STORED XYLEM SAP FROM WHEAT AND BARLEY IN DRYING SOIL CONTAINS A TRANSPIRATION INHIBITOR WITH A LARGE MOLECULAR-SIZE

Xylem sap was collected from wheat and barley growing in a drying soil, and the effect of the sap on transpiration was detected by a bioassay with detached wheat leaves. The inhibitory activity of fresh sap was small, and could be largely accounted for by the abscisic acid content (about 2 x 10(-5) mol m-3). When fresh sap was stored at -20-degrees-C for several days, the activity increased. Maximum activity developed after a week. This increase in activity was due to a compound that increased in size with storage at -20-degrees-C. When fresh sap was fractionated with filters of different molecular size exclusion characteristics, and the separated fractions stored at -20-degrees-C for a week, activity developed only in the fraction containing compounds smaller than 0.3 kDa. However, when sap already stored at -20-degrees-C was fractionated, activity was only in fractions containing compounds larger than 0-3 kDa. The increase in activity and in size did not occur with storage in liquid nitrogen (-196-degrees-C) or at -80-degrees-C. These results suggest that storage at -20-degrees-C causes the aggregation or polymerization of a small compound with low activity to form a large compound with high activity. This change is not catalysed by an enzyme because it can occur in a fraction from which molecules larger than 0.3 kDa are removed. It is probably promoted by high solute concentrations when ice crystals form. Sap collected from plants in soils of high water potential had little or no activity after storage at -20-degrees-C.