Phytochrome‐mediated closure of Albizzia lophantha leaflets as affected by sorbitol, mannitol and fusicoccin

Abstract

The effect of sorbitol and mannitol and their interaction with the effect of fusiccocin has been investigated in phytochrome‐mediated nyctinastic closure of Albizzia lophantha leaflets. Treatments were commenced 2 h into the photoperiod. Pairs of leaflets were excised, floated on test solutions, and transferred to darkness immediately after red or far red irradiation. Leaflet angles were measured in darkness for 3 h. Fusicoccin (1 to 100 μM) inhibited nyctinastic closure and enhanced reopening in the presence of Pft (far‐red‐absorbing form of phytochrome). Fusicoccin effects on far red treated leaflets were not constant and whichever fusicoccin concentration was applied, the effects were small. These results confirm the role of the plasma membrane proton pump in extensor cells during opening. Sorbitol and mannitol (25–100 mM) increased nyctinastic closure for about 2 h and then enhanced reopening in the presence of Pfr. When sugar alcohols were applied together with fusicoccin, sorbitol and mannitol enhanced the fusicoccin inhibition of closure and a marked synergistic effect was observed. These results indicate that both fusiccocin and the reduction of external osmotic potential caused by sorbitol and mannitol supply may stimulate the activity of the extensor H+ pump promoting the reopening.