C_3植物稳定碳同位素组成与盐分的关系

Relationship between salinity and stable carbon isotope composition of C_3 plants

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。

There are two components of variation in carbon isotope composition（expressed as the ratio of stable carbon isotope relative to PDB,denoted δ^13C）of plant tissues under salinity conditions：one is the effect of physiological processes;the other is the high salinity-induced switch from C3-photosynthesis to Crassulacean acid metabolism or C4-photosynthesis.The former is a main factor affecting plant δ^13C values,whereas the later generally has no significant effect on whole tissue δ^13C.The relationship between δ^13C and salinity is relevant to intrinsic salt-tolerance,salinity level and the period when plants grow under saline conditions.Non-halophyte and halophytes have different response patterns in δ^13C to salinity in terms of the mechanism of carbon isotope discrimination.For non-halophytes,δ^13C values will increase with increasing salinity when the stomatal closure is the major factor in restricting photosynthesis.As non-stomatal limitation becomes the key factor with the stress intensified,however,the values of plant δ^13C may decline.It should be noted that the correlationship between δ^13C and salinity is not exhibited by mild stress treatment.For halophytes,the lowest δ^13C occurs at a favourable salinity,and the values increase when the salinity is lower or higher than the optimum level.In dense woodlands,the CO2 derived from respiration recaptured by leaves using photosynthesis may alter their plant＇s δ^13C due to the different δ^13C from that of the air.So,further investigations are necessary covering a large range of salinity and duration of treatment,to explore the response of plant δ^13C to salinity,and the salt-tolerance of species.