The Influence Of Environmental Gradients And Leaf Type On The Relationship Between Leaf Water Supply And Demand

The plant functional traits such as leaf area, veins, stomata and stem conduits exhibit coordination and adjustments enabling plants to adapt to different water availability and climatic conditions. However, there is still lack of a broad consensus on the balance between water supply and demand in leaf level, the relationship between hydraulic structure of branches and leaves, and stomatal adjustment, which results in many questions requiring further investigations. In this study, we looked into plant water relations between supply and demand in both the natural conditions and the shade house. Our investigations are important to understand the species strategies for maintaining the function of hydraulic systems.This thesis mainly investigated three different aspects of the water supply and demand. The mainly contents and findings were as follows:In the first part, we explored whether the balance of leaf water supply and demand maintains along a large altitudinal gradient. We measured leaf area, stomatal density, stomatal length and minor vein density in 105 angiosperm tree species across two altitudinal ranges in tropical mountain forests (800 m-1400 m a.s.l.) and subtropical mountain forests (2000 m-2600 m a.s.l.) in Yunnan. We found that the altitudinal average stomatal density at community-level was independent of altitude in both the subtropical mountain forests and tropical mountain forests. There was no significant difference in the minor vein density across four altitudes in either subtropical mountain forests or tropical mountain forests. But the average minor vein density in tropical mountain forests was significantly higher than that in subtropical mountain forests. We found a positive correlation between minor vein density and stomatal density in the species in subtropical mountain forests, while a weak correlation across all 105 tree species together. Our results not only further extended the leaf water balance theory to the angiosperm tree species across diverse families, but also indicated a decoupled adaptation of stomatal density and minor vein density in these tree species along altitudinal gradient.In the second part, we investigated the change trend and coordination between stomata and minor veins along the stem of peanut (Arachis hypogaea Linn.) plants by a pot-cultivation experiment. Moreover, we demonstrated that the stable stomatal number per minor vein length could be as the indicator showing leaf water balance between supply and demand. We measured the area of apical leaflets, their minor vein density and thickness, stomatal density and length and calculated the stomatal number per minor vein length and total stomatal number and minor vein length per leaflet from plants grown under three different light regimes. We found that stomata number per minor vein length of peanut did not change significantly with leafing position on the stem and leaflet area within each light treatment of peanut plants. There was no significant difference in average stomatal length between light regimes while the average minor vein thickness showed significant difference. Minor vein density was positively correlated with leaflet area within each light level. The correlation between minor vein density and stomatal density became weaker under highly shaded conditions. We found a strong correlation between minor vein length per leaflet and stomatal number per leaflet under each of the three light conditions, and the slopes of the regressions under the three light conditions were not significantly different. The stable stomata number per minor vein length under a certain light regime indicated the leaf water balance between hydraulic supply and demand. Under the lowest light treatment (shading 96%), peanut plants tended to decrease stomatal density and minor vein thickness rather than stomatal length and minor vein density, which led to the lowest stomata number per minor vein length than that in full sun.To study the impact of leaf type on water supply and demand between branches and leaves, in the third part, we measured the sapwood conductance, water potential, and relative water content of branches, rachis, and leaf at midday and the stomatal conductance in 11 species with compound leaves and 6 species with palmate leaves in Fabaceae. Midday water potential of compound leaf rachis was closer to that of branches, although a significant difference between them was found. However, both rachis and branch water potential were significantly higher than midday leaf water potential. At a given leaf water potential at noon, the stem water potential of compound leaf species was significantly higher than that of palmate leaf species. We did not find the correlation between midday stomatal conductance and rachis water potential at midday, as well as the midday leaf and stem water potential. We found highly positive correlation between leaf water potential and stem water potential in all the 17 species, and the rachis water potential at midday and the stem water potential at midday showed highly positive correlation in 11 compound leaf species. We found that leaf hydraulic traits had close relationship with stomata conductance, for example, the leaf hydraulic conductance showed positive correlation with stomata conductance at midday, and negative correlation with the decrease in stomata conductance at midday. Along the strong correlation between leaf and stem water potential at midday, the results showed that the leaf and rachis water potentials at midday also had negative correlation with the change in stem relative water content in compound leaf species. Through those coordination of leaf and stem, plants could maintain the leaf water status.Over all, we conclude that our results not only provide new evidences to support the hypothesis of leaf-level balance between water supply and demand but also show the uncoupled change between them when plants response to the environmental changes along altitudes and under different light regimes. When extending the perspective of leaf water balance between supply and demand to individual plant level, we found that compound leaf rachis had obvious effect on water relations between branches and leaves. The stomatal conductance was more affected by leaf hydraulic conductance and its water status, and there was no direct link of stomata conductance to the water status of branches.