Data from: Seed set variation in wild Clarkia populations: teasing apart the effects of seasonal resource depletion, pollen quality, and pollen quantity

In habitats where resource availability declines during the growing season, selection may favor early-flowering individuals. Under such ephemerally favorable conditions, late-blooming species (and individuals) may be particularly vulnerable to resource limitation of seed production. In California, a region prone to seasonal drought, members of the annual genus Clarkia are among the last to flower in the spring. We compared pollen limitation (PL) of seed set and outcrossing rates between early- and late-flowering individuals in two mixed-mating Clarkia taxa to detect whether flowering time is associated with changes in seed set due to resource depletion, PL, or increased selfing. In 2008–2010, we hand-pollinated one flower on a total of 1855 individual plants either Early (near the onset of flowering) or Late (near the end of flowering) in the flowering season and compared seed set to adjacent, open-pollinated flowers on the same stem. To assess the contribution of pollen quality to reproduction, we first (2008) used allozymes to estimate outcrossing rates of seeds produced by Early and Late open-pollinated flowers. Second (2009), we conducted an anther-removal experiment to estimate self-pollen deposition. Seed set in Clarkia unguiculata was not pollen-limited. Clarkia xantiana ssp. xantiana was pollen-limited in 2008 and 2010, but not 2009. PL did not differ between Early and Late treatments. In both taxa, seed set of Early flowers was greater than Late flowers, but not due to PL in the latter. Reproduction was generally pollinator-dependent. Most pollen deposition was xenogamous, and outcrossing rates were >0.7 – and similar between Early and Late periods. These results suggest that pollen receipt and pollen quality remain seasonally consistent. By contrast, the resources necessary to provision seeds decline, reducing the fitness benefits associated with resource allocation to ovules.

在生长季期间资源可获得性下降的生境中，自然选择可能更青睐早开花的个体。在这类短暂有利的环境条件下，晚开花物种（及个体）可能尤其容易受到种子生产的资源限制影响。在易发生季节性干旱的加利福尼亚地区，一年生克拉克草属（Clarkia）植物是春季开花最晚的类群之一。我们针对两个具有混合交配系统的克拉克草属类群，比较了早、晚开花个体的种子生产花粉限制（pollen limitation, PL）水平与异交率，以探究开花时间是否会因资源耗竭、花粉限制或自交率提升而导致种子产量发生变化。2008年至2010年间，我们对1855株个体植物的一朵花进行人工授粉：分别在开花季早期（接近开花始期）与晚期（接近开花末期）开展操作，并将其种子产量与同一茎秆上相邻的开放授粉花朵的种子产量进行对比。为评估花粉质量对繁殖的影响，我们首先在2008年通过同工酶技术估算了早、晚花期开放授粉花朵所结种子的异交率；其次在2009年开展去雄实验，以估算自花花粉的沉积量。有爪克拉克草（Clarkia unguiculata）的种子产量不受花粉限制。黄枝克拉克草原亚种（Clarkia xantiana ssp. xantiana）在2008年与2010年存在花粉限制，但2009年无此现象。早、晚花期处理间的花粉限制水平并无显著差异。在两个类群中，早花期花朵的种子产量均高于晚花期花朵，但晚花期的产量下降并非由花粉限制所致。植物繁殖整体上依赖传粉者。绝大多数花粉沉积属于异花授粉，且异交率均大于0.7，早、晚花期的异交率水平相近。上述结果表明，花粉接收量与花粉质量在整个开花季保持相对稳定。与之相反，为种子发育提供养分所需的资源量持续下降，从而降低了将资源分配至胚珠所带来的适合度收益。