两种集合繁殖体形态及间歇性萌发特性——以蒺藜和欧夏至草为例

集合繁殖体(synaptospermy)是荒漠植物为适应恶劣环境条件而演化出的关键性特征,其作为植物生活史特性之一,有助于了解在荒漠环境下集合繁殖体植物的适应策略。蒺藜(Tribulus terrestris)和欧夏至草(Marrubium vulgare)分别为一年生及多年生草本植物,蒺藜扩散单位由一朵花发育而成的集合繁殖体组成,欧夏至草扩散单位由多朵花发育的集合繁殖体和种子组成,以这两种植物为材料,对其集合繁殖体形态、活力、吸水及萌发特性进行初步研究。结果显示:(1)蒺藜集合繁殖体依照发育成熟先后位置在质量、附属物刺的长度、附属物占质量百分比、种子数、活力、吸水量、萌发率及萌发速率均存在显著差异;欧夏至草集合繁殖体质量及吸水量明显大于种子,而活力、萌发率及萌发速率不存在显著差异。(2)萌发位置上,蒺藜集合繁殖体仅在长刺端位置的种子萌发,属非随机萌发,而欧夏至草集合繁殖体萌发不具有规律性,属随机萌发;萌发时间上,蒺藜和欧夏至草集合繁殖体在当季下均只萌发部分种子,具间歇性萌发特性。(3)对不同生活型植物而言,其集合繁殖体附属物对种子的保护、保水、扩散及萌发行为的功能相同,但一年生草本植物的萌发行为侧重于种群繁衍,因此蒺藜集合繁殖体在当季萌发率及萌发速率较高;多年生草本植物的萌发行为侧重于种群扩散,因此欧夏至草扩散单位多样化,且萌发率及萌发速率较低。

Morphology and intermittent germination characteristics of two types of synaptospermy: a case study of Tribulus terrester and Marrubium vulgare

Female plants protect and adjust their offspring through the morphology of synaptospermy after withering. This adjustment provides elasticity and guarantees heredity and reproduction for a given species. Synaptospermy is a critical characteristic of desert plants that evolved as an adaptation to harsh environmental conditions, and may have important ecological significance for the protection of seed function and cross-pollination between plants. Knowledge about this characteristic of the plant life cycle would increase our understanding about the adaptation strategies of synaptospermic plants in desert environments. Synaptospermy involves the joining of two or more seeds or one-seeded fruit to form a compound dispersal unit. This adaptation is common throughout the plant kingdom, but is largely restricted to arid regions. Out of the many morphological-ecological types that have been distinguished among synaptospermic plants, only a few are mentioned here. Synaptospermy is primarily derived from monospermy. Thus, this adaptation represents an advanced evolutionary stage associated with (1) the degeneration of separation tissues in pods, legumes, loments, and capsules, (2) the formation of abscission tissue in the joints of peduncles and pedicels, and (3) a series of other characteristics. In fact, synaptospermic species are found in genera belonging to phylogenetically advanced groups within their respective families, such as the Brassicinae, Galegeae, Hordeae, and Paronychieae. Fruit that contains synaptospermic seeds are able to establish physiological gradients;thus, spreading germination over time (a major advantage). The dispersal and spreading of germination over time (polyphenic germination) allows a mother plant to reduce competition among its offspring. Polyphenic germination is especially pronounced in synaptospermic seeds. Synaptospermy may be divided into two morphologically different types: (1) diaspores develop from the joined seeds of the carpels of one flower, and (2) diaspores develop from the units of different flowers. None of these dispersal units, which contain up to several seeds in a single diaspore, develops more than two seedlings in one season, indicating that all seeds remain together in the dispersal unit, but are ready to germinate in different years;this phenomenon is called intermittent germination. The annual Tribulus terrestris and the perennial Marrubium vulgare are synaptospermic plants with dispersal units belonging to the two morphologically different diaspore types;M. vulgare also disperses using seeds. Here, we conducted preliminary comparisons and observations on the morphology, vigor, water absorption, and germination characteristics of each dispersal unit. The results showed that mass, length of burs, percentage of appendages accounting for mass, number of seeds, vigor, and water absorption all significantly differed among the synaptospermies of T. terrestris. Conversely, while mass and water absorption significantly differed among M. vulgare dispersal units, no significant differences were observed for vigor, germination, and germination rate. The position of germination in synaptospermy differed between plants. For instance, seeds only germinated on the long thorns of synaptospermids in T. terrestris, indicating nonrandom germination, but no regularity in germination was observed for M. vulgare, indicating random germination. In addition, under the conditions with the highest germination percentage, only part of the seeds germinated in one season for both species, indicating intermittent germination. For different plant life forms, the synaptospermic appendage has the same functions, e.g., protecting seeds, retaining water, dispersing, and germinating. However, the germination behavior of annual plants focuses on population reproduction;therefore, the germination percentage of synaptospermy was high over the season for T. terrestris. Conversely, the germination behavior of perennial plants focuses on population dispersal;therefore, the dispersal units of M. vulgare showed diverse characteristics, with a low germination percentage for the dispersal units over the season.