Unusual underwater flowering of Utricularia australis populations: a botanical enigma?

• Published in: Aquatic botany 2022-04, Vol.178, p.103487, Article 103487
• Main Authors: Ceschin, S., Pelella, E., Azzella, M.M., Bellini, A., Ellwood, N.T.W.
• Format: Article
• Language: English
• Subjects: Aquatic plant; Bladderwort; Chasmogamous flower; Lake environment; Stress induced flowering; Underwater flowering
• ISSN: 0304-3770; 1879-1522
• DOI: 10.1016/j.aquabot.2021.103487

The free-floating aquatic plant Utricularia australis has typically subsurface shoots and subaerial showy inflorescences. To find it growing with large populations among benthic Chara stands at 4–6 m depth in two Central Italian lakes, is a phenomenon that was never previously reported. Production of showy chasmogamous flowers at this depth makes such finding yet increasingly intriguing. Here we make quantitative-qualitative comparisons of morphological characters among subaerial and underwater flowers taken from diverse sites in Central Italy. Environmental data were used to delineate differences between superficial and underwater habitats where flowering U. australis populations were found, and to help explain these extraordinary findings. Although similar, the subaerial and underwater flowers did show some differences, the most important being the length of the floral scape. Underwater scapes were around three times longer than those emerged. This might be a typical phototropic-response or a failed attempt to get flowers above water surface for allowing entomophilous pollination, or a redundant morpho-physiological trait as a result of a recent ecological move. Underwater flowers were generally smaller, but they did have longer styles, nectar signalling was less obvious, thinner petals, mucous-coated pollen (subaerial pollen is dry and grainy). Both types of flower were sterile, producing abortive fruits. Possibility of underwater ecological conditions causing stress-induced flowering is also proposed. A lack of water movement is an important ecological requirement for U. australis growth and could explain why large populations have been found in deep water far from its typical habitat. •Utricularia australis large benthic populations found at 4–6 m depth at the first time.•Surprising production of U. australis showy chasmogamous flowers at this depth.•Morphological differences among U. australis typical emerged and underwater flowers.•Underwater inflorescences had longer scapes and flowers smaller and longer styles.•They had thinner petals, nectar signalling less obvious and mucous-not grainy pollen.