The influence of (photo)respiration on carbon isotope discrimination in plants

The contribution which (photo)respiration makes to carbon isotope discrimination (Δ¹³C) was examined by conducting simultaneous gas exchange measurements and isotopic analysis of carbon dioxide passing over leaves of Triticum aestivum and Phaseolus vulgaris, via manipulations of the carbon isotope composition (δ¹³C) of source CO₂ during growth and measurement. Dark respiration only altered net Δ¹³C (Δ_obs) at low CO₂ assimilation, and was sensitive to source CO₂δ¹³C during measurement. Photorespiration reduced Δ_obs relative to Δ¹³C predicted from p_i/p_a (Δ_i) over the full range of CO₂ assimilation, to a greater degree under elevated oxygen partial pressure (pO₂), indicating fractionation during photorespiration (f) in T. aestivum. For P. vulgaris, Δ_obs was insensitive to elevated pO₂ at higher assimilation rates, suggesting that f was minimal. A model was developed to calculate gross discrimination (Δ_ps), independent of (photo)respiration, from which estimates of f were obtained for T. aestivum (3.3‰) and P. vulgaris (0.5‰). Because photorespiratory fractionation varies interspecifically, and influences net Δ¹³C which is directly reflected in leaf δ¹³C, consideration of (photo)respiratory fractionation is necessary when interpreting δ¹³C of leaf material, especially under conditions where (photo)respiratory CO₂ losses make a large relative contribution to total plant carbon budgets.     

The ecology of Lagarostrobos franklinii (Hook.f.) Quinn (Podocarpaceae) in Tasmania. 2. Population structure and spatial pattern

Analysis of stand structure and spatial patterning indicates that Lagarostrobos franklinii (Huon pine) regenerates intermittently in response to canopy disturbance. Its ability to resprout rapidly from fallen trees is likely to be important in the continued dominance of gaps by this species in competition with faster-growing tree species. The largely riverine distribution pattern of this species is best explained by rapid vegetative spread down river systems and slow dispersal into canopy gaps away from river edges. The probability of recurrent disturbance within the lifespan of this species is high, implying that competitive exclusion of Huon pine is unlikely once it establishes at a site.     

The ecology of Lagarostrobos franklinii (Hook.f.) Quinn (Podocarpaceae) in Tasmania. 1. Distribution,floristics and environmental correlates

A state-wide survey of the endemic Tasmanian podocarp Lagarostrobos franklinii (Huon pine) resulted in the recognition of four major community types largely restricted to the river systems of western and southern Tasmania. Continuous variation was found between most of these communities. The floristic variability was found to be correlated with temperature, rainfall and geological gradients. It is suggested that the restricted nature of Huon pine distribution in Tasmania is associated with a slow terrestrial dispersal rate rather than a narrow fundamental niche. There is some evidence to suggest that the species has been further restricted by fire.     

Variation and inheritance of carbon isotope discrimination in wheat (Triticum aestivum)

Significant small variations exist in the proportion of the heavy isotope (¹³C) in organic and inorganic materials. The amounts of variations depend on the discrimination against or (in favor of) the heavy isotope through equilibrium reactions or kinetic processes particularly during CO₂ fixation in plants. These variations have also paved the way to measure carbon isotope discrimination in plant materials as a surrogate for water-use efficiency. Thus looking for variation and understanding the inheritance of this character is a prerequisite for improving wheat genotypes showing high water use efficiency and tolerance to drought. The main objectives of this review is to provide an overview of various aspects in which inheritance of ? is documented and of variation for carbon isotope discrimination reported in wheat under different circumstances. The relationship between carbon isotope discrimination and drought tolerance is also discussed exclusively in wheat.     

