Convergent size coordination of stomata and xylem from whole-plant level to single-leaf level
The convergent size coordination of stomata and xylem in the case of entire plants and individual leaves implies that individual leaves have a tight control over the whole-plant water conductance (Fig. 2; Fig. 3; Table 1). Previously we showed, based on the same seedling populations, that the xylem vessels widen basipetally from the tip to the base for both single leaf (midvein) and the whole plant (stem) and that the remarkably tight covariation in vessel diameter between different organs (especially between leaf and stem) (Zhong et al. , 2019). When we now combine all three findings, we can conclude that natural selection has led to rather tight regulation of water-related architecture featuring similar size-driven variation across seedlings of diverse woody species, both for single leaves and entire plant individuals. Their xylem vessels widen basipetally from the tip to the base, from leaves to the entire individuals, in a way that maintains a constant leaf-specific conductance (Sterck & Zweifel, 2016, Zhong et al. , 2019) and a constant xylem-stomatal size scaling. Using hydraulic properties of single leaves to predict the entire plant water transport is an alternative choice, as numerous studies have done (Brodribbet al. , 2017, Carins Murphy et al. , 2014, Meinzer, 2002), especially when the conductance-related parameters of entire plants are difficult to acquire, for example in adult trees. Specifically, knowing leaf size (i.e. leaf area) is of the utmost importance, not only to predict photosynthetic productivity precisely (Li et al. , 2020), but also to understand plant water transport (Echeverría et al. , 2019), from the single leaf, to the branch, to the whole-tree, and even to the forest level.
The slopes of the ln-scaling regression lines between stomatal and xylem traits are notably larger than the slope of 1 (Fig. 2; Fig. 3; Table 1), which means that stomata do not scale linearly to xylem but exponentially. In actual fact, it should be the stem xylem conductance area, rather than stem xylem area, that scales with the total stomatal area, while we gave the pattern for both in order to enable comparison with a previous study which used the same seedling population (Zhonget al. , 2019). In that study, the total leaf area (LA ) scaled with stem xylem area (Xstem ) at mid stem height as LAXstem1.25 (Zhong et al. , 2019). Together with isometric scaling of leaf area and total stomatal area, we could elicit that total stomatal area should scale with Xstem with an exponent approximating 1.25. Our finding in the current paper (exponents 1.29 and 1.30 for entire plants and individual leaves respectively) is in line with this theoretical prediction. When considering the total water path length (e.g. by sampling the anatomical cross-section at the stem base), our results are in line with our prediction that there should be isometric scaling both between total leaf area and xylem conductance area and between total stomatal conductance area and xylem conductance area (see also Echeverríaet al. , 2019, Fiorin et al. , 2016, Lechthaler et al. , 2019, Meinzer & Grantz, 1990). Further studies are needed to integrate the relations between leaf area, stomatal area and xylem conductance area from the perspective of the (3-dimensional) water transport system from single-leaf level to whole-plant level. Ideally, such studies should be carried out also on adult woody plants and across different biomes.