COMPENSATORY RESPONSES OF 2 MELAMPYRUM SPECIES AFTER DAMAGE

1. Compensatory responses after damage were studied in two closely rel ated annual hemiparasites of the genus Melampyrum. We followed the gro wth and the seed set of individuals subjected to clipping of the main stem, defoliation and removal of flowers and unripe fruits. 2. Defolia tion resulted in decreased seed set and growth. Despite the hemiparasi tic life-style of the species, the leaves are thus important, either f or photosynthesis or because transpiration makes possible the acquisit ion of resources from a host. 3. There was a trade-off between early a nd late season reproduction: when all the flowers or unripe fruits wer e removed in early season, the plants enhanced their seed production i n late season, compensating totally (in Melampyrum pratense) or partia lly (in Melampyrum sylvaticum) for the loss of the seeds from early se ason flowers and fruits. This compensation was mainly brought about th rough changes in the abortion regimes of reproductive structures, not through regrowth. Thus, flower production substantially outstripped th e ability of the plants to provision all their seeds. 4. After main st em clipping the branches produced more fruits and seeds per flower in M. sylvaticum and fewer in M. pratense than the branches of the contro l plants. The timing of the damage determined the compensation mechani sm in M. sylvaticum: plants compensated by regrowth (increased flower number) after main stem clipping in May and by reducing seed abortion after main step clipping in June. 5. A large part of the findings can be accounted for in terms of resource allocation and the timing of the damage. The different responses to main step clipping, however, sugge st that differences in the architecture of the species are also import ant in compensatory responses.