分散贮藏后期植物对鼠类贮藏行为的调控：种子营养及气味衰减假说

Until recently, rodent-mediated seed dispersal via scatter hoarding was assumed to result from incidental activity of rodents. In this view, dispersed seeds are those that are buried in favorable situations and forgotten, and the mother tree has little or no control over these events. However, more recently, it has been suggested that plants can increase the likelihood of seed dispersal by producing seed traits that direct animals to increase probability of scatter hoarding during the caching stage, and to decrease the likelihood of cache retrieval during the post-caching period. Several studies have observed that plants increase scatter-hoarding intensity of rodents through regulating seed crop size, nutritional content, handling cost and/or physical or chemical defense. However, little is known about how plants may decrease rodent discovery of buried seeds. Here, we will test the seed nutrition degradation and odor attenuation hypotheses that seed nutrition contents will degrade to a certain level and seed odor will drop to a minimum in a short time after the seeds are cached in the soil. As a result, seedling establishment of plants will be promoted due to the reduction in seed cache recovery. We will conduct a series of experiments both in the field and within controlled enclosures to test these hypotheses using the plant–rodent seed dispersal system in the Donglingshan area, western Beijing city, and Wuhan urban area, central China. The subjects are systems of [Quercus wutaishanica, Armeniaca sibirica, Amygdalus davidiana + Niviventer confucianus, Apodemus peninsulae, Sciurotamias davidianus] in the Donglingshan and [Q. chenii, Q. variabilis + N. confucianus, N. fulvescens, Dremomys pernyi] in the Wuhan urban area. We will test four inter-related questions: 1) what nutritional contents and volatile chemicals are there in the focal seed species? 2) do the nutritional contents and volatile chemicals of seeds degrade shortly after caching by rodents? 3) does nutrition degradation and attenuation of volatile chemicals of buried seeds affect seed-recovery behavior of rodents? And 4) what nutrients and volatile chemicals of seeds most significantly determine these hoarding behavior of rodents? We predict that nutritional contents and volatile chemicals of seeds will quickly degrade or reduce to a certain level when the seeds are buried in the soil by rodents. Nutritional degradation and volatile chemicals attenuation may reduce the probability of seed retrieval by rodents, and thus improve seed survival and plant regeneration. Our ideas are novel and will help develop new hypotheses of seed nutrition degradation and odor attenuation, and further develop a comprehensive theory of how plants promote seed dispersal by manipulating animals to behave in a way that increases seed scatter-hoarding and reduces cache-retrieving.

早期观点认为“植物–鼠类”种子传播系统中，种子命运主要决定于鼠类。近期观点认为植物能通过结实特定特征的种子调控鼠类采取减少取食、增加分散贮藏等行为，促进种子传播。但种子被埋藏于土壤后，植物能否/如何调控鼠类行为以降低种子找到几率，提高种子传播效率少有报道。本项目拟以暖温带、亚热带次生林 “植物–鼠类”种子传播系统为例，运用野外跟踪、围栏实验及生化分析等方法，针对分散贮藏后期植物对鼠类贮藏行为的调控，提出并验证种子营养及气味衰减假说：种子被埋藏于土壤后，营养成分含量和挥发性化学物质会迅速衰减，种子的利用价值降低，嗅觉线索减弱，被鼠类找到几率降低，传播效率提高。预期结果可以补充对种子贮藏后期植物调控鼠类行为认识的不足，便于从整个种子传播过程认识“植物–鼠类”互惠关系。本研究沿袭植物能够调控鼠类行为促进种子传播的新观点，针对种子贮藏后期植物如何调控鼠类行为提出营养及气味衰减假说，具有一定创新性。

鼠类介导的植物种子扩散分贮藏前和贮藏后两个阶段。针对贮藏前阶段，已有许多理论和假说解释植物在进化过程中获得了调控种子的特征，操控鼠类行为，提高种子扩散适合度。针对贮藏后阶段，植物是否也获得操控鼠类贮藏行为，减少种子找回，提高种子扩散适合度，则鲜有研究。如果种子营养含量、气味水平随埋藏时间下降，鼠类将难以找回种子，从而增加种子存留量，提高扩散适合度（营养及气味衰减假说）。本研究以“栓皮栎/小叶栎—鼠类”传播系统为对象，通过种子营养及气味物质检测、结合人工种子、野外埋藏等验证植营养及气味衰减假说。结果显示随埋藏时间增加，栓皮栎种子中营养物质相对含量呈下降趋势，防御物质呈增加趋势；随淀粉浓度降低，种子被找回的几率显著下降；随单宁浓度增加，种子取食率显著下降。栓皮栎和小叶栎种子中挥发性气味物质相对含量均随埋藏时间增加而下降，栓皮栎种子存留率随雪松醇和右旋柠檬烯浓度下降而增加；小叶栎种子存留率随β-蒎烯浓度下降呈拱形不变化，随雪松醇、4-萜品醇、右旋柠檬烯浓度下降而增加，邻苯二甲酸二甲酯浓度下降而增加。栓皮栎种子烯烃类的相对含量小于小叶栎种子，但酯类相对含量高于小叶栎种子，醇类和酮类挥发物近存在于小叶栎种子。添加醇类物质、烯烃类物质、烷烃类物质和酯类物质的栓皮栎种子存留率显著低于小叶栎种子。虫蛀率随种子埋藏时间而增加，鼠类优先取食虫蛀种子，贮藏完好种子。结果表明在种子埋藏后阶段，植物可以调控营养含量、气味水平和虫蛀率，迫使鼠类减少找回/放弃种子，增加种子存留率，提高种子扩散适合度，有利于植物更新，一定程度上支持营养及气味衰减假说。未来尚需在其他物种及种子扩散系统中进行深入的研究。