How do roots control xylem sap ABA concentration in response to soil drying?

Maize (Zea mays L.) and Leucaena leucocephala (a tropical tree) were subjected to different degrees of soil drying and changes in their root and xylem ABA concentrations were measured simultaneously. Additional treatments such as defoliation, light sheltering and stem girdling were applied so that phloem transport into roots was manipulated. When soil was dried, ABA concentration in xylem sap increased almost linearly with that of roots in both species. Such an increase was relatively plateaued at high root ABA concentrations, suggesting that severe soil drying might have hindered some parts of roots from delivering ABA into xylem flow. When ³H-ABA was loaded into roots and monitored for its disappearance, it was found that the capability of roots to catabolize ABA was much lower than that reported for leaves, with a half-life of 1.15 h and a considerable amount (46%) of fed ³H-ABA was remained unmodified during 21 h of incubation. Previous soil drying significantly reduced such a capability of catabolism and extended the half-life of fed ³H-ABA to 2.27 h, and the amount remaining unmodified was elevated to 50.7%. Defoliation and light-sheltering treatments significantly reduced xylem ABA concentration of waterstressed plants. Using stem girdling approach, it was estimated that about 25 to 30% of ABA in the xylem sap might come from shoot through phloem under soil drying condition. It was concluded that in addition to an enhanced ABA biosynthesis in roots during soil drying, a slowed ABA catabolism in roots and an increased import from shoots were also important contributions to the sensitive changes of xylem ABA concentration in response to such a stress. Copyright © 1997 JSPP.