Lazy
days of summer
The overall response of a plant to its
environment is an integration of the different effects of different factors on
different plant processes. This is very nicely illustrated by Poa bulbosa, the subject of the research
of Ofir and Kigel, Rehovot, Israel
(pp. 659–666). This grass, an inhabitant of the Mediterranean region, is
summer dormant and exists as ecotypes with different drought tolerances. The
plants exist in the dormant phase as bulbs formed at the base of tillers and
new growth arises from these bulbs in autumn. The authors gathered plants from
several sites along a rainfall gradient and grew them outdoors in a
‘net-house’. From this range four ecotypes were selected: a flowering ecotype
from an arid site, a flowering and a non-flowering ecotype from a semi-arid
site (non-flowering populations reproduce only via bulbs) and a non-flowering
ecotype from a mesic site. In the net-house, the ecotypes maintained their
normal flowering and dormancy behaviour (the more arid the site, the earlier
was the entry into dormancy; non-flowering ecotypes entered dormancy later than
flowering ecotypes). The plants were then grown under three different
temperature regimes (16°/10°, 22°/16°, 28°/22°C), under long or short days,
with or without pre-chilling. Here we concentrate on the effects of daylength
and temperature. Short-day treatments promoted flowering and inhibited
dormancy. Indeed, under short days, even non-flowering ecotypes were induced to
flower, especially in the lowest temperature regime. In the flowering ecotypes,
higher temperatures induced earlier flowering, but in all ecotypes fewer
panicles were produced at higher temperatures such that even the flowering
ecotypes produced very few flowers at 28°/22° C. Under long days, very few plants
flowered and then only at the lower temperatures. By contrast, long-day
treatments accelerated the onset of dormancy, which was further accelerated by
higher temperatures with the actual timing of dormancy again varying with
ecotype (as already seen in net-house-grown plants).
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
