Effect of Seed Damage on Germination in the Common Vetch (Vicia Sativa L.)

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Effect of Seed Damage on Germination in the Common Vetch (Vicia saliva L.)

ABSTRACT.-Larvae of the tortricid moths Cydia lunulana and C. nigricana feed inside developing pods of Vicia spp. (Fabaceae: Papilionoideae) in North Yorkshire, England. In the common vetch Vicia sativa, larvae commonly consume or at least damage every single seed in the pod in which they develop. Seeds that are completely consumed cannot perpetuate the vetch species, but what about partially damaged seeds? Naturally damaged seeds were obtained from field-collected pods, and sorted into five categories: (1) no damage, (2) <10% damage (volume missing), (3) 10-25% damage, (4) 25-50% damage, and (5) 5075% damage. Seeds were soaked and planted individually to monitor germination. While scarification of the seed coat of perfect seeds is beneficial to imbibition and germination, the percent of germination decreased with each successive damage category. That a substantial proportion of damaged seeds do germinate suggests that seed damage is not always a death sentence; but growing conditions in North Yorkshire are likely to favor the survival of seeds with seed coat intact to prevent germination in the autumn rains and plant death (before reproduction) in the winter frost.

INTRODUCTION

Seed predation takes different forms in plant/herbivore systems (Janzen, 1971). Large seed eaters, like parrots, may consume and kill entire seeds by eating many of them at a time; specialist insects, like bruchid weevils, complete their development by burrowing into and eating the vital parts of a single seed, each damaged seed yielding one adult weevil. Many insects, such as some tortricid moths, whose larvae develop by feeding on seeds inside developing fruit, devour some of the seeds, but only partially damage others (Koptur and Lawton, 1988).

Just how much damage a seed can sustain and still yield a viable seedling has not been the subject of many investigations (Janzen, 1976). If the embryo is not killed, many seeds can germinate and develop normally (Sonesson, 1994), though perhaps with fewer initial reserves. With reduced initial reserves, the seedling may be a poorer competitor than its better provisioned counterparts.

Many temperate herbs in the legume family have seeds that dry out and/or become physically dormant (Baskin and Baskin, 1989), and have a seed coat that is impermeable to water unless some force compromises the integrity of the seed coat (freezing, cracking, abrading, fire, etc.). For such species, a bite from a seed predator during the seed's development thwarts this seed coat integrity, and will render this physical seed dormancy nonfunctional even though the seed may be functional in all other ways. Baskin and Baskin (1989) argued that scarification by microbes, insects or physical factors is relatively unimportant in germination of legume seeds because many legumes have evolved a specific anatomical region of the seed coat that becomes permeable to water in response to certain temperature regimes. Such responses to temperature are adaptive mechanisms to permit seeds to respond to seasons, or to microclimatic changes such as gaps in the forest canopy. I have observed, however, that scarification by seed predators also takes place, and can compromise the adaptive response of seeds to the prevailing climate. This study addresses the effects of biotic scarification and seed damage.

Defoliation of the common vetch, Vicia sativa, significantly reduces the number of fruit and seeds, as well as total seed mass and individual seed mass (Koptur et al., 1996). Vicia sativa enjoys some protection against surface-feeding herbivores from ants visiting its stipular nectaries in both alien (Koptur, 1979) and native (Koptur and Lawton, 1988) habitats. But the extrafloral nectaries also enhance predation by internally feeding pod predators by protecting these herbivores against their natural enemies in areas where both ants and these herbivores are abundant; the resulting damage to seeds is substantial (Koptur and Lawton, 1988). …