Physiological And Ecological Mechanisms Of Leaf Shape Evolution In Oxytropis Diversifolia(Fabaceae)

Leaf shape is one of the most variable plant traits.To understand the evolutionary drivers of diversification in leaf shape and the ecological significance therein has been of continuing interest for many years.As the leaf is the primary photosynthetic organ and therefore of paramount functional importance for plants,leaf shape has long been hypothesized to be adaptive.Yet few studies have detected genetic signatures of natural selection on leaf shape and related variation to putative adaptive function,the adaptive feature of leaf shape still needs to be further elucidated.In this study,Oxytropis diversifolia,which has interspecific leaf shape variation(1-leaflet,1-3 leaflets,3leaflets)and shows clinal variation along geographic gradients,was taken as the study object.Firstly,leaf morphological traits were measured,and environmental association analysis was conducted,to test whether leaf shape is adaptive;then,the photosynthetic physiological characteristics and vein density of leaves were analyzed,to test whether leaf shape adapts to the environment through the mechanism of temperature and water regulation.The main results are as follows:1)Compound leaves of O.diversifolia have smaller effective leaf sizes than single leaves.For the phenotypes of 1-leaflet,1-3 leaflets and 3 leaflets,with the increase of leaflet number,the length,width,perimeter,and area of leaflet tended to decrease,while the perimeter area ratio tended to increase.For the 1-3 leaflets phenotype,the same trend was observed in three leaf types of 1-leaflet,2 leaflets,and 3 leaflets.2)The clinal variation of leaf morphological traits were significantly correlated with the spatial variation of selected bioclimatic variables.At population level,for both early leaves and mature leaves,leaf length,width,perimeter,and area were all positively correlated with annual precipitation(Bio 12),but negatively correlated with mean diurnal range(Bio02).The perimeter/area ratio was negatively correlated with annual precipitation(Bio 12)and annual mean temperature(Bio01).At the individual level,annual precipitation(Bio 12)was still the most significant explanatory variable,followed by isothermality(Bio03),temperature seasonality(Bio04)and precipitation seasonality(Bio15),as well as annual mean temperature(Bio01)and altitude.When using aridity index instead of annual precipitation(Bio 12),the results were basically the same.3)Leaf photosynthetic physiological characteristics of four representative populations of O.diversifolia were measured for two consecutive years.In 2021,we found that the net photosynthetic rate of the 1-3 leaflets phenotype was higher than that of other phenotypes,and the difference between leaf temperature and air temperature in 3 leaflets is lower than that in 1-leaflet and 1-3 leaflets.Other photosynthetic physiological characteristics,such as transpiration rate,stomatal conductance,instantaneous water-use efficiency,and intrinsic water-use efficiency,did not show any differences among different phenotypes.4)Using dried leaf samples collected in the field,leaflet area and total vein length were measured after removing mesophyll tissue chemically.We found that,with the increase of leaflet number,the leaflet area and the total vein length became smaller,while vein density became higher.The difference was mainly found in early leaves,in mature leaves the pattern was not that clear.Our study shows that the leaf shape cline of O.diversifolia is significantly correlated with bioclimatic gradients.Combined with our previous population genetic analysis using microsatellites,we suggest that leaf shape variation in this specie is adaptive.The compound leaves have smaller effective leaf size,higher leaf vein density and smaller difference between leaf temperature and air temperature,which preliminarily support the hypothesis that compound leaves have advantages in temperature regulation and hydraulic conductance.The higher net photosynthetic rate of the meso-phenotype indicates that it may have higher fitness,and we speculate that the leaf shape variation in this species may be subject to balancing selection.Our study can provide empirical evidence for the adaptive hypothesis of leaf shape,and also provide a basis for understanding the evolution of plants in arid steppe desert regions.