Patterns of intramolecular carbon isotopic heterogeneity within amino acids of autotrophs and heterotrophs

A survey of the intramolecular C isotopic composition of a variety of organisms was conducted to investigate the potential of intramolecular isotopic measurements as a tracer of biological or geochemical processes. Based on a consideration of inorganic C sources and enzymatic fractionations, contrasting predictions were made for the relative ¹³C enrichments of the -carboxyl carbons fixed by the anapleurotic ()-carboxylation pathway during amino acid synthesis by photoautotrophs and heterotrophs. To test the model predictions, the stable C isotopic compositions of the acid hydrolyzable C fraction, the total amino acid -carboxyl C fraction and the -carboxyl C of glutamate from a variety of autotrophic and heterotrophic organisms were compared. The relative ¹³C enrichments of carboxyl carbons in the bulk amino acid fraction and in glutamate conformed qualitatively to model predictions. Macroalgal taxa possessed a significantly less enriched carboxyl C fraction than did either C3 or C4 vascular plants, indicating the presence of a different -carboxylation pathway operating in these organisms. In most multicellular heterotrophs, the isotopic composition of the amino acid carboxyl carbons closely resembled that of their food sources. Amino acids are apparently assimilated into tissue proteins directly from their diets without significant metabolic modification. However, shifts in the isotopic composition of the carboxyl C fractions in some organisms were detected that were consistent with the occurrence of significant resynthesis of amino acids from non-amino acid precursors. Comparison of plant leaves and roots provided evidence of environmentally controlled assimilate partitioning. Intramolecular isotopic measurements of biological molecules provide unique insights into the origins and transformations of bio-molecules.     

Drought-inhibition of photosynthesis in C3 plants: stomatal and non-stomatal limitations revisited

There is a long-standing controversy as to whether drought limits photosynthetic CO2 assimilation through stomatal closure or by metabolic impairment in C3 plants. Comparing results from different studies is difficult due to interspecific differences in the response of photosynthesis to leaf water potential and/or relative water content (RWC), the most commonly used parameters to assess the severity of drought. Therefore, we have used stomatal conductance (g) as a basis for comparison of metabolic processes in different studies. The logic is that, as there is a strong link between g and photosynthesis (perhaps co-regulation between them), so different relationships between RWC or water potential and photosynthetic rate and changes in metabolism in different species and studies may be 'normalized' by relating them to g. Re-analysing data from the literature using light-saturated g as a parameter indicative of water deficits in plants shows that there is good correspondence between the onset of drought-induced inhibition of different photosynthetic sub-processes and g. Contents of ribulose bisphosphate (RuBP) and adenosine triphosphate (ATP) decrease early in drought development, at still relatively high g (higher than 150 mmol H20 m(-2) s(-1)). This suggests that RuBP regeneration and ATP synthesis are impaired. Decreased photochemistry and Rubisco activity typically occur at lower g (<100 mmol H20 m(-2) s(-1)), whereas permanent photoinhibition is only occasional, occurring at very low g (<50 mmol H20 m(-2) s(-1)). Sub-stomatal CO2 concentration decreases as g becomes smaller, but increases again at small g. The analysis suggests that stomatal closure is the earliest response to drought and the dominant limitation to photosynthesis at mild to moderate drought. However, in parallel, progressive down-regulation or inhibition of metabolic processes leads to decreased RuBP content, which becomes the dominant limitation at severe drought, and thereby inhibits photosynthetic CO2 assimilation.     

Nonlinearity of photosynthetic responses to growth in rising atmospheric CO2: an experimental and modelling study

Nonlinear responses of photosynthesis to the CO₂ concentration at which plants were grown (C_g) have been often reported in the literature. This study was designed to develop mechanistic understanding of the nonlinear responses with both experimental and modelling approaches. Soybean (Glycine max) was grown in five levels of C_g (280, 350, 525, 700, 1000 ppm) with either a high or low rate of nitrogen fertilization. When the rate of nitrogen fertilization was high, the photosynthetic rate measured at C_g was highest in plants from the 700 ppm CO₂ treatment. When the rate of nitrogen fertilization was low, little variation was observed in the photosynthetic rates of plants from the different treatments measured at their respective C_g. Measurements of CO₂-induced changes in mass-based leaf nitrogen concentration (n_m, an index of changes in biochemical processes) and leaf mass per unit area (h, an index of morphological properties) were used in a model and indicate that the nonlinearity of photosynthetic responses to C_g is largely determined by relative changes in photosynthetic sensitivity, biochemical downregulation, and morphological upregulation. In order to further understand the nonlinear responses, we compiled data from the literature on CO₂-induced changes in n_m and h. These compiled data indicate that h generally increases and n_m usually decreases with increasing C_g, but that the trajectories and magnitudes of the changes in h and n_m vary with species and growth environments. Integration of these variables (n_m and h) into a biochemically based model of photosynthesis enabled us to predict diverse responses of photosynthesis to C_g. Thus a general mechanism is suggested for the highly variable, nonlinear responses of photosynthesis to C_g reported in the literature.     

