| The mangrove ecosystem has a large number of ecological functions,rich application prospects,and excellent use value.Exploring how mangrove plants adapt to complex and ever-changing environments and maintain high productivity as well as high photosynthetic capacity has always been one of the topics in the field of mangrove plant research.In recent years,more and more studies have revealed the importance of the mesophyll conductance(gm)in plant photosynthesis.It has even become the primary internal factor that affects the level of plant photosynthesis.But such studies on the mesophyll conductance of mangrove plants are still scarce.In addition,due to the unique mechanisms of salt tolerance of mangrove plants,some of the conclusions of mangrove plant response to environmental factors may be significantly different from those of previous non-salt-tolerant plants.The plant stable isotope technology has been applied widely to for mesophyll conductance measurements.But few studies were reported to use this technique in mangroves.Therefore,this study includes two parts:a)field experiments to reveal the differences in the stable C and N isotopic compositions(?13C,?15N)of various mangrove plant leaves,and to explore the effects of various environmental factors on?13C and?15N of mangrove plant leaves;and b)indoor control experiments to quantify the gm of different mangrove plants and their contribution to photosynthesis,to reveal the changes of gm of different mangrove plants under different salinity conditions,to explore the specific reasons for the difference of gm in mangrove plants,and to establish the physiological mechanism on how gm of mangrove plants respond to salinity.The main findings of the current study are as follows:(1)The ranges and averages of leaf?13C values of seven studied mangrove species are:Bruguiera gymnorrhiza(-32.18?-29.71,-30.55),Avicennia marina(-31.11?-29.16,-30.25),Sonneratia caseolaris(-31.19?-28.82,-29.80),Acanthus ilicifolius(-28.48?-25.81,-26.91),Sonneratia apetala(-31.28?-29.46,-30.47),Aegiceras corniculatum(-31.99?-30.05,-30.66),and Kandelia obovata(-31.22?-28.57,-29.50).Most leaf?13C values were between-31and-29,and the averages were close to the medians.The order of the?13C averages of the seven mangrove plants is:Acanthus ilicifolius>Kandelia obovata>Sonneratia caseolaris>Avicennia marina>Sonneratia apetala>Bruguiera gymnorrhiza>Aegiceras corniculatum.The range and average value of leaf?15N values of the 7 mangrove species are:Bruguiera gymnorrhiza(0.05?5.60,3.25),Avicennia marina(3.76?6.37,5.04),Sonneratia caseolaris(5.55?9.05,7.47),Acanthus ilicifolius(4.31?10.99,7.68),Sonneratia apetala(2.92?7.64,5.20),Aegiceras corniculatum(2.05?7.56,4.50),Kandelia obovata(0.18?5.14,3.05).The leaf?15N value of different mangrove plants was quite different.The average order of the leaf?15N values of the seven mangrove plants was as follows:Acanthus ilicifolius>Sonneratia caseolaris>Sonneratia apetala>Avicennia marina>Aegiceras corniculatum>Bruguiera gymnorrhiza>Kandelia obovata.(2)The leaf?13C values of mangrove plants were significantly different,which was mainly due to the difference in the degree of response of different mangrove plants to environmental factors.Salinity had a good correlation with the leaf?13C values of 7mangrove plants,indicating that salinity had the potential to become a major driver of the leaf?13C values of mangrove plants.The leaf?15N of mangrove plant also differed from each other,but this phenomenon was not caused by the different degree of response of mangrove plants to various environmental factors,but by the different element content of mangrove plant leaves(mainly mangrove plant leaves N element content).It should be noted that the leaf?15N values of mangrove plants correlated with the N content of leaves positively,contrary to the conclusions of other studies in this area.(3)The present study demonstrated that net photosynthetic rate(An)in mangrove plants under salinity stress was predominately limited by gm rather than stomatal conductance(gsc)or biochemistry.This also explained to a certain extent why the changing trends of mangrove plants An and gm with salinity are first rising and then decreasing and the changes are very similar.(4)Salinity-induced changes of gm was associated with leaf anatomy.Specifically,the lower chloroplast surface area exposed to intercellular air space per leaf area(Sc/S)and higher the volume fraction of intercellular air space(fias)caused reduction in gm under salinity treatments.Surprisingly,reduction in gm of K.obovate with salinity was significantly affected by lowered cell wall thickness(Tcw),while was not the case in A.corniculatum.These structural differences may be related to different salt management strategies of the two mangrove species. |
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