Microautoradiographic localisation of [3H]sucrose and [3H]mannitol in Robinia pseudoacacia pulvinar tissues during phytochrome-mediated nyctinastic closure

Summary. We have analysed the incorporation of ³H]sucrose and ³H]mannitol in pulvinar motor cells of Robinia pseudoacacia L. during phytochrome-mediated nyctinastic closure. Pairs of leaflets, excised 2 h after the beginning of the photoperiod, were fed with 50 mM ³H]sucrose or ³H]mannitol, irradiated with red (15 min) or far-red (5 min) light and placed in the dark for 2–3 h. Label uptake was measured in whole pulvini by liquid scintillation counting. The distribution of labelling in pulvinar sections was assessed by both light and electron microautoradiography. ³H]Sucrose uptake was twice that of ³H]mannitol incorporation in both red- and far-red-irradiated pulvini. In the autoradiographs, ³H]sucrose and ³H]mannitol labelling was localised in the area from the vascular bundle to the epidermis, mainly in vacuoles, cytoplasm, and cell walls. Extensor and flexor protoplasts displayed a different distribution of ³H]sucrose after red and far-red irradiation. Far-red light drastically reduced the ³H]sucrose incorporation in extensor protoplasts and caused a slight increase in internal flexor protoplasts. After red light treatment, no differences in ³H]sucrose labelling were found between extensor and flexor protoplasts. Our results indicate a phytochrome control of sucrose distribution in cortical motor cells and seem to rule out the possibility of sucrose acting as an osmoticum. Correspondence and reprints: Unidad de Fisiología Vegetal, Facultad de Biología, Universidad de Barcelona, Avenida Diagonal 645, 08028 Barcelona, Spain.