STOMATAL CONDUCTANCE, GROWTH AND ROOT SIGNALING IN BETULA-PENDULA SEEDLINGS SUBJECTED TO PARTIAL SOIL DRYING

Seedlings of Betula pendula Roth were grown with their root systems se parated between two soil compartments. Four treatments were imposed: ( i) adequate irrigation in both compartments (WW, controls); (ii) adequ ate irrigation in one compartment and drought in the other compartment (ND); (iii) drought in both compartments (DD); and (iv) half of the r oot system severed and the remainder kept well-watered (root excision, RE). Predawn leaf water potential, stomatal conductance, soil-to-leaf specific hydraulic conductance, and root and leaf growth decreased in DD-treated seedlings, which also displayed severe leaf shedding (30% loss in leaf area). The DD treatment also resulted in increased concen trations of abscisic acid (ABA) and its glucose ester in the xylem sap of roots and shoots compared to concentrations in control seedlings ( about 200 Versus 20 nM). Despite the difference in xylem sap concentra tions, total ABA flux to the shoots was similar in the two treatments (1-2 pmol ABA m(-2) leaf area s(-1)) as a result of reduced transpirat ion in the DD-treated seedlings. Compared with root growth in control plants, root growth increased in the RE-treated plants and decreased i n the drying compartment of the WD treatment; however, the RE and WD t reatments only slightly reduced leaf expansion, and had no detectable effects on shoot water relations or ABA concentrations of the root and shoot xylem sap. We conclude that short-term soil water depletion aff ecting only 50% of the root system does not cause a measurable stress response in birch shoots, despite root growth cessation in the fractio n of drying soil.