GRAVIRESPONSIVENESS OF CRESS SEEDLINGS AND STRUCTURAL STATUS OF PRESUMPTIVE STATOCYTES FROM THE HYPOCOTYL

Graviresponse kinetics of roots and shoots (hypocotyls) were determine d on cress seedlings (Lepidium sativum L.) 36, 48, 60 and 72 h after a ctivation of the seeds. Presumptive statocytes of the hypocotyl were i nvestigated after rhodamine-phalloidin labelling and by thin sectionin g and freeze-fracture electron microscopy. Within a continuous stimula tion time of 420 min response kinetics of roots developed asymptotical ly whereas that of hypocotyls showed a double sigmoidal course. On the other hand the process of reorientation in hypocotyls was more effect ive than in roots. Hypocotyls always reached the vertical line within the stimulation time. This exact reorientation was never observed with roots. Rhodamine-phalloidin labelling revealed a pronounced network o f filaments running mostly unoriented in apical statocytes and showing a more transversal orientation in basal statocytes. Electron microsco py showed that statoliths that are characterized by amyloplasts were l ocated in an extremely thin layer of cytoplasm, sometimes not thicker than the surrounding vacuole membrane. This result indicates that the vacuole membrane under-went a strong deformation by the extremely low force exerted by displacing statoliths. After freeze-fracture electron microscopy disintegration of the vacuole membrane was observed, espec ially in areas of deformation. The plasma membrane of longitudinal cel l walls was characterized by paired or clustered plasmodesmata. As a w orking hypothesis it is proposed that stimulus transduction occurs by deformation of the vacuole membrane that surrounds one prominent stora ge site of calcium ions in plant cells.