Symplastic entry and phloem translocation of phosphonate

Abstract

Symplastic entry and phloem transport of phosphonate, the active metabolite of the systemic fungicide fosetyl-Al, was evaluated in sugar beet ( Beta vulgaris L.) leaf discs and in castor bean plants ( Ricinus communis L. var. Gibsonii). Entry of phosphonate into leaf discs was linear for concentration and time; accumulation occurred after 24 hr when discs were floated on solutions ranging from 0.03 to 4 m M. Uptake was sensitive to metabolic inhibitors, but not to external phosphate concentration. It was affected by external pH, with maximum uptake occurring at pH 5. With the exception of efflux from the free space, negligible efflux of phosphonate from leaf discs occurred, indicating that phosphonate does not readily diffuse across the plasmalemma. The translocation profile of phosphonate in castor bean plants was similar to [ 14C]sucrose and was detected in phloem exudate 30 min after application. Entry of both sucrose and phosphonate into the phloem was reduced or eliminated in the presence of 2,4-dinitrophenol, indicating involvement of an active uptake process. Using a hydroponic growth system, phosphonate could be detected in root exudates of tomato ( Lycopersicon esculentum Mil.) and Persea indica L. which had been treated with potassium phosphonate.