The impact of pollen tube growth on stigma lobe curvature in Kosteletzkya virginica: The best of both worlds

Kosteletzkya virginica (Malvaceae) is an herbaceous species native to North America that was introduced into China in 1993. Flowers are viable for one day and contain five independent lobes with curving stigmas that are pollinated either by insects or by stylar movement. If pollinated, stigma lobes remain erect, but if pollination fails, stigmas curve towards the anther, eventually making contact and self-fertilizing. We measured 1) the effect of pollination on stigma lobe curvature, 2) the effect of self and outcrossed pollen on pollen tube growth, and 3) the fertility of flowers in which the number of pollinated stigmas varied. Results indicate that the most important signal to halt stigma curvature occurs when the pollen tube grows beyond the stigma lobe. This response was not influenced by the amount of pollen received. The stigma lobes (n = 121) halted curvature after receiving from 1 to 162 pollen grains. Experimental pollination from one to four stigmas did not inhibit curvature of adjacent un-pollinated lobes. Under open-pollination conditions, the flowers with five stigmas produced significantly more seeds than flowers with one to four stigmas (U = 61.00, W = 881.00, P < 0.001). These results not only provide an experimental test that curvature of individual lobes is independent, it represents the first demonstration that pollen tube growth rather than pollen quantity is the signal that halts stigma lobe curvature. The ability to independently regulate curvature in each lobe allows the plant to produce a mixture of outcrossed and selfed seeds, which can maximize fitness benefits and may represent a “best of both worlds” mating system.