Physiological determinants of genotypic differences in carbon isotope discrimination in potato grown in well-watered conditions

Carbon isotope discrimination (A), leaf conductance (g_s), photosynthetic capacity, and plant growth were measured in well-watered, glasshouse-grown potato plants of clones from a cross made between diploid Solanum tuberosum and Solanum vernei. Clones showed significant differences (P < 0.001) in g_s, Δ, stomatal density, root growth, and total dry matter production. Carbon isotope discrimination of genotypes was positively correlated (P < 0.001) with g_s. There was no correlation between g_s and stomatal density indicating that differences in g_s reflected differences in stomatal aperture. Differences in rooting characteristics or in root/shoot ratio did not contribute to differences in g_s or A. Genotypic differences in photosynthetic capacity were not statistically significant, and there was no correlation between A and photosynthetic capacity. Total dry matter production and A were positively correlated (P < 0.001) when differences in the time of plant emergence were included in the regression model. It is concluded that differences in A among potato genotypes is largely determined by g_s, but confounding of g_s and photosynthetic capacity reduces genotypic variation in A compared with that in g_s. Total dry matter production is largely determined by processes other than carbon assimilation rate per unit area in individual leaves. Effective use of A as a character for selection in plant breeding depends on elucidating the effects that differences in stomatal characteristics have on crop production both in well-watered and in water-limited crops.     

Physiological characterization of recombinant inbred lines of barley with contrasting levels of carbon isotope discrimination

Background and Aims

Carbon isotope discrimination (Δ¹³C) in C₃ plants used as an indirect measure of water-use efficiency (WUE) provides a tool for selecting crops with high WUE under dry environments.

Methods

We evaluated the physiology and Δ¹³C of a set of 8 F₅ recombinant inbred lines (RILs) with contrasting levels of leaf Δ¹³C derived from two parents, ‘W89001002003’ (low Δ¹³C) and ‘I60049’ (high Δ¹³C) of six-row barley (Hordeum vulgare L.) in a greenhouse and under field conditions in three locations (Lacombe, Vegreville and Castor). In the greenhouse experiment, seven days of water deficit was imposed at the stem elongation stage followed by re-watering to pre-deficit level.

Results

A significant negative relationship between WUE and leaf Δ¹³C was observed. Under water-deficit conditions, both photosynthetic rate (A) and stomatal conductance (g _s ) were significantly reduced with a strong positive correlation (r = 0.89) between the two, and the variation in g _s was proportionally greater than A. The low leaf-Δ¹³C RIL ‘147’ maintained the highest A and g _s among ten genotypes (RILs and parents) under water-deficit conditions. Leaf Δ¹³C was positively correlated with biomass and grain yield in the field trials. Multivariate analysis of leaf Δ¹³C, harvest index and plant height discriminated genotypes into three clusters: drought sensitive, drought tolerant and an intermediate type.

Conclusions

The study suggests that it is possible to select low Δ¹³C lines such as RIL ‘147’, which is able to maintain or produce high yields under low moisture conditions on the Canadian Prairies     

Water availability and carbon isotope discrimination in conifers

The stable C isotope composition ('¹³C) of leaf and wood tissue has been used as an index of water availability at both the species and landscape level. However, the generality of this relationship across species has received little attention. We compiled literature data for a range of conifers and examined relationships among landscape and environmental variables (altitude, precipitation, evaporation) and '¹³C. A significant component of the variation in '¹³C was related to altitude (discrimination decreased with altitude in stemwood, 2.53‰ km^-1 altitude, r²=0.49, and in foliage, 1.91‰ km^-1, r²=0.42), as has been noted previously. The decrease in discrimination with altitude was such that the gradient in CO₂ partial pressure into the leaf (P_a-P_i) and altitude were generally unrelated. The ratio of precipitation to evaporation (P/E) explained significant variation in P_a-P_i of stemwood (r²=0.45) and foliage (r²=0.27), but only at low (<0.8) P/E. At greater P/E there was little or no relationship, and other influences on '¹³C probably dominated the effect of water availability. We also examined the relationship between plant drought stress (O) and '¹³C within annual rings of stemwood from Pinus radiata and Pinus pinaster in south-western Australia. Differential thinning and fertiliser application produced large differences in the availability of water, nutrients and light to individual trees. At a density of 750 stems ha^-1, O and '¹³C were less (more negative) than at 250 stems ha^-1 indicating greater drought stress and less efficient water use, contrary to what was expected in light of the general relationship between discrimination and P/E. The greater '¹³C of trees from heavily thinned plots may well be related to an increased interception of radiation by individual trees and greater concentrations of nutrients in foliage - attributes that increase rates of photosynthesis, reduce P_i and increase '¹³C. '¹³C was thus modified to a greater extent by interception of radiation and by nutrient concentrations than by water availability and the '¹³C-O relationship varied between thinning treatments. Within treatments, the relationship between '¹³C and O was strong (0.38<r²<0.58). We conclude that '¹³C may well be a useful indicator of water availability or drought stress, but only in seasonally dry climates (P/E<1) and where variation in other environmental factors can be accounted for.     

