Are orchid pollens recalcitrant?

It has been suggested that Orchidaceae species may have recalcitrant (desiccation sensitive) pollen. We explored drying responses in 23 species from the subfamily Epidendroideae by sampling the Kew living collection and drying the pollen to two conditions (70% RH, control for drying time) and 33% RH. Drying was for 3 days and the pollen germinated subsequently in vitro on a medium containing 1% sucrose, based on the response of 23 species. All species had pollen that tolerated, to different levels, short-term drying to 70% RH. This means the pollen is not recalcitrant in the sense of recalcitrant seeds, which die below 90% RH. But drying to 33% RH was drastic enough to reduce germination and tube growth close to zero in four species. Orchid pollen size was small, about 10-20  m in diameter, and much less than that in monocot Arecaceae and Poaceae (about 50  m). Variable drying tolerances across the three families indicates there is no simple relationship between pollen size, taxonomy and phylogeny. Although the sample size for species investigated remains small in relation to the orchid family, quite good levels of drying tolerance means wide-scale preservation (banking) for horticulture and conservation should be possible.