C3和C4植物光合途径的适应性变化和进化

高等植物大多为C₃植物, C₄植物和景天酸代谢(Crassulacean acid metabolism, CAM)植物是由C₃植物进化而来的。C₄途径的多源进化表明, 光合途径由C₃途径向C₄途径的转变相对简单。该文分析研究了植物光合途径的进化前景, 指出C₄植物是从C₃植物进化而来的高光效种类, 且地质时期以来降低的大气CO₂浓度和升高的大气温度以及干旱和盐渍化是C₄途径进化的外部动力。C₃植物的C₄途径的发现说明植物的光合途径并非是一成不变的, C₃和C₄植物的光合特征具有极大的可塑性, 某些环境的变化会引起植物光合途径在C₃和C₄途径之间转变。C₃植物具有的C₄途径是环境调控的产物, 是对逆境的适应性进化结果, 因而光合途径的转变也适用于干旱地区植被的适应性生存机理研究。该文还利用国外最新的C₄光合进化模型介绍了植物在进化C₄途径中所经历的7个重要时期(从分子基础到形态基础、结构基础, 再到物质代谢水平、光合酶活水平, 直到C₃和C₄途径协调运转时期, 最后达到形态与功能最优化阶段), 并结合全球气候变化的特点对国内外相关领域的研究进行了分析, 总结了植物光合途径的适应性转变和进化的研究成果, 为今后的相关工作提出建议。     

不同光合类型牧草对干旱-复水的光合生理响应及生长适应策略

基于干旱频率增加、强度增大这一全球降水变化背景, 探究干旱-复水条件下不同功能群(C₃和C₄)植物的光合生理响应及生长适应策略有助于预测降水格局变化条件下草地的植被组成和生态系统功能。该研究采用盆栽实验, 以松嫩草地生长的一年生C₃ (4种)和C₄ (3种)牧草为实验材料, 设置了对照、中度干旱和重度干旱3个水分处理水平, 在干旱末期及复水期对植物进行气体交换、生物量和比叶质量的测量。在干旱条件下, 各物种净光合速率和气孔导度均呈下降趋势, 水分利用效率呈上升趋势。干旱对不同植物光合指标的影响存在功能群差异, 随干旱程度的增加C₄植物逐渐丧失光合优势, 重度干旱对C₄植物净光合速率的影响较C₃植物更加明显。由于干旱条件下C₃植物光合固碳主要受气孔限制而C₄植物主要受代谢限制, 因此复水后C₄植物净光合速率恢复速度较C₃植物慢。干旱条件下, 各物种的生物量降低, 根冠比和比叶质量升高, 干旱对C₃植物各生长指标的影响均大于C₄植物; 复水处理后, C₃植物生物量随干旱强度增加呈下降趋势, 而C₄植物的生物量与对照相比无显著差异。     

C3和C4植物的氮素利用机制

提高植物的氮素利用效率(NUE)不仅有利于保障全球粮食安全, 也是实现农业可持续发展的重要途径。近半个世纪以来, 植物氮素利用机理研究已取得重要进展, 但NUE的调控机制仍不明确, NUE的提高仍然十分有限。高等植物集光合碳素同化和氮素同化于一体, 只有碳氮代谢相互协调, 才能维持植物体内的碳氮平衡, 保证植物正常生长发育。由于C₃和C₄植物的光合氮素利用率(PNUE)存在差异, 对氮素的利用效率也会存在差异。为了更有效地提高作物的NUE, 须更全面地了解C₃和C₄植物对氮素吸收、转运、同化和信号转导等关键因子的功能和调控机制。此外, 面对大气CO₂浓度增高和全球气候变暖条件下的植物碳氮同化及其机理的研究也不容忽视。该文综述了C₃和C₄植物氮素利用关键因素的差异及其调控机制, 并对提高C₃禾本科作物氮素利用效率的遗传改良途径进行了展望。     

开放式空气CO2浓度升高与作物/杂草的竞争关系

CO₂浓度升高对植物的光合作用、呼吸作用和水分利用等生理过程产生直接影响,进而影响植物的生长和繁殖.CO₂浓度升高对于具有C₃光合途径的植物较具C₄光合途径的植物更为有益.由于许多重要的杂草是C₄植物,而许多重要的作物是C₃植物,CO₂浓度升高对杂草/作物的相互关系将有重要影响.本文就全球CO₂浓度升高和气候变化对杂草/作物之间竞争关系影响进行综述,同时针对目前研究现状和可持续农业的需要,提出CO₂浓度升高条件下杂草/作物之间竞争关系及未来农田杂草治理方面理论与实践中有待解决的问题.     

