Ecophysiological significance of nonhydraulic root-to-shoot signaling in control of stomatal behavior during soil drying

The objectives of this study were to: (1) characterize stomatal response of six deciduous tree species to nonhydraulic root-to-shoot signals of soil drying, and (2) test whether species sensitivity to nonhydraulic signaling is allied with their drought avoidance and tolerance profiles. Saplings Ac6r rubrum, ChionBnthus virginicus, Cornus florida, Halasid Carolina, Liriodendron tulipifera, and Oxydendrum arboreum were grown with roots divided between two pots. Three treatments were compared: half of the root system watered and half draughted (WD), half of the root system watered and half severed (WS), and both halves watered (WW). Partial soil drying caused nonhydraulic declines in stomatal conductance (g₂) in all species, with maximum declines ranging from 31% to 57% of WS controls. Declines in stomatal conductance were closely related to declining soil matric potential at soilψ_m below -0.10 MPa. Soil ψ_m required to cause declines in WD g_s, to 80% of WS controls varied from -0.013 to -0.044 MPa. Stomatal conductance of some species declined and remained low as soil dried, while g_s of other species declined initially with declining soil moisture and then increased as soil dried further. Leaf osmotic potentials during soil drying were mostly similar among treatments. Stomatal responses were not correlated with previously identified lethal leaf water potentials or osmotic adjustment, suggesting that stomatal sensitivity to nonhydraulic root signals may not be mechanistically linked to other characteristics defining relative species drought tolerance.