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The CO2 respired by leaves is13C-enriched relative to leaf biomass and putative respiratory substrates (Ghashghaieet al.,Phytochemistry Reviews2, 145–161, 2003), but how this relates to the13C content of root, or whole plant respiratory CO2 is unknown. The C isotope composition of respiratory CO2 (δR) from shoots and roots of sunflower (Helianthus annuusL.), alfalfa (Medicago sativaL.), and perennial ryegrass (Lolium perenneL.) growing in a range of conditions was analysed. In all instances plants were grown in controlled environments with CO2 of constant concentration andδ13C. Respiration of roots and shoots of individual plants was measured with an open CO2 exchange system interfaced with a mass spectrometer. Respiratory CO2 from shoots was always13C-enriched relative to that of roots. Conversely, shoot biomass was always13C-depleted relative to root biomass. Theδ-difference between shoot and root respiratory CO2 was variable, and negatively correlated with theδ-difference between shoot and root biomass (r2 = 0.52,P = 0.023), suggesting isotope effects during biosynthesis.13C discrimination in respiration (R) of shoots, roots and whole plants (eShoot,eRoot,ePlant) was assessed ase = (δSubstrate − δR)/(1 + δR/1000), where root and shoot substrate is defined as imported C, and plant substrate is total photosynthate. Estimates were obtained from C isotope balances of shoots, roots and whole plants of sunflower and alfalfa using growth and respiration data collected at intervals of 1 to 2 weeks.eplant andeShoot differed significantly from zero.eplant ranged between−0.4 and−0.9‰, whereaseShoot was much greater (−0.6 to−1.9‰).eRoot was not significantly different from zero. The present results help to resolve the apparent conflict between leaf- and ecosystem-level13C discrimination in respiration. [ABSTRACT FROM AUTHOR]

A 13C/12C mass spectrometer was interfaced with a open gas exchange system including four growth chambers to investigate CO2 exchange components of perennial ryegrass ( Lolium perenne L.) stands. Chambers were fed with air containing CO2 with known δ13C ( δCΟ2−2.6 or −46.8‰). The system did not fractionate C isotopes and no extraneous CO2 leaked into chambers. The on-line 13C discrimination (Δ) of ryegrass stands in light was independent of δCΟ2 when δCΟ2 was constant. The δ of CO2 exchanged by the stands in light ( δNd) and darkness ( δRn) differed by 0.7‰, suggesting some Δ in dark respiration at the stand-level. However, Δ decreased by ∼ 10‰ when δCΟ2 was switched from −46.8 to −2.5‰, and increased by ∼ 10‰ following a shift from −2.6 to −46.7‰ due to isotopic disequilibria between photosynthetic and respiratory fluxes. Isotopic imbalances were used to assess (non-photorespiratory) respiration in light and the replacement of the respiratory substrate pool(s) by new photosynthate. Respiration was partially inhibited by light, but increased during the light period and decreased in darkness, in association with temperature changes. The labelling kinetics of respiratory CO2 indicated the existence of two major respiratory substrate pools: a fast pool which was exchanged within hours, and a slow pool accounting for ∼ 60% of total respiration and having a mean residence time of 3.6 d. [ABSTRACT FROM AUTHOR]

ABSTRACTThe effect of defoliation on leaf elongation rate (LER) and on the spatial distribution of epidermal cell lengths in the leaf growth zone was studied in vegetative main tillers of perennial ryegrass (Lolium perenne L. cv Modus) grown in a controlled environment. A new material approach was used to analyse the responses of epidermal cell expansion and production during the initial, non-steady growth phase following defoliation. The analysis involved assigning an identity to individual expanding cells, assessing the displacement and estimating the expansion of cells with assigned identity during day 1 and day 2 after defoliation. LER decreased by 34% during the first 2 d after defoliation and did not recover to the pre-defoliation rate within the 14 day regrowth period. Decreased LER on day 1 and day 2 after defoliation was associated with (i) a decrease in the length of the leaf growth zone; (ii) a decrease in the length at which epidermal cells stopped expanding; (iii) a reduced expansion of cells at intermediate growth stages; and (iv) a reduction in cell production (i.e. division) and an associated decrease in the number of expanding cells in the growth zone. However, defoliation had no effect on the expansion of cells located in the proximal part of the growth zone. Reduced LER at 14 d after defoliation was associated with a reduced cell production rate (27% lower than the pre-defoliation rate) and decreased final cell size (- 28%). [ABSTRACT FROM AUTHOR]

