Pollen morphology, viability, and stigma receptivity of Canistrum (Bromelioideae: Bromeliaceae): implications for taxonomy and conservation

Abstract Canistrum (Bromeliaceae) comprises 13 species endemic to the Atlantic Forest and is subdivided into two subgenera ( Canistrum and Cucullatanthus ). This study aimed to characterize the morphology of reproductive structures (pollen and stigma), assess pollen viability and stigma receptivity across all species of the genus, and evaluate the taxonomic, reproductive, and conservation relevance of these characters. Analyses were conducted using light microscopy (with pollen grains prepared by lactic acetolysis) and scanning electron microscopy. Pollen viability was assessed using histochemical tests and in vitro germination assays at three floral stages. Stigma receptivity was evaluated through enzymatic assays. Pollen grains are dispersed as monads and show variation in shape (oblate, suboblate, and oblate-spheroidal), polarity (apolar and subisopolar), amb (circular and elliptical), aperture (pantoporate and biporate), and exine ornamentation, consistently semitectate and reticulate-heterobrochate to foveolate, with differences in lumen structure depending on the presence of granules. The occurrence of distinct pollen types proved informative for infrageneric delimitation within Canistrum . Stigmas are conduplicate-spiral, with stigmatic lobes ranging from crenulate to laciniate, and coloration varying from white to yellow. Pollen viability was high in all species, especially during anthesis, a stage that also showed the highest in vitro germination rates. Stigma receptivity peaked at anthesis and remained high during post-anthesis, indicating synchrony between pollen and stigma and suggesting the absence of dichogamy. Overall, pollen aperture type and exine ornamentation are stable characters of taxonomic value, whereas viability, germination, and receptivity patterns provide insights into reproductive strategies, supporting conservation and reproductive management of endemic and threatened Canistrum species.