Opposite effects of daylength and temperature on flowering and summer dormancy of Poa bulbosa

BACKGROUND AND AIMS: The timing of flowering and summer dormancy induction plays a central role in the adaptation of Mediterranean geophytes to changes in the length of the growth season along rainfall gradients. Our aim was to analyse the role of the variation in the responses of flowering and summer dormancy to vernalization, daylength and growth temperature for the adaptation of Poa bulbosa, a perennial geophytic grass, to increasing aridity. METHODS: Flowering and dormancy were studied under controlled daylengths [9 h short day (SD) vs. 16 h long day (LD)] and temperatures (16/10, 22/16 and 28/22 degrees C day/night) in four ecotypes originating in arid, semi-arid and mesic habitats (110, 276 and 810 mm rain year(-1), respectively) and differing in flowering capacity under natural conditions: arid-flowering, semi-arid-flowering, semi-arid-non-flowering and mesic-non-flowering. KEY RESULTS: Flowering and dormancy were affected in opposite ways by daylength and growth temperature. Flowering occurred almost exclusively under SD. In contrast, plants became dormant much earlier under LD than under SD. In both daylengths, high temperature and pre-chilling (6 weeks at 5 degrees C) enhanced dormancy imposition, but inhibited or postponed flowering, respectively. Induction of flowering and dormancy in the different ecotypes showed differential responsiveness to daylength and temperature. Arid and semi-arid ecotypes had a higher proportion of flowering plants and flowering tillers as well as more panicles per plant than mesic ecotypes. 'Flowering' ecotypes entered dormancy earlier than 'non-flowering' ecotypes, while the more arid the site of ecotype origin, the earlier the ecotype entered dormancy. CONCLUSIONS: Variation in the flowering capacity of ecotypes differing in drought tolerance was interpreted as the result of balanced opposite effects of daylength and temperature on the flowering and dormancy processes.     

Photothermal control of the imposition of summer dormancy in Poabulbosa, a perennial grass geophyte

Poa bulbosa L., like many other Mediterranean geophytes, grows in the winter and enters a phase of summer dormancy in the spring. Summer dormancy enables these plants to survive the hot and dry summer. Long days are the main environmental factor active in the induction of summer dormancy in P . bulbosa and elevated temperatures accelerate dormancy development. P . bulbosa becomes dormant earlier than most other species that grow actively in the winter. Previous studies suggested that pre-exposure of P . bulbosa to short days and low temperatures during the autumn and early winter increased its sensitivity to photoperiodic induction in late winter, and thus enabled the early imposition of dormancy. To study this hypothesis, experiments were carried out under controlled photothermal conditions in the phytotron, under natural daylight extended with artificial lighting. The critical photoperiod for induction of summer dormancy at an optimal temperature (22/17°C day/night) was between 11 and 12 h. Photoperiods shorter than 12 h were noninductive, while 14- and 16-h days were fully inductive. A night break of 1 h of light given at the middle of the dark period of an 8-h photoperiod also resulted in full induction of dormancy. Pre-exposure to either low temperature (chilling at 5°C) or to short days of 8 h (SD) enhanced the inductive effect of subsequent 16-h long days (LD). The enhancing effect of chilling and SD increased with longer duration, i.e. fewer LDs were required to impose dormancy. However, the day-length during the low-temperature pretreatment had no effect on the level of induction at the following LD. Chilling followed by SD did not induce dormancy. The relevance of these responses to the development and survival of P . bulbosa in its natural habitat is discussed.     

