In vivo and in vitro effects of boron on the plasma membrane proton pump of sunflower roots

Abstract

The effect of boron excess and deficiency on H+ efflux from excised roots from sunflower (Heliarahus annuus L. cv. Enano) seedlings and on plasma membrane H+‐ATPase (EC 3.6.1.35) in isolated KI‐washed microsomes has been investigated. When seedlings were grown in media with toxic levels of H3BO3 (5 mM) or without added boron and exposed to light conditions, an inhibition of the capacity for external acidification by excised roots was observed as compared to roots from seedlings grown with optimal H3BO3 concentration (0.25 mM). Toxic and deficient boron conditions also inhibited the vanadate‐sensitive H+‐ATPase of microsomes isolated from the roots. The mechanism of boron toxicity was investigated in vitro with microsorne vesicles. A strong effect of boron on the vanadate‐sensitive, ATP‐dependent H+ transport was found, but the vanadate‐sensitive phospho‐bydrolase activity was not affected. These results suggest that boron could exert an effect on the plasma membrane properties, directly or indirectly regulating, proton transport.