基于稳定碳同位素技术研究大气CO2浓度升高对植物-土壤系统碳循环的影响

大气CO₂浓度升高影响植物光合作用过程和生物量积累,改变植物地上和地下生物量的动态分配.土壤有机质的形成和周转依赖于植物组分的输入,因此,CO₂浓度升高所造成的植物生理和代谢的变化对土壤碳库收支平衡具有重要影响.采用稳定碳同位素(¹³C)技术研究土壤-植物系统的碳循环可阐明大气CO₂浓度升高条件下光合碳在植物各器官的分配特征和时间动态,明确光合碳在土壤中的积累、分解与迁移转化过程以及对土壤有机碳库周转的影响.本文综述了基于¹³C自然丰度法或¹³C示踪技术研究大气CO₂浓度升高对土壤-植物系统碳循环的影响,主要包括:1)对植物光合作用的同位素分馏的影响;2)对植物光合碳(新碳)分配动态的影响;3)对土壤有机碳新老碳库动态以及微生物转化过程的影响.明确上述过程及其调控机制可为预测CO₂浓度升高对陆地生态系统碳循环及源汇效应的长期影响奠定基础.     

基于稳定碳同位素技术研究大气CO2浓度升高对植物-土壤系统碳循环的影响

大气CO₂浓度升高影响植物光合作用过程和生物量积累,改变植物地上和地下生物量的动态分配.土壤有机质的形成和周转依赖于植物组分的输入,因此,CO₂浓度升高所造成的植物生理和代谢的变化对土壤碳库收支平衡具有重要影响.采用稳定碳同位素(¹³C)技术研究土壤-植物系统的碳循环可阐明大气CO₂浓度升高条件下光合碳在植物各器官的分配特征和时间动态,明确光合碳在土壤中的积累、分解与迁移转化过程以及对土壤有机碳库周转的影响.本文综述了基于¹³C自然丰度法或¹³C示踪技术研究大气CO₂浓度升高对土壤-植物系统碳循环的影响,主要包括:1)对植物光合作用的同位素分馏的影响;2)对植物光合碳(新碳)分配动态的影响;3)对土壤有机碳新老碳库动态以及微生物转化过程的影响.明确上述过程及其调控机制可为预测CO₂浓度升高对陆地生态系统碳循环及源汇效应的长期影响奠定基础.     

Construction of CO2-response model of electron transport rate in C4 crop and its application

准确估算光合电子流对CO₂响应的变化趋势对深入了解光合过程具有重要意义。该研究在植物光合作用对CO₂响应新模型(模型I)的基础上构建了电子传递速率(J)对CO₂的响应模型(模型II), 并对用LI-6400-40便携式光合仪测量的玉米(Zea mays)和千穗谷(Amaranthus hypochondriacus)的数据进行了拟合。结果表明, 模型II可以很好地拟合玉米和千穗谷叶片J对CO₂浓度的响应曲线(J-C_a曲线), 得到玉米和千穗谷的最大电子传递速率分别为262.41和393.07 mmol·m ^-2·s ^-1, 与估算值相符合。在此基础上, 对光合电子流分配到其他路径进行了探讨。结果显示, 380 mmol·mol ^-1 CO₂浓度下玉米和千穗谷碳同化所需的电子流为247.92和285.16 mmol·m ^-2·s ^-1, 分配到其他途径的光合电子流为14.49和107.91 mmol·m ^-2·s ^-1(考虑植物CO₂的回收利用)。比较两种植物的其他途径光合电子流分配值发现, 两者相差6倍之多。分析认为这与千穗谷和玉米的催化脱羧反应酶种类以及脱羧反应发生的部位不同密切相关。该发现为人们研究C₄植物中烟酰胺腺嘌呤二核苷磷酸苹果酸酶型和烟酰胺腺嘌呤二核苷酸苹果酸酶型两种亚型之间的差异提供了一个新的视角。此外, 构建的电子传递速率对CO₂的响应模型为人们研究C₄植物的光合电子流的变化规律提供了一个可供选择的数学工具。     