Abscisic acid involvement in the induction of summer‐dormancy in Poa bulbosa, a grass geophyte

Summer‐dormancy occurs in geophytes that inhabit regions with a Mediterranean climate (mild, rainy winters and hot, dry summers). The environmental control of summer‐dormancy and the involvement of phytohormones in its induction have been little studied. Poa bulbosa L. is a perennial grass geophyte in which summer‐dormancy is induced by long days and by high temperature. Prolonged treatment with ABA (0.1‐1.0 m M ) under non‐inductive 8‐h short days (SD) resulted in cessation of leaf and tiller production and in the development of typical features of dormancy: bulbing at the base of the tillers and leaf senescence. Short‐term applications of ABA had similar effects but dormancy was transient, i.e. after a short while, leaf growth from the formed bulbs was resumed. ABA treatment of plants growing under an inductive 16‐h photoperiod (LD) enhanced the onset of dormancy. Endogenous levels of ABA in leaf blades and at the tiller base (where the bulb develops) increased markedly after the plants were transferred from SD to LD. This increase was greater in the tiller base, and concomitant with bulb maturation. High temperature (27/22 vs 22/17°C) accelerated both bulb development and ABA accumulation in leaf blades.
These results suggest that ABA plays a key role in the photoperiodic induction and development of summer‐dormancy in P. bulbosa .     

Regulation of summer dormancy by water deficit and ABA in Poa bulbosa ecotypes

BACKGROUND AND AIMS: Survival of many herbaceous species in Mediterranean habitats during the dry, hot summer depends on the induction of summer dormancy by changes in environmental conditions during the transition between the winter (growth) season to the summer (resting) season, i.e. longer days, increasing temperature and drought. In Poa bulbosa, a perennial geophytic grass, summer dormancy is induced by long days, and the induction is enhanced by high temperature. Here the induction of summer dormancy in a Mediterranean perennial grass by water deficit under non-inductive photoperiodic conditions is reported for the first time. METHODS: Plants grown under 22/16 degrees C and non-inductive short-day (9 h, SD) were subjected to water deficit (WD), applied as cycles of reduced irrigation, or sprayed with ABA solutions. They were compared with plants in which dormancy was induced by transfer from SD to inductive long-day (16 h, LD). Responses of two contrasting ecotypes, from arid and mesic habitats were compared. Dormancy relaxation in bulbs from these ecotypes and treatments was studied by comparing sprouting capacity in a wet substrate at 10 degrees C of freshly harvested bulbs to that of dry-stored bulbs at 40 degrees C. Endogenous ABA in the bulbs was determined by monoclonal immunoassay analysis. KEY RESULTS: Dormancy was induced by WD and by ABA application in plants growing under non-inductive SD. Dormancy induction by WD was associated with increased levels of ABA. Bulbs were initially deeply dormant and their sprouting capacity was very low, as in plants in which dormancy was induced by LD. Dormancy was released after 2 months dry storage at 40 degrees C in all treatments. ABA levels were not affected by dormancy relaxation. CONCLUSIONS: Summer dormancy in P. bulbosa can be induced by two alternative and probably additive pathways: (1) photoperiodic induction by long-days, and (2) water deficit. Increased levels of endogenous ABA are involved in both pathways.     

Phenology and polyploidy in annual Brachypodium species (Poaceae) along the aridity gradient in Israel

Local adaptation of plants along environmental gradients provides strong evidence for clinal evolution mediated by natural selection. Plants have developed diverse strategies to mitigate stress, for example, drought escape is a phenological strategy to avoid drought stress, while polyploidy was proposed as a genomic adaptation to stress. Polyploidy as an adaptation to aridity (an environmental parameter integrating temperature and precipitation) was previously documented in annual Brachypodium spp. (Poaceae) in the Western Mediterranean. Here, we examined whether polyploidy or phenology are associated with aridity in annual Brachypodium spp. along the aridity gradient in the Eastern Mediterranean. Using flow cytometry, we determined ploidy levels of plants from natural populations along the Israeli gradient, spanning ∼424 km from mesic Mediterranean to extreme desert climates. In a common garden we recorded time of seedling emergence, flowering and senescence. We tested whether the proportion of allotetraploids in the populations and phenological traits were associated with aridity. Contrary to a previous study in the Western Mediterranean, we found no effect of aridity on the proportion of allotetraploids and diploids within populations. Interestingly, phenology was associated with aridity: time of emergence was later, while flowering and senescence were earlier in desert plants. Our results indicate that in the Eastern Mediterranean, adaptation of Brachypodium to aridity is mediated mainly by phenology, rather than ploidy level. Therefore, we suggest that genome duplication is not the main driver of adaptation to environmental stress; rather, phenological change as a drought escape mechanism may be the major adaptation.     

