Characterization of photosynthetic pathway of plant species growing in the eastern Tibetan plateau using stable carbon isotope composition

The photosynthetic pathway of plant species collected at Menyuan, Henan, and Maduo sites, east of Tibetan Plateau, China, during the growing season were studied using stable carbon isotopes in leaves. The 232 samples leaves analyzed belonged to 161 species, 30 families, and 94 genera. The delta(13)C values (from -24.6 to -29.2 %o) indicated that all the considered species had a photosynthetic C-3 pathway. The absence of plant species with C-4 photosynthetic pathway might be due to the extremely low air temperature characterizing the Tibetan Plateau. The average delta(13)C value was significantly (p < 0.05) different between annuals and perennials at the three considered study sites. Hence the longer-lived species had greater water-use efficiency (WUE) than shorter-lived species, that is, longer-lived species are better adapted to the extreme environmental conditions of the Tibetan Plateau.

浑善达克沙地优势豆科植物 光合生理特性研究

　　氮素是植物光合生产的决定性因素，尤其是在沙地草地生态系统中，氮素贫乏往往限制植物的生长发育。因此，研究沙地植物光合作用与叶片N含量之间的关系，以及不同植物功能型氮素利用效率，有助于理解不同植物资源利用效率的差异。以浑善达克沙地分布的80种植物为研究对象，对不同生境(固定沙丘、丘间低地和湿地)、不同生活型(乔、灌、草)、不同光合途径(C3和C4)以及豆科和非豆科植物等功能型进行研究，结果表明：无论在单位叶面积水平还是单位干重水平上的叶片氮含量，均与光合速率成极显著正相关，但单位氮素的光合利用效率在不同生境以及不同功能型之间差异很大;光合氮素利用效率表现为：湿地植物>沙丘>丘间低地植物;草本植物>灌木>乔木;C4草本>C3草本植物，非豆科植物>豆科植物。 　　为了验证浑善达克沙地豆科植物是否比非豆科植物具有更高的光合潜力，我们比较研究了3种优势豆科植物小叶锦鸡儿(Caragana microphylla)、木岩黄芪(Hedysarum fruticosum var. lignosum)、披针叶黄华(Thermopsis lanceolata)和2种非豆科植物羊草(Leymus chinensis)和黄柳(Salix gordejeviii)，结果表明并非所有豆科植物都比非豆科植物有着显著高的光合速率，仅木岩黄芪表现出较高的光合速率，其它两种豆科植物的光合速率和羊草、黄柳的差异并不显著(P>0.05)，甚至低于后者，这是因为氮素利用效率(PNUE)在其中起关键作用，通过对影响PNUE的几个主要因素进行分析得出：叶绿素对光能的吸收、光化学转换效率和CO2分压并不是构成豆科和非豆科植物PNUE差异的主要因素，而Rubisco羧化效率决定了所实验的5种植物对氮素利用效率的高低。 　　木岩黄芪在浑善达克沙地的沙丘上为优势种，甚至成为流动沙丘的先锋种。除了其显著高的氮含量外，对沙丘胁迫生境的光合适应性是我们关注的另一个重点。通过对木岩黄芪和其伴生种黄柳的光反应曲线以及光合日动态的研究，发现木岩黄芪具有显著高的光合速率、水分利用效率和PSII 光化学效率，其忍受中午强光和高温的能力较强(即“光合午休”现象不明显)。另外，该物种还表现出了显著高的光饱和点和低光补偿点。 对木岩黄芪的模拟降雨试验结果表明：气体交换参数以及叶绿素荧光参数均受到干旱和模拟降雨的影响，其中气孔因素和非气孔因素共同决定了干旱条件下木岩黄芪光合速率的降低;但降雨解除干旱后，气孔导度恢复较快，而PSII 潜在活性和PSII 光能转换效率的恢复却比较缓慢。在0-15mm的降雨量范围内，随降雨量的增加各项生理指标不断升高，但大于15mm的降雨量对木岩黄芪影响不大，因此木岩黄芪可被视为低耗水型植物。 　　对木岩黄芪光合酶的研究结果表明，其C4光合酶的活性很高，磷酸稀醇式丙酮酸羧化酶(PEPcase)、NAD-苹果酸酶(NAD-ME)、NADP-苹果酸酶(NADP-ME)、NAD-苹果酸脱氢酶(NAD-MDH)、NADP-苹果酸脱氢酶(NADP-MDH) 和丙酮酸磷酸双激酶(PPDK)等酶的活性，在整个生育期内为黄柳的5倍以上，但稳定性碳同位素测定结果却表明木岩黄芪为C3植物。因此，我们认为C3豆科植物木岩黄芪体内可能存在着C4光合途径，这种机制使得其对于流动沙丘的胁迫环境有着很强的适应性和很高的资源利用效率。