植物高光效基因工程育种

C₄植物所具有的C₄光合途径赋予其较高的光合作用效率,而一些主要的农作物如水稻、小麦、大豆等均为C₃作物,光合效率低下。随着生物技术的发展,通过基因工程手段利用C₄光合特性来改善C₃植物的光合效率进而提高其生物产量逐渐成为植物高光效育种的一个研究热点。综述了目前这一领域的研究进展及存在问题,预测了这一领域的发展前景。     

东北草原区C3、C4植物的生态分布及其适应盐碱环境的生理特性

用5种实验方法对东北草原区233种植物光合类型进行鉴定,并对其相对分布随纬度变化关系及其与土壤含盐量和PH值的关系进行分析.在此基础上对几种典型C₃、C₄牧草适应于盐碱环境的生理特点进行深入研究结果表明,在所鉴定的233种植物中,C₃植物有144种,隶属于28科94属,C₄植物有89种;隶属于17科55属,在高纬度地区C₃植物表现出更高的生长优势,在纬度较低和盐碱化区域,C₄植物分布具相对优势.尤其在盐碱化程度较重的地区,C₄植物成为明显的优势种,分布上的差别决定于它们对环境适应机制上的差异C₃植物对盐碱环境适应机制主要通过积累脯氨酸等有机溶质进行渗透调节,而C₄植物主要通过液泡中离子区域化积累作用进行调节,并且与C₃植物相比对盐碱环境具更强的适应能力.     

全球变化对森林土壤甲烷吸收的影响及其机制研究进展

大气CO₂浓度升高、降水格局改变、全球氮沉降增加和土地覆盖变化等全球变化不仅改变了森林土壤理化性质,而且影响了植物的生长和微生物活性,导致森林土壤碳、氮循环发生改变,进而影响土壤CH₄的吸收.本研究综述了森林土壤CH₄吸收的重要性,森林土壤CH₄吸收对大气CO₂浓度升高、降水格局改变、全球氮沉降增加和土地覆盖变化等全球变化的响应差异及驱动机制.大气CO₂浓度升高抑制土壤CH₄吸收;降水减少倾向于促进土壤CH₄吸收;外源氮输入抑制富氮森林土壤CH₄吸收,而对贫氮森林土壤CH₄吸收则表现为促进或不影响;森林转化为草地、农田或人工林会减少土壤CH₄的吸收量,而植树造林则会增加土壤CH₄的吸收量.今后的研究重点是探讨全球变化对森林土壤CH₄吸收产生长期影响和综合效应,并借助分子生物学方法进一步探究土壤CH₄吸收的微生物学机制.     

Individual and interactive effects of warming and nitrogen supply on CO2 fluxes and carbon allocation in subarctic grassland

Climate warming has been suggested to impact high latitude grasslands severely, potentially causing considerable carbon (C) losses from soil. Warming can also stimulate nitrogen (N) turnover, but it is largely unclear whether and how altered N availability impacts belowground C dynamics. Even less is known about the individual and interactive effects of warming and N availability on the fate of recently photosynthesized C in soil. On a 10-year geothermal warming gradient in Iceland, we studied the effects of soil warming and N addition on CO₂ fluxes and the fate of recently photosynthesized C through CO₂ flux measurements and a ¹³CO₂ pulse-labeling experiment. Under warming, ecosystem respiration exceeded maximum gross primary productivity, causing increased net CO₂ emissions. N addition treatments revealed that, surprisingly, the plants in the warmed soil were N limited, which constrained primary productivity and decreased recently assimilated C in shoots and roots. In soil, microbes were increasingly C limited under warming and increased microbial uptake of recent C. Soil respiration was increased by warming and was fueled by increased belowground inputs and turnover of recently photosynthesized C. Our findings suggest that a decade of warming seemed to have induced a N limitation in plants and a C limitation by soil microbes. This caused a decrease in net ecosystem CO₂ uptake and accelerated the respiratory release of photosynthesized C, which decreased the C sequestration potential of the grassland. Our study highlights the importance of belowground C allocation and C-N interactions in the C dynamics of subarctic ecosystems in a warmer world.     