Seasonal carbon isotope discrimination in a grassland community

Summary Grassland communities of arid western North America are often characterized by a seasonal increase in ambient temperature and evaporative demand and a corresponding decline in soil moisture availability. As the environment changes, particular species could respond differently, which should be reflected in a number of physiological processes. Carbon isotope discrimination varies during photosynthetic activity as a function of both stomatal aperture and the biochemistry of the fixation process, and provides an integrated measure of plant response to seasonal changes in the environment. We measured the seasonal course of carbon isotope discrimination in 42 grassland species to evaluate changes in gas exchange processes in response to these varying environmental factors. The seasonal courses were then used to identify community-wide patterns associated with life form, with phenology and with differences between grasses and forbs. Significant differences were detected in the following comparisons: (1) Carbon isotope discrimination decreased throughout the growing season; (2) perennial species discriminated less than annual species; (3) grasses discriminated less than forbs; and (4) early flowering species discriminated more than the later flowering ones. These comparisons suggested that (1) species active only during the initial, less stressful months of the growing season used water less efficiently, and (2) that physiological responses increasing the ratio of carbon fixed to water lost were common in these grassland species, and were correlated with the increase in evaporative demand and the decrease in soil moisture.     

Modelling of carbon isotope discrimination by vegetation

The paper presents a simple box model simulating the temporal variation of atmospheric ¹³CO₂ concentration, atmospheric CO₂ mixing ratio and ¹³C content of plant material. The model is driven by observed meteorological and measured biosphere-atmosphere CO₂ exchange data. The model was calibrated and validated using measurements from a Hungarian atmospheric monitoring station. The simulated atmospheric stable carbon isotope ratio data agreed well with the measured ratios considering both the magnitude and the seasonal dynamics. Observed deviations between the measured and simulated δ¹³C_air values were systematically negative in winters, while deviations were random in sign and smaller by an order of magnitude during periods when the vegetation was photosynthetically active. This difference, supported by a significant correlation between the deviation and modeled fossil fuel contributions to CO₂ concentration, suggests the increased contribution of ¹³C-depleted fossil fuel CO₂ from heating and the lower boundary layer heights during winter.     

Short-term changes in leaf carbon isotope discrimination in salt- and water-stressed c(4) grasses

Online carbon isotope discrimination (Δ) and leaf gas exchange measurements were made with control and salt-stressed Zea mays and Andropogon glomeratus, two NADP-ME type C₄ grasses. Linear relationships between Δ and p_i/p_a (the ratio of intercellular to atmospheric CO₂ partial pressure) were found for control plants which agreed well with theoretical models describing carbon isotope discrimination in C₄ plants. These data provided estimates of , the proportion of CO₂ fixed by phosphoenolpyruvate carboxylase which leaks out of the bundle sheath and the component of fractionation due to diffusion in air. Salt-stressed plants had wider variation in Δ for the same or less range in p_i/p_a. Additional work indicated Δ changed independently of p_i/p_a in both water- and salt-stressed plants, suggesting a possible diurnal change in as plant water status changed linked to a decrease in the activity of the C₃ photosynthetic pathway relative to C₄ pathway activity. The possible effect of stress-induced changes in on organic matter δ¹³ C of C₄ plants is apt to be most apparent in chronically stressed environments.     

