Anti-transpiration and anti-growth activities of xylem sap
were studied for maize, sunflower, cotton and castor
bean, which were subjected to several stress treatments: soil drying,
flooding and salinity. All samples of xylem
sap showed an increased concentration of proteins when plants were
either soil-dried, salt-treated or flooded. The
protein transportation flux in xylem sap, i.e. the product of protein
concentration and volume flux, was also
increased as a result. Electrophoresis using SDS–PAGE analysis
showed that xylem proteins were of variable size.
When samples of xylem sap were used to feed detached leaves or
shoots in a bioassay system for their anti-transpiration and
anti-growth activities, such xylem proteins in their original
concentrations apparently showed
neither direct inhibition nor enhanced effects of ABA. All samples
of xylem sap from plants subjected to different
stresses showed anti-transpiration and anti-growth activities that
could be fully accounted for by their ABA
concentration. In cases where such activities were more than could
be explained by the ABA concentrations,
filtration with a 0·20 μm microfilter removed the discrepancies.