Natural 13C abundance as a tracer for studies of soil organic matter dynamics

A method for measuring the long- and medium-term turnover of soil organic matter is described. Its principle is based on the variations of 13C natural isotope abundance induced by the repeated cultivations of a plant with a high 13C/12C ratio (C4 photosynthetic pathway) on a soil which has never carried any such plant. The 13C/12C ratio in soil organic matter being about equal to the 13C/12C ratio of plant materials from which it is derived, changing the 13C content of the organic inputs to the soil (by altering vegetation from C3 type into C4 type) is equivalent to a true labelling in situ of the organic matter. Two cases of continuous corn cultivation (Zea mays: δ13C = −12%.) on soils whose initial organic matter average δ13C is −26%. were studied. The quantity of organic carbon originating from corn (that is the quantity which had turned-over since the beginning of continuous cultivation) was estimated using the 13C natural abundance data. After 13 yr, 22% of total organic carbon had turned-over, in the system studied. Particle size fractions coarser than 50μm on the one hand, and finer than 2μm on the other. contained the youngest organic matters. The turnover rate of silt-sized fractions was slower

Effects of afforestation on soil carbon turnover in China's subtropical region

Afforestation in China's subtropics plays an important role in sequestering CO2 from the atmosphere and in storage of soil carbon (C). Compared with natural forests, plantation forests have lower soil organic carbon (SOC) content and great potential to store more C. To better evaluate the effects of afforestation on soil C turnover, we investigated SOC and its stable C isotope (delta C-13) composition in three planted forests at Qianyanzhou Ecological Experimental Station in southern China. Litter and soil samples were collected and analyzed for total organic C, delta C-13 and total nitrogen. Similarly to the vertical distribution of SOC in natural forests, SOC concentrations decrease exponentially with depth. The land cover type (grassland) before plantation had a significant influence on the vertical distribution of SOC. The SOC delta C-13 composition of the upper soil layer of two plantation forests has been mainly affected by the grass biomass C-13 composition. Soil profiles with a change in photosynthetic pathway had a more complex C-13 isotope composition distribution. During the 20 years after plantation establishment, the soil organic matter sources influenced both the delta C-13 distribution with depth, and C replacement. The upper soil layer SOC turnover in masson pine (a mean 34% of replacement in the 10 cm after 20 years) was more than twice as fast as that of slash pine (16% of replacement) under subtropical conditions. The results demonstrate that masson pine and slash pine plantations cannot rapidly sequester SOC into long-term storage pools in subtropical China.

No C_4 Plants Found at the Haibei Alpine Meadow Ecosystem Research Station in Qinghai, China: Evidence from Stable Carbon Isotope Studies

Using the measurement of stable carbon isotopes in leaves as a tool to investigate photosyn-thetic pathway of 102 plant species grown at an alpine meadow ecosystem, at the foot of the Qilian Mountain, Qinghai, China. The results indicate that the δ~(3)C values of plants have a narrow range from -28.24‰ to -24.84‰, which means that none of the species examined belongs to C_4 and crassulaceous acid metabolism (CAM) photosynthetic pathway and all of these species perform photosynthesis through the C_3 pathway. This is likely due to a long-term adaptation to environments at the alpine meadow ecosystem.

Fatty acid biosynthesis in eukaryotic photosynthetic microalgae: Identification of a microsomal delta 12 desaturase in Chlamydomonas reinhardtii

Polyunsaturated fatty acids (PUFAs) are important components of infant and adult nutrition because they serve as structural elements of cell membranes. Fatty acid desaturases are responsible for the insertion of double bonds into pre-formed fatty acid chains in reactions that require oxygen and reducing equivalents. In this study, the genome-wide characterization of the fatty acid desaturases from seven eukaryotic photosynthetic microalgae was undertaken according to the conserved histidine-rich motifs and phylogenetic profiles. Analysis of these genomes provided insight into the origin and evolution of the pathway of fatty acid biosynthesis in eukaryotic plants. In addition, the candidate enzyme from Chlamydomonas reinhardtii with the highest similarity to the microsomal Delta 12 desaturase of Chlorella vulgaris was isolated, and its function was verified by heterologous expression in yeast (Saccharomyces cerevisiae).