Kang Han, Jaganathan Ganesh K, Han Yingying, Li Jiajin, Liu Baolin
Germplasm Conservation Laboratory, Department of Health Science and Engineering, University of Shanghai for Science and Technology, Shanghai, People's Republic of China.
Planta. 2023 Jan 7;257(2):33. doi: 10.1007/s00425-022-04061-4.
Fully mature acorns of Quercus variabilis, Q. aliena, Q. mongolica, and Q. glandulifera are desiccation-sensitive. X-ray computer tomography showed that cotyledons shrink during drying, but embryos are protected. Information available on recalcitrant acorns of tropical and sub-tropical species of Quercus suggests that an impermeable pericarp, which limits the entry and loss of water only through the hilum (scar), is the underlying mechanism that prevents drying of the embryo axis following dispersal until the germination season. However, there is a lack of consensus supporting this proposition across species, and it is not well understood if such mechanisms occur in temperate Quercus species. This study investigated the significance of the acorn pericarp for temperate oak species and presents an ecological framework based on the post-dispersal climatic conditions. Using Quercus variabilis, Q. aliena, Q. mongolica, and Q. glandulifera acorns, the relationship between moisture content (MC) and germination was established, and X-ray computed tomography (X-ray CT) was used to understand the internal structural changes of cotyledons and embryonic axis occurring during desiccation. Water entry and exit routes through the scar, pericarp and apex were determined by imbibition and drying experiments. Climatic data and acorn morphological characteristics and germination were subjected to a principal component analysis (PCA). Freshly dispersed acorns of all species had a moisture content (MC) above 35% fresh weight (FW) basis, but drying to 15-10% MC resulted in complete loss of viability, implying recalcitrance behaviour. X-ray CT images suggested that the pericarp offers some protection to cotyledons and embryonic axis during desiccation, but it is contingent on MC. Extensive drying to a low MC with the scar and apex covered with vaseline resulted in internal tissues shrinkage, corresponding with viability loss. Water could enter or exit through the pericarp, albeit at a much slower rate than through the scar. A combination of factors including acorn anatomy, moisture content at the time of dispersal, microhabitat, the position of acorns in the soil prevent embryo desiccation below the critical MC and thus promotes survival of acorns on/in the soil during winter in temperate regions. Pericarp anatomy, to some extent, prevents excessive drying of the embryonic axis by slowing water movement, but prolonged drying or predatory pressure could result in pericarp cracks, favouring the absorption of water during sporadic rain. In the latter case, the survival of acorns possibly depends extensively on the continuous erratic rainfall, i.e. continuous wet-dry cycle, but in-situ experiments are yet to be performed to test this hypothesis.
栓皮栎、辽东栎、蒙古栎和麻栎完全成熟的橡子对脱水敏感。X射线计算机断层扫描显示，子叶在干燥过程中会收缩，但胚受到保护。关于热带和亚热带栎属顽拗性橡子的现有信息表明，不透水的果皮是防止胚轴在传播后直至萌发季节干燥的潜在机制，该机制仅通过种脐（疤痕）限制水分的进出。然而，对于这一观点，不同物种间缺乏共识，且对于温带栎属物种是否存在此类机制也尚未完全了解。本研究调查了橡子果皮对温带栎树种的重要性，并基于传播后的气候条件提出了一个生态框架。利用栓皮栎、辽东栎、蒙古栎和麻栎的橡子，建立了水分含量（MC）与萌发之间的关系，并使用X射线计算机断层扫描（X射线CT）来了解干燥过程中子叶和胚轴的内部结构变化。通过吸胀和干燥实验确定了水分通过种脐、果皮和顶端的进出途径。对气候数据、橡子形态特征和萌发进行了主成分分析（PCA）。所有物种刚传播的新鲜橡子的水分含量（MC）以鲜重（FW）计均高于35%，但干燥至15 - 10%的MC会导致活力完全丧失，这意味着其具有顽拗性行为。X射线CT图像表明，果皮在干燥过程中对子叶和胚轴提供了一定保护，但这取决于MC。用凡士林覆盖种脐和顶端并将其大量干燥至低MC会导致内部组织收缩，这与活力丧失相对应。水分可以通过果皮进出，尽管其速率比通过种脐慢得多。包括橡子解剖结构、传播时的水分含量、微生境、橡子在土壤中的位置等多种因素的组合，可防止胚在关键MC以下脱水，从而促进温带地区冬季橡子在土壤上/中的存活。果皮解剖结构在一定程度上通过减缓水分移动来防止胚轴过度干燥，但长时间干燥或捕食压力可能导致果皮开裂，有利于在零星降雨期间吸收水分。在后一种情况下，橡子的存活可能在很大程度上依赖于持续不稳定的降雨，即持续的干湿循环，但尚未进行原位实验来验证这一假设。
