Changes in 13C/12C of oil palm leaves to understand carbon use during their passage from heterotrophy to autotrophy

Estimation of intrinsic water-use efficiency from δ13C signature of C3 leaves: Assumptions and uncertainty

Carbon isotope composition (δ¹³C) has been widely used to estimate the intrinsic water-use efficiency (iWUE) of plants in ecosystems around the world, providing an ultimate record of the functional response of plants to climate change. This approach relies on established relationships between leaf gas exchange and isotopic discrimination, which are reflected in different formulations of ¹³C-based iWUE models. In the current literature, most studies have utilized the simple, linear equation of photosynthetic discrimination to estimate iWUE. However, recent studies demonstrated that using this linear model for quantitative studies of iWUE could be problematic. Despite these advances, there is a scarcity of review papers that have comprehensively reviewed the theoretical basis, assumptions, and uncertainty of ¹³C-based iWUE models. Here, we 1) present the theoretical basis of ¹³C-based iWUE models: the classical model (iWUE_sim), the comprehensive model (iWUE_com), and the model incorporating mesophyll conductance (iWUE_mes); 2) discuss the limitations of the widely used iWUE_sim model; 3) and make suggestions on the application of the iWUE_mes model. Finally, we suggest that a mechanistic understanding of mesophyll conductance associated effects and post-photosynthetic fractionation are the bottlenecks for improving the ¹³C-based estimation of iWUE.

Carbon and nitrogen stable isotope variations in leaves of two grapevine cultivars (Chasselas and Pinot noir): Implications for ecophysiological studies

We investigated the within- and between-leaf variability in the carbon and nitrogen isotope composition (δ¹³C and δ¹⁵N) and total nitrogen (TN) content in two grapevine cultivars (Vitis vinifera cv. Chasselas and Pinot noir) field-grown under rain-fed conditions. The within-leaf variability was studied in discs sampled from base-to-tip and left and right regions from the margin to midrib. The intra- and interplant variability was studied by comparing leaves at different positions along the shoot (basal, median, apical). In leaves from both cultivars, a decrease in δ¹³C from base to tip was observed, which is in line with an upward gradient of stomatal density and chlorophyll concentration. Less important, but still significant differences were observed between the right and left discs. The leaf TN and δ¹⁵N values differed between cultivars, showed smaller variations than the δ¹³C values, and no systematic spatial trends. The intraleaf variations in δ¹³C, δ¹⁵N, and TN suggest that stomatal behavior, CO₂ fixation, chlorophyll concentrations, and the chemical composition of leaf components were heterogeneous in the leaves. At the canopy scale, the apical leaves had less ¹³C and more ¹⁵N and TN than the basal leaves, indicating differences in their photosynthetic capacity and remobilizations from old, senescing leaves to younger leaves. Overall, this study demonstrates patchiness in the δ¹³C and δ¹⁵N values of grapevine leaves and species-specificity of the nitrogen assimilation and ¹⁵N fractionation. These findings suggest that care must be taken not to overinterpret foliar δ¹³C and δ¹⁵N values in studies based on fragmented material as markers of physiological and biochemical responses to environmental factors.

Potassium deficiency reconfigures sugar export and induces catecholamine accumulation in oil palm leaves

Metabolic effects of potassium (K) deficiency have been described for nearly 70 years but specific effects of low K availability on sugar composition, sugar export rate and its relationship with other leaf metabolites are not very well documented. Having such pieces of information is nevertheless essential to identify metabolic signatures to monitor K fertilization. This is particularly true in oil-producing crop species such as oil palm (Elaeis guineensis), which is strongly K-demanding and involves high sugar dependence for fruit formation because of low carbon use efficiency in lipid synthesis. Here, we used metabolic analyses, measured sugar export rates with ¹³C isotopic labeling and examined the effects of K availability on both leaflet and rachis sugar metabolism in oil palm seedlings. We show that low K leads to a modification of sugar composition mostly in rachis and decreased sucrose and hexose export rates from leaflets. As a result, leaflets contained more starch and induced alternative pathways such as raffinose synthesis, although metabolites of the raffinose pathway remained quantitatively minor. The alteration of glycolysis by low K was compensated for by an increase in alternative sugar phosphate utilization by tyrosine metabolism, resulting in considerable amounts of tyramine and dopamine.

Effects of Potassium Fertilization on Oil Palm Fruit Metabolism and Mesocarp Lipid Accumulation

Potassium fertilization is commonly practiced in oil palm (Elaeis guineensis) plantations to increase yield. However, its effects on fruit oil content and composition are not well documented. Here, we conducted bunch, metabolomics, and oil composition analyses in two contrasting crosses (Deli × La Mé and Deli × Yangambi) grown under different K fertilization conditions. K availability impacted bunch oil content, resulting in lower water content and higher oil proportion in fruit mesocarp, in Deli × La Mé only, thus showing differential responses of crosses to K. Oil composition at maturity did not significantly change under low K conditions despite clear alterations in fruit metabolism associated with lipid production during maturation, demonstrating the resilience of oil biosynthetic metabolism. However, the analysis of variance in oil content (across K treatments and crosses) demonstrates that sugar availability, lipid synthesis rates, and metabolic recycling are all important in determining the oil content.

