Pollinia-borne chemicals that induce early postpollination effects in Dendrobium flowers move rapidly into agar blocks and include ACC and compounds with auxin activity

Abstract

The early visible effects of pollination in orchids are likely due to pollinia-borne chemicals. In Dendrobium we tested whether such compounds were water soluble and would diffuse in solid-aqueous phase, and determined both 1-aminocyclopropane-1-carboxylic acid (ACC) concentrations and auxin activity. Following pollination, the flower peduncle showed epinastic movement, followed by yellowing of the flower lip, flower senescence and ovary growth. Placing pollinia on agar blocks for 3, 6, 9 or 12h, prior to transferring them to the stigma, increased the time to these early postpollination effects or prevented them. Placing agar blocks that had been used for contact with the pollinia on the stigma also induced the early postpollination effects. The concentrations of ACC, the direct precursor of ethylene, in pollinia was lower the longer the pollinia had been in contact with the agar blocks, whilst the ACC content in the agar blocks increased with the period of contact. The auxin activity of the agar blocks also increased with the time of contact with pollinia. It is concluded that chemicals in the pollinia are responsible for the early visible postpollination effects, and that these (a) rapidly diffuse in aqueous media, and (b) comprise at least ACC and compounds with auxin activity. The idea is discussed that ACC plus auxin is adequate for the production of the early postpollination effects.

Keywords: Dendrobium; Epinasty; Ethylene; Postpollination effects; Senescence.