Light quality effects on subsequent dark 14CO2-fixation in Fucus serratus

The intensity and fate of ¹⁴CO₂-fixation in the dark are studied on Fucus serratus apices previously maintained under low illumination conditions using white, blue, red or yellow isoquantic lights.In the case of a 180 s pulse, light quality affected dark carbon-fixation, with a higher level of incorporation into ethanol-soluble organic matter in the case of yellow light cultivated apices. After a 30 s pulse ¹⁴C was mainly fixed into glycerate and aspartate-malate pools whatever the pre-treatment light conditions, with a higher level into glycerate when apices were pre-illuminated with blue or yellow light. After a 180 s pulse, ¹⁴C was mainly transferred into amino acids (glutamate and alanine) at the expense of aspartate and malate in red and yellow pre-illumination conditions, as found in the white light reference experiment, and only at the expense of glycerate in blue light pre-illumination conditions.The metabolic pathway of glycerate formation, principally enhanced by blue light preillumination, remains unexplained under these non-photosynthetic conditions. Results are discussed with reference to CO₂-fixation via phosphoenolpyruvate carboxykinase and light quality effects on its in vitro activity.     

High irradiance can minimize the negative effect of exogenous sucrose on the photosynthetic capacity of <Emphasis Type="Italic">in vitro</Emphasis> grown coconut plantlets

There is increasing evidence that the sucrose normally added to the culture medium affects negatively the photosynthetic capacity of plantlets. At the same time, however, sucrose cannot be eliminated from the medium, as it is required for normal in vitro growth. We argue that this is true only under the conventional light conditions of growth-rooms. In the present paper irradiance of growth-rooms was increased 10 times and although the sucrose-inhibitory effect was found at high sucrose concentrations, it was possible to grow coconut (Cocos nucifera L.) plantlets without sucrose. Those plantlets showed both high photosynthetic capacity and comparable in vitro growth to those grown with sucrose in the medium under conventional growth-room irradiance. Nevertheless, the best growth was achieved under mixotrophic conditions where at high irradiance and moderate sucrose concentrations plantlets accumulated 27 % more biomass than plantlets grown without sucrose under high irradiance and 43 and 73 % more biomass than their counterparts at low irradiance with or without sucrose, respectively.     

Enhanced Thermal Energy Dissipation Depending on Xanthophyll Cycle and D1 Protein Turnover in Iron-Deficient Maize Leaves Under High Irradiance

Pigment contents of chloroplasts and net photosynthetic rate were dramatically reduced in maize leaves suffering from iron deficiency. However, the reduction in photosynthesis was probably not caused by decreased contents of chlorophylls and carotenoids and by photon absorption; the primary limiting factor for photosynthesis may rather be the decrease of electron transport activity in photosystem 1. Iron-deficient leaves suffered serious acceptor-side photoinhibition, and more than 60 % of absorbed photons were dissipated, while less than 40 % was used in photochemical reaction. Thermal energy dissipation depending on xanthophyll cycle and D1 protein turnover was enhanced when acceptor-side photoinhibition occurred in iron-deficient maize leaves.     

Temperature-Dependent Gas Exchange and Stomatal/Non-Stomatal Limitation to CO2 Assimilation of Quercus Liaotungensis under Midday High Irradiance

The effects of varying leaf temperature (T ₁) on some ecophysiological characteristics of photosynthesis for Quercus liaotungensis Koiz. under ambient radiation stress around midday on clear summer days were investigated using an IRGA equipped with a temperature-controlled cuvette. Net photosynthetic rate (P _N) decreased as T ₁ increased from 30 to 35 °C as a result of stomatal closure, whereas non-stomatal limitation led to decreased P _N in the T ₁ range of 35–45 °C. Decreased transpiration rate (E) and stomatal conductance (g _s) at leaf temperatures above 30 °C were interpreted as a combined feedward effect as a result of enhanced leaf-air vapour pressure deficit (VPD) and stomatal closure. Changes in E from T ₁ 30 to 20 °C depended on VPD when g _s was maintained constant. Water use efficiency (WUE) varied inversely with T ₁ by following a hyperbola. A decrease in intercellular CO₂ concentration (C _i) occurred as a result of stomatal closure and a relatively high carboxylation capacity, whereas inactivation of mesophyll carboxylation in combination with photorespiration might be associated with the observed increase in C _i in the T ₁ range of 40 to 45 °C.     

