STOMATAL CONDUCTANCE, GROWTH AND ROOT SIGNALING IN YOUNG OAK SEEDLINGS SUBJECTED TO PARTIAL SOIL DRYING

Leaf conductance, water relations, growth, and abscisic acid (ABA) con centrations in xylem sap, root apices and leaves were assessed in oak seedlings (Quercus robur L.) grown with a root system divided between two compartments and subjected to one of four treatments: (a) well wat ered, WW; (b) half of root system exposed to soil drying and half kept well watered, WD; (c) whole root system exposed to drought, DD; and ( d) half of root system severed, RE. Sharp decreases in plant stomatal conductance, leaf water potential, hydraulic conductance and leaf grow th were observed during DD treatment. No significant differences in pl ant leaf water potential and stomatal conductance were detected betwee n the WW and WD treatments. Nevertheless, the WD treatment resulted in inhibition of leaf expansion and stimulation of root elongation only in the well-watered compartment. Abscisic acid concentrations did not change in leaves, root tips, or xylem sap of WD-compared to WW-treated plants. Increased concentrations of ABA were observed in xylem sap fr om DD-treated plant roots, but the total flux of ABA to shoots was red uced compared to that in WW-treated plants, because of decreases in tr anspiration flux. Similar plant responses to the WD and RE treatments indicate that the responses observed in the WD-treated plants were pro bably not triggered by a positive signal originating from drying roots .