Breeding system and inbreeding depression in the clonal plant species Knautia arvensis (Dipsacaceae):: implications for survival in abandoned grassland

Larger clonal sizes observed in abandoned than in managed grassland could result in increased geitonogamy in Knautia arvensis. The species was investigated for self-compatibility, seed set without pollinators, and inbreeding depression in a greenhouse experiment. Seed set was equivalent in hand-selfed and hand-outcrossed plants (c. 53%), indicating full self-compatibility. Seed set was low (2.8%) in caged, unmanipulated flower heads. There was no significant difference in seed weight, germination, and survival between selfed and outcrossed progeny. Juvenile biomass was significantly lower in selfed than in outcrossed progeny (on average 45% lower). Cumulative fitness calculations for germination, survival, and juvenile biomass showed that the mean inbreeding depression was 58% in offspring produced by selfing. The results suggest that in K. arvensis, which is protected against self-pollination within flower heads through protandry, the likely mechanism for selfing is via geitonogamy among flower heads. Geitonogamy among ramets within the larger clones observed in abandoned grassland could reduce offspring viability in K. arvensis since the fitness of selfed offspring was low. Inbreeding depression could therefore add to the recruitment problems experienced by plants in abandoned grassland, both in K. arvensis and probably in other clonal grassland species, depending on their growth structure and breeding system. (C) 2002 Elsevier Science Ltd. All rights reserved.