Transport and accumulation rates of abscisic acid and aldehyde oxidase activity in Pisum sativum L. in response to suboptimal growth conditions

Pea plants (Pisum sativum L.) grown initially in nutrient solutions with adequate nitrogen supply (4 mM NO₃^-) were transferred to solutions containing salt (50 or 100 mM NaCl), ammonium (4 mM) or a low nitrogen supply (0.4 mM NO₃^-). No changes of abscisic acid (ABA) content were found in roots of stressed pea plants 9 d after the beginning of the treatments; however, accumulation of ABA in the leaves was observed. Old leaves accumulated ABA to a higher extent than young leaves. Accumulation of ABA in leaves of ammonium-fed plants and plants grown under low nitrogen supply occurred in the absence of both increased ABA xylem loading rate and enhanced aldehyde oxidase (AO, EC 1.2.3.1) activity in roots. Enhanced leaf AO activity was observed in all treatments, with the highest increase in old leaves. Among the three AO isoforms (AO-1, AO-2 and AO-3) detected in extracts of pea leaves, the lowest one AO-3 (highest mobility in the gel) correlated with ABA production and showed the highest increment in response to the treatments. The increase of AO activity detected in leaves after 2 weeks of stress application was less prominent than after 9 d, suggesting a transient enhancement of ABA production following the onset of stress. An increase of ABA xylem loading rate as well as AO root activity 4 d and 9 d after application of the treatments was observed only in salt-treated plants followed by a decrease after 14 d in 100 mM NaCl. Decreased cytokinin (trans-zeatin riboside) delivery rate into the xylem sap was observed in all treatments. The role of abscisic acid and cytokinins as positive and negative growth signals, as well as the involvement of root-generated ABA on ABA accumulation in leaves is discussed.