Patterns of seed after-ripening inBromus tectorumL

Abstract

For grass seeds that lose dormancy through after-ripening in dry storage, the probability of germination following a particular wetting event can be predicted only if the relationship between storage temperature and change in after-ripening status is known. This study examined patterns of seed dormancy loss in Bromus tectorum L., quantifying changes in germination percentage, speed, and uniformity through time. Seed collections from three semi-arid habitats were stored at temperatures from 10-40°C. At monthly intervals, subsamples were incubated at 5/15, 10/20, 15/25, and 20/30°C. For recently harvested seeds, germination percentage, mean germination time, and days between 10% and 90% of total germination (D90-D10) ranged from 1-75%, 10-24 d, and 10-20 d, respectively. Recently harvested seeds were generally most dormant, slowest to germinate and least uniform at high incubation temperatures. In contrast, after-ripened seeds for all collections had nearly 100% germination, mean germination times <5 d, and D90-D10 values <5 d. Three indices were used to characterize after-ripening rates for each seedlot at each incubation temperature. The mean dormancy period, the mean rate index, and the mean uniformity index defined the storage period required for seedlots to become half as dormant as at harvest, to progress half-way to the fastest speed, and to progress half-way to the greatest uniformity, respectively. Seeds required longer storage to germinate uniformly than to germinate completely or quickly, because germination time-course curves for incompletely after-ripened seeds were positively skewed rather than sigmoidal. Mathematically, the three indices were described as negative exponential functions of storage temperature, which suggests that after-ripening is likely completed in late summer or early autumn regardless of summer conditions.