Differential grazing by the mollusc Arion hortensis Fér. on cyanogenic and acyanogenic seedlings of the white clover, Trifolium repens L.

Abstract

The average amount of releasable cyanide in cyanogenic seedlings of white clover was found to increase by a third from age 5 days to age 35 days. Over this period levels of cyanide decrease in cotyledons, but increase in stems and young leaves.The slug, Anon hortensis was introduced to mixed populations of mostly cyanogenic and mostly acyanogenic. seedlings of white clover of known age for 12 hour periods, and grazing levels of known phenotype seedlings was scored according to whether damage to the seedlings was lethal or not. Slugs did not discriminate between seedling phenotypes with respect to grazing. No difference in the level of damage between phenotypes was detected at 5 days age. At 16, 23 and 35 days age, very few grazed cyanogenic seedlings suffered lethal damage, although most grazed acyanogenic seedlings were lethally damaged. At these ages, most seedlings suffering non-lethal damage were cyanogenic.It is concluded that older seedlings of cyanogenic white clover are protected from lethal grazing damage by the slug Anion hontensis. This may influence the frequency of cyanogenic morphs in populations more than discriminatory grazing on adult plants.

The effects of altitude, aspect, grazing and time on the proportion of cyanogenics in neighbouring populations of Trifolium repens L. (white clover)

“…This suggested that a cohort with a distinctive proportion of cyanogenics had increased in diameter between years. If selection for the proportion of cyanogenics occurs mostly at genet or ramet birth as suggested by Ennos (1981bEnnos ( , 1982 and Horrill and Richards (1986), this finding suggests that the environment at birth may determine the proportion of cyanogenics for that cohort so that this proportion persists in that cohort as it matures. If this interpretation is correct, to account for the relationship between January minima 2. years previously, we must suppose that the 2 to 3-year-old cohort predominates in the population.…”

“…Remarkably, this seems to have been the only study of vertebrate grazing apart from a passing comment by Jones (1962) that the vole Microtus agrestis selectively eats acyanogenic Lotus corniculatus. Adult clover recovers well after grazing, so that selection for cyanogenics by herbivores might be mostly restricted to the seedling phase (Ennos, 1981b(Ennos, , 1982, and so will be greatest at sites where population cycling is common (Horrill and Richards, 1986).…”

Altitudinal clines for cyanogenesis in introduced populations of white clover near Vancouver, Canada

“…Most recent investigations of the cyanogenesis polymorphism in white clover have focussed on the selective factors themselves. There is conclusive evidence that herbivory by slugs and snails, and perhaps aphids, selects against the acyanogenic phenotype (Angseesing, 1974;Angseesing and Angseesing, 1973;Crawford-Sidebotham, 1972;Dirzo and Harper, 1982a;Dritschilo et al, 1979;Horrill and Richards, 1986;Jones, 1962Jones, , 1966.…”

Biologically active secondary metabolites in white clover (Trifolium repens L.) – a review focusing on contents in the plant, plant–pest interactions and transformation

“…Cyanogen synthesis varies with plant age; increasing with seedling age (by 37 %, day 5-35) (Horrill and Richards, 1986), but decreasing with mature plant age (by 90 %, day 19-159) (Vickery et al, 1987). The greater synthesis in seedlings than in mature plants probably results from the fact that the cyanogenic defence mechanism is more important for seedlings, as the fitness of mature plants will be only marginally reduced if mature plants are attacked by herbivores (Crawford-Sidebotham, 1972).…”

“…Cyanogenesis in white clover is effective against fungi (Levin, 1976), herbivores (Hughes, 1991;Stochmal and Oleszek, 1997) including slugs and snails (Angseesing, 1974;Angseesing and Angseesing, 1973;Crawford-Sidebotham, 1972;Horrill and Richards, 1986;Raffaelli and Mordue, 1990), weevils (Ellsbury et al, 1992;Raffaelli and Mordue, 1990) and mammals (sheep, cattle, water voles) (Coop and Blakley, 1949;Lehmann et al, 1990;Moran, 1954;Vickery et al, 1987;Viette et al, 2000). The suppressing effect of sudan grass (Sorghum vulgare var.…”