Correlated patterns of variation in phenology and seed production in populations of two annual grasses along an aridity gradient

I applied a comparative approach to reveal correlated patterns of variation in phenology and seed production in four populations of two annual grasses Hordeum spontaneum and Avena sterilis, sampled in the same environments distributed along an aridity gradient in Israel. The steep aridity gradient in Israel represents two parallel clines of environmental productivity (annual rainfall) and predictability (variation in amount and timing of annual rainfall) that is likely to induce similar responses in natural plant populations distributed along the gradient, if (1) selection is strong, (2) species share the same ecological niche, and (3) there is genetic variation for ecologically important traits. I found in plants of both species (1) ultimate advance in onset of flowering, and (2) more but smaller seeds, with increasing aridity. The broad sense heritabilities of onset of flowering, seed size and seed yield in both species were very high, moderate and low, respectively. It appears that the observed adaptive complex of traits have evolved in both species in response to this specific array of environments.     

Cost of reproduction in the pink lady's slipper orchid (Cypripedium acaule, Orchidaceae): an eleven-year experimental study of three populations

An 11-yr experimental study of the cost of reproduction in three wild populations of the perennial orchid Cypripedium acaule contrasted experimental plants that were repeatedly hand-pollinated and often made fruits with control plants that were not hand-pollinated and only rarely made fruits. Repeated flowering without subsequent fruit production resulted in no detectable reduction in either plant size or probability of flowering in subsequent years. A cost of fruit production was evident in experimental plants in all three populations in terms of a reduced probability of flowering and smaller leaf area in subsequent years, but was not evident in terms of mortality rate. Experimental effects of fruit production reached maximum values at 3-7 yr, depending on the population. The probability of remaining dormant below ground in a given year was strongly dependent on plant size in the previous year. Furthermore, the length of the dormancy period (one to several years) was a significant and inverse function of plant size just prior to dormancy. Sample sizes and the consequent ability to detect experimental effects declined over time as more plants died or stopped flowering. Four to seven years appears to be an optimal duration for studies of the cost of reproduction in perennial herbs similar to this species. Studies lasting less than 4 yr may be too brief to reveal experimental effects, whereas those lasting more than 7 yr may fail to reveal new insights.     

Dormancy and Flowering in Anemone coronaria L. as Affected by Photoperiod and Temperature

The effects of day-length and temperature on flowering and dormancyinduction were studied in Anemone coronaria L., with plantsraised either from corms or achenes. An Israeli hybrid sourcewas used (de Caen cv. Hollandia x Israeli wild type). Dormancy onset is characterized by the cessation of foliageleaf production, the appearance of leaf scales protecting theperennating bud, and leaf senescence. Dormancy was induced byhigh temperature and long days but increasing temperatures (from17/12 °C to 32/12 °C) induced earlier dormancy thanprolonging the photoperiod (range 8–16 h). A significant(P = 0.01) interaction was found between these factors, withsmaller photoperiodic effects the higher the temperature. At22/17 °C the critical day-length for dormancy inductionwas between 11 and 12 h. The transition from the vegetative to the reproductive stageappears to be an autonomous process that occurs with developmentin plants raised from either corms or achenes and does not requireenvironmental induction. Photo- and thermoperiodic effects onflowering were indirect, being mediated through their influenceon dormancy induction. Anemone coronaria L., dormancy, flowering, photoperiod, thermoperiod     

Bumble-bee learning selects for both early and long flowering in food-deceptive plants

