Do metabolic fluxes matter for interpreting isotopic respiratory signals?

δ13C as a tool for iron and phosphorus deficiency prediction in crops

Many studies proposed the use of stable carbon isotope ratio (δ¹³C) as a predictor of abiotic stresses in plants, considering only drought and nitrogen deficiency without further investigating the impact of other nutrient deficiencies, that is, phosphorus (P) and/or iron (Fe) deficiencies. To fill this knowledge gap, we assessed the δ¹³C of barley (Hordeum vulgare L.), cucumber (Cucumis sativus L.), maize (Zea mays L.), and tomato (Solanum lycopersicon L.) plants suffering from P, Fe, and combined P/Fe deficiencies during a two-week period using an isotope-ratio mass spectrometer. Simultaneously, plant physiological status was monitored with an infra-red gas analyzer. Results show clear contrasting time-, treatment-, species-, and tissue-specific variations. Furthermore, physiological parameters showed limited correlation with δ¹³C shifts, highlighting that the plants' δ¹³C, does not depend solely on photosynthetic carbon isotope fractionation/discrimination (Δ). Hence, the use of δ¹³C as a predictor is highly discouraged due to its inability to detect and discern different nutrient stresses, especially when combined stresses are present.

Two distinct plant respiratory physiotypes might exist which correspond to fast-growing and slow-growing species

The origin of the carbon atoms in CO2 respired by leaves in the dark of several plant species has been studied using 13C/12C stable isotopes. This study was conducted using an open gas exchange system for isotope labeling that was coupled to an elemental analyzer and further linked to an isotope ratio mass spectrometer (EA-IRMS) or coupled to a gas chromatography-combustion-isotope ratio mass spectrometer (GC-C-IRMS). We demonstrate here that the carbon, which is recently assimilated during photosynthesis, accounts for nearly ca. 50% of the carbon in the CO2 lost through dark respiration (Rd) after illumination in fast-growing and cultivated plants and trees and, accounts for only ca. 10% in slow-growing plants. Moreover, our study shows that fast-growing plants, which had the largest percentages of newly fixed carbon of leaf-respired CO2, were also those with the largest shoot/root ratios, whereas slow-growing plants showed the lowest shoot/root values.

Species-specific differences in temporal and spatial variation in δ(13)C of plant carbon pools and dark-respired CO (2) under changing environmental conditions

Stable carbon isotope signatures are often used as tracers for environmentally driven changes in photosynthetic δ(13)C discrimination. However, carbon isotope signatures downstream from carboxylation by Rubisco are altered within metabolic pathways, transport and respiratory processes, leading to differences in δ(13)C between carbon pools along the plant axis and in respired CO(2). Little is known about the within-plant variation in δ(13)C under different environmental conditions or between species. We analyzed spatial, diurnal, and environmental variations in δ(13)C of water soluble organic matter (δ(13)C(WSOM)) of leaves, phloem and roots, as well as dark-respired δ(13)CO(2) (δ(13)C(res)) in leaves and roots. We selected distinct light environments (forest understory and an open area), seasons (Mediterranean spring and summer drought) and three functionally distinct understory species (two native shrubs-Halimium halimifolium and Rosmarinus officinalis-and a woody invader-Acacia longifolia). Spatial patterns in δ(13)C(WSOM) along the plant vertical axis and between respired δ(13)CO(2) and its putative substrate were clearly species specific and the most δ(13)C-enriched and depleted values were found in δ(13)C of leaf dark-respired CO(2) and phloem sugars, ~-15 and ~-33 ‰, respectively. Comparisons between study sites and seasons revealed that spatial and diurnal patterns were influenced by environmental conditions. Within a species, phloem δ(13)C(WSOM) and δ(13)C(res) varied by up to 4 ‰ between seasons and sites. Thus, careful characterization of the magnitude and environmental dependence of apparent post-carboxylation fractionation is needed when using δ(13)C signatures to trace changes in photosynthetic discrimination.

(12)C/(13)C fractionations in plant primary metabolism

Natural (13)C abundance is now an unavoidable tool to study ecosystem and plant carbon economies. A growing number of studies take advantage of isotopic fractionation between carbon pools or (13)C abundance in respiratory CO(2) to examine the carbon source of respiration, plant biomass production or organic matter sequestration in soils. (12)C/(13)C isotope effects associated with plant metabolism are thus essential to understand natural isotopic signals. However, isotope effects of enzymes do not influence metabolites separately, but combine to yield a (12)C/(13)C isotopologue redistribution orchestrated by metabolic flux patterns. In this review, we summarise key metabolic isotope effects and integrate them into the corpus of plant primary carbon metabolism.

The 13C/12C isotopic signal of day-respired CO2 in variegated leaves of Pelargonium × hortorum

In leaves, although it is accepted that CO(2) evolved by dark respiration after illumination is naturally (13) C-enriched compared to organic matter or substrate sucrose, much uncertainty remains on whether day respiration produces (13) C-depleted or (13) C-enriched CO(2). Here, we applied equations described previously for mesocosm CO(2) exchange to investigate the carbon isotope composition of CO(2) respired by autotrophic and heterotrophic tissues of Pelargonium × hortorum leaves, taking advantage of leaf variegation. Day-respired CO(2) was slightly (13) C-depleted compared to organic matter both under 21% O(2) and 2% O(2). Furthermore, most, if not all CO(2) molecules evolved in the light came from carbon atoms that had been fixed previously before the experiments, in both variegated and green leaves. We conclude that the usual definition of day respiratory fractionation, that assumes carbon fixed by current net photosynthesis is the respiratory substrate, is not valid in Pelargonium leaves under our conditions. In variegated leaves, total organic matter was slightly (13) C-depleted in white areas and so were most primary metabolites. This small isotopic difference between white and green areas probably came from the small contribution of photosynthetic CO(2) refixation and the specific nitrogen metabolism in white leaf areas.