Do increases in xylem sap pH and/or ABA concentration mediate stomatal closure following nitrate deprivation?

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Abstract
Stomatal conductance (gs) of pepper (Capsicum annuum L.) plants decreased during the second photoperiod (day 2) after withholding nitrate (N). Stomatal closure of N‐deprived plants was not associated with a decreased shoot water potential (Ψshoot); conversely Ψshoot was lower in N‐supplied plants. N deprivation transiently (days 2 and 3) alkalized (0.2–0.3 pH units) xylem sap exuded from de‐topped root systems under root pressure, and xylem sap expressed from excised shoots by pressurization. The ABA concentration of expressed sap increased 3–4‐fold when measured on days 2 and 4. On day 2, leaves detached from N‐deprived and N‐supplied plants showed decreased transpiration rates when fed an alkaline (pH 7) artificial xylem (AX) solution, independent of the ABA concentration (10–100 nM) supplied. Thus changes in xylem sap composition following N deprivation can potentially close stomata. However, the lower transpiration rate of detached N‐deprived leaves relative to N‐supplied leaves shows that factors residing within N‐deprived leaves also mediate stomatal closure, and that these factors assume greater importance as the duration of N deprivation increases.