Changes and their possible causes in δ13C of dark-respired CO2 and its putative bulk and soluble sources during maize ontogeny

The issues of whether, where, and to what extent carbon isotopic fractionations occur during respiration affect interpretations of plant functions that are important to many disciplines across the natural sciences. Studies of carbon isotopic fractionation during dark respiration in C3 plants have repeatedly shown respired CO2 to be (13)C enriched relative to its bulk leaf sources and (13)C depleted relative to its bulk root sources. Furthermore, two studies showed respired CO2 to become progressively (13)C enriched during leaf ontogeny and (13)C depleted during root ontogeny in C3 legumes. As such data on C4 plants are scarce and contradictory, we investigated apparent respiratory fractionations of carbon and their possible causes in different organs of maize plants during early ontogeny. As in the C3 plants, leaf-respired CO2 was (13)C enriched whereas root-respired CO2 was (13)C depleted relative to their putative sources. In contrast to the findings for C3 plants, however, not only root- but also leaf-respired CO2 became more (13)C depleted during ontogeny. Leaf-respired CO2 was highly (13)C enriched just after light-dark transition but the enrichment rapidly decreased over time in darkness. We conclude that (i) although carbon isotopic fractionations in C4 maize and leguminous C3 crop roots are similar, increasing phosphoenolpyruvate-carboxylase activity during maize ontogeny could have produced the contrast between the progressive (13)C depletion of maize leaf-respired CO2 and (13)C enrichment of C3 leaf-respired CO2 over time, and (ii) in both maize and C3 leaves, highly (13)C enriched leaf-respired CO2 at light-to-dark transition and its rapid decrease during darkness, together with the observed decrease in leaf malate content, may be the result of a transient effect of light-enhanced dark respiration.

Natural (13) C distribution in oil palm (Elaeis guineensis Jacq.) and consequences for allocation pattern

Oil palm has now become one of the most important crops, palm oil representing nearly 25% of global plant oil consumption. Many studies have thus addressed oil palm ecophysiology and photosynthesis-based models of carbon allocation have been used. However, there is a lack of experimental data on carbon fixation and redistribution within palm trees, and important C-sinks have not been fully characterized yet. Here, we carried out extensive measurement of natural (13) C-abundance (δ(13) C) in oil palm tissues, including fruits at different maturation stages. We find a (13) C-enrichment in heterotrophic organs compared to mature leaves, with roots being the most (13) C-enriched. The δ(13) C in fruits decreased during maturation, reflecting the accumulation in (13) C-depleted lipids. We further used observed δ(13) C values to compute plausible carbon fluxes using a steady-state model of (13) C-distribution including metabolic isotope effects ((12) v/(13) v). The results suggest that fruits represent a major respiratory loss (≈39% of total tree respiration) and that sink organs such as fruits are fed by sucrose from leaves. That is, glucose appears to be a quantitatively important compound in palm tissues, but computations indicate that it is involved in dynamic starch metabolism rather that C-exchange between organs.

Changes in δ(13)C of dark respired CO2 and organic matter of different organs during early ontogeny in peanut plants

Carbon isotope composition in respired CO2 and organic matter of individual organs were measured on peanut seedlings during early ontogeny in order to compare fractionation during heterotrophic growth and transition to autotrophy in a species with lipid seed reserves with earlier results obtained on beans. Despite a high lipid content in peanut seeds (48%) compared with bean seeds (1.5%), the isotope composition of leaf- and root-respired CO2 as well as its changes during ontogeny were similar to already published data on bean seedlings: leaf-respired CO2 became (13)C-enriched reaching -21.5‰, while root-respired CO2 became (13)C-depleted reaching around -31‰ at the four-leaf stage. The opposite respiratory fractionation in leaves vs. roots already reported for C3 herbs was thus confirmed for peanuts. However, contrarily to beans, the peanut cotyledon-respired CO2 was markedly (13)C-enriched, and its (13)C-depletion was noted from the two-leaf stage onwards only. Carbohydrate amounts being very low in peanut seeds, this cannot be attributed solely to their use as respiratory substrate. The potential role of isotope fractionation during glyoxylate cycle and/or gluconeogenesis on the (13)C-enriched cotyledon-respired CO2 is discussed.

