C3、C4和CAM途径的生态学意义

本文通过C_3,C_4和CAM植物在不同环境中的分布,从3个方面阐述了这3种碳同化途径在利用环境资源时的生态学意义——资源分隔: 1.光合途径的空间分异:(1)不同光合类型植物的气候带(水平和垂直带)和地理区域分布;(2)不同光合类型植物在微环境中的分布。 2.光合途径的时间分异。 3.光合途径在资源分隔中的重叠。     

C3植物稳定碳同位素组成与盐分的关系

植物在盐生环境中δ13C值的改变可能包含两个成分:一个是盐分对CO2的扩散、传递或光合速率的影响而引起的δ13C值的改变;另一个是光合途径的转换引起的δ13C值的变化,δ13C值的大小与诱导发生CAM或C4代谢的程度有关.植物组织的δ13C值随盐度的变化趋势除了与植物本身固有的耐盐性有关以外,盐度和胁迫时间是影响植物δ13C的重要因素.根据盐生条件下同位素分馏特点可知,盐生植物与非盐生植物的δ13C随盐度的变化趋势有所不同.对非盐生植物而言,在低盐度和短期的盐处理下,随盐度的增加和胁迫时间的延长植物的δ13C值增大,这个阶段限制光合作用的主要因素是气孔导度;但是如果盐度过低,δ13C变化很小,则难以表现出应有的相关性;随着胁迫的加强,当限制光合作用的非气孔因素成为主导因素时,由于光合作用受到强烈抑制(光合结构遭到破坏),δ13C将随之降低.对盐生植物而言,其δ13C与最适盐度有关.最适盐度下,植物的δ13C低于其它盐度条件下的δ13C值.盐生条件下,有些C3植物可能发生光合途径的转换,无论诱导发生的是C4代谢还是CAM代谢,δ13C值均趋于增大.但是,一般情况下,盐处理诱导的光合途径的改变对植物组织整体的δ13C的影响很小.在密闭环境中或郁闭林地,植物和土壤呼吸释放的CO2再次参与光合作用,也会改变植物的δ13C值.为了更加全面地考察植物δ13C与盐度的关系,需要设置较大的盐度范围和进行长期的胁迫处理,才能够获得相对充分的数据,才有利于全面分析植物δ13C值与耐盐性的关系.     

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.     

玉米C4光合叶不同部位解剖结构和光抑制特性的比较

以玉米第5位全展叶(C4光合叶)为材料,分别测定基部、中部和顶部的光合速率后,将叶片置于强光(2000μmol·m-2·s-1)下处理3h和暗中恢复3h,再测定这3个部位在处理期间的叶绿素荧光参数变化;然后分别从叶片的基部、中部和顶部取样观察显微结构和超微结构,测定叶绿素含量。结果表明,3个部位光合速率和叶绿素含量的大小依次为:中部>顶部>基部。基部的维管束鞘细胞叶绿体数量少,体积小,排列无规律,类囊体膜有部分垛叠;中部和顶部维管束鞘细胞叶绿体数量多,体积大,大部分围绕维管束呈离心排列,类囊体膜垛叠消失。在强光下,基部、中部和顶部均发生光抑制,但光抑制程度不同,根据严重度依次为:基部>顶部>中部,3个部位在暗中的光抑制恢复能力依次为:中部>顶部>基部。与叶基部相比,叶中部在强光下能维持较高的电子传递效率(φEo)和较低的热耗散比率(φDo)。这表明,C4光合循环是保持较高电子传递效率、减轻光抑制的重要因子。     

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     

中国东北草原植物中的C3和C4光合作用途径

以光合作用关键羧化酶ＰＥＰＣ和ＲｕＢＰＣ活性化,并且参照叶片ＣＯ补偿浓度,δ＾１３Ｃ值和叶片解剖结构特点来鉴定东北草原区２３３种植物的Ｃ３,或Ｃ４光全作用途径,这些植物隶属于１４４属７３科,其中１３７种为首次鉴定。８９种具有Ｃ４光合作用途径,隶属于５５属１７科;１４４种人有Ｃ３光俣作用途径;隶属于９４属２８在多数Ｃ４种分布在禾本科、莎草科、苋科和藜科。苋属、地肤属、狗昌属和虎尾草属中的均为Ｃ４植物     

