Embryo size as a tolerance trait against seed predation: Contribution of embryo-damaged seeds to plant regeneration

Abstract

Embryo damage in seed predation is a common occurrence and has been generally considered equivalent to seed death. We hypothesize that seeds with proportionally larger embryos (radicle plus plumule) provide greater tolerance to seed damage by rodents, allowing successful germination. To test this hypothesis, we examined germination and estimated the contribution of embryo-damaged seeds to plant regeneration by comparing the dispersal patterns of intact and embryo-damaged seeds and the ecophysiological responses of their emerged seedlings in two oak species of contrasting embryo size.Our results show that embryo size was positively correlated with seed size in both oak species, but one (Quercus lobata) had proportionally longer embryos than the other (Q. agrifolia), revealing inter-specific differences in embryo size. Probability of embryo excision behavior (partial seed damage to extirpate the embryo) was positively associated with seed size but intensity of damage was relatively constant across all sizes. The species with proportionally longer embryos showed a stronger capacity to regenerate from embryo damage because longer embryos experienced reduced probability of total (lethal) embryo damage by rodents. Seed size and thus intraspecific embryo size only increased germination success in the species with shorter embryos, allowing larger seeds to escape full embryo damage.Seedlings from intact seeds performed better (greater plant biomass and higher chlorophyll and nitrogen index) than those from embryo-damaged seeds. However, seedlings from embryo-damaged seeds showed higher anthocyanin content, a possible response to seed damage. Intact and embryo-damaged acorns showed similar dispersal patterns by rodents (e.g. seed caching behavior and dispersal distances). Importantly, seed retrieval by rodents was significantly lower for embryo-damaged seeds, allowing greater seed survival and supporting the expectation that embryo excision behavior is a strategy to store seeds for longer periods. We conclude that tolerance to embryo damage is an important reproductive strategy that enables plant recruitment. Embryo size could play a crucial role in the evolutionary and ecological responses of seeds to animal predation.