Discussion
Woody seedlings are a convenient life stage to acquire water conductance
parameters at the whole-plant level because of their size advantage,
even though patterns of seedlings may could not completely reflect the
water relation in adult woody plants. Nevertheless, how woody seedlings
regulate water relations, in terms of the xylem-stomatal covariation, is
important for their survival and growth into adulthood. Based on an
anatomical analysis across ontogenetically comparable seedlings of 53
diverse woody species (Table S1), we have presented here key new
findings on xylem-stomata coordination from a previously neglected
aspect: we started from the water balance between the liquid (water
delivery) and vapor (water loss) phase at the whole-plant level by
scaling plant total stomatal area to stem xylem (conductance)
cross-sectional area (Fig. 2; Table 1). This area-scaling pattern was
driven by the covariation of stomata numbers and minor vessel numbers
per plant (Fig. 5a; Table 1). We then zoomed in on the water exchange in
individual leaves by showing the coordination of leaf total stomatal
area and midvein xylem area (Fig. 3; Table 1). We also found that plant
size (or leaf size) scales with stomatal (or minor vessel) number, while
it does not scale with individual stomatal (or minor vessel) area (Fig.
4; Table 1), which has an important implication for our understanding of
the design of xylem structure and stomatal distribution.