ACCUMULATION OF PHYLLODULCIN IN SWEET-LEAF PLANTS OF HYDRANGEA-SERRATA AND ITS NEUTRALITY IN THE DEFENSE AGAINST A SPECIALIST LEAFMINING HERBIVORE

Among wild plants of Hydrangea serrata (Hydrangeaceae) in Japan, there are sweet plants whose leaves contain a kind of isocoumarin, phyllodu lcin, which happens to be 350 times as sweet as sucrose to the human t ongue. In a primary beech forest in Ashu, Kyoto, the spatial distribut ion of sweet plants and temporal and the spatial distribution of phyll odulcin within and among plants were investigated using a high perform ance liquid chromatograph. The distribution of sweet plants was confin ed within a valley and was parapatric with non-sweet plants. A plant's characteristic phyllodulcin accumulation did not change, even when tr ansplanted into the different habitats. The phyllodulcin content of th e sweet plants varied greatly among plants, and the population mean pe aked in July when the plants flowered. Within a plant, phyllodulcin co ntent was elevated by partial defoliation. We examined the possible ef fect of phyllodulcin on herbivory by a specialist leafmining herbivore , Antispila hydrangifoliella (Lepidoptera: Heliozelidae). We transplan ted sweet and nonsweet plants reciprocally between their original habi tats, excluded attacks by parasitoids, and compared performance of the leafminer. Leafminer colonization and larval survivorship on transpla nted and in situ plants was not significantly different between sites. The fact that accumulation of phyllodulcin did not augment a defensiv e function, at least against herbivory by the leafminer, and the spora dic distribution of phyllodulcin-accumulating plants, suggest that the genotypes synthesizing phyllodulcin emerged independently at separate localities by mutation, and that the genotypes are almost adaptively neutral in defence against the specialist herbivore.