In red clover seed production, low seed yield is limiting the commercial exploitation of tetraploid red clover. To explore if pollination is the limiting factor for the seed yield in tetraploid red clover, we investigated pollinator behaviour and plant reproductive success in diploid (2x) cultivar ‘Rajah' and tetraploid (4x) cultivar ‘Amos' using honey bee and hand pollination approaches. We measured the seed set at the flower head level with the increasing visitation rate of honey bees (one bee, two bees and open pollination) and pollen diversity (one and two pollen donors) of hand pollination. We found no difference in honey bee pollination behaviour (visitation rate) between the diploid and tetraploid red clover. Surprisingly, the seed set in two bees was significantly lower than that of one bee in ‘Rajah'. We suggest that honey bees prefer to visit already tripped florets or to visit fewer florets due to scent marks left by the previous honey bees. In open pollination, seed number per flower head in ‘Amos' (on average 11) was half of ‘Rajah' (on average 20). Seed set in ‘Amos' was lower than 50% of ‘Rajah' in honey bee (one bee treatment) and hand pollination (one and two pollen donors treatments). There were no differences in stigma receptivity whereas pollen viability in ‘Amos' was lower than ‘Rajah' after the peak pollen viability was reached (six days after the onset of flowering). We suggest that the fertility problems such as low pollen viability may be the reason for the low seed set in tetraploid red clover. This conclusion should be verified by investigating other fertility traits during the ovary, embryo and seed development with a broader range of diploid and tetraploid cultivars.

中文翻译：

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比较二倍体和四倍体红三叶草 Trifolium pratense L 种子组的不同授粉方法。

在红三叶草种子生产中，种子产量低限制了四倍体红三叶草的商业开发。为了探讨授粉是否是四倍体红三叶草种子产量的限制因素，我们使用蜜蜂和人工授粉方法研究了二倍体 (2x) 品种“Rajah”和四倍体 (4x) 品种“Amos”的授粉行为和植物繁殖成功率. 我们测量了随着蜜蜂访问率（一只蜜蜂、两只蜜蜂和开放授粉）和人工授粉的花粉多样性（一个和两个花粉供体）的增加而在花头水平上的种子集。我们发现二倍体和四倍体红三叶草之间的蜜蜂授粉行为（访问率）没有差异。令人惊讶的是，两只蜜蜂的种子明显低于“Rajah”一只蜜蜂的种子。我们建议蜜蜂更喜欢访问已经绊倒的小花或由于之前蜜蜂留下的气味痕迹而访问较少的小花。在开放授粉中，'Amos' 每个花头的种子数（平均 11 个）是 'Rajah'（平均 20 个）的一半。在蜜蜂（一种蜜蜂处理）和人工授粉（一种和两种花粉供体处理）中，'Amos' 中的种子低于 'Rajah' 的 50%。柱头可受性没有差异，而在达到峰值花粉活力后（开花开始后 6 天），'Amos' 的花粉活力低于 'Rajah'。我们认为，低花粉活力等生育问题可能是四倍体红三叶草结实率低的原因。这个结论应该通过调查卵巢期间的其他生育性状来验证，