A seasonal strategy for pollen tube growth and ovule development to overcome winter in Japanese stone oak (Lithocarpus edulis)

Delayed fertilization is commonly observed in many acorn-producing Fagaceae trees, yet its underlying mechanisms and adaptive significance remain poorly understood. In recent years, a new hypothesis has been proposed suggesting that the nearly year-long delay in fertilization is driven by an overwintering strategy, wherein female gametophyte development is delayed, and pollen tube growth is arrested before winter. This mechanism allows ovules to be fertilized and seeds to develop during more favorable seasons while avoiding adverse winter conditions. However, empirical evidence for this overwintering strategy has been limited. To address this, we observed the seasonal progression of pollen tube growth and ovule development in Lithocarpus edulis, a species with spring and autumn flowering seasons. Monthly observations of pistillate flowers from both seasons were conducted using microtome techniques and scanning confocal microscopy. Our findings revealed that pollen tubes were arrested at the style joining site, and ovules remained immature in both spring and autumn flowers prior to winter. Following winter, pollen tube regrowth and ovule maturation were synchronized in the subsequent spring, regardless of the flowering season. These results support the hypothesis that ovule development is delayed, leading to delayed fertilization, until after winter. This study highlights the importance of temporally coordinating fertilization phenology with flowering and fruiting phenology in seasonal environments to avoid unfavorable winter conditions.