AMBIENT AIR HUMIDITY AFFECTS PHLOEM MOBILITY OF FOLIAR-APPLIED BORON

Foliar fertilization is a common method for the correction of B deficiency. However, plant parts not directly hit by the B-spray (e.g. roots) often do not benefit from this measure. The reason for B immobility is not fully understood. We studied the effect of ambient air humidity on phloem mobility of foliar-applied B. In Ricinus communis seedlings with cut hypocotyls B concentrations in phloem sap reached up to 100% of the B concentrations applied to the cotyledons. In a second experiment B mobility was studied using intact Ricinus seedlings. At high relative air humidity (r.h.) B applied to the cotyledons was transported to hypocotyls and roots, whereas at low r.h. no translocation of B was detectable. It is concluded that ambient air humidity controls phloem mobility of B via its effect on the xylem flow rate: if the xylem flow rate is low or xylem flow is interrupted foliar-applied B becomes mobile.