Effect of pollen load, self-pollination and plant size on seeds and germination in the endangered pink-lipped spider orchid, Caladenia behrii

Abstract

Caladenia behrii Schltdl. (Orchidaceae) (syn. Arachnorchis behrii) is a sexually deceptive, endangered orchid that produces aggregated pollen as pollinia. It is pollinated by a thynnine wasp, and may also be pollinated incidentally by other insects. Pollinator effectiveness may depend on the number of pollinia that pollinators carry and deposit, and on whether they mediate cross-pollination or self-pollination. To understand the role of pollinators and guide conservation programs, we determined the effect of pollen load (one pollinium v. two pollinia) and self-pollination on seed number, seed (embryo) size and germination at 35 days. We also examined the effect of plant size on seed size and seed number. By using partial correlations with leaf width, seed size, seed number, capsule volume and stem length, we found that leaf width was a good predictor for seed number, and that seed size was not correlated with any of the variables examined. Flowers pollinated with one pollinium and two pollinia did not produce seeds that differed in size or number. Cross-pollinated flowers produced fewer but larger seeds, which germinated faster than did seeds from self-pollinated flowers. We conclude that seed production in the field may be estimated from leaf size, that pollinators carrying one pollinium are as effective as those carrying two pollinia and that selfing affects germination negatively, partly because of the smaller size of selfed seeds. Conservation programs aiming to perform hand-pollination of this species should use crossing with a single pollinium.