JXB Advance Access originally published online on March 3, 2003
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Journal of Experimental Botany, Vol. 54, No. 385, pp. 1281-1288,
April 1, 2003
© 2003 Oxford University Press
Do increases in xylem sap pH and/or ABA concentration mediate stomatal closure following nitrate deprivation?
Received 10 December 2002; Accepted 19 December 2002
1 Department of Botany, The University of Queensland, Brisbane, QLD 4072, Australia
2 Natural Sciences Academic Group, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore 637616
3 Present address and to whom correspondence should be sent: Department of Biological Sciences, IENS, University of Lancaster, Lancaster LA1 4YQ, UK. Fax: +44 (0)1524 843854. E-mail: I.Dodd{at}lancaster.ac.uk
Abbreviations: AX, artificial xylem (solution); gs, stomatal conductance; 
shoot, shoot water potential.
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 (shoot); conversely shoot 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.
Key words: Abscisic acid, Capsicum, nitrate, stomata, water status, xylem pH.
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