| In this study,two mangrove species Bruguiera gymnoihiza and Avicennia marina were undertaken to explore the ecologically adaptive mechanism of the variable anatomical characteristics of secondary xylem of mangrove plants changing with the dynamic changes of soil physicochemical indexes.Soil physicochemical factors of two sites were determined to analyze the relationship between soil indexes and climate factors at different times of one year by various statistical analysis methods.Morphological characters of secondary xylem of two mangrove plants including vessels,rays and fibers in different continuous time spans within one year were surveyed by means of light microscopy and the vestured pits of secondary xylem of two mangrove plants in different times of one year were surveyed by means of scanning electron microscopy,which aims to analyse the relationships between changes in secondary xylem structures and changes in soil factors.The main conclusions are as follows:1.The soil factor indexes of 12 months in two sampling sites were measured to respond to the change of monthly climate factors to explore the relationship between soil and climate factors in different months of the year.Stepwise regression analysis found that soil factors Mg2+,Mn2+,Fe3+,Zn2+,Ca2+ decreased significantly or extremely with the increase of monthly temperature in two sites.In addition,there was a significant negative correlation between soil pH and Na+ content and monthly average rainfall,a significant positive correlation between soil total salt content and Cl-content and monthly mean temperature in the site of B.gynoihiza.So the research showed that there was a significant correlation between the change of soil seasonal factors and climate conditions,and the reasons of the different changes of the soil factors in two sites could be the soil factors affected by different seawater,tides,topography and vegetation environments.2.The new growth layers was demarcated from B.gymnoihiza branches collected in 11 different continuous time spans within one year under a microscope since B.gymnoihiza was characterized by growth ring,and the average values of soil factors measured in successive months correspond to the changes of soil factors in 11 different periods.7 groups typical variables were obtained from the canonical correlation analysis between the microstructure characteristics of secondary xylem and soil factors in 11 different time spans of one year,which showed that soil total salt content,ion content,total phosphorus content and pH value effected ray area,pore area,pore density,pore frequency,vessel cluster size,vessel wall thickness and others anatomical characteristics of B.gymnoihiza.The research showed that under the high salt environment with the increase of Mg2+,Mn2+,Ca2+and the decrease of pH,the water transport safety will be reduced because of the "large size" of the vessel,but the increase of vessel cluster size and ray areas are benefit to water transport safety;with the increase of soil total salt content,Fe3+,K+,pH and decrease of total phosphorus,The "miniaturization" of vessel and decrease of the pore frequency will ensure the safety of water transport,and vessel wall thickness can strengthen the mechanical strength of the vessel wall and enhance the resistance of the plants to the wind,waves and tides,which showed that the variation of the quantitative anatomical characteristics of the secondary xylem with soil factors might be adaptive to the heterogeneous habitats in the intertidal areas.3.The new growth layers was demarcated from A.marina branches collected in 11 different continuous time spans within one year under a microscope since A.marina was characterized by growth ring,and the average values of soil factors measured in successive months correspond to the changes of soil factors in 11 different periods.4 groups typical variables were obtained from the canonical correlation analysis between the microstructure characteristics of secondary xylem and soil factors in 11 different time spans of one year,which showed that soil total salt content,ion content(Cl-,Na+,Mg2+,K+,Mn2+,Fe3+,Zn2+,Ca2+),soil organic matter effected ray area,pore area,pore density,pore frequency,vessel cluster size,contact wall length between two vessels and others anatomical characteristics of A.marina.The research showed that under the high salt environment with the decrease of Mg2+,Mn2+,Ca2+ and organic matter,the "miniaturization" of vessel can reduce the probability of vessel embolization and ensures normal and effective transportation of water;under the high salt environment with the increase of organic matter,the water transport safety will be reduced because of the "large size" of vessel,but the increase of ray areas are benefit to the repair of vessel embolism and pore density can improve the efficiency of water transport and The increase of vessel cluster size provides multiple paths for water transport,which are benefits to the safety and effectiveness of water transportation.4.Some fibriform vessel elements,parenchyma cells contain lots of starch grains,vessel vestures and helical structures on vessel walls were found with observing the outermost part of structural characteristics of the two mangrove plants branches and included phloem in the secondary xylem of the A.marina was found in 12 months of the year based on new growth layers demarcated under a microscope and these structures may be beneficial to the safety of water transport in the salt environment based on functional explanation.Stepwise regression analysis indicated the results that there was a significant negative correlation between frequency of vestured vessel outer aperture of B.gymnoihiza and soil Na+content and a significant positive correlation between frequency of vestured vessel pit chamber of B.gymnoihiza and organic matter,a significant negative correlation between the frequency of vestured vessel wall of two plants and soil total nitrogen content,which is able to the adaptation characteristics of vestured pits in high salt habitats.There was a significant positive correlation between soil Cl-and long axis of outer aperture and long axis of pit membrane of B.gymnoihiza in the year,and a significant positive correlation between soil Na+ and longest axis of outer aperture and shortest axis of outer aperture of the A.marina in the year,a significant positive correlation between soil total salt content and longest axis of inner aperture and longest axis of pit membrane of the A.marina in the year,which showed that the increase of the pits may be beneficial to improve the efficiency of water transport.The results showed that the pit characteristics of secondary xylem in high salt habitats varied with soil factors,and the adaptability of vestured pits structure to environmental factors should be further studied. |
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