Leaf Venation Characteristics And Functional Traits Of Chinese Jujube

Leaf veins play an important role in the transport of water, nutrients, and minerals and in support and protection of plant leaves. Leaf vein characteristics and functional traits jointly reflected the adaptation of a given plant to local environments. Leaf vein characteristics can be considered as indicators for ecological adaptation strategies. Jujubes(Ziziphus jujuba Mill.) are an endemic species of fruit tree and distribute widely in China. In recent years, the living environments of jujubes have changed a lot with the transition of jujube plantation areas. Especially with the east-to-west transition of jujube varieties to desert and gobi areas, new jujube plantation regions have more sunshine and heat resources, but less precipitation.In this study, a total of 223 samples of jujube leaves(including 188 samples cultivated jujubes, 28 samples of wild jujubes, and 7 transitional types) were collected in jujube plantation regions in northern China. These samples were analyzed with the LEAF GUI, a software for leaf venation analysis, to study the leaf venation characteristics, such as leaf vein density(VLA), distance between veins(IVD), loopiness(VAA), and node number(Nodes). Several leaf functional traits, including maximum net photosynthetic rate(Am) per unit leaf weight and leaf nitrogen concentration(Nm) were also analyzed. Then, relationship models were established between venation characteristics and functional traits of jujube leaves. The variations of leaf veins and functional traits of Chinese jujube(including the wild and transitional varieties) have been investigated in a relatively large scale, as well as their responses and adaptations to the environments in different jujube plantation regions. This study will benefit the utilization of germplasm resource and long-distance introduction of new cultivars of jujubes in China. Some main conclusions have been drawn as following.(1) A total of 104 leaf samples of jujubes were collected in the hilly areas of the Loess Plateau in Shaanxi and Shanxi provinces or the Yellow Rive Canyon. Then cultivated, wild jujube and transitional type were compared for their leaf venation and functional traits. The results showed that wild jujube had relatively higher values of VLA, loopiness, specific leaf mass and longer longevity than cultivated jujube. This indicated that wild jujubes had stronger resistance to drought and infertility, better utilization efficiency and storage capacity of natural resources. However, it could take longer time for wild jujube to compensate the higher establishment consumption and to survive through slow nutrient cycle. Compared with wild jujube, cultivated jujube had higher values of Am and Nm, which indicated that cultivated jujube could synthesize enough organic matters in a shorter time due to higher photosynthetic rate. Thus, cultivated jujube had high biomass and yield. Wild jujube took a position in the economic spectrum more close to the end of high specific leaf mass, low nitrogen concentration, weak photosynthetic capacity, slow growth rate, and long longevity. On the contrary, cultivated jujube was more close to the end of low specific leaf mass, high nitrogen concentration, strong photosynthetic capacity, fast growth rate, and short longevity. Hence, cultivated and wild jujubes could take different adaptation strategies after judging the relationships between leaf venation characteristics and leaf functional traits. In addition, wild jujube had wider ecological amplitude than cultivated jujube, which reveals the survival mechanism of wild jujube to sufficiently utilize the limited environmental resources.(2) The influences of climatic factors on leaf venation characteristics of jujubes were analyzed based on historic weather data of 62 years and 116 representative jujube varieties in 32 counties and districts in northern China. The results showed that leaf venation of jujubes changed with environmental gradient. Leaf vein traits were strongly correlated with mean annual temperature(MAT) and mean annual precipitation(MAP). This illustrated that temperature and precipitation were the main climatic factors that influenced leaf venation of jujubes. The parameter of leaf vein density was negatively correlated with MAP and MAT. This parameter had relatively higher correlations with each climatic factor than other leave vein traits. Vein density showed different adaptations to short-term and long-term environmental changes. On the one hand, jujube vein density was insensitive to short-term environmental changes and had high heredity and low plasticity; on the other hand, jujube leaf vein density changed with climatic gradient to adapt to local different climatic conditions through long-term evolution. This means jujube vein density was a result of long-term adaptation to local climatic factors, but insensitive to short-term environmental changes.Based on principal component analysis(PCA) analysis and hierarchical clustering of eight parameters of jujube leaf vein, the jujube varieties in jujube regions of northern China were divided into three main groups(Groups 1, 2, and 3). This kind of division just coincided with the division of three main jujube planting regions in northern China. Group 1 included the alluvial soils in the middle and lower reaches of the Yellow River and Haihe River(or ASYH area); Group 2 included the hills in the Loess Plateau(or HLP area); Group 3 included arid valleys and hills in Gansu, Ningxia, and Xinjiang provinces. This illustrated that leaf venation reflected the adaptation of jujube varieties to local environments. Particularly, leaf vein density and distance between veins could be used as important indicators for introduction of new jujube varieties. The introduction of new jujube varieties would be more possibly successful if there were great similarities between jujube varieties introduced and native varieties in the introduction area for their leaf venation characteristics.(3) Based on the data above, we investigated the influences of climatic factors(MAP and MAT) on leaf functional traits of jujubes(including specific leaf mass, longevity, Am and Nm) in jujube plantation regions in northern China. The results showed that MAP, MAT, and mean average sunshine hours(MASH) were the main influential factors for leaf functional traits. Jujube cultivars tended to have higher specific leaf mass and longevity and lower Am and Nm, with the decrease of precipitation and temperature and increase of sunshine hours and sunshine percentage. The relationships between leaf functional traits and environments were not single. The adaptation of jujube varieties to environments was collectively determined through interactions between leaf functional traits. The differences among leaf functional traits of jujubes were the results of long-term adaptation to local environments.(4) For the three main cultivated jujube varieties in Xinjiang Province, including Junzao(originating from Shanxi Province), Huizao(originating from Henan Province), and Qiyuexian(originating from Shaanxi Province), the responses of their leaf venation and functional traits to the environments of jujube plantation areas in Xinjiang were studied. The results showed that with the higher solar radiation and temperature and lower precipitation in Xinjiang Province, the parameter of leaf vein density increased remarkably for the three main jujube varieties above. This further illustrated that leaf vein density of jujubes was a result of long-term adaptation to the environments. On the contrary, nodes number and leaf vein width varied limitedly and were insensitive to environments. Additionally, specific leaf mass, longevity, assimilatory efficiencies of carbon and nitrogen per unit area increased dramatically. Compared with their origin areas, the values of Am increased by 13.5% for the three main jujube varieties above, while Nm increased by 14.2%. Hence, the jujube plantation areas in Xinjiang Province had more photosynthetic assimilates in jujube leaves or sinks, which might partially illustrate why the same jujube variety could have higher yield and better quality in Xinjiang than in their origin areas. At the same time, higher specific leaf mass and leaf vein density were the response and adaptation of jujube cultivars to the dry and little rain environments of Xinjiang. It confirmed that jujube is an economically important fruit tree to adapt to desert environments.