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 bioassa y with detached wheat leaves. The inhibitory activity of fresh sap was small, and could be largely accounted for by the abscisic acid conten t (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 develope d after a week. This increase in activity was due to a compound that i ncreased in size with storage at -20-degrees-C. When fresh sap was fra ctionated with filters of different molecular size exclusion character istics, 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 f ractionated, activity was only in fractions containing compounds large r than 0-3 kDa. The increase in activity and in size did not occur wit h storage in liquid nitrogen (-196-degrees-C) or at -80-degrees-C. The se results suggest that storage at -20-degrees-C causes the aggregatio n or polymerization of a small compound with low activity to form a la rge compound with high activity. This change is not catalysed by an en zyme because it can occur in a fraction from which molecules larger th an 0.3 kDa are removed. It is probably promoted by high solute concent rations when ice crystals form. Sap collected from plants in soils of high water potential had little or no activity after storage at -20-de grees-C.