浑善达克沙化草地C4植物资源及其与植被演替的关系

根据野外调查和文献资料研究了浑善达克沙化草地 C4 植物的种类组成及其与沙化草地植被演替的关系。浑善达克沙化草地共有野生 C4 植物 2 7种 ,分属于 7科、2 2个属 ,其中以禾本科最多 ,有 16种 ,藜科次之 ,有 6种。该区域的 C4植物中 1年生植物约占 6 3% ,且多为沙化演替的先锋物种。C4 植物数量和 C4 / C3比基本随沙化演替的进行而增加 ,尤其在弃耕地和流动沙丘阶段 C4 / C3比均在 2 0 %以上 ,体现了 C4 植物抗逆性强的生物学特点     

森林生态系统碳循环的基本概念和野外测定方法评述

全球气候变化与森林生态系统碳循环息息相关,定量评估森林碳收支是生态系统与全球变化研究的重要任务。30年来森林生态系统碳循环研究已经取得了长足的进展,但全球和区域森林碳收支仍然存在很大的不确定性。这一方面与森林生态系统本身的复杂性有关,另一方面也与具体研究方法有关。评述了森林生态系统碳循环的基本概念和主要野外测定方法,为我国森林生态系统碳循环研究提供可参考的方法论。从生态系统碳浓度、密度、通量、分配和周转5个方面回顾了碳循环相关概念,指出碳浓度和碳储量是对碳库的静态描述,而碳通量和碳周转是对碳库的动态描述。净初级生产力是测量最普遍的碳通量组分,但大多数情况下因忽略了一些细节而被系统低估。普遍使用的净生态系统生产力,由于没有包含非CO2形式的水文、气象和干扰过程产生的碳通量,通常情况下高于生态系统净碳累积速率。在详细介绍碳通量组分的基础上,改进了森林生态系统碳循环的概念模型。重点讨论了碳通量的3种地面实测方法:测树学方法、箱法和涡度协方差法,并指出了其注意事项和不确定性来源。针对当前碳循环研究的突出问题,建议从4个方面减小碳循环测定的不确定性:(1)恰当运用生物量方程估算乔木生物量;(2)尽可能全面测定生态系统碳组分;(3)给出碳通量估算值的不确定性;(4)多种途径交互验证。     

Carbon input by roots into the soil: Quantification of rhizodeposition from root to ecosystem scale

Despite its fundamental role for carbon (C) and nutrient cycling, rhizodeposition remains ‘the hidden half of the hidden half’: it is highly dynamic and rhizodeposits are rapidly incorporated into microorganisms, soil organic matter, and decomposed to CO₂. Therefore, rhizodeposition is rarely quantified and remains the most uncertain part of the soil C cycle and of C fluxes in terrestrial ecosystems. This review synthesizes and generalizes the literature on C inputs by rhizodeposition under crops and grasslands (281 data sets). The allocation dynamics of assimilated C (after ¹³C‐CO₂ or ¹⁴C‐CO₂ labeling of plants) were quantified within shoots, shoot respiration, roots, net rhizodeposition (i.e., C remaining in soil for longer periods), root‐derived CO₂, and microorganisms. Partitioning of C pools and fluxes were used to extrapolate belowground C inputs via rhizodeposition to ecosystem level. Allocation from shoots to roots reaches a maximum within the first day after C assimilation. Annual crops retained more C (45% of assimilated ¹³C or ¹⁴C) in shoots than grasses (34%), mainly perennials, and allocated 1.5 times less C belowground. For crops, belowground C allocation was maximal during the first 1–2 months of growth and decreased very fast thereafter. For grasses, it peaked after 2–4 months and remained very high within the second year causing much longer allocation periods. Despite higher belowground C allocation by grasses (33%) than crops (21%), its distribution between various belowground pools remains very similar. Hence, the total C allocated belowground depends on the plant species, but its further fate is species independent. This review demonstrates that C partitioning can be used in various approaches, e.g., root sampling, CO₂ flux measurements, to assess rhizodeposits’ pools and fluxes at pot, plot, field and ecosystem scale and so, to close the most uncertain gap of the terrestrial C cycle.     

