Flowering and dormancy in P. bulbosa
were affected in opposite ways by daylength and growth temperature. Flowering was rare under LD, but readily occurred after 3 months of growth in SD and was enhanced by low temperature ( and ). Dormancy, in contrast, was induced by LD and postponed by low growth temperature ( and ), as previously reported (Ofir and Kerem, 1982
; Ofir and Dorenfeld, 1992
; Ofir and Kigel, 1999
). Plants also became dormant under SD, but at a much later stage and mostly under high temperature (). Thus, high growth temperature inhibited flowering under SD () and enhanced dormancy under both LD and SD ( and ). Furthermore, exposure of plants to low temperature pre-treatment (6 weeks at 5
°C) accelerated dormancy imposition in LD (, and ; Ofir and Kigel, 1999
) and in some ecotypes also in SD (), but lacked consistent effects on flowering ( and ).
Similar trends of earlier flowering in short days, minor or no effect of vernalization and a delay of flowering (i.e. more leaves to flowering) due to increasing growth temperature have been reported for high latitude and alpine populations of perennial types of P. annua
) and for perennial P. annua
’ populations (Johnson and White, 1997a
). However, in contrast to P. bulbosa
that did not reach flowering in long days, perennial types of P. annua
flowered in long days at a later plant age compared with short days, particularly under relatively low growth temperatures, or after vernalization treatments (Heide, 2001
). In P. bulbosa
, long days induced early dormancy and only a few plants ‘escaped’ dormancy and reached flowering at low growth temperatures. The fact that flowering mostly occurred in short days, together with the lack of clear response to vernalization, suggests that in regard to flowering, P. bulbosa
is a short day plant.
It may also be argued that P. bulbosa
is a facultative long day plant like most Festucoid grasses, but the early transition to the dormant state under LD arrests reproductive development prematurely, before any morphological changes at the shoot apex have occurred. Moreover, even if a shift from short day to long day is required for flowering, as in other temperate grasses (Heide, 1994
), in P. bulbosa
this shift will enhance the induction of dormancy (Ofir and Kigel, 1999
), thus strengthening the inhibition of flowering under these conditions. Suppression of flowering due to dormancy enhancement by the same environmental factors required for flowering induction and inflorescence development has probably also occurred in garlic (A. sativum
) and onion (A. cepa
), due to selection since historical times for increased storage in the bulbs and for earliness of maturation (Brewster, 1990
; Etoh and Simon, 2002
). In some bolting types of garlic, the transition of the shoot apex to the reproductive state can occur under both short and long days. However, under short days, the inflorescence fails to elongate, while under long days the fate of the inflorescence depends on the strength of long day induction. Increasing the number of long days leads from normal elongation and flowering to development of dormant vegetative bulbils, instead of flowers (Kamenetsky et al., 2004
). Similarly, in the short day C4
perennial grass Bouteloua eriopoda
, continued exposure to inductive short days leads to early abortion of the inflorescence (Schwartz and Koller, 1975
). This response is probably related to the fact that in this species the rest period occurs during the dry winter. Altogether, it can be argued that natural or artificial selection for increased responsiveness to environmental factors inducing flowering could result in the loss of flowering capacity that is associated with morphological and physiological processes leading to dormancy. Moreover, since similar environmental factors induce summer dormancy in P. bulbosa
as well as flowering in many other Festucoid grasses (i.e. long day, pre-exposure to short day or to low temperature), it is conceivable that the control of these phenological transitions can be channelled through inter-related developmental pathways. Thus, the inhibition of flowering in P. bulbosa
by high temperature can be attributed to enhanced responsiveness to induction of dormancy by long days or to direct inhibition of flowering (Heide, 1994
It is noteworthy that flowering does not depend on vernalization but is promoted by short days in perennial types of Poa
from contrasting habitats, such as alpine and high latitude populations of P. annua
that grow during the summer (Heide, 2001
), as well as in Mediterranean populations of P. bulbosa
that grow in the winter season. Despite the large climatic differences, both habitats are characterized by a relatively short growth season. In the Mediterranean and adjoining arid phytogeographic regions, P. bulbosa
grows on shallow soils with a low moisture-holding capacity, imposing a short growth period in the winter that is followed by a long dry summer. These were probably factors in the evolution of the ability to achieve flowering in short days at low temperatures, together with the early transition to the dormant state.
Flowering of P. bulbosa
ecotypes along the steep rainfall gradient occurs during late winter and early spring, when the daylength ranges between 11 and 12
h. This range is also the threshold daylength for dormancy induction (Ofir and Kigel, 1999
). Since the rainfall gradient along which the ecotypes were collected spans a narrow latitude range, it may be assumed that the ecotypes in the field are exposed to the same daylength and quite similar temperatures along the gradient. Thus, the ecotypic variation in the capacity and timing of flowering with increasing aridity can be interpreted as the result of balanced opposite effects of daylength and temperature on the flowering and dormancy processes.
In the non-flowering mesic and semi-arid ecotypes, flowering did not occur during 3 years in outdoor conditions in a net-house (), but they flowered in the phytotron under SD at the lower temperatures (M–NF at 16/10
°C; SA–NF at 16/10 and 22/16
°C, ). Thus, lack of flowering in the field doees not necessarily indicate a loss of flowering capacity. These non-flowering ecotypes probably have an increased responsiveness to the inhibitory effect of higher temperature on flowering. It is conceivable that in the field, these ecotypes could reach flowering in years with early onset of the rainy season and relatively cold and prolonged winters. Thus, they may produce seeds or inflorescence bulbils only in particularly favourable years, relying on tiller bulbs for reproduction in the less favourable years. In this case, the balance between local reproduction and persistency by basal bulbs vs. dispersible seeds or bulbils produced by inflorescences is determined by the responsiveness of the ecotype to the relevant environmental conditions and the frequency with which these conditions occur. The facts that arid and semi-arid ecotypes (A–F and SA–F) flowered every year (), had a larger proportion of flowering plants than the mesic ecotype (M–NF) even at the intermediate temperature (i.e. 22/16
°C) and also underwent earlier flowering ( and ) support the idea that a higher level of dispersal by sexual and/or asexual propagules (i.e. seeds and bulbils) has an advantage for P. bulbosa
populations under arid conditions (Ofir and Kigel, 2003
). At the same time, higher responsiveness of these populations to long day and high temperature induction of dormancy reduces the risk of death due to early drought.
We conclude that the variation in the flowering capacity of P. bulbosa ecotypes differing in drought tolerance can be seen as the result of balanced opposite effects of daylength and temperature on the flowering and dormancy processes, thus regulating reproductive capacity, the balance between seed and vegetative reproduction and adaptation to prolonged drought during the summer.