UNEQUAL DISTRIBUTION OF OSMIOPHILIC PARTICLES IN THE EPIDERMAL PERIPLASMIC SPACE OF UPPER AND LOWER FLANKS OF GRAVI-RESPONDING RYE COLEOPTILES

In various studies, auxin (IAA)-induced coleoptile growth has been rep orted to be closely correlated with an increased occurrence of osmioph ilic particles (OPs) at the inner surface of the outer, growth-limitin g epidermal cell wall, indicating a possible function related to the m echanism of IAA-induced wall loosening. In order to test whether chang es in cell elongation rates of upper and lower flanks (UFs, LFs, respe ctively) during gravi-responsive growth are reflected in appropriate c hanges in the occurrence of OPs, rye (Secale cereale L.) coleoptiles, either as segments or as part of intact seedlings, were gravitropicall y stimulated by positioning them horizontally for 2 h. Ultrastructural analyses within the UFs and LFs of the upward-bending coleoptiles rev ealed a distinct imbalance in the occurrence of OPs. The number of OPs per transverse epidermal cell section of the elongation-inhibited UF on average amounted to twice the number of OPs counted in epidermal ce ll sections of the faster-growing LE As a hypothesis, the results lead us to suggest that Ops are involved in the mechanism of wall loosenin g and that temporary growth inhibition of epidermal cells of the UF du ring upward bending is mediated by inhibition of OP entry into the cel l walls. Thereby, more OPs accumulate near the inner surface of the ou ter wall of epidermal cells of the UF compared with the LF.