Physiological And Ecological Characteristics And Molecular Mechanisms Of Populus Euphratica Leaves With Different Shapes

Populus euphratica Oliv.(P.euphratica)is a tertiary relict species and a deciduous tree belonging to Populus of Salicaceae.It has heterophyllous leaves and grows leaves of different shapes at different stages of development.P.euphratica is a specie of desert riparian forest in northwest China,which plays an important role in maintaining regional ecosystem balance,biodiversity conservation and economic and social development.In this study,P.euphratica individuals at different developmental stages and different growing seasons were taken as the research object to carry out studies on morphological and physiological traits,dynamic changes in different leaves of P.euphratica and the related transcriptional regulation mechanisms.The main findings are as follows:(1)The leaf morphology of P.euphratica had significant spatiotemporal variation and showed certain regularity in the time and space of its development.With the growth and development of P.euphratica,leaves of different shapes such as striate,lanceolate,ovate and broad leaves appeared successively.Leaf length and leaf shape index showed a decreasing trend from striate leaves to broad leaves,while leaf width and leaf area showed an increasing trend,and there were significant differences between different leaf shapes at different developmental stages.The leaf length and leaf shape index of P.euphratica decreased gradually,while the leaf width and leaf area increased gradually,and the difference between different growing seasons was significant.The leaf length and leaf shape index of P.euphratica leaves decreased from the bottom of the tree crown to the top,while the leaf width and leaf area increased.(2)The physiological characteristics of P.euphratica leaves have obvious spatiotemporal variation.The photosynthetic characteristics of P.euphratica leaves were significantly different at different growth stages.The striate leaves of young trees showed higher photosynthetic rate,while the broad leaves of mature trees had higher transpiration rate and stomatal conductance.With the change of growing season,the physiological function of leaves of various forms of P.euphratica was constantly improved,which showed the trend of increasing photosynthetic capacity and photosynthesis.The photosynthesis of P.euphratica changed with the spatial distribution of leaves,and the photosynthetic capacity of the leaves of different shapes growing from bottom to top gradually increased.(3)The light response and the CO2 response of P.euphratica have obvious spatiotemporal variation.Among the leaves of different forms in different development stages of P.euphratica,the striate leaves of seedling plants have strong weak light utilization ability and high photosynthetic efficiency.From the bar leaves in the initial stage to the broad leaves in the later stage,P.euphratica showed increasing adaptability to strong light.In the aspect of seasonal development,from May to July,the leaves of each shape kept maturing,and the maximum photosynthetic rate,enzyme activity,electron transfer efficiency and so on were all continuously improved,showing the changing rule that the photosynthetic capacity increased successively.For P.euphratica leaves growing in different spaces of the tree crown,the striate leaves growing on the lower part of the plants in the early stage of development in May had higher photosynthetic rate and stronger ability to utilize weak light.Later,with the gradual development and maturity of the leaves,the photosynthetic capacity of the leaves of different forms increased successively from bottom to top,and the broad leaves growing on the upper part of the plants adapted to strong light conditions and had higher photosynthetic efficiency.(4)Through the comparative analysis of transcriptome sequencing,it can be seen that in different developmental stages of P.euphratica,the differential genes in leaves of different shapes were significantly different.The number of differential genes ranged from more to least was lanceolate leaf of P.euphratica young trees and middle stage trees(16162),broad leaf of P.euphratica middle stage trees and mature trees(6500),stripe leaf of P.euphratica seedlings and young trees(870),lanceolate leaf and ovate leaf of P.euphratica middle stage trees(480),ovate leaf and broad leaf of P.euphratica middle stage trees(12).The above results showed that the transcriptional regulation of genes in different developmental stages was obvious,while the transcriptional regulation of genes in leaves of different shapes of the same plant was relatively weak.The GO function classification and enrichment analysis of differential genes in the leaves of P.euphratica at different developmental stages showed that,328 genes associated with development process were identified.Through the analysis,it was found that there were more differential genes between leaves of the same morphology of P.euphratica in different tree ages,while there were fewer differential genes between leaves of different morphology of P.euphratica in the same tree age.The differential genes of leaves of different morphology of P.euphratica were significantly enriched in photoperiod regulation,flowering and photoperiod morphogenesis.The strip-leaf differential genes were significantly enriched in the stem-axis system and root development,and the lanceolate and broad-leaf differential genes were significantly enriched in the cell differentiation and multicellular organ development.According to KEGG enrichment analysis,the differential genes in different leaves of P.euphratica were significantly enriched in carbon metabolism,photosynthetic carbon sequestration,glyoxylate and dicarboxylic acid metabolism.The strip-leaf differential genes were significantly enriched in flavonoid biosynthesis,and the lanceolate and oval-shaped genes were significantly enriched in the metabolic pathways of glycine,serine and threonine,indicating that these genes were related to the growth and development of P.euphratica.Leaves of P.euphratica of different tree ages have differential genes in photosynthetic pathway.The lanceolate leaves had the most amount of differential gene expression,and there were more differential genes related to photosynthetic system ?.There were more down-regulated genes in the strip-leaf,indicating that the photosynthetic capacity of the strip-leaf was reduced.There were many differential genes in photosynthetic electron transport in leaves of different morphology,which indicated that photosynthetic electron transport rate directly affected the performance of photosynthetic physiological function.These differential genes are the main reason why the photosynthetic physiology of leaves of different forms in different developmental stages of P.euphratica was different,and the regulation of the differential genes related to the photosynthetic system ? were most significant.