Role of pulvinar chloroplasts in light-driven leaf movements of the trifoliate leaf of bean (Phaseolus vulgarisL.)

Abstract

Laminar pulvini of bean (Phaseolus vulgaris L.) contain numerous chloroplasts in cells of their motor tissue. The quantitative relationships of the chloroplast pigments, chlorophyll a and b, β-carotene, lutein, neo-xanthin as well as the xanthophyll cycle carotenoids (violaxanthin, antheraxanthin and zeaxanthin) were similar to those of mesophyll chloroplasts from leaflet laminae. Exposure of pulvinules to light caused de-epoxidation of violaxanthin to zeaxanthin, showing that the xanthophyll cycle is functioning. Chlorophyll fluorescence analysis of pulvinules confirmed that their chloroplasts are capable of both photosynthetic electron transport and non-photochemical fluorescence quenching, showing that they build up a considerable transthylakoid proton gradient in the light. Application of DCMU to excised pulvinules and laminar discs, as well as to pulvinules of intact, attached terminal leaflets blocked electron transport and fluorescence quenching. Application of the uncoupler CCCP to intact pulvinules also prevented non-photochemical fluorescence quenching. The rate of movement of the low-light-adapted terminal leaflet in response to exposure of its pulvinule to overhead red light (500 μmol m -2 s -1 ) was reduced when the pulvinule was pretreated with DCMU. The pulvinar response to overhead blue light (50 μmol m -2 s -1 ), which is more pronounced than to red light, was not affected by similar pretreatment with DCMU. Pretreatment with CCCP caused a short lag in the response to red light, but did not affect its subsequent rate. The results suggest that the pulvinar response to red, but not to blue light, requires non-cyclic electron transport and the resulting generation of ATP.