Tracking the paths of residual conductance during leaf expansion in <i>Tilia americana</i> and <i>Fagus grandifolia</i>

The rate of residual water loss is a major determinant of plant survival during drought, yet how the major paths of residual water flow develop as leaves expand is poorly understood. Here, we tracked the rate of residual water loss, the compositional development of cuticular wax, stomatal differentiation, pore formation, and xylem development as leaves expand in two co-occurring, deciduous tree species Tilia americana and Fagus grandifolia. As leaves expanded, residual conductance declined rapidly, primarily driven by decreases in cuticular conductance, which was the main pathway for residual water loss from branches with young leaves. Very little water was lost through stomatal pores as leaves expanded, because the outer cuticular ledge only formed above the majority of stomata once leaves approached complete expansion. Similar development of residual conductance was observed between the two species despite differences in cuticle composition and stomatal development timing as leaves expanded. Our work suggests that the cuticle is the primary pathway through which water is lost from the youngest expanding leaves, and residual conductance is minimized only when cuticular wax deposition is complete, stomata have formed, and leaves have fully expanded.