Stable carbon isotope discrimination in the smut fungusUstilago violacea

Haploid strains 15.10, 1.C429, and 1.C2y and diploid strain JK2 ofUstilago violacea were grown on one or more of the following carbon sources: glucose, sucrose, maltose, inulin, starch, inositol, glycerol, casein, and yeast extract. The media, both before and after fungal growth, and the fungal cells were analyzed for ¹³C/¹²C content (δ¹³C values) using an isotope ratio mass spectrometer after combustion to CO₂. In all cases, the used and unused media had identical δ¹³C values. Strain 15.10 had significantly less¹³C than the media when grown on glucose, sucrose, maltose, and inositol; significantly more¹³C when grown on inulin, starch, and glycerol; and no significant difference in δ¹³C values when grown on casein and yeast extract media. Other haploid strains responded similarly to 15.10. Diploid strain JK2 was also depleted in¹³C when grown on glucose and enriched in¹³C when grown on glycerol; however, JK2 was slightly depleted in¹³C when grown on casein, whereas all the tested haploid strains were enriched in¹³C.     

QTLs for grain carbon isotope discrimination in field-grown barley

In several crops including cereals, carbon isotope discrimination (Delta) has been associated with drought tolerance in terms of water-use efficiency and yield stability in drought-prone environments. By using a complete genetic map generated from 167 recombinant inbred lines from a cross between Tadmor and Er/Apm, QTLs associated with grain Delta have been detected in barley grown in three Mediterranean field environments, two differing only in water availability. Ten QTLs were identified: one was specific to one environment, two presented interaction with the environment, six presented main effects across three or two environments and one presented both effects. Heading date did not contribute to the environment (E) and G x E effects acting on Delta. Seasonal rainfall and the ratio of rainfall to evapo-transpiration made large contributions to the environmental effect, but their influence on G x E was weaker. Eight QTLs for Delta co-located with QTLs for physiological traits related to plant water status and/or osmotic adjustment, and/or for agronomic traits previously measured on the same population. Some perspectives in terms of characterising drought tolerance are evoked.     

Carbon isotope discrimination varies genetically in c(4) species

Carbon-isotope discrimination (Δ) is used to distinguish between different photosynthetic pathways. It has also been shown that variation in Δ occurs among varieties of C₃ species, but not as yet, in C₄ species. We now report that Δ also varies among genotypes of sorghum (Sorghum bicolor Moench), a C₄ species. The discrimination in leaves of field-grown plants of 12 diverse genotypes of sorghum was measured and compared with their grain yields. Discrimination varied significantly among genotypes, and there was a significant negative correlation between grain yield and Δ. The variation in Δ may be caused by genetic differences in either leakiness of the bundle-sheath cells or by differences in the ratio of assimilation rate to stomatal conductance. At the leaf level, the former should be related to light-use efficiency of carbon fixation and the latter should be related to transpiration efficiency. Both could relate to the yield of the crop.     

Spatial and temporal variation in carbon isotope discrimination in prairie graminoids

We present the results of a 5-year examination of variation in the stable carbon isotope composition () of three C₃ graminoid species from a Sandhills prairie: Agropyron smithii, Carex heliophila and Stipa comata. Although consistent species-specific patterns for mean were seen, there was also significant and substantial among-year and within-season variation in . A smaller contribution to variation in came from topographic variation among sampling sites. Effects of species, year, season and topography contribute to variation in in an additive manner. An association between climate and exists that is consistent with previous work suggesting that reflects the interplay between photosynthetic gas exchange and plant water relations. Within the growing season, the time over which integrates plant response to the environment ranges from days to months.     

木荷稳定碳同位素分辨率的种源差异

利用设置在福建建瓯、浙江淳安和浙江庆元3个区试点的5年生木荷种源试验林,选取18个代表性种源测定叶片的稳定碳同位素分辨率(Δ值),研究其在种源间的差异和地理变异模式,以及造林立地环境和种源生长对其的影响.结果表明：木荷种源叶片稳定碳同位素分辨率存在很大差异,福建建瓯、浙江淳安和庆元3个区试点的叶片Δ值最高和最低种源分别相差6.9％、3.0％和3.7％.种源叶片Δ值与其产地纬度呈显著负相关,而与产地经度的相关性较小,表现为典型的纬向渐变模式,来自木荷分布区南部的种源叶片Δ值较大,说明其长期水分利用效率较低.随着造林立地环境的改善和年降雨量的增多,木荷叶片Δ值显著增加.种源叶片Δ值与其树高、胸径、一级侧枝总数和最大侧枝长等皆呈显著正相关,树冠浓密的速生种源具有较高的Δ值.此外, 在木荷中心产区(福建建瓯)和边缘产区(浙江淳安)分别初选出2个和4个水分利用效率高的速生优良种源,供推广应用.     

