The contribution of internode and mesocotyl tissues to root-to-shoot signalling of abscisic acid

The xylem of first internode of runner bean and of previously etiolated maize mesocotyl segments was perfused with media containing abscisic acid (ABA) or abscisic acid glucose ester (ABA-GE) in concentrations as they occur under stress conditions. ABA-GE passed through the internode and mesocotyl segments unchanged. Within 10 min the concentration of ABA-GE(xyl) rose to a level similar to that in the external perfusion medium. By contrast, 30-40 min passed before the concentration of free ABA in the xylem sap [ABA(xyl)] reached the level in the external medium. When ABA-free media were used, ABA was released from the xylem parenchyma to the xylem vessels resulting in an [ABA(xyl)] of 13-23 nM (runner bean internode) or 1-6 nM (maize mesocotyl). The total perimeter and, hence surface area, of the xylem elements was measured microscopically and from these measurements it was estimated that, in both bean internodes and maize hpyocotyls, the flux of ABA to the xylem was 1 pmol m(-2) s(-1). The ABA efflux from the stem and mesocotyl parenchyma into the xylem could be increased when the tissues were treated with tetcyclacis, an inhibitor of ABA degradation, but also by changing the pH from its normal value of about pH 5.8 to pH 7.0 and by adding 100 mM NaCl to the perfusion medium. If 100 nM ABA was added to the perfusion medium the above treatments had only small effects on the release of ABA from the tissues into the xylem.