Rhizosphere processes are quantitatively important components of terrestrial carbon and nutrient cycles

While there is an emerging view that roots and their associated microbes actively alter resource availability and soil organic matter (SOM) decomposition, the ecosystem consequences of such rhizosphere effects have rarely been quantified. Using a meta‐analysis, we show that multiple indices of microbially mediated C and nitrogen (N) cycling, including SOM decomposition, are significantly enhanced in the rhizospheres of diverse vegetation types. Then, using a numerical model that combines rhizosphere effect sizes with fine root morphology and depth distributions, we show that root‐accelerated mineralization and priming can account for up to one‐third of the total C and N mineralized in temperate forest soils. Finally, using a stoichiometrically constrained microbial decomposition model, we show that these effects can be induced by relatively modest fluxes of root‐derived C, on the order of 4% and 6% of gross and net primary production, respectively. Collectively, our results indicate that rhizosphere processes are a widespread, quantitatively important driver of SOM decomposition and nutrient release at the ecosystem scale, with potential consequences for global C stocks and vegetation feedbacks to climate.     

Warmer winters increase the rhizosphere carbon flow to mycorrhizal fungi more than to other microorganisms in a temperate grassland

A decisive set of steps in the terrestrial carbon (C) cycle is the fixation of atmospheric C by plants and the subsequent C‐transfer to rhizosphere microorganisms. With climate change winters are expected to become milder in temperate ecosystems. Although the rate and pathways of rhizosphere C input to soil could be impacted by milder winters, the responses remain unknown. To address this knowledge‐gap, a winter‐warming experiment was established in a seminatural temperate grassland to follow the C flow from atmosphere, via the plants, to different groups of soil microorganisms. In situ ¹³CO₂ pulse labelling was used to track C into signature fatty acids of microorganisms. The winter warming did not result in any changes in biomass of any of the groups of microorganisms. However, the C flow from plants to arbuscular mycorrhizal (AM) fungi, increased substantially by winter warming. Saprotrophic fungi also received large amounts of plant‐derived C—indicating a higher importance for the turnover of rhizosphere C than biomass estimates would suggest—still, this C flow was unaffected by winter warming. AM fungi was the only microbial group positively affected by winter warming—the group with the closest connection to plants. Winter warming resulted in higher plant productivity earlier in the season, and this aboveground change likely induced plant nutrient limitation in warmed plots, thus stimulating the plant dependence on, and C allocation to, belowground nutrient acquisition. The preferential C allocation to AM fungi was at the expense of C flow to other microbial groups, which were unaffected by warming. Our findings imply that warmer winters may shift rhizosphere C‐fluxes to become more AM fungal‐dominated. Surprisingly, the stimulated rhizosphere C flow was matched by increased microbial turnover, leading to no accumulation of soil microbial biomass.     

东北草原区的C3,C4牧草及其生态分布的初步研究

本文用RuBPCase活性/PEPCase活性的比值,鉴定东北草原区牧草资源中C_3、C_4植物。供测定45种中35种为首次鉴定。辅助观察禾草类Kranz结构。在此基础上,对其中11种C_3、C_4优势牧草的地理分布、物候谱及其相对优势度与土壤pH值和含盐量之间的关系进行了研究。结果表明,C_3植物30种,隶属于10科24属,C_4植物15种,隶属于5科9属。与C_3植物相比,在高温、强光照和降水量大的季节,C_4植物显示其更高的相对生长优势,在纬度较低和盐碱化区域,C_4植物具有相对分布优势。C.4草相对优势度与土壤pH值和含盐量的相关系数分别为0.826和0.760,而C_3植物均为负值。     

