Watsonia

Abstract: Summary Some orchids have been proposed to be Batesian floral mimics imitating flowers of sympatric rewarding species to attract pollinators. It is not yet well understood which traits are critical for pollinator attraction, although colour, shape and scent have all been implicated. We conducted field-based behavioural experiments using pairwise combinations of plastic flowers differing in spectral and shape properties offered to long-proboscid tabanid flies (Philoliche aethiopica) at two sites – one where the fly-pollinated orchid mimic Disa pulchra occurs with its pink-flowered model, the iris Watsonia lepida and another where the flies forage on a blue-flowered rewarding plant Agapanthus campanulatus. Flies intensively visited and probed plastic flowers of colours indistinguishable in a fly vision model from those of the rewarding plants. Inflorescence architecture and brightness of plastic flowers made little difference to fly attraction, but those that matched the shape and nectar guides of Watsonia flowers were significantly more attractive. Flowers of the three focal plant species are weakly scented and divergent in scent chemistry. This study shows that traits that mimic, in order of importance, the spectra, shape and nectar guide patterns of flowers of rewarding plants would be under strong selection in food-deceptive orchids as they maximize attractiveness to their pollinators. Our experiments also help to explain why deceptive orchids in general often seem to match the flowers of sympatric rewarding plants more in visual attributes than in scent chemistry.

Abstract: The genus Watsonia has a number of species with potential to be developed as new ornamental crop plants, but to date there are no reports on in vitro propagation of any member of this genus. Seeds from four Watsonia species, Watsonia gladioloides, Watsonia lepida, Watsonia laccata, and Watsonia vanderspuyiae were decontaminated and germinated on one-tenth strength MS media without hormones or sucrose. Shoots were induced from seedling hypocotyl segments when both an auxin [α-naphthaleneacetic acid (NAA)] and cytokinin [N6−benzylaminopurine (BA)] were present in the medium, while root and leaf explants failed to produce shoots. Multiplication of axillary shoots was greatest when only BA (0.5 mg l−1) was added to the medium. Shoot explants propagated in a ‘liquid-shake’ culture exhibited greater growth rates than those on agar-solidified medium, but shoot production varied between species. Meristemoids were induced in all species, but no significant trend was found between growth index (GI) and meristemoid formation, suggesting that reduction in GI may not necessarily be a prerequisite for producing meristemoids. Corm formation was inconsistent and storage organs could only be induced in one of the four species, W. vanderspuyiae. This occurred best at 25°C with 3% sucrose and an agar level of 15%. Indole-3-acetic acid (IAA) and NAA at 1 mg l−1 significantly increased mean number of roots per shoot explant on all four species, while indole-3-butyric acid (IBA) was more effective when applied at 0.1 mg l−1. Plantlet survival ex vitro was negatively affected when NAA and 2,4-dichlorophenoxyacetic acid (2,4-D) were used to root shoot explants for all species. In W. laccata, all auxin treatments [IAA, IBA, NAA, phenylacetic acid (PAA), and 2,4-D] at a concentration of 1 mg l−1 significantly reduced ex vitro survival of plantlets. Successful micropropagation of Watsonia is an important step in the further development of this genus as a horticultural crop.