The 18 O enrichment (Δ 18 O) of cellulose (Δ 18 O Cel ) is recognized as a unique archive of past climate and plant function. However, there is still uncertainty regarding the proportion of oxygen in cellulose (p ex ) that exchanges post-photosynthetically with medium water of cellulose synthesis. Particularly, recent research with C 3 grasses demonstrated that the Δ 18 O of leaf sucrose (Δ 18 O Suc , the parent substrate for cellulose synthesis) can be much higher than predicted from daytime Δ 18 O of leaf water (Δ 18 O LW ), which could alter conclusions on photosynthetic versus post-photosynthetic effects on Δ 18 O Cel via p ex . Here, we assessed p ex in leaves of perennial ryegrass (Lolium perenne) grown at different atmospheric relative humidity (RH) and CO 2 levels, by determinations of Δ 18 O Cel in leaves, Δ 18 O LGDZW (the Δ 18 O of water in the leaf growth-and-differentiation zone) and both Δ 18 O Suc and Δ 18 O LW (adjusted for ε bio , the biosynthetic fractionation between water and carbohydrates) as alternative proxies for the substrate for cellulose synthesis. Δ 18 O LGDZW was always close to irrigation water, and p ex was similar (0.53 ± 0.02 SE) across environments when determinations were based on Δ 18 O Suc . Conversely, p ex was erroneously and variably underestimated (range 0.02-0.44) when based on Δ 18 O LW . The photosynthetic signal fraction in Δ 18 O Cel is much more constant than hitherto assumed, encouraging leaf physiological reconstructions., (© 2024 The Authors. Plant, Cell & Environment published by John Wiley & Sons Ltd.)

The 13C isotope composition (δ13C) of leaf dry matter is a useful tool for physiological and ecological studies. However, how post-photosynthetic fractionation associated with respiration and carbon export influences δ13C remains uncertain. We investigated the effects of post-photosynthetic fractionation on δ13C of mature leaves of Cleistogenes squarrosa, a perennial C4 grass, in controlled experiments with different levels of vapour pressure deficit and nitrogen supply. With increasing leaf age class, the 12C/13C fractionation of leaf organic matter relative to the δ13C of atmosphere CO2 (ΔDM) increased while that of cellulose (Δcel) was almost constant. The divergence between ΔDM and Δcel increased with leaf age class, with a maximum value of 1.6‰, indicating the accumulation of post-photosynthetic fractionation. Applying a new mass balance model that accounts for respiration and export of photosynthates, we found an apparent 12C/13C fractionation associated with carbon export of -0.5‰ to -1.0‰. Different ΔDM among leaves, pseudostems, daughter tillers, and roots indicate that post-photosynthetic fractionation happens at the whole-plant level. Compared with ΔDM of old leaves, ΔDM of young leaves and Δcel are more reliable proxies for predicting physiological parameters due to the lower sensitivity to post-photosynthetic fractionation and the similar sensitivity in responses to environmental changes., (© The Author(s) 2023. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

