Pollination density - effects on pollen germination and pollen tube growth in Betula pubescens Ehrh. in northern Sweden.

The aim of this study was to test the hypothesis that increased pollination enhances the probability of'pollen competition'in Betula pubescent. In an in vitro test, the germination of pollen from three B. pubescens trees, increased with increasing pollen density (r = 0·82, 0·81 and 0·82, respectively). Pollen tubes grew at different rates in styles when testing different paternal/maternal combinations. Evidence was obtained to suggest that these differences might be attributed to both maternal and paternal effects. Further, increasing the number of tubes per style tended to increase the length of the longest tube per style, but had no significant effect on the mean length of the tubes. The r values, for the correlations of lengths with numbers of pollen tubes per style, were lower in two natural B pubescens populations than in the data from a controlled pollination experiment, is concluded that selection among male gametes may occur after supplementary pollination in B. pubescens, but in natural situations, pollen deposition is usually too low for pollen Selection to be a common occurrence.