Temperature response of carbon isotope discrimination and mesophyll conductance in tobacco

The partial pressure of CO₂ at the sites of carboxylation within chloroplasts depends on the conductance to CO₂ diffusion from intercellular airspace to the sites of carboxylation, termed mesophyll conductance (g_m). We investigated the temperature response of g_m in tobacco (Nicotiana tabacum) by combining gas exchange in high light, ambient CO₂ in either 2 or 21% O₂ with carbon isotope measurements using tuneable diode laser spectroscopy. The g_m increased linearly with temperature in 2 or 21% O₂. In 21% O₂, isotope discrimination associated with g_m decreased from 5.0 ± 0.2 to 1.8 ± 0.2‰ as temperature increased from 15 to 40 °C, but the photorespiratory contribution to the isotopic signal is significant. While the fractionation factor for photorespiration (f = 16.2 ± 0.7‰) was independent of temperature between 20 and 35 °C, discrimination associated with photorespiration increased from 1.1 ± 0.01 to 2.7 ± 0.02‰ from 15 to 40 °C. Other mitochondrial respiration contributed around 0.2 ± 0.03‰. The drawdown in CO₂ partial pressure from ambient air to intercellular airspaces was nearly independent of leaf temperature. By contrast, the increase in g_m with increasing leaf temperature resulted in the drawdown in CO₂ partial pressure between intercellular airspaces and the sites of carboxylation decreasing substantially at high temperature.     

Evaluation of isotope discrimination in (13)C-based metabolic flux analysis

This paper describes a modified noninvasive microtest electrophysiological technology (NMT) for vacuolar H⁺ flux detection. In this NMT system, the vacuole isolation procedure and buffer slope were modified, and the measuring errors from small spherical geometry were corrected. The trends in changes of vacuolar H⁺ flux (ΔH⁺ flux) after ATP or PP_i supply calculated by NMT were consistent with the activities of V-ATPase and PPase measured by traditional methods. These findings indicate that our modified NMT is an appropriate method for vacuolar H⁺ flux detection.     

Genetic differentiation in carbon isotope discrimination and gas exchange in Pseudotsuga menziesii

Patterns of genetic variation in gas-exchange physiology were analyzed in a 15-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantation that contains 25 populations grown from seed collected from across the natural distribution of the species. Seed was collected from 33°30 to 53°12 north latitude and from 170 m to 2930 m above sea level, and from the coastal and interior (Rocky Mountain) varieties of the species. Carbon isotope discrimination () ranged from 19.70() to 22.43() and was closely related to geographic location of the seed source. The coastal variety (20.50 (SE=0.21)) was not significantly different from the interior variety (20.91 (0.15)). Instead, most variation was found within the interior variety; populations from the southern Rockies had the highest discrimination (21.53 (0.20)) (lowest water-use efficiency). Carbon isotope discrimination (), stomatal conductance to water vapor (g), the ratio of intercellular to ambient CO₂ concentration (c_i/c_a), and intrinsic water-use efficiency (A/g) were all correlated with altitude of origin (r=0.76, 0.73, 0.74, and –0.63 respectively); all were statistically significant at the 0.01 level. The same variables were correlated with both height and diameter at age 15 (all at P0.0005). Observed patterns in the common garden did not conform to our expectation of higher WUE, measured by both A/g and , in trees from the drier habitats of the interior, nor did they agree with published in situ observations of decreasing g and with altitude. The genetic effect opposes the altitudinal one, leading to some degree of homeostasis in physiological characteri tics in situ.