Clonal Structure, Seed Set, and Self-Pollination Rate in Mass-Flowering Bamboo Species during Off-Year Flowering Events

Bamboos are typical examples of highly synchronized semelparous species. Their mass-flowering events occur at supra-annual intervals but they sometimes flower on a small scale in off-years. If some bamboo ramets (culms) of a genet flower and die in off-years, whereas other culms of the same genet do not flower synchronously, the genet can still survive blooming in an off-year and could participate in the next mass-flowering event. At genet level, the effect might be similar to that achieved by synchronously reproducing iteroparous plants. In addition, if multiple genets flower simultaneously in off-years, cross-pollination will be promoted. However, it is not known whether all the culms in a genet flower synchronously and whether multiple genets flower in off-years. We determined the clonal structure of three temperate dwarf bamboo species, i.e., Sasa senanensis, S. kurilensis, and S. palmata, at 24 off-year flowering sites and the surrounding areas in northern Japan using seven microsatellite markers. We also estimated seed set at seven of the sites and self-pollination rates at five sites to determine off-year reproductive success. Next, we investigated whether seed sets at the culm level were related to flowering area and/or number of flowering genets, using generalized linear mixed-effect models (GLMMs). Multiple genets flowered at 9/24 flowering sites. We found that 40/96 of the genets identified had some flowering culms. Non-flowering culms were present in 24/40 flowering genets. Seed set was in the range 2.2%–12.5% and the self-pollination rate was 96.3%. In the best GLMM, seed set increased with flowering area. Seeds were produced in off-years, but cross-pollination was rare in off-years. We suggest that some dwarf bamboos may exhibit iteroparity or imperfectly synchronized semelparity at the genet level, a characteristic similar to that of other reproductively synchronous plants. We also found synchronous flowering of a few genets even in off-years.

竹子是高度同步单次结实（semelparous）物种的典型代表。它们的大规模开花事件间隔多年发生，但有时会在非开花年小规模开花。若某一基株（genet）的部分分株（ramet，即竹秆）在非开花年开花死亡，而该基株的其余竹秆未同步开花，则该基株仍可在非开花年开花存活，并参与下一次大规模开花事件。在基株层面，这种效应可能与同步繁殖的多次结实（iteroparous）植物所实现的效果相似。此外，若多个基株在非开花年同时开花，将促进异花授粉。然而目前尚不清楚：基株内的所有竹秆是否会同步开花，以及多个基株是否会在非开花年开花。我们利用7个微卫星标记（microsatellite markers），在日本北部的24个非开花年开花样地及周边区域，对3种温带矮竹物种——Sasa senanensis、S. kurilensis和S. palmata的克隆结构进行了测定。我们还在其中7个样地评估了结实率，并在5个样地估算了自花授粉率，以明确非开花年的繁殖成功率。随后，我们采用广义线性混合效应模型（generalized linear mixed-effect models, GLMMs），探究了竹秆水平的结实率是否与开花面积和/或开花基株数量相关。在24个开花样地中，有9个样地存在多个基株开花的情况。我们发现，在所鉴定的96个基株中，有40个基株带有开花竹秆；在这40个开花基株中，有24个基株仍存在未开花的竹秆。结实率介于2.2%至12.5%之间，自花授粉率为96.3%。在最优广义线性混合效应模型中，结实率随开花面积的增大而升高。非开花年虽可产生种子，但异花授粉在该时期较为罕见。我们认为，部分矮竹在基株层面可能表现出多次结实性，或存在不完全同步的单次结实性，这一特征与其他繁殖同步性植物相似。我们还发现，即便在非开花年，仍有少量基株会同步开花。