Most rewardless orchids engage in generalized food-deception, exhibiting floral traits typical of rewarding species and exploiting the instinctive foraging of pollinators. Generalized food-deceptive (GFD) orchids compete poorly with rewarding species for pollinator services, which may be overcome by flowering early in the growing season when relatively more pollinators are naive and fewer competing plant species are flowering, and/or flowering for extended periods to enhance the chance of pollinator visits. We tested these hypotheses by manipulating flowering time and duration in a natural population of Calypso bulbosa and quantifying pollinator visitation based on pollen removal. Both early and long flowering increased bumble-bee visitation compared with late and brief flowering, respectively. To identify the cause of reduced visitation during late flowering, we tested whether negative experience with C. bulbosa (avoidance learning) and positive experience with a rewarding species, Arctostaphylos uva-ursi, (associative learning) by captive bumble-bees could reduce C. bulbosa's competitiveness. Avoidance learning explained the higher visitation of early- compared with late-flowering C. bulbosa. The resulting pollinator-mediated selection for early flowering may commonly affect GFD orchids, explaining their tendency to flower earlier than rewarding orchids. For dissimilar deceptive and rewarding sympatric species, associative learning may additionally favour early flowering by GFD species.     

Germination behaviour in populations ofCapsella bursa-pastoris (Cruciferae)

Germination behaviour of variousCapsella bursa-pastoris populations collected from Scandinavia, Middle Europe and the Alps, was tested in unheated, non-illuminated greenhouses (46 populations) and in growth chambers using 5–7 alternating temperature regimes (16 populations). For all populations, the influence of temperature on germination rate is straightforward: the higher the temperature, the greater the germination. Germination capacity, however, may depend on the geographical region. There is also a strong seed age effect on both, rate and capacity of germination. Once dormancy was broken, seeds from all populations were able to germinate over the entire range of temperatures. Some populations revealed a more or less pronounced temperature optimum for germination capacity, others germinated equally well over the entire temperature range. This indicates genetic heterogeneity between populations. However, no correlation between germinability and any environmental pattern was detected. The data indicate thatCapsella bursa-pastoris has adopted a germination strategy which includes a broad temperature tolerance. Germination of wildCapsella plants seems to be regulated by the factors contributing to the inception and breaking of dormancy which depend on pre- and postharvest conditions. Adaptation in germination behaviour inCapsella bursa-pastoris is different from that in other life history traits (flowering behaviour, growth form parameters).     

The fission yeast mitotic activator cdc25 and sucrose induce early flowering synergistically in the day-neutral Nicotiana tabacum cv. Samsun

Here, the tobacco (Nicotiana tabacum) day-neutral (DN) cv. Samsun transformed with the Schizosaccharomyces pombe mitotic activator gene Spcdc25 was used to study the onset of flowering. Wild type (WT) and cdc25 plants were grown from seeds in vitro until they were 20 cm high. Apical and basal nodes were then subcultured repeatedly and the regenerated plants were used to document time to flowering and the number of leaves formed before flowering. Three sucrose treatments (3, 5 or 7% (weight/volume)) were used and measurements of leaf endogenous soluble carbohydrates were performed. In the 3% treatment, cdc25 plants flowered but WT plants did not. The higher sucrose treatments enabled WT flowering; two-thirds of the plants flowered at 5%, while all plants flowered at 7% sucrose. However, in all treatments, cdc25 plants exhibited significantly earlier flowering and fewer leaves compared with wild type. Remarkably, a typical acropetal flowering gradient in WT plants did not occur in cdc25 plants. In cdc25 leaves, there were significantly higher amounts of endogenous sugars with a higher proportion of sucrose compared with WT. Our data demonstrate that Spcdc25 expression and sucrose act synergistically to induce precocious flowering.     

The influence of air temperature on the starting date of Quercus pollination in the South of Europe

In temperate zones the air temperature influences many aspects of the plant growth and also the time of flowering is often correlated with this environmental parameter. It is a generally accepted idea that higher temperatures in the period preceding ripening of the flowers determine earlier pollination. To verify if a correlation between the air temperature and the date of onset of the pollination period of Quercus spp. exists, a comparative study was carried out over 7 years (1995–2001) in two South-European towns: Vigo (Spain) and Perugia (Italy). Quercus pollen is released in the atmosphere of Perugia on average in the last two weeks of April while in the Spanish region the pollination occurs on average one month before.

