Analysis of the reproductive ecology of white clover and its application to the restoration of grazed swards

本研究旨在加深对白三叶种子繁殖生态的认识。用两个品种Kopu(大叶)和Tahora(小叶)进行了一系列试验。主要结果描述如下。普通环境实验表明，Kopu比Tahora表现出更大的花头和更宽的每一基因的花头排列。采用析因试验，同时考虑了去叶胁迫和资源水平，考察了品种的可塑性反应。结果表明，不同品种种子繁殖性状的反应程度不同，甚至每荚粒数(决定结实率)和单株种子大小都因强烈的落叶胁迫而降低。结果还表明，不同品种对中度落叶胁迫具有复杂的可塑性反应模式。资源水平微妙地影响塑料对落叶胁迫的反应。本研究和前人的研究结果表明，在个体水平上开花头数多、花期长、种子萌发不同步的基因型将有利于种子的持续。我还在野外进行了生殖生态的观察。种群大小、结实率与传粉者访花次数呈显著正相关。体型极小的种群，授粉者访花频率低，结实率降低。结实率的显著降低会严重限制恢复的成功。在有小片孤立的白三叶草的放牧牧场中，这是非常有可能的。成虫的丰度和分布格局适合结实，这是三叶草含量自我恢复的关键。

The aims of this research are to deepen our understanding of the seed reproductive ecology of white clover. A series of experiments were conducted using two cultivars, Kopu (large-leaved) and Tahora (small-leaved). Major results are described below. A common environment experiment showed that Kopu exhibited larger flower-heads and wider arrangement of flower-heads per genet than Tahora. A factorial experiment, in which both of defoliation stress and resource level were manipulated, was conducted to examine the plastic responses of the cultivars. The experiment showed that the seed reproductive characters differed in the extent of the responses, and even seed number per pod (which determine seed set rate) and individual seed size decreased due to strong defolation stress. It also showed that the cultivars had complex patterns of the plastic responses to moderate defoliation stress. Resource level affected the plastic responses to defoliation stresses subtly. The results obtained in this research and the previous studies suggest that the genotypes which have many flower-heads and long periods of flowering at individual level and asynchronous seed germination would be advantageous in persisting. I also conducted the observation on reproductive ecology in fields. It clearly showed positive relationships among population size, seed set rate and visitation frequency of pollinators. The populations with extremely small size had low visitation frequency of pollinators, resulting in reduction in seed set rate. Marked reduction in seed set rate can impose serious limitation to successful restoration. It is highly possible in the grazed swards with small isolated patches of white clover. It would be essential to self-restoration of clover content that the abundance and distributional pattern of adults are proper for the seed set.