Seed heteromorphism in Triticum dicoccoides: association between seed positions within a dispersal unit and dormancy.

Variation in seed size and dormancy can take the form of seed heteromorphism, i.e., production of different kinds of seeds by a single individual. In this paper, I tested for the effect of seed position within a spikelet on its germination over time, and the contribution of this effect to population differentiation along an aridity gradient in an annual grass, Triticum dicoccoides. The results show that the upper grain in a spikelet is larger than the bottom grain, and either germinates in the season following dispersal, or dies. In contrast, a substantial fraction of the bottom grains do not germinate in the first season, but remain dormant in the soil seed bank for 1 and, very rarely, 2 years. This pattern was observed in seeds of all origins, but the bottom grains from the most arid location had the lowest, and from the least arid location, the highest germination fraction in the 1st year and vice versa in the 2nd year. This difference in germination fraction was observed under controlled irrigation conditions but not in the field experiment. These mixed results suggest that seed dimorphism is a life history trait with a complicated evolutionary history and wide adaptive implications. Seed dimorphism in T. dicoccoides could initially be an adaptation for reducing competition in productive (i.e., high precipitation) environments. In addition to this, seed dimorphism under increasing aridity could become a bet-hedging trait allowing a population to survive periods of insufficient rainfall through dormancy.