H+ Uptake and Release during Circadian Rhythmic Movements of Excised Samanea Motor Organs 1: Effects of Mannitol, Sorbitol, and External pH

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We investigated H+ fluxes during circadian rhythmic movements of Samanea saman leaflets by monitoring the pH of a weakly buffered medium bathing extensor or flexor motor tissue excised at different times during 51 hours of darkness. Experiments were made in media of two different osmotic potentials: −0.3 megapascal (control medium) and −1.2 megapascals (control medium supplemented with 0.4 molar mannitol or sorbitol). Both extensor and flexor tissue took up H+ from the control medium at all times when the initial pH was 5.5. Rates of uptake by the extensor varied rhythmically in phase with the leaflet movement rhythm, whereas rates for the flexor were similar at all times. Addition of 0.4`molar mannitol (or sorbitol) to the medium magnified the amplitude of the rhythm in H+ uptake and release by extensor tissue and revealed a rhythm with flexor tissue. In the flexor, mannitol promoted H+ release (or reduced H+ uptake) at all times. We propose that mannitol reduces flexor cell turgor, and that low turgor activates the H+ pump. The magnitude and/or direction of pH changes varied with the initial pH of the medium. The pH values after 60 minutes converged to a narrow range, suggesting that cell wall pH might be regulated.

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