In order to overcome the dormancy period Perugia requires 1110 Chilling Hours (CH)-884 Growth Degree-Days (GDD°C) and Vigo 709 CH-861 GDD°C. With the Ashcroft method Perugia needs 1075 CH-1000 GDD°C and Vigo 625 CH-1512 GDD°C. Heat accumulation from the end of winter dormancy to the onset of pollination, showed the highest significance when mean temperature in Perugia and maximum temperature in Vigo were used. Every year we have found that the colder station needed a lower heat accumulation: Perugia required a higher quantity of chilling and heat than Vigo. However, the correlation detected between temperature and flowering was, on average, less significant that those found in the same regions for other arboreal taxa that present winter pollination (e.g. Corylus, Alnus). This preliminary study suggests that there is an effect of air temperature on Quercus pollination, but other environmental factors, such as photoperiod, hours of light, rainfall, relative humidity, may be of great influence in determining the onset of pollination in plants with a spring flowering.     

Tiller dynamics and assimilate partitioning in Lolium perenne with particular reference to flowering

The fate of 100 seedling plants of Lolium perenne L. was studied over a period of 2 years in a field plot. The birth and death of tillers and the production of inflorescences was followed, and the components of seed yield were recorded in detail in the first year. The pattern of distribution of ¹⁴CO₂ assimilated by the main shoot was examined at monthly intervals and during the first flowering season the distribution of ¹⁴C-assimilate from individual leaves and from the inflorescence was also studied. The capacity of individual tillers to assimilate ¹⁴CO₂ prior to flowering and the re-distribution of previously accumulated assimilate during seed growth were also assessed. Plants died at a more or less constant rate with time and only 54 survived to the end of the 2–yr period. First year mortality was associated with severe grazing or cutting but in the second year the death of ungrazed plants was observed. There was great variability in the production of tillers by surviving plants. In both years the number of live tillers per plant increased from July to the end of April with particularly rapid tillering in March and April establishing the maximum value for each year. There was a similar phase of rapid tillering after flowering in July. The number of live tillers per plant declined by 50% during stem elongation and inflorescence emergence and the majority of dead tillers were young secondary (in the first year) and tertiary (in the second year) tillers with a mean age of 40 days. Such tillers had poor assimilatory capacity prior to the onset of death and were not supplied with assimilate from the main shoot. Most of the plants surviving at the end of the experiment flowered in both years and one quarter of the maximum number of live tillers per plant recorded in April of each year produced inflorescences. The earlier a tiller was produced the greater was its chance of flowering and the greater its production of seed. The greater weight of seed produced was associated with the development of more seed-bearing florets per spikelet. There was relatively little export of “C-assimilate from the flowering main shoot, and the lower internodes formed the major sink for post-anthesis assimilate. The growth of seeds appeared to be relatively independent of the leaves for current assimilate. There was some evidence that assimilate accumulated in lower internodes was remobilised and utilised in the growth of seeds and new tillers. Overall, the results confirm the view that the grass plant is a dynamic population of short-lived tillers and indicate that increasing competition for assimilate at flowering exerts a major influence on the production and survival of tillers.     

Determinants of seed production in Geranium maculatum

Summary We experimentally examined factors limiting seed production in two populations of the perennial woodland herb Geranium maculatum in central Illinois, USA. To test the pollinator-limitation hypothesis, we compared the seed production of plants whose flowers were supplementarily pollinated with outcross pollen to that of control plants receiving natural pollination only. To test if fruit production by early flowers suppresses fruit and seed formation by late flowers, a third group of plants was prevented from producing seed from the first 50% of the flowers to open (stigmas were excised at flower opening). Finally, to test if seed maturation and flower initiation are correlated with photosynthetic capacity, we performed a defoliation experiment in which either the stem leaves within the inflorescence, the stem leaves below the inflorescence, or the rosette leaves were removed during late flowering. Plants that reccived supplemental pollination produced 1.5–1.6 times more seeds than control plants. We found no difference between hand-pollinated plants and controls in mortality, flowering frequency or number of flowers produced in the year following the experiment. In both control and hand-pollinated plants, the fruit set and total seed production of early flowers were more than twice as high as those of late flowers. In one of the two populations, plants whose early flowers were prevented from setting seed produced significantly more seeds from their late flowers than did control plants. Seed predation was low and did not differ between early and late flowers. Leaf removal did not affect seed number or size in the year of defoliation, nor did it reduce survival or flower production in the subsequent year. This suggests that the plants were able to compensate for a partial defoliation by using stored resources or by increasing photosynthetic rates in the remaining leaves. Taken together, the results demonstrate that both pollinator activity and resource levels influence patterns of seed production in G. maculatum. While seed production was pollinatorlimited in both populations, a seasonal decline in resource availability was apparently responsible for the low seed production by late flowers.     