Opposite carbon isotope discrimination during dark respiration in leaves versus roots - a review

In general, leaves are (13) C-depleted compared with all other organs (e.g. roots, stem/trunk and fruits). Different hypotheses are formulated in the literature to explain this difference. One of these states that CO2 respired by leaves in the dark is (13) C-enriched compared with leaf organic matter, while it is (13) C-depleted in the case of root respiration. The opposite respiratory fractionation between leaves and roots was invoked as an explanation for the widespread between-organ isotopic differences. After summarizing the basics of photosynthetic and post-photosynthetic discrimination, we mainly review the recent findings on the isotopic composition of CO2 respired by leaves (autotrophic organs) and roots (heterotrophic organs) compared with respective plant material (i.e. apparent respiratory fractionation) as well as its metabolic origin. The potential impact of such fractionation on the isotopic signal of organic matter (OM) is discussed. Some perspectives for future studies are also proposed .

Proteomic analysis of grapevine (Vitis vinifera L.) leaf changes induced by transition to autotrophy and exposure to high light irradiance

Using a proteomics approach, we evaluated the response of heterotrophic and autotrophic leaves of grapevine when exposed to high light irradiation. From a total of 572 protein spots detected on two-dimensional gels, 143 spots showed significant variation caused by changes in the trophic state. High light treatment caused variation in 90 spots, and 51 spots showed variation caused by the interaction between both factors. Regarding the trophic state of the leaf, most of the proteins detected in the heterotrophic stage decreased in abundance when the leaf reached the autotrophic stage. Major differences induced by high light were detected in autotrophic leaves. In the high-light-treated autotrophic leaves several proteins involved in the oxidative stress response were up-regulated. This pattern was not observed in the high-light-treated heterotrophic leaves. This indicates that in these types of leaves other mechanisms different to the protein antioxidant system are acting to protect young leaves against the excess of light. This also suggests that these protective mechanisms rely on other sets of proteins or non-enzymatic molecules, or that differences in protein dynamics between the heterotrophic and autotrophic stages makes the autotrophic leaves more prone to the accumulation of oxidative stress response proteins.

BIOLOGICAL SIGNIFICANCE

Transition from a heterotrophic to an autotrophic state is a key period during which the anatomical, physiological and molecular characteristics of a leaf are defined. In many aspects the right functioning of a leaf at its mature stage depends on the conditions under what this transition occurs. This because apart of the genetic control, environmental factors like mineral nutrition, temperature, water supply, light etc. are also important in its control. Many anatomical and physiological changes have been described in several plant species, however in grapevine molecular data regarding changes triggered by this transition or by light stress are still scarce. In this study, we identify that the transition from heterotrophic to autotrophic state in grapevine triggers major changes in the leaf proteome, which are mainly related to processes such as protein synthesis, protein folding and degradation, photosynthesis and chloroplast development. With the exception of proteins involved in carbon fixation, that increased in abundance, most of the proteins detected during the heterotrophic stage decreased in abundance when the leaf reached its autotrophic stage. This is most likely because leaves have reached their full size and from now they have to work as a carbon source for sink organs located in other parts of the plant. Despite the potential control of this transition by light, to date, no studies using a proteomics approach have been conducted to gain a broader view of the effects of short-term high light stress. Our results indicate that short-term high light exposure has a major impact on the proteome of the autotrophic leaves, and trigger a differential accumulation of several proteins involved in the oxidative stress response. Surprisingly, heterotrophic leaves do not display this pattern which can be attributed to a lower sensitivity of these leaves to high light stimulus. In fact we discovered that heterotrophic leaves are more tolerant to light stress than autotrophic leaves. This finding is of high biological significance because it helps to understand how young leaves are able to evolve to autotrophy in areas where high light intensities are predominant. This also reveals in this type of leaves the existence of alternative mechanisms to address this stressful condition. These observations provide new insights into the molecular changes occurring during transition of leaves to autotrophy particularly when this transition occurs under high light intensities. This for example occurs during the springtime when the grapevine buds burst and the young leaves are suddenly exposed to high light intensities.

Profiling of metabolites in oil palm mesocarp at different stages of oil biosynthesis

Oil palm is one of the most productive oil producing crops and can store up to 90% oil in its fruit mesocarp. However, the biosynthetic regulation and drivers of palm mesocarp development are still not well understood. Multiplatform metabolomics technology was used to profile palm metabolites during six critical stages of fruit development in order to better understand lipid biosynthesis. Significantly higher amino acid levels were observed in palm mesocarp preceding lipid biosynthesis. Nucleosides were found to be in high concentration during lipid biosynthesis, whereas levels of metabolites involved in the tricarboxylic acid cycle were more concentrated during early fruit development. Apart from insights into the regulation of metabolites during fruit development in oil palm, these results provide potentially useful metabolite yield markers and genes of interest for use in breeding programs.