The 18 O enrichment (Δ 18 O) of leaf water affects the Δ 18 O of photosynthetic products such as sucrose, generating an isotopic archive of plant function and past climate. However, uncertainty remains as to whether leaf water compartmentation between photosynthetic and nonphotosynthetic tissue affects the relationship between Δ 18 O of bulk leaf water (Δ 18 O LW ) and leaf sucrose (Δ 18 O Sucrose ). We grew Lolium perenne (a C 3 grass) in mesocosm-scale, replicated experiments with daytime relative humidity (50% or 75%) and CO 2 level (200, 400 or 800 μmol mol -1 ) as factors, and determined Δ 18 O LW , Δ 18 O Sucrose and morphophysiological leaf parameters, including transpiration (E leaf ), stomatal conductance (g s ) and mesophyll conductance to CO 2 (g m ). The Δ 18 O of photosynthetic medium water (Δ 18 O SSW ) was estimated from Δ 18 O Sucrose and the equilibrium fractionation between water and carbonyl groups (ε bio ). Δ 18 O SSW was well predicted by theoretical estimates of leaf water at the evaporative site (Δ 18 O e ) with adjustments that correlated with gas exchange parameters (g s or total conductance to CO 2 ). Isotopic mass balance and published work indicated that nonphotosynthetic tissue water was a large fraction (~0.53) of bulk leaf water. Δ 18 O LW was a poor proxy for Δ 18 O Sucrose , mainly due to opposite Δ 18 O responses of nonphotosynthetic tissue water (Δ 18 O non-SSW ) relative to Δ 18 O SSW , driven by atmospheric conditions., (© 2023 The Authors. Plant, Cell & Environment published by John Wiley & Sons Ltd.)

Altering plant water use efficiency (WUE) is a promising approach for achieving sustainable crop production in changing climate scenarios. Here, we show that WUE can be tuned by alleles of a single gene discovered in elite maize (Zea mays) breeding material. Genetic dissection of a genomic region affecting WUE led to the identification of the gene ZmAbh4 as causative for the effect. CRISPR/Cas9-mediated ZmAbh4 inactivation increased WUE without growth reductions in well-watered conditions. ZmAbh4 encodes an enzyme that hydroxylates the phytohormone abscisic acid (ABA) and initiates its catabolism. Stomatal conductance is regulated by ABA and emerged as a major link between variation in WUE and discrimination against the heavy carbon isotope (Δ13C) during photosynthesis in the C4 crop maize. Changes in Δ13C persisted in kernel material, which offers an easy-to-screen proxy for WUE. Our results establish a direct physiological and genetic link between WUE and Δ13C through a single gene with potential applications in maize breeding., (© The Author(s) 2022. Published by Oxford University Press on behalf of American Society of Plant Biologists.)

Background: The anthropogenic increase of atmospheric CO 2 concentration (c a ) is impacting carbon (C), water, and nitrogen (N) cycles in grassland and other terrestrial biomes. Plant canopy stomatal conductance is a key player in these coupled cycles: it is a physiological control of vegetation water use efficiency (the ratio of C gain by photosynthesis to water loss by transpiration), and it responds to photosynthetic activity, which is influenced by vegetation N status. It is unknown if the c a -increase and climate change over the last century have already affected canopy stomatal conductance and its links with C and N processes in grassland., Results: Here, we assessed two independent proxies of (growing season-integrating canopy-scale) stomatal conductance changes over the last century: trends of δ 18 O in cellulose (δ 18 O cellulose ) in archived herbage from a wide range of grassland communities on the Park Grass Experiment at Rothamsted (U.K.) and changes of the ratio of yields to the CO 2 concentration gradient between the atmosphere and the leaf internal gas space (c a - c i ). The two proxies correlated closely (R 2  = 0.70), in agreement with the hypothesis. In addition, the sensitivity of δ 18 O cellulose changes to estimated stomatal conductance changes agreed broadly with published sensitivities across a range of contemporary field and controlled environment studies, further supporting the utility of δ 18 O cellulose changes for historical reconstruction of stomatal conductance changes at Park Grass. Trends of δ 18 O cellulose differed strongly between plots and indicated much greater reductions of stomatal conductance in grass-rich than dicot-rich communities. Reductions of stomatal conductance were connected with reductions of yield trends, nitrogen acquisition, and nitrogen nutrition index. Although all plots were nitrogen-limited or phosphorus- and nitrogen-co-limited to different degrees, long-term reductions of stomatal conductance were largely independent of fertilizer regimes and soil pH, except for nitrogen fertilizer supply which promoted the abundance of grasses., Conclusions: Our data indicate that some types of temperate grassland may have attained saturation of C sink activity more than one century ago. Increasing N fertilizer supply may not be an effective climate change mitigation strategy in many grasslands, as it promotes the expansion of grasses at the disadvantage of the more CO 2 responsive forbs and N-fixing legumes.