Acid-efflux patterns of primary and secondary roots of Ricinus communis (Euphorbiaceae)

Graviresponding primary roots of castor bean (Ricinus communis) were characterized by more acid efflux on their upper (i.e. rapidly growing) sides than their lower sides. Acid-efflux patterns of the upper and lower sides of horizontally oriented lateral roots were symmetrical. The onset of graviresponsiveness by secondary roots correlated positively with the development of asymmetric patterns of acid efflux similar to those of graviresponding primary roots. The addition of 1 mm sodium orthovanadate (an inhibitor of auxin-induced proton efflux) to the growth medium abolished gravicurvature and the development of acid-efflux asymmetry by primary and secondary roots. These results suggest that (i) the absence of an auxin-induced asymmetry of acid efflux may be the factor responsible for uncoupling graviperception from gravitropism in lateral roots, and (ii) the development of an auxin-induced asymmetry of acid efflux may mediate the onset of graviresponsiveness by secondary roots of R. communis.