Carbon Stable Isotope Composition In Three Mangrove Species Under Salinity

Mangrove plant,as a typical halophytic species,their ecological response to salinity has been widely studied.But little is know of the correlations between stable carbon isotope composition(presented by convention asδ¹³C) and salinity,even though plantδ¹³C have been thought to be an index of reflection of the integrated response of physiological characteristic to environmental factors.δ¹³C in plant tissues is correlated with their intrinsic salt tolerances,leading to varied patterns ofδ¹³C in different plants species or different parts of a certain plant.Previous studies showed that halophytic species had a similar pattem ofδ¹³C variation as compared with nonhalophytic species.From the model of Farquhar et al.(1982) expression for discrimination in leaves of C₃ plants,variation in plantδ¹³C is mainly associated with stomatal and photosynthetic effects.However,the pattems of CO₂ exchange properties have been shown to be different between halophytic and nonhalophytic species as affected by salinity.Therefore,I hypothesized that the correlation betweenδ¹³C and salinity for mangrove plants differ from nonhalophytic species.To test the hypothesis,δ¹³C variation of three dominant mangrove species in Fujian province under salinity were studied.Based on the primary results,I discussed whetherδ¹³C could be used to compare the salt tolerance between mangrove plants belonging of different species.In addition,I tried to explore the mechanism ofδ¹³C changes of mangrove plants induced by salinity,and to determine the photosynthetic pathway of mangrove species under salinity using the stable isotope technique combined with gas exchange measurements and other relative technique.Following are the major conclusion:â…°) Leafδ¹³C exhibited special patterns in A.corniculatum and K.candel as response to salinity.In A.comiculatum,an increase in salinity from 0 to 15‰induced reduction in leafδ¹³C;while as salinity above 15‰,leafδ¹³C increased.The similar pattern was also found in K.candel,but the maximum values of leafδ¹³C occurred at 20‰.These changes became more pronounced for long time treated with salinity,although the differences between salt treatments were non-significant for A. corniculatum.These results indicate that the patterns of leafδ¹³C in A.corniculatum and K.candel differ from those in nonhalophyte,these two species showed the lowestδ¹³ at the optimum salinity.In A.corniculatum,a great lower leafδ¹³C were observed in 130-d samples relative to 60-d samples at high salinity(30‰).â…±) The patterns of photosynthetic parameters including net photosynthetic rate(Pn), stomatal conductance(Gs),transpiration rate(Tr),and intercellular CO₂ concentration(Ci) were opposite to leafδ¹³C.As salinity increased from 0 to 20‰,Pn,Gs and Tr increased, revealing that the decrease in leafδ¹³C was caused by stomatal opening;while as salinity higher than 20‰,these parameters decreased,which resulted in an increase in leafδ¹³C.Therefore,it could be concluded that changes in leafδ¹³C of these two mangrove species mainly resulted by stomatal adjustment as affected by salinity.â…²) There was no significant differences in malic acid content of A.corniculatum or pH value of K.candel between day and night,revealing that theses two species have not been induced into crassulacean acid metabolism by salinity.The values ofδ¹³C in A.corniculatum and K.candel were-30.36～-26.55‰and -31.96～-28.13‰respectively,which were within the range of C₃ plants.Therefore,it could be concluded that these two mangrove species are typical C₃ plants.â…³) Unlike in the case of A.corniculatum and K.candel,the changes in leafδ¹³C of A.marina always increased as salinity increased from 0 to 35‰,but the differences between salinity were not significant.Pn decreased with increasing salinity,while Gs increased.An increase in Gs paralleled the increase of stomatal density,indicating that the increase of Gs was the result of increasing stomatal density but not of the stomatal opening.The reduction in carboxylation efficiency(CE) was also detected and which maybe one reason for Pn prohibition.In addition,the decrease of photorespiration(Rp) and CO₂ compensation point(Γ) indicated that C₄ photosynthetic metabolism increased to some extent.The differences in pH between day and night were not significant,indicating no CAM occurred.Summary,δ¹³C variation is correlated with depression of Pn and CE,and the enhanced C₄ photosynthesis which maybe also an important factor in contribution to the tolerance of the species to high salinity. â…´) In A.corniculatum,root and leaf exhibited quite similar response patterns inδ¹³C to salinity,butδ¹³C in stem showed less variation.δ¹³C in both stem and leaf were significant lower than in root.δ¹³C in seedling parts in A.marina increased followed the order of leaf＜above-stem＜below-stem＜above-root＜below-root.The similar variation patterns inδ¹³C were observed in various seedling parts.It is interesting noted that different parts of one seedling organ showed significant difference inδ¹³C; while,δ¹³C in different organs linked together showed non-significant differences.It is suggested that the differences between plant parts are correlated with the discrimination to C-13 during transport processes.â…µ) The difference in optimum salinity of A.corniculatum and K.candel inferred by leafδ¹³C provides a feasible method for comparing salt tolerance between plants of different species,and it is useful for mangrove restoration.