Nutlet dimorphism in individual flowers of two cold desert annual Lappula species (Boraginaceae): implications for escape by offspring in time and space

摘要

Most studies on seed heteromorphism have been done on species that produce one diaspore per flower. In Lappula duplicicarpa and L. semiglabra, the four nutlets per flower are dimorphic. We determined the ratio of nutlets with long (LN) and short (SN) glochids (stiff bristles) produced by individual flowers and compared their morphology, anatomy, dispersal, and germination characteristics. Nutlet morph ratios in all flowers in infructescences of field-grown plants were recorded, and morphology, anatomy, dispersal ability, and germination of LN and SN compared. The LN/SN ratio in different flowers varied; the proportion of LN in a single flower increased from base to top of infructescence. In both species, LN had greater mass and was more easily detached and dispersed than SN. LN of both species had higher overall germination percentages and rates than SN. The proportion of SN and LN produced at different positions in the infructescence changed through time. Thus, SN are produced first and then LN, resulting in an investment in long-distance dispersal after the safe sites of mother plants potentially are filled.