脊椎动物传播植物肉质果中的次生物质及其生态作用

在种子植物-动物的互惠关系中,植物果实成熟后需要吸引种子传播者取食果实,传播其种子至适宜萌发的生境,同时又要防御种子捕食者过度消耗种子。果实内的次生物质(如:配糖生物碱、大黄素、辣椒素)在此过程中起到重要的调控作用。依赖脊椎动物传播的肉质果中往往含有与植物茎、叶内相同的次生物质,其种类繁多,主要分为含氮化合物、酚类化合物和萜类化合物。未成熟果实内富含次生物质(如:单宁、大黄素),主要保护未成熟种子不被潜在的捕食者和食果动物取食,这些次生物质的含量通常随果实成熟而降低;其它次生物质(如:脱辅基类胡萝卜素)的含量随果实成熟而增多,可能起到吸引食果动物的作用。在对脊椎动物捕食的抵御中,果实内不同类型的次生物质促使成熟果实对所有脊椎动物都有毒性(专毒性)或者仅对种子捕食者有毒性(泛毒性)。肉质果内的次生物质对植物-食果动物相互关系的调控作用,还可以通过调节动物取食频次和数量、抑制和促进种子萌发、改变种子在肠道的滞留时间、吸引传播者等生态作用而实现。某种次生物质往往集多种生态作用于一身。目前对肉质果内次生物质与脊椎动物相互关系的探讨还不够深入。未来研究需要综合考虑植物次生物质与果实生理生化、形态学等特征对食果者的综合调控机理;次生物质在种子传播后的调控作用对植物种群或群落结构和分布格局的影响;从动植物协同进化角度探讨植物次生物质的产生、防御和吸引策略与脊椎动物对果实的选择和消费之间的关系等。开展脊椎动物传播肉质果实中次生物质的研究,对完善种子传播机制、植物繁殖和更新格局,丰富动植物相互作用、协同进化理论具有重要的意义。

Secondary substances and their ecological effects on seed dispersal in vertebrate-dispersed fleshy fruit plants

Plant species which depend on vertebrates for dispersing their seeds in flesh fruits to safe sites for establishment often face the trade-off between attracting seed dispersers and preventing their seeds from over consumption by seed predators. Secondary substances such as glycoalkaloids, emodin, and capsaicinoids in fleshy fruits play an important role for regulating this process. There are diverse secondary substances in vertebrate-dispersed fleshy fruits, but they are often the same as those found in the stems and leaves, and mostly belong to the three major categories: nitrogen-containing compounds (e.g. glycoalkaloids and capsaicinoids), phenolic compounds (e.g. emodins and tannins), and terpenoids (e.g. saponins). The content of some secondary substances such as tannins and emodins is abundant in immature fruits, which deters the potential predators or pre-mature disposal by frugivores; the content decreases with the fruit maturity. Other secondary substances (e.g. apocarotenoids) increase with the maturation, which may promote the consumption by dispersers. Secondary substances in vertebrate-dispersed fleshy fruits may modify the relationship between animals and plants by altering the fruit consumability: depending on the type of the secondary substances, the fleshy fruits could be toxic to all vertebrates (non-specific or general toxic) or only to some species (specific or directed toxic). Secondary compounds might be able to regulate animal feeding frequency and the amount of consumption by causing frugivores to leave the fruit plant sooner in a foraging bout; early departure from the plant might reduce the number of fruits consumed by a particular animal, but may increase the overall the likelihood of seed dispersal away from parent plant by other dispersers. Secondary substances in ripe fruit pulp can inhibit pre-dispersal seed germination or enhance post-dispersal seed germination. Gut retention time of seeds in vertebrates could be altered by some secondary substances, permitting either more rapid passage via laxative effects or slower passage rates by constipating effects. The colors, odors, and flavors of the fleshy fruits can be modified by secondary substances, which can attract dispersers by providing foraging and rewarding cues recognized by frugivores. A single secondary substance can have multiple ecological functions. Our knowledge of secondary substances in flesh fruits and their interactions with vertebrate animals are still limited. Future research need to consider diverse aspects of plant secondary substances such as the comprehensive regulatory mechanisms of fruit physiological biochemistry and morphology characteristics to the frugivores; the regulatory mechanisms of secondary substances for plant population and community structure and distribution patterns post dispersal; and the coevolution between the production of secondary substances and their defense and attraction strategies of plants and the selection and consumption of fleshy fruits by vertebrate animals. The research of secondary substances in vertebrate-dispersed fleshy fruits will enhance our knowledge of seed dispersal mechanisms, plant generation and recruitment patterns, and animal-plant interaction and their coevolution.