Dioecy and pollen dimorphism in Vitis riparia (Vitaceae)

The breeding system of Vitis riparia Michx. hitherto has not been investigated. Field observations indicated that two morphs exist, one that produces fruit, and another that does not. Through experimental cross- and self-pollinations it was found that V. riparia is functionally dioecious in southern Ontario. Further, it was found that male plants produce functional spherical tricolporate pollen, whereas female plants produce inaperturate pollen of the same diameter, 23 μm, that cannot bring about fertilization. The sex ratios found at seven locations were skewed slightly, but significantly, in favour of male plants (average 0.567). Pollination is brought about by wind, probably supplemented to a greater or lesser extent by insects which feed on the cytoplasm-filled pollen of both sexes.