Micha Ofir

INDUCTION OF SUMMER-DORMANCY IN POA BULBOSA L. UNDER NATURAL ENVIRONMENT AND SUBSEQUENT CONTROLLED PHOTO-THERMAL CONDITIONS

The development of summer-dormancy under natural conditions was studied in Poa Bulbosa L., transferring plants from a sandstone hill in the central coastal plain of Israel to controlled photo-thermal conditions. Plant transfers were done several times in 1981–2 and 1987–8 during autumn and winter, the active growing season. The proportion of plants exhibiting dormancy (presence of bulbs, leaf senescence, and cessation of leaf appearance) increased the later the transfer to controlled conditions. Under controlled conditions of 16-h photoperiods (long days, LD), this increase started by the end of autumn, while under controlled conditions of 8-h photoperiods (short days, SD), dormancy was observed only in plants transferred after mid-winter. Plants in the field showed external signs of dormancy (leaf senescence) by the end of winter. At higher temperature regimes plants became dormant faster than at lower ones. Dormancy-enhancing effect by high temperature was found in LD for all transfers, and at SD only after the plants in the field had attained such levels of induction, by mid-winter, that dormancy development continued even under this non-inductive photoperiod.The results indicate that induction of summer-dormancy in Poa bulbosa under natural field conditions starts quite early, probably during early winter, since an increasing proportion of the plants becomes dormant even under SD at later dates of transfer. Plants transferred from the field by late autumn, when days are shortest and temperatures are decreasing, show an increased responsiveness to induction of dormancy by LD, compared to plants transferred earlier. Hence, in P. bulbosa the induction of summer-dormancy by LD might be enhanced by a pre-exposure to SD alone or in combination with low temperatures.

Ecotypic variation of summer dormancy relaxation associated with rainfall gradient in the geophytic grass Poa bulbosa.

BACKGROUND AND AIMS Summer dormancy is an adaptive trait in geophytes inhabiting regions with a Mediterranean climate, allowing their survival through the hot and dry summers. Summer dormancy in Poa bulbosa is induced by increasing day-length and temperature and decreasing water availability during spring. Populations from arid habitats became dormant earlier than those from mesic habitats. Relaxation of dormancy was promoted by the hot, dry summer conditions. Here we test the hypothesis that dormancy relaxation is also delayed in ecotypes of P. bulbosa inhabiting arid regions, as a cautious strategy related to the greater unpredictability of autumn rains associated with decreasing precipitation. METHODS Ecotypes collected across a precipitation gradient (100-1200 mm year(-1)) in the Mediterranean climate region were grown under similar conditions in a net-house in Israel. Differences among ecotypes in dormancy induction and dormancy relaxation were determined by measuring time to dormancy onset in spring, and time to sprouting after the first effective rain in autumn. Seasonal and ecotype variation in dormancy relaxation were assessed by measuring time to sprouting initiation, rate of sprouting and maximal sprouting of resting dry bulbs sampled in the net-house during late spring, and mid- and late summer, and planted in a wet substrate at temperatures promoting (10 degrees C) or limiting (20 degrees C) sprouting. KEY RESULTS Earlier dormancy in the spring and delayed sprouting in autumn were correlated with decreasing mean annual rainfall at the site of ecotype origin. Seasonal and ecotype differences in dormancy relaxation were expressed in bulbs planted at 20 degrees C. During the summer, time to sprouting decreased while rate of sprouting and maximal sprouting increased, indicating dormancy relaxation. Ecotypes from more arid sites across the rainfall gradient showed delayed onset of sprouting and lower maximal sprouting, but did not differ in rate of sprouting. Planting at 10 degrees C promoted sprouting and cancelled differences among ecotypes in dormancy relaxation. CONCLUSIONS Both the induction and the relaxation of summer dormancy in P. bulbosa are correlated with mean annual precipitation at the site of population origin. Ecotypes from arid habitats have earlier dormancy induction and delayed dormancy relaxation, compared with those from mesic habitats.

EFFECTS OF INDUCTION LEVEL ON MORPHOGENETIC ASPECTS OF ONSET OF SUMMER DORMANCY AND FLOWERING IN HORDEUM BULBOSUM L.

ABSTRACT Hordeum bulbosum is a vernalizable, facultative long-day plant that enters summer dormancy after flowering in spring. Onset of dormancy is characterized by development of a bulb with several dormant regeneration buds, and is closely linked with onset of reproductive development. This has been demonstrated quantitatively by the similarity between the responses of both processes to the combined effects of different levels of thermoinduction and photoinduction. In fully thermoinduced plants, only photoperiods longer than 10 h accelerated apex development, diminished the final number of leaves and spikelet-bearing nodes and resulted in earlier onset of dormancy. A strong positive interaction was observed between the effects of thermo- and photoinduction on the phenomena associated with onset of dormancy and initiation of flowering. The number of reproductive nodes was reduced by raising the level of either factor. Duration of the reproductive phase became shorter as photoinduction was increased, but ...

Temporal and intraclonal variation of flowering and pseudovivipary in Poa bulbosa

BACKGROUND AND AIMS Versatility in the reproductive development of pseudoviviparous grasses in response to growth conditions is an intriguing reproduction strategy. To better understand this strategy, this study examined variation in flowering and pseudovivipary among populations, co-occurring clones within populations, and among tillers in individual clones of Poa bulbosa, a summer-dormant geophytic grass that reproduces sexually by seed, and asexually by basal tiller bulbs and bulbils formed in proliferated panicles. METHODS Clones were collected from 17 populations across a rainfall gradient. Patterns of reproduction were monitored for 11 years in a common garden experiment and related to interannual differences in climatic conditions. Intraclonal variation in flowering and pseudovivipary was studied in a phytotron, under daylengths marginal for flowering induction. KEY RESULTS Clones showed large temporal variability in their reproductive behaviour. They flowered in some years but not in others, produced normal or proliferated panicles in different years, or became dormant without flowering. Proliferating clones did not show a distinct time sequence of flowering and proliferation across years. Populations differed in incidence of flowering and proliferation. The proportion of flowering clones increased with decreasing rainfall at the site of population origin, but no consistent relationship was found between flowering and precipitation in the common garden experiment across years. In contrast, flowering decreased at higher temperatures during early growth stages after bulb sprouting. Pulses of soil fertilization greatly increased the proportion of flowering clones and panicle production. High intraclonal tiller heterogeneity was observed, as shown by the divergent developmental fates of daughter plants arising from bulbs from the same parent clone and grown under similar conditions. Panicle proliferation was enhanced by non-inductive 8 h short days, while marginally inductive 12 h days promoted normal panicles. CONCLUSIONS Interannual variation in flowering and proliferation in P. bulbosa clones was attributed to differences in the onset of the rainy season, resulting in different daylength and temperature conditions during the early stages of growth, during which induction of flowering and dormancy occurs.