Does zinc concentration in the substrate influence the onset of flowering in Arabidopsis arenosa (Brassicaceae)?

We investigated the impact of low zinc (Zn) concentrations in the substare on the onset of flowering in Arabidopsis arenosa (Brassicaceae). Experiments were carried out in controlled conditions using plants from four different populations. The research was aimed to verify experimentally the following hypotheses: (1) Zn content in the growth medium promote the onset of flowering in A. arenosa, (2) Changes in the onset of flowering induced by Zn depend on Zn concentration employed; (3) Zn-induced early onset of flowering is an universal plant response present within the species and is not an effect of stress or physiological adaptation to high Zn content in the environment. Investigated plants were subjected to four different Zn concentrations: 0.4 (control), 155, 775 and 1,550???M Zn²⁺. To asses stress level in investigated plants we calculated biomass accumulation and employed fluorometric methods. Zn content was estimated in shoots using atomic absorption spectroscopy. Differences in the onset of flowering were assessed using Kaplan?CMeier curves. Our results showed that Zn was transported form growth medium to roots and shoots of investigated plants and that the content of Zn increased with the increase of Zn concentration in the growth medium. We evidenced that apart from one (1,550???M Zn²⁺) applied Zn concentrations did not caused stress in investigated plants what was confirmed by two independent experimental approaches: measurement of biomass accumulation and chlorophyll a fluorescence. Flowering curves obtained on the basis of calculation of Kaplan?CMeier estimator showed that: (1) control plants originating from four different populations did not differ in terms of the onset of flowering, (2) plants from each population tested tends to enter flowering phase earlier in response to applied Zn concentrations than control plants, (3) plants treated with the lowest tested Zn concentration (155???M Zn²⁺) tend to flower earlier than plants treated with the higher concentration (775???M Zn²⁺), (4) the impact of Zn on the onset of flowering did not depend on the origin on the plant material used (Zn-rich or Zn-poor soils). Our results indicate that Zn ions present in the growth medium promote early flowering in A.arenosa and that this effect may depend on Zn concentration used. Zn-induced early flowering in A. arenosa seems to be an universal plant response present within the species and is not an effect of stress or physiological adaptation to high Zn content in the environment.     

The Effects of Temperature and Photoperiod on the Onset of Summer Dormancy in Poa bulbosa L.

Poa bulbosa L. plants became dormant in long days (16 h), whilein short days (8 h) they remained non-dormant for extended periods.Morphologically, the onset of dormancy was expressed by theformation of a true bulb at the base of every tiller, by thecessation of tillering and leaf emergence and, finally, by thedrying-up of the leaves. Low temperature delayed the onset ofdormancy but did not prevent it. This effect of temperaturemay explain the delayed onset of dormancy observed in naturalpopulations under a cool climate at a hilly habitat, comparedto plants growing under a warmer climate, at a lower, coastal-plainhabitat. Dormancy could be induced under short days by pre-exposureof the plants to a limited number of long days. The responsewas proportional to the number of long days given. The adaptivesignificance of the results for plant survival in a Mediterraneanclimate is discussed. Poa bulbosa L., summer dormancy, photoperiod, temperature, leaf emergence, bulbs, tillers     

Pattern of growth regulating substances in the leaves of vernalized sugar-beet during flowering period

The changes in the levels of growth regulating substances using the wheat coleoptile straight growth test were determined in the leaves of vernalized (flowering) and non-vernalized (non-flowering) plants of sugar-beet, cv. Poly-AG-Poland at two stages; the end of vernalization treatment (210 days from planting) and full-flowering stage. IAA was detected only in the extracts of the leaves of non-vernalized plants after210 days from planting. No inhibitory activity was detected, except in the case of the concentrated extract of the leaves of non-vernalized plants. This growth promoting zone was found at R_f 0.5–0.8 in the leaves of flowering plants after cold treatment and at flowering time. This zone of growth promoting action was suggested to have a major role in the flowering of sugar-beet.     

