Abstract: Vascular tissues inside the mesophyll and peripheral mechanical tissues constitute the veins. The diverse orders and network structures of veins contribute to their functional diversification and differentiation. In this review, we summarized the research progresses on the structure and function of the leaf vein system. We reviewed three aspects of veins and critically evaluated the characteristics of the leaf vein hierarchy system and its important role in leaf economic spectrum (LES), and explained the mechanisms linking vein traits and other functional traits of the leaf. Leaf veins of different orders show obvious functional differentiation in terms of hydraulic conduction and mechanical support. Among them, the first three orders of veins (major veins) play a major role in maintaining leaf shape, leaf surface area and physical support, and which is conductive to the growth of leaves with the largest light-receiving area. The higher order veins (minor veins) have the function of water regulation, and their coordination with the stomata determines the rate of leaf water transport, transpiration and photosynthesis. The patterns of dynamic variation in leaf spread and leaf vein development explain the relationship between vein density and leaf size. Leaf surface area is negatively correlated with the density of main veins and positively correlated with the diameter of main veins, but independent of the density of minor veins. The framework model of LES linking with vein traits predicts that leaves with higher vein density have short lifespan and smaller leaf mass per area, which explains the better leaf carbon assimilation rate, metabolism rate and resource acquisition strategy with higher leaf vein density.

Key words: leaf vein orders, plant function, leaf economic spectrum, leaf hydraulic properties, leaf shape