MICROELECTRODE-RECORDED DEVELOPMENT OF THE SYMPLASMIC AUTONOMY OF THESIEVE ELEMENT COMPANION CELL COMPLEX IN THE STEM PHLOEM OF LUPINUS-LUTEUS L

From the cambial stage onwards, the symplasmic autonomy of sieve eleme nt/companion cell complexes (SE/CC-complexes) was followed in stems of Lupinus luteus L. by microinjection techniques. The membrane potentia l and the symplasmic autonomy of the mature SE/CC-complex was measured in successive internodes. A microelectrode was inserted into SE/CC-co mplexes or phloem parenchyma cells (PPs) and, after stabilization of t he membrane potential, the membrane-impermeant fluorescent dye Lucifer Yellow CH (LYCH) was injected intracellullary. The plasmodesmata of t he cambial SE/CC precursor were gradually shut off at all interfaces b eginning at the walls to be transformed into sieve plates. In the cour se of maturation, symplasmic discontinuity was maintained at the longi tudinal walls of the complex. In the transverse walls of the SE, wide sieve pores were formed giving rise to longitudinal multicellular symp lasmic domains of SE/CC-complexes. Symplasmic isolation of the files o f mature SE/CC-complexes was demonstrated in several ways: (i) the mem brane potential of the SE/CC-complexes (between -100 mV and -130 mV) w as consistently more negative than that of the PPs (between -50 and -1 00 mV). (ii) No exchange of LYCH was observed between SE/CC-complexes and the PPs. Lucifer Yellow CH injected into the SEs exclusively moved to the associated CCs and to other SE/CC-complexes whereas LYCH injec ted into the PPs was only displaced to other PPs. (iii) The electrical coupling ratio between adjacent PPs was ten times higher than that be tween SE/CC-complex and PP. A gradient in the membrane potential of th e SE/CC-complexes along the stem was not conclusively demonstrated.