How Do Roots Control Xylem Sap ABA Concentration in Response to Soil Drying?

Abstract

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 3 H-ABA was load