Carbon allocation in forest ecosystems

Carbon allocation plays a critical role in forest ecosystem carbon cycling. We reviewed existing literature and compiled annual carbon budgets for forest ecosystems to test a series of hypotheses addressing the patterns, plasticity, and limits of three components of allocation: biomass, the amount of material present; flux, the flow of carbon to a component per unit time; and partitioning, the fraction of gross primary productivity (GPP) used by a component. Can annual carbon flux and partitioning be inferred from biomass? Our survey revealed that biomass was poorly related to carbon flux and to partitioning of photosynthetically derived carbon, and should not be used to infer either. Are component fluxes correlated? Carbon fluxes to foliage, wood, and belowground production and respiration all increased linearly with increasing GPP (a rising tide lifts all boats). Autotrophic respiration was strongly linked to production for foliage, wood and roots, and aboveground net primary productivity and total belowground carbon flux (TBCF) were positively correlated across a broad productivity gradient. How does carbon partitioning respond to variability in resources and environment? Within sites, partitioning to aboveground wood production and TBCF responded to changes in stand age and resource availability, but not to competition (tree density). Increasing resource supply and stand age, with one exception, resulted in increased partitioning to aboveground wood production and decreased partitioning to TBCF. Partitioning to foliage production was much less sensitive to changes in resources and environment. Overall, changes in partitioning within a site in response to resource supply and age were small (<15% of GPP), but much greater than those inferred from global relationships. Across all sites, foliage production plus respiration, and total autotrophic respiration appear to use relatively constant fractions of GPP – partitioning to both was conservative across a broad range of GPP – but values did vary across sites. Partitioning to aboveground wood production and to TBCF were the most variable – conditions that favored high GPP increased partitioning to aboveground wood production and decreased partitioning to TBCF. Do priorities exist for the products of photosynthesis? The available data do not support the concept of priorities for the products of photosynthesis, because increasing GPP increased all fluxes. All facets of carbon allocation are important to understanding carbon cycling in forest ecosystems. Terrestrial ecosystem models require information on partitioning, yet we found few studies that measured all components of the carbon budget to allow estimation of partitioning coefficients. Future studies that measure complete annual carbon budgets contribute the most to understanding carbon allocation.     

Carbon flow in an upland grassland: effect of liming on the flux of recently photosynthesized carbon to rhizosphere soil

The effect of liming on the flow of recently photosynthesized carbon to rhizosphere soil was studied using ¹³CO₂ pulse labelling, in an upland grassland ecosystem in Scotland. The use of ¹³C enabled detection, in the field, of the effect of a 4‐year liming period of selected soil plots on C allocation from plant biomass to soil, in comparison with unlimed plots. Photosynthetic rates and carbon turnover were higher in plants grown in limed soils than in those from unlimed plots. Higher δ¹³C‰ values were detected in shoots from limed plants than in those from unlimed plants in samples clipped within 15 days of the end of pulse labelling. Analysis of the aboveground plant production corresponding to the 4‐year period of liming indicated that the standing biomass was higher in plots that received lime. Lower δ¹³C‰ values in limed roots compared with unlimed roots were found, whereas no significant difference was detected between soil samples. Extrapolation of our results indicated that more C has been lost through the soil than has been gained via photosynthetic assimilation because of pasture liming in Scotland during the period 1990–1998. However, the uncertainty associated with such extrapolation based on this single study is high and these estimates are provided only to set our findings in the broader context of national soil carbon emissions.     

Partitioning of belowground C in young sugar maple forest

Background and aims

Trees allocate a high proportion of assimilated carbon belowground, but the partitioning of that C among ecosystem components is poorly understood thereby limiting our ability to predict responses of forest C dynamics to global change drivers.

Methods

We labeled sugar maple saplings in natural forest with a pulse of photosynthetic ¹³C in late summer and traced the pulse over the following 3 years. We quantified the fate of belowground carbon by measuring ¹³C enrichment of roots, rhizosphere soil, soil respiration, soil aggregates and microbial biomass.

Results

The pulse of ¹³C contributed strongly to root and rhizosphere respiration for over a year, and respiration comprised about 75 % of total belowground C allocation (TBCA) in the first year. We estimate that rhizosphere carbon flux (RCF) during the dormant season comprises at least 6 % of TBCA. After 3 years, 3.8 % of the C allocated belowground was recovered in soil organic matter, mostly in water-stable aggregates.

Conclusions

A pulse of carbon allocated belowground in temperate forest supplies root respiration, root growth and RCF throughout the following year and a small proportion becomes stabilized in soil aggregates.