C3和C4植物叶片对光氧化响应的日变化

田间生长的C3植物花生和C4植物玉米分别于晴天上午9：00、中午12：00、下午15：00取样。中午12：00花生叶片的Fv／Fm较早上9：见下降16％,出现了光抑制现象,玉米叶片的Fv／Fm则未下降。不同时间取样的花生和玉米叶片经甲基紫精（MV） 强光的人为光氧化处理,叶绿素和类胡萝卜素出现不同程度的氧化降解,中午12：00降解幅度最大,15时降幅最小。植物叶片的抗氧化能力与其SOD活性相关,而与PEPCase的活性没有明显的相关性。光氧化处理后,花生和玉米的叶绿素荧光参数FV／Fm、qp、pSII都下降,花生在12：00的降幅最小,玉米的降幅最大。光氧化引起花生的qN和热耗散系数（KD）上升,玉米则都下降．结果显示C3植物花生和C4植物玉米对光氧化的响应可能存在不同的机制。     

中国东北样带羊草群落C3和C4植物功能群生物量及其对环境变化的响应

在全球动态植被模型的发展中,受限于人力、物力和财力使得在物种水平上的研究变得既不可能也无必要。植物功能群的划分是从生态学的,而不是系统发育的角度来相互比较地对待不同地区的植物,从而削减了植被变化研究中植物分类群的数量,已成为研究植被变化及生物多样性对生态系统功能作用的重要单位。植物的不同光合途径（C3、C4和CAM）从叶片组织结构到生理功能,从生态适应到地理分布均表现出对不同水、热、光环境的响应,是理想的植物功能群分类。为此,分析了中国东北样带以羊草（Leymus chinensis）为建群种或共建种的草原群落植物光合类型功能群生物量及其与群落初级生产力和环境变化的关系。结果表明：（1）C4植物生物量具有明显的变化规律,且对环境变化的响应显著,其变异性较高,更能反映样地问环境变化的差异;（2）C4与C3植物变化具有明显的互补性,并且多数C4植物常在逆境中起到更大作用,如干旱化、盐碱化和放牧干扰;（3）C4植物种类少,在所有调查样方中仅出现7种,占总出现种类的9．72％。这些特点说明C4植物可以考虑作为评估和预测我国温带草原植被及其生态系统变化的重要植物功能群。     

高浓度二氧化碳对植物影响的研究进展

工业革命后全球大气CO2浓度持续上升,不仅对全球气候的变迁产生重大影响,而且对植物的形态、水分利用、蛋白质合成、光合、抗性、生长及生物量等都有不同程度的影响。高浓度CO2促进植物根、幼苗的生长,叶片增厚,降低气孔密度、气孔导度及蒸腾速率,增加水分利用效率、作物的产量及生物量,促进乙烯生物合成,增强植物的抗氧化能力。不同光合途径(C3、C4及CAM)及不同植被类型的植物对高浓度CO2的响应不同。长期和短期的高浓度CO2处理,植物响应方式有很大的差异,如短期高CO2处理使光合能力增强,而长期处理则使光合能力下调。     

大麦和玉米叶片叶绿体中Rubisco及其活化酶的免疫金标记定位

运用免疫金标记电镜技术研究了禾本科C3植物大麦(Hordeum vulgare L.)和C4植物玉米(Zea mays L.)叶片中Rubisoo及其活化酶(RCA)的细胞定位,结果表明:两种植物叶片解剖结构及叶绿体超微结构差别明显.在大麦叶细胞中,只有一种叶肉细胞叶绿体,Rubisoo和RCA主要分布于叶绿体的间质中.在玉米叶细胞中,存在着维管束鞘细胞和叶肉细胞两种类型叶绿体,Rubisco主要分布于鞘细胞叶绿体的基质中,但在叶肉细胞叶绿体中亦有少量特异性标记;RCA在鞘细胞叶绿体和叶肉细胞叶绿体的基质中都有分布.两种植物叶绿体结构及光合作用关键酶定位的不同,体现了C3植物和C4植物在光合器结构与功能上的差异.     

