Experimental Studies on Slug-Plant Interactions: IV. The Performance of Cyanogenic and Acyanogenic Morphs of Trifolium Repens in the Field

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Abstract
The performance of cyanogenic and acyanogenic morphs of T. repens (white clover) was investigated by means of a field experiment in which cuttings of both morphs were transplanted into a permanent grassland and observed for 1 yr. During the growing season (spring and summer), slightly more acyanogenic plants survived and their rate of leaf and stolon production was slightly greater than that of cyanogenic plants. These differences were not apparent during the rest of the year. Observations of leaf scars left by several herbivores showed that only mollusks consistently preferred the acyanogenic plants. Weevils appeared not to be deterred by cyanogenesis and sheep appeared to change their preference from 1 morph to the other at different times of the year. There was damage by pathogens to the leaves of both morphs, but infection by the systemic rust Uromyces trifolii appeared almost exclusively on cyanogenic plants. This association of cyanogenesis and rust susceptibility was very marked in the field in which the studies were concentrated but was absent from another sampled field in the neighborhood. During the winter, frost damage caused tissue necrosis and death of leaflets; recordings for the early part of the winter showed that frost damage was significantly greater on the cyanogenic leaves. Few of the transplanted cuttings flowered. Proportionately fewer cyanogenic plants flowered than acyanogenic ones. Apparently, mollusk grazing is only 1 of several forces affecting the cyanogenesis polymorphism in white clover. The events responsible for this polymorphism seem to be more complex than can be accounted for simply by mollusk grazing or by any other single factor (for example, winter temperature). The ecological differences between the morphs of the single species T. repens may be as significant as are those commonly found between different species growing together in the same area.