Bhardwaj M, Eckert C G
Department of Biology, Queen's University, Kingston, Ontario, Canada K7L 3N6.
Am J Bot. 2001 Dec;88(12):2204-13.
Dichogamy is one of the most widespread floral mechanisms in flowering plants and is thought to have evolved to reduce interference between pollen import and export within flowers, especially self-pollination. Self-pollination between flowers may also be reduced if dichogamy is synchronous among flowers on an inflorescence. The analysis of dichogamy at both levels requires that the sexual phases of individual flowers be defined functionally in terms of pollen deposition and removal. We conducted morphological and functional analyses to investigate the degree of dichogamy within flowers and the synchronicity of dichogamy between flowers within inflorescences in an emergent, aquatic monocot, flowering rush (Butomus umbellatus). Based on daily observations of the development of marked flowers, data on the schedule of anther dehiscence within flowers, and repeat surveys of floral sex ratios in three populations, individual flowers appear to be strictly protandrous. On average, each flower spends ∼1 d in each of male and female phases with an intervening 1-d neuter phase during which there is no available pollen in anthers and stigmas are not yet exposed to receive pollen. Morphological criteria used to delimit the beginning and end of each of these three sex phases were validated by quantifying the temporal schedule of pollen removal from anthers and pollen deposition on stigmas. Experimental pollinations showed that the morphological changes marking the end of female phase are hastened by pollen deposition. At the umbel level, synchronous development within sequential cohorts of flowers reduced overlap of male and female sexual phases between flowers. On average (±1 SE), 72 ± 3% of flowers completed their female phase while no other flowers on the same umbel were in male phase. Computer simulations of umbel development showed that this value is significantly higher than expected if the timing of flower development within umbels was random (30 ± 1%). Surveys of floral sex ratios in three populations revealed that 87% of umbels were either unisexual male or female at any given time. Pollinators usually visited more than one flower in sequence when foraging on umbels, suggesting that synchronous dichogamy may be an adaptation to avoid geitonogamy. The adaptiveness of both flower- and umbel-level dichogamy is also suggested because both traits are expressed to a lesser extent in obligately clonal, triploid populations, where flowers do not make seeds and hence floral adaptations are not maintained by natural selection.
雌雄异熟是开花植物中最普遍的花部机制之一，被认为是为了减少花朵内部花粉输入和输出之间的干扰，尤其是自花授粉。如果花序上的花朵之间雌雄异熟是同步的，那么花朵之间的自花授粉也可能会减少。对两个层面的雌雄异熟进行分析，需要根据花粉的沉积和去除情况，从功能上定义单个花朵的性阶段。我们进行了形态学和功能分析，以研究一种挺水水生单子叶植物花蔺（Butomus umbellatus）花朵内部的雌雄异熟程度以及花序内花朵之间雌雄异熟的同步性。基于对标记花朵发育的每日观察、花朵内花药开裂时间的数据以及对三个种群花性比例的重复调查，单个花朵似乎严格为雄性先熟。平均而言，每朵花在雄性和雌性阶段各花费约1天，中间有1天的中性阶段，在此期间花药中没有可用的花粉，柱头也尚未暴露以接收花粉。通过量化花药上花粉去除和柱头上花粉沉积的时间安排，验证了用于界定这三个性阶段开始和结束的形态学标准。实验授粉表明，标记雌性阶段结束的形态变化会因花粉沉积而加速。在伞形花序层面，连续几批花朵的同步发育减少了花朵之间雄性和雌性阶段的重叠。平均而言（±1标准误差），72±3%的花朵完成其雌性阶段时，同一伞形花序上没有其他花朵处于雄性阶段。伞形花序发育的计算机模拟表明，如果伞形花序内花朵发育的时间是随机的，这个值会显著高于预期（30±1%）。对三个种群花性比例的调查显示，在任何给定时间，87%的伞形花序要么是单性雄花要么是单性雌花。传粉者在伞形花序上觅食时通常会依次访问不止一朵花，这表明同步雌雄异熟可能是一种避免同株异花授粉的适应方式。花朵层面和伞形花序层面雌雄异熟的适应性也得到了暗示，因为在专性克隆的三倍体种群中，这两个特征的表达程度较低，在这些种群中花朵不产生种子，因此花部适应无法通过自然选择得以维持。
