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Recent work has shown that stomatal conductance (gs) and net photosynthesis (Anet) are responsive to the hydraulic conductance of the soil to leaf pathway (Xp). However, no study has compared xylem anatomical differences under controlled conditions to directly evaluate the whole plant hydraulic and chemical response to manipulations of XP. Two tree species with differing xylem structures were used to study the effect of systematic manipulations in Xp that elevated xylem hydraulic resistance. Simultaneous measures of gs, Anet, bulk leaf abscisic acid concentration (ABAL), leaf water potential (L), and whole plant transpiration (Ew) were taken under controlled environment conditions. Quercus shumardii Buckl. (shumard oak), a ring porous species, and Acer rubrum L. ‘Summer Red’ (red maple), a diffuse porous species, were studied to investigate the short term hydraulic and chemical messenger response to water stress. Both species decreased Anet, gs, L, and Ew in response to an immediate substrate moisture alteration. However, ABAL was not significantly different before or after drought stress exposure. Relative to initial well-watered values, red maple Anet, gs, and Ew declined more than shumard oak. However, gs and Anet versus whole plant leaf specific hydraulic resistance was greater in shumard oak. In addition, the larger hydraulic resistance in red maple was attributed to higher shoot system resistance. Stomatal conductance, Anet, and L of both species had a strong correlation with substrate volumetric water content (Vw), where the L of shumard oak declined to a lower level than red maple, and Anet of red maple decreased more than that of shumard oak. The results indicate hydraulic

hortscience

(142) Hydraulic Resistance: A Determinant of Short-term Stomatal Conductance Signaling in Disparate Xylem Anatomy of Red Maple (Acer rubrum L.) and Shumard Oak (Quercus shumardii Buckl.)