Reproductive ecology and the persistence of an endangered plant

Abstract

Amsinckia grandiflora (large-flowered fiddleneck) is an extremely rare California annual wildflower, known only from three populations. We conducted field and greenhouse experiments to compare the rare heterostyle with a cryptic self-incompatibility system (A. grandiflora) to a common, self-compatible, homostylous, sympatric congener (A. tessellata). Inter-species comparisons of adult plants suggested that in the greenhouse, A. grandiflora balances low floral seed set (seeds per flower) with increased floral output (flowers / plant) and a greater number of flowers per inflorescence. Seed set from active self-, intra- and inter-morph pollinations was high in A. grandiflora, indicating that the cryptic self-incompatibility system does not prevent seed set in the species. In the field, A. grandiflora floral output was only slightly greater than for A. tessellata, and did not fully balance lower floral seed set. Amsinckia tessellata average seed weight was lower than that of A. grandiflora, which, along with the lower number of flowers produced, indicated lower maternal investment per nutlet than for A. grandi- flora. Under conditions of unlimited resources, it appears that A. grandiflora fitness is not intrinsically limited when compared to its weedy relative A. tessellata. The differences in nutlet output between A. grandiflora and A. tessellata under field conditions are more likely due to differential responses to extrinsic factors such as competition and pollinator availability.