澳门城市栖息地斑块中鸟类群落功能和谱系多样性

了解群落构建过程是生态学的核心和基本问题,这对于解释物种共存和物种多样性的维持,完善生物多样性保护政策至关重要。与传统的物种多样性相比,功能和谱系多样性能够提供更多的信息,特别是在群落构建过程方面提供深刻见解。2014年5月—2017年1月,采用样线法对澳门地区5个城市栖息地斑块中(生态一区、鹭鸟林、赛马场滩涂、关闸口岸滩涂和莲花桥侧红树林)的鸟类进行繁殖季和越冬季的调查,共调查14次。利用鸟类丰富度和多度来表征群落的物种多样性,其中多度以最大值保留法计;功能和谱系多样性分别采用功能丰富度(functional richness:FRic)和Faith'PD。同时,利用基于多度加权的平均成对功能距离(mean pairwise functional distance,MFD)和平均成对谱系距离(mean pairwise phylogenetic distance,MPD)来表征群落的功能和谱系结构。结果表明:共记录98种鸟类,隶属于14目32科,目、科、种中数量最多的依次为,雀形目鸟类最多为47种,鹭科物种数为11种,白鹭多度为849只;鸟类物种丰富度(68种)和PD在生态一区最高,多度在莲花桥侧红树林最高为2940只,MFD和FRic在赛马场滩涂最高,MPD在生态一区最高;斑块面积在鸟类丰富度、多度、FRic和PD中具有普遍重要性,植被面积和滩涂面积则分别强烈影响着MFD和MPD;关闸口岸滩涂和莲花桥侧红树林的鸟类群落SES.MFD显著低于随机值(P <0. 05),鹭鸟林和赛马场滩涂的鸟类群落SES. MPD显著低于随机值(P<0.05),表明关闸口岸滩涂和莲花桥侧红树林鸟类群落功能聚集(functional clustering),可能与生境过滤驱动群落构建有关。栖息地斑块的特征差异导致了鸟类群落构建机制的不同,这是仅仅考虑物种多样性难以提供的信息,说明了研究群落构建机制时考虑多维度多样性的必要性。     

Ontogeny and leaf gas exchange mediate the carbon isotopic signature of herbaceous plants

Values (Δ(i)) predicted by a simplified photosynthetic discrimination model, based only on diffusion through air followed by carboxylation, are often used to infer ecological conditions from the 13C signature of plant organs (δ13C(p)). Recent studies showed that additional isotope discrimination (d that includes mesophyll conductance, photorespiration and day respiration, and post-carboxylation discrimination) can strongly affect δ13C(p); however, little is known about its variability during plant ontogeny for different species. Effect of ontogeny on leaf gas exchange rates, Δ(i) , observed discrimination (Δ(p)) and d in leaf, phloem and root of seven herbaceous species at three ontogenetic stages were investigated under controlled conditions. Functional group identity and ontogeny significantly affected Δ(i) and Δ(p). However, predicted Δ(i) did not match Δ(p). d, strongly affected by functional group identity and ontogeny, varied by up to 14 ‰. d scaled tightly with stomatal conductance, suggesting complex controls including changes in mesophyll conductance. The magnitude of the changes in δ13C(p) due to ontogeny was similar to that due to environmental factors reported in other studies. d and ontogeny should, therefore, be considered in ecosystem studies, integrated in ecosystem models using δ13C(p) and limit the applicability of δ13C(leaf) as a proxy for water-use efficiency in herbaceous plants.     

Modelling ¹⁸O₂ and ¹⁶O₂ unidirectional fluxes in plants: I. regulation of pre-industrial atmosphere

In closed systems, the O₂ compensation point (Γ_O) was previously defined as the upper limit of O₂ level, at a given CO₂ level, above which plants cannot have positive carbon balance and survive. Studies with ¹⁸O₂ measure the actual O₂ uptake by photorespiration due to the dual function of Rubisco, the enzyme that fixes CO₂ and takes O₂ as an alternative substrate. One-step modelling of CO₂ and O₂ uptakes allows calculating a plant specificity factor (Sp) as the sum of the biochemical specificity of Rubisco and a biophysical specificity, function of the resistance to CO₂ transfer from the atmosphere to Rubisco. The crossing points (Cx, Ox) are defined as CO₂ and O₂ concentrations for which O₂ and CO₂ uptakes are equal. It is observed that: (1) under the preindustrial atmosphere, photorespiration of C3 plants uses as much photochemical energy as photosynthesis, i.e. the Cx and Ox are equal or near the CO₂ and O₂ concentrations of that epoch; (2) contrarily to Γ_C, a Γ_O does not practically limit the plant growth, i.e. the plant net CO₂ balance is positive up to very high O₂ levels; (3) however, in a closed biosystem, Γ_O exists; it is not the limit of plant growth, but the equilibrium point between photosynthesis and the opposite respiratory processes; (4) a reciprocal relationship exists between Γ_O and Γ_C, as unique functions of the respective CO₂ and O₂ concentrations and of Sp, this invalidates some results showing two different functions for Γ_O and Γ_C, and, consequently, the associated analyses related to greenhouse effects in the past; (5) the pre-industrial atmosphere levels of O₂ and CO₂ are the Γ_O and Γ_C of the global bio-system. They are equal to or near the values of Cx and Ox of C3 plants, the majority of land plants in preindustrial period. We assume that the crossing points represent favourable feedback conditions for the biosphere-atmosphere equilibrium and could result from co-evolution of plants-atmosphere-climate. We suggest that the evolution of Rubisco and associated pathways is directed by an optimisation between photosynthesis and photorespiration.