Photosynthetic traits in C3 and C4 grassland species in mesocosm and field environments

The North American tallgrass prairie is composed of a diverse mix of C₃ and C₄ plant species that are subject to multiple resource limitations. C₄ grasses dominate this ecosystem, purportedly due to greater photosynthetic capacity and resource-use efficiency associated with C₄ photosynthesis. We tested the hypothesis that intrinsic physiological differences between C₃ and C₄ species are consistent with C₄ grass dominance by comparing leaf gas exchange and chlorophyll fluorescence variables for seven C₄ and C₃ herbaceous species (legumes and non-legumes) in two different settings: experimental mesocosms and natural grassland sites. In the mesocosms, C₄ grasses had higher photosynthetic rates, water potentials and water-use efficiency than the C₃ species. These differences were absent in the field, where photosynthetic rates declined similarly among non-leguminous species. Thus, intrinsic photosynthetic advantages for C₄ species measured in resource-rich mesocosms could not explain the dominance of C₄ species in the field. Instead, C₄ dominance in this ecosystem may depend more on the ability of the grasses to grow rapidly when resources are plentiful and to tolerate multiple limitations when resources are scarce.     

C4 photosynthesis in a single C3 cell is theoretically inefficient but may ameliorate internal CO2 diffusion limitations of C3 leaves

Attempts are being made to introduce C₄ photosynthetic characteristics into C₃ crop plants by genetic manipulation. This research has focused on engineering single‐celled C₄‐type CO₂ concentrating mechanisms into C₃ plants such as rice. Herein the pros and cons of such approaches are discussed with a focus on CO₂ diffusion, utilizing a mathematical model of single‐cell C₄ photosynthesis. It is shown that a high bundle sheath resistance to CO₂ diffusion is an essential feature of energy‐efficient C₄ photosynthesis. The large chloroplast surface area appressed to the intercellular airspace in C₃ leaves generates low internal resistance to CO₂ diffusion, thereby limiting the energy efficiency of a single‐cell C₄ concentrating mechanism, which relies on concentrating CO₂ within chloroplasts of C₃ leaves. Nevertheless the model demonstrates that the drop in CO₂ partial pressure, pCO₂, that exists between intercellular airspace and chloroplasts in C₃ leaves at high photosynthetic rates, can be reversed under high irradiance when energy is not limiting. The model shows that this is particularly effective at lower intercellular pCO₂. Such a system may therefore be of benefit in water‐limited conditions when stomata are closed and low intercellular pCO₂ increases photorespiration.     

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。     

干旱胁迫对2种光合类型C4荒漠植物叶片光合特征酶和抗氧化酶活性的影响

以荒漠C₄草本植物蔷薇猪毛菜(NADP苹果酸酶型,NADP-ME）和粗枝猪毛菜（NAD苹果酸酶型,NAD-ME）为研究对象,采用盆栽控水试验设置正常供水和轻度、中度、重度干旱处理（土壤含水量分别为田间持水量80%、60%、45%和35%）,通过测定不同程度干旱胁迫下叶片含水量、C₄光合特征酶和抗氧化酶活性等指标,探讨不同类型C₄荒漠植物光合特征酶和抗氧化系统对干旱逆境的适应机制。结果显示：（1）2种植物叶片含水量均随干旱胁迫的加剧不同程度降低。（2）叶片磷酸烯醇式丙酮酸羧化酶（PEPC）活性在中度干旱胁迫下显著增加而在重度干旱胁迫下急剧下降;蔷薇猪毛菜NAD-ME活性和粗枝猪毛菜NADP-ME活性都很低,且它们基本不受干旱胁迫的影响;随干旱胁迫的加剧,蔷薇猪毛菜NADP-ME活性呈下降趋势,而粗枝猪毛菜NAD-ME活性先显著增加而在重度干旱胁迫下显著降低。（3）随着干旱胁迫的加剧,叶片超氧化物歧化酶（SOD）活性呈下降趋势,过氧化物酶（POD）活性在不同程度干旱胁迫下均有不同程度增加;过氧化氢酶（CAT）活性在中度干旱胁迫下均有不同程度的增加,但在重度干旱胁迫下蔷薇猪毛菜CAT活性降低,而粗枝猪毛菜CAT活性显著增加;丙二醛（MDA）含量随干旱胁迫的加剧均有不同程度的增加。研究认为,一定程度干旱胁迫下,2种荒漠植物的PEPC活性均有增加;不同光合类型C₄植物叶片脱羧酶（NADP-ME和NAD-ME）对干旱胁迫的响应有明显的差异。POD和CAT是这两种C₄植物适应干旱胁迫的主要抗氧化酶,但蔷薇猪毛菜CAT在重度干旱胁迫下没有起到积极保护作用。     

Integration and expression of Sorghum C4 phosphoenolpyruvate carboxylase and chloroplastic NADP-malate dehydrogenase separately or together in C3 potato plants

