on a mass and an area basis) and N(area) (on an area basis), as well as NSC-N ratios, but significantly lower levels of N(mass) (on a mass basis) concentration and specific leaf area (SLA), compared to the current-year needles. Azimuthal (south-facing vs. north-facing crown side) effects were found to be significant on starch [both on a mass (ST(mass)) and an area basis (ST(area))], δ(13)C values, and N(area), with higher levels in needles on the S-facing crown side than the N-facing crown side. Needle N(mass) concentrations significantly decreased but needle ST(mass), ST(area), and δ(13)C values significantly increased with increasing vertical crown levels. Our results suggest that the sun-exposed crown position related to photosynthetic activity and water availability affects starch accumulation and carbon isotope discrimination. Needle age associated with physiological activity plays an important role in determining carbon and nitrogen physiology. The present study indicates that across-scale sampling needs to carefully select tissue samples with equal age from a comparable crown position.

Why are non-photosynthetic tissues generally 13C enriched compared with leaves in C3 plants? Review and synthesis of current hypotheses

Non-photosynthetic, or heterotrophic, tissues in C₃ plants tend to be enriched in ¹³C compared with the leaves that supply them with photosynthate. This isotopic pattern has been observed for woody stems, roots, seeds and fruits, emerging leaves, and parasitic plants incapable of net CO₂ fixation. Unlike in C₃ plants, roots of herbaceous C₄ plants are generally not ¹³C-enriched compared with leaves. We review six hypotheses aimed at explaining this isotopic pattern in C₃ plants: (1) variation in biochemical composition of heterotrophic tissues compared with leaves; (2) seasonal separation of growth of leaves and heterotrophic tissues, with corresponding variation in photosynthetic discrimination against ¹³C; (3) differential use of day v. night sucrose between leaves and sink tissues, with day sucrose being relatively ¹³C-depleted and night sucrose ¹³C-enriched; (4) isotopic fractionation during dark respiration; (5) carbon fixation by PEP carboxylase; and (6) developmental variation in photosynthetic discrimination against ¹³C during leaf expansion. Although hypotheses (1) and (2) may contribute to the general pattern, they cannot explain all observations. Some evidence exists in support of hypotheses (3) through to (6), although for hypothesis (6) it is largely circumstantial. Hypothesis (3) provides a promising avenue for future research. Direct tests of these hypotheses should be carried out to provide insight into the mechanisms causing within-plant variation in carbon isotope composition.

Variation in root-zone CO2 concentration modifies isotopic fractionation of carbon and nitrogen in tomato seedlings

•  The contribution to the carbon budget and growth by root acquisition of inorganic carbon and the influence that this has on NO ₃ ^- and NH ₄ ⁺ uptake and assimilation has not been adequately quantified. •  The influence of varying root-zone CO ₂ concentrations on tissue δ ¹³ C and δ ¹⁵ N was used to estimate the contribution to the carbon budget of root-assimilated carbon in tomato ( Lycopersicon esculentum ) seedlings. •  Biomass accumulation was greater at 0.5% and 1% (v/v) root-zone CO ₂ in NO ₃ ^- and NH ₄ ⁺ -fed plants than with 0% root-zone CO ₂ . The plant δ ¹³ C values were not altered by 1% CO ₂ with δ ¹³ C = -29.00‰, but they were increased when supplied with 1% CO ₂ with δ ¹³ C = -10.91‰. The δ ¹⁵ N values of NO ₃ ^- -fed plants were unchanged by variation in root-zone CO ₂ concentration. In NH ₄ ⁺ -fed plants the δ ¹⁵ N values were c.  1.5‰ higher at 1% CO ₂ . •  Changes in δ ¹³ C values with increased CO ₂ concentration (δ ¹³ C = -10.91‰) were ascribed to root incorporation of CO ₂ . Less than 5% of carbon was derived from root dark fixation and thus cannot explain increases in growth on a mass basis. Reduced discrimination with NH ₄ ⁺ nutrition at 1% CO ₂ could be related to increased exudation of NH ₄ ⁺ and organic nitrogen and also reduced uptake.