Reproductive limits of a late-flowering high-mountain Mediterranean plant along an elevational climate gradient

Mountain plants are particularly sensitive to climate warming because snowmelt timing exerts a direct control on their reproduction. Current warming is leading to earlier snowmelt dates and longer snow-free periods. Our hypothesis is that high-mountain Mediterranean plants are not able to take advantage of a lengthened snow-free period because this leads to longer drought that truncates the growing season. However, reproductive timing may somewhat mitigate these negative effects through temporal shifts. We assessed the effects of flowering phenology on the reproductive success of Silene ciliata, a Mediterranean high-mountain plant, across an altitudinal gradient during two climatically contrasting years. The species showed a late-flowering pattern hampering the use of snowmelt water. Plant fitness was largely explained by the elapsed time from snowmelt to onset of flowering, suggesting a selective pressure towards early flowering caused by soil moisture depletion. The proportion of flowering plants decreased at the lowest population, especially in the drier year. Plants produced more flowers, fruits and seeds at the highest population and in the mild year. Our results indicate that water deficit in dry years could threaten the lowland populations of this mountainous species, while high-altitude environments are more stable over time.     

Involvement of the ethylene response pathway in dormancy induction in chrysanthemum

Temperature plays a significant role in the annual cycling between growth and dormancy of the herbaceous perennial chrysanthemum (Chrysanthemum morifolium Ramat.). After exposure to high summer temperatures, cool temperature triggers dormancy. The cessation of flowering and rosette formation by the cessation of elongation are characteristic of dormant plants, and can be stimulated by exogenous ethylene. Thus, the ethylene response pathway may be involved in temperature-induced dormancy of chrysanthemum. Transgenic chrysanthemums expressing a mutated ethylene receptor gene were used to assess this involvement. The transgenic lines showed reduced ethylene sensitivity: ethylene causes leaf yellowing in wild-type chrysanthemums, but leaves remained green in the transgenic lines. Extension growth and flowering of wild-type and transgenic lines varied between temperatures: at 20 degrees C, the transgenic lines showed the same stem elongation and flowering as the wild type; at cooler temperatures, the wild type formed rosettes with an inability to flower and entered dormancy, but some transgenic lines continued to elongate and flower. This supports the involvement of the ethylene response pathway in the temperature-induced dormancy of chrysanthemum. At the highest dosage of ethephon, an ethylene-releasing agent, wild-type plants formed rosettes with an inability to flower and became dormant, but one transgenic line did not. This confirms that dormancy is induced via the ethylene response pathway.     

基于数字化标本和种群形态变异特征试论块茎堇菜的分类地位

块茎堇菜是青藏高原及其邻近地区的一种特有两型闭花植物,目前其分类地位存在诸多争议。据此,本文对块茎堇菜及其近缘类群鳞茎堇菜的主要数字化标本以及同域分布的几个自然种群的叶片形态变异特征进行了分析。结果显示,所有参试数字化标本被分为两个明显不同的集群,一个集群符合块茎堇菜的叶片形态,另一集群则符合鳞茎堇菜;同样,同域分布的6个自然种群也分为两个集群。此外,叶片形态变异分析还显示叶片先端和叶基部形态参数ΔW、AL、BL和LWR在块茎堇菜/鳞茎堇菜同一类群不同种群间或种群内变异幅度较小且较为稳定,然而它们在不同类群间差异却极为显著（P<0.01）。因此,块茎堇菜和鳞茎堇菜理应作为两个不同的类群处理,这也表明叶片形态特征完全可以作为鳞茎堇菜和块茎堇菜的重要分类学依据。