We have integrated two cDNAs expressing Sorghum photosynthetic phosphoenolpyruvate carboxylase (C₄-PEPC) and NADP-malate dehydrogenase (cpMDH), two key enzymes involved in the primary carbon fixation pathway of NADP-malic enzyme-type C₄ plants, separately or together into a C₃ plant (potato). Analysis of the transgenic plants showed a 1.5-fold increase in PEPC and cpMDH activities compared to untransformed plants. Immunolocalization confirmed an increase at the protein level of these two enzymes in the transgenic plants and indicated that the Sorghum cpMDH was specifically addressed to the chloroplasts of potato mesophyll cells. However, integration of either or both of the cDNAs into the potato genome did not appear to significantly modify either tuber starch grain content or the rate of photosynthetic O₂ production compared to control untransformed plants. The low level of transgene expression probably explains the lack of influence on carbon metabolism and photosynthetic rates. This general observation suggests that some complex mechanism may regulate the level of production of foreign C₄ metabolism enzymes in C₃ plants.     

Relationship between allocation of absorbed light energy in PSII and photosynthetic rates of C3 and C4 plants

Two C₃ dicotyledonous crops and five C₄ monocotyledons treated with three levels of nitrogen were used to evaluate quantitatively the relationship between the allocation of absorbed light energy in PSII and photosynthetic rates (P _N) in a warm condition (25–26°C) at four to five levels [200, 400, 800, 1,200 (both C₃ and C₄) and 2,000 (C₄ only) μmol m⁻² s⁻¹] of photosynthetic photon flux density (PPFD). For plants of the same type (C₃ or C₄), there was a linear positive correlation between the fraction of absorbed light energy that was utilized in PSII photochemistry (P) and P _N, regardless of the broad range of their photosynthetic rates due to species-specific effect and/or nitrogen application; meanwhile, the fraction of absorbed light energy that was dissipated through non-photochemical quenching (D) showed a negative linear regression with P _N for each level of PPFD. The intercept of regression lines between P and P _N of C₃ and C₄ plants decreased, and that between D and P _N increased with increasing PPFD. With P and D as the main components of energy dissipation and complementary to each other, the fraction of excess absorbed light energy (E) was unchanged by P _N under the same level of PPFD. At the same level of P _N, C₄ plants had lower P and higher D than C₃ plants, due to the fact that C₄ plants with little or no photorespiration is considered a limited energy sink for electrons. Nevertheless there was a significant negative linear correlation between D and P when data from both C₃ and C₄ plants at varied PPFD levels was merged. The slope of regression lines between P and D was 0.85, indicating that in plants of both types, most of the unnecessary absorbed energy (ca. 85%) could dissipate through non-photochemical quenching, when P was inhibited by low P _N due to species-specific effect and nitrogen limitation at all levels of illumination used in the experiment.     

Photochemical systems in mesophyll and bundle sheath chloroplasts of C4 plants

1. The agranal bundle sheath chloroplasts of Sorghum bicolor possess very low Photosystem II activity compared with the grana-containing mesophyll chloroplasts.

2. Sorghum mesophyll chloroplasts have a chlorophyll (chl) and carotenoid composition similar to that of spinach chloroplasts. In contrast, the sorghum bundle sheath chloroplasts have a higher chl a/chl b ratio; they are enriched in β-carotene and contain relatively less xanthophylls as compared to sorghum mesophyll or spinach chloroplasts.

3. Sorghum mesophyll chloroplasts with 1 cytochrome f, 2 cytochrome b₆ and 2 cytochrome b-559 per 430 chlorophylls have a cytochrome composition similar to spinach chloroplasts. Sorghum bundle sheath chloroplasts contain cytochrome f and cytochrome b₆ in the same molar ratios as for the mesophyll chloroplasts, but cytochrome b-559 is barely detectable.

4. The chl/P700 ratios of mesophyll chloroplasts of S. bicolor and mesophyll and bundle sheath chloroplasts of Atriplex spongiosa are similar to that of spinach chloroplasts suggesting that these chloroplasts possess an identical photosynthetic unit size to that of spinach. The agranal bundle sheath chloroplasts of S. bicolor possess a photosynthetic unit which contains only about half as many chlorophyll molecules per P700 as found in the grana-containing chloroplasts.

5. The similarity of the composition of the bundle sheath chloroplasts of S. bicolor with that of the Photosystem I subchloroplast fragments, together with their smaller photosynthetic unit and low Photosystem II activities suggests that these chloroplasts are highly deficient in the pigment assemblies of Photosystem II.