绒毛白蜡二态型果实的扩散特征

多态型果实或种子的出现对植物种群的扩散具有重要的意义。绒毛白蜡(Fraxinus velutina)的果实具有二态型特征,主要表现在果翅数量上不同,分别定义为二翅型和三翅型果实,为了比较两类果实在风传扩散时的差异,研究了两类果实的形态、果翅结构和扩散距离及扩散时长。在大型封闭地下室内,以电扇在不同速度档位产生的气流作为风源,分别从2、1.5、1m处手动释放果实,对风速为0、4.6、6.5、7.3m/s时的果实扩散距离及扩散时长进行了比较;并在此基础上对果实的形态特征与扩散特征进行了线性相关分析。结果表明:在同一高度及相同风速下,三翅型果实的水平扩散距离都极显著的大于二翅型,但其相应的扩散时长都小于二翅型。在相同情况下,三翅型的果实沉降速度显著高于二翅型。两类果实随着释放高度的增加,其扩散距离和扩散时长都相应的增加;随着风速的升高,其扩散距离及扩散时长都相应的增加。三翅型果实质量显著高于二翅型,相反,三翅型果翅长与宽都显著小于二翅型。两种翅型的果翅细胞结构都一样,细胞内部都呈现气囊状,果翅表面沿纵轴方向有流线型的纵棱。通过直线相关分析发现,翅型是对扩散距离和扩散时长影响最显著的形态特征;与果实释放高度相比,风速是影响绒毛白蜡果实扩散距离与扩散时长最显著的环境因素。绒毛白蜡三翅型果实比二翅型果实传播的远,关键在于其具有三翅,三翅对阵风瞬间响应,使得沉降速度较二翅型高,可以在瞬时风的作用下,快速传播到较远的距离。三翅型与二翅型在扩散方式上的结合增强了绒毛白蜡的生存与定殖机会。

Study on dispersal traits of dimorphic fruits of Fraxinus velutina

Seed dispersal is a key stage in the life cycle of plants, and thus plays an important role in the colonization and extension of plant populations. Fruit dimorphism is a phenomenon widely reported in plants. There are differences in seed dispersal between normal fruits and heteromorphic fruits, such as in dispersal distance and in-flight behavior. Dimorphism of fruits allows different dispersal modes for plant species, thus facilitates the dispersal efficiency, dispersal range, reproduction, and ultimate evolution of the plant species. Many factors, such as fruit features, insertion height of the fruits, and wind speed, will affect the spread of fruits or seeds. To Fraxinus velutina, dimorphism of fruits is characterized by the number of wings on the samara:two-winged, which is the normal samara type, and three-winged. However, few studies have investigated the difference between these two kinds of samara types. The aim of the present research was to compare the dispersal performance of these two samara types under different wind speeds and release heights to better understand how their dispersal was affected by samara types and morphological and structural characteristics. In a large basement, air current was generated by an electric fan with a speed regulator that could be set at different levels as the wind source. By being placed close the fan, the fruits were manually released from 2, 1.5, and 1m high under wind speeds of 0, 4.6, 6.5, and 7.3m/s respectively. Then, dispersal distance and in-flight duration of the fruits were measured. The linear correlations among samara traits (wing type, fruit weight, wing width, and wing length) and dispersal parameters of the fruits (release height, wind speed, dispersal distance, and in-flight duration) were explored. The results showed that the horizontal dispersal distance of three-winged fruits was significantly greater than that of the two-winged fruits under the same height and wind speed, but the corresponding in-flight duration was shorter than that of the two-winged fruits. The terminal velocity of the three-winged fruits significantly had also exceeded that of the two-winged fruits under the same situation, while, the in-flight distance and in-flight duration increased correspondingly with increasing height and wind speed. The fruit weight of the three-winged type was significantly higher than that of the two-winged type, but wing length and width were lower. The cellular structure of the wings was the same between three-winged and two-winged fruits. In both samara types, a lot of air sacs were permeated the wings of samara, and ridges along the longitudinal axes of wings were streamlined. Correlation analysis showed that the phenotypic traits greatly correlated with the dispersal performance of fruits. Wing shape was the main morphological factor that had significantly affected the dispersal distance and duration. Compared to release height, wind speed also had significantly affected the dispersal distance and duration. Faster responses to gusts of the three-winged fruits than two-winged fruits was the key to its greater dispersal distance, which had resulted in higher terminal velocity. The combination of three-winged and two-winged fruits dispersal mechanisms improves chances of survival and colonization of this species.