Variation in Onset of Summer Dormancy and Flowering Capacity Along an Aridity Gradient in Poa bulbosa L., a Geophytic Perennial Grass

Sleeping in the sun

The remarkable intraspecific variation exhibited by many species in respect of response to environmental factors has been noted previously in this column. A questions that arises is the origin of that variation: is it inherited or does it represent developmental and/or physiological plasticity within one relatively homogeneous genotype? Ofir and Kigel (Hebrew University of Jerusalem, pp. 391-400) have investigated this in a perennial grass, Poa bulbosa, and have worked with plants in habitats where the annual rainfall varies from 110 to 810 mm. All these populations are dormant in summer; dormancy is induced in spring (before the end of the wet season) by increasing day length and increasing temperature. However, in the wild there is a clear correlation between rainfall and dormancy: the higher the annual rainfall, the later is the onset of dormancy. The authors transplanted P. bulbosa from several sites along this rainfall gradient to a single outdoor site. After 2 years� growth (to eliminate carryover effects), basal tiller bulbs were removed and planted under one controlled growth regime in the phytotron. The age at which these �daughter� plants became dormant was strongly correlated with the amount of rainfall at the original collection site (the lower the rainfall, the earlier the onset of dormancy), as was the timing of dormancy onset in the parental plants grown at the outdoor site. This clear association between rainfall at the original site and dormancy onset was mirrored in other aspects of plant growth. For example, plants with the earliest dormancy onset, and hence the shortest growing season, produced the most flowers, while those with the longest growing season were more likely to reproduce solely by vegetative means. These results suggest that the specific traits exhibited by different populations growing in habitats with differing rainfall do indeed represent genetic differences between those populations.