Search

Background and Aims Photoperiod contains information about the progress of seasons. Plants use the changing photoperiod as a cue for the correct timing of important life history events, including flowering. Here the effect of photoperiod on flowering in four Arabidopsis lyrata populations originating from different latitudes was studied, as well as expression levels of candidate genes for governing the between-population differences. Methods Flowering of plants from four A. lyrata populations was studied in three different photoperiods after vernalization. Flowering development was separated into three steps: flower primordia formation, inflorescence shoot elongation and opening of the first flower. Circadian expression rhythms of the A. lyrata homologues of GIGANTEA (GI), FLAVIN-BINDING, KELCH REPEAT, F-BOX 1 (FKF1), CONSTANS (CO) and FLOWERING LOCUS T (FT) were studied in three of the populations in the intermediate (14 h) photoperiod treatment. Key Results Most plants in all populations formed visible flower primordia during vernalization. Further inflorescence development after vernalization was strongly inhibited by short days in the northern European population (latitude 61°N), only slightly in the central European population (49°N) and not at all in the North American populations (36°N and 42°N). In the 14 h daylength, where all plants from the three southernmost populations but only 60 % of the northernmost population flowered, the circadian expression rhythm of the A. lyrata FT was only detected in the southern populations, suggesting differentiation in the critical daylength for activation of the long-day pathway. However, circadian expression rhythms of A. lyrata GI, FKF1 and CO were similar between populations. Conclusions The results indicate that in A. lyrata, transition to flowering can occur through pathways independent of long days, but elongation of inflorescences is photoperiodically regulated.

Background and Aims Climate change is advancing the leaf-out times of many plant species and mostly extending the growing season in temperate ecosystems. Laboratory experiments using twig cuttings from woody plant species present an affordable, easily replicated approach to investigate the relative importance of factors such as winter chilling, photoperiod, spring warming and frost tolerance on the leafing-out times of plant communities. This Viewpoint article demonstrates how the results of these experiments deepen our understanding beyond what is possible via analyses of remote sensing and field observation data, and can be used to improve climate change forecasts of shifts in phenology, ecosystem processes and ecological interactions. Scope The twig method involves cutting dormant twigs from trees, shrubs and vines on a single date or at intervals over the course of the winter and early spring, placing them in containers of water in controlled environments, and regularly recording leaf-out, flowering or other phenomena. Prior to or following leaf-out or flowering, twigs may be assigned to treatment groups for experiments involving temperature, photoperiod, frost, humidity and more. Recent studies using these methods have shown that winter chilling requirements and spring warming strongly affect leaf-out and flowering times of temperate trees and shrubs, whereas photoperiod requirements are less important than previously thought for most species. Invasive plant species have weaker winter chilling requirements than native species in temperate ecosystems, and species that leaf-out early in the season have greater frost tolerance than later leafing species. Conclusions This methodology could be extended to investigate additional drivers of leaf-out phenology, leaf senescence in the autumn, and other phenomena, and could be a useful tool for education and outreach. Additional ecosystems, such as boreal, southern hemisphere and sub-tropical forests, could also be investigated using dormant twigs to determine the drivers of leaf-out times and how these ecosystems will be affected by climate change.

Rice (Oryza sativa) and Arabidopsis thaliana have been widely used as model systems to understand how plants control flowering time in response to photoperiod and cold exposure. Extensive research has resulted in the isolation of several regulatory genes involved in flowering and for them to be organized into a molecular network responsive to environmental cues. When plants are exposed to favourable conditions, the network activates expression of florigenic proteins that are transported to the shoot apical meristem where they drive developmental reprogramming of a population of meristematic cells. Several regulatory factors are evolutionarily conserved between rice and arabidopsis. However, other pathways have evolved independently and confer specific characteristics to flowering responses.This review summarizes recent knowledge on the molecular mechanisms regulating daylength perception and flowering time control in arabidopsis and rice. Similarities and differences are discussed between the regulatory networks of the two species and they are compared with the regulatory networks of temperate cereals, which are evolutionarily more similar to rice but have evolved in regions where exposure to low temperatures is crucial to confer competence to flower. Finally, the role of flowering time genes in expansion of rice cultivation to Northern latitudes is discussed.Understanding the mechanisms involved in photoperiodic flowering and comparing the regulatory networks of dicots and monocots has revealed how plants respond to environmental cues and adapt to seasonal changes. The molecular architecture of such regulation shows striking similarities across diverse species. However, integration of specific pathways on a basal scheme is essential for adaptation to different environments. Artificial manipulation of flowering time by means of natural genetic resources is essential for expanding the cultivation of cereals across different environments.

†Background and Aims The timing of flowering has a direct impact on successful seed production in plants. Flowering of soybean (Glycinemax) is controlled by severalE loci, and previous studies identified the genes responsible for the flowering lociE1,E2,E3 andE4. However, natural variation in these genes has not been fully elucidated. The aims of this study were the identification of new alleles, establishment of allele diagnoses, examination of allelic combinations for adaptability, and analysis of the integrated effect of these loci on flowering. †Methods The sequences of these genes and their flanking regions were determined for 39 accessions by primer walking. Systematic discrimination among alleles was performed using DNA markers. Genotypes at the E1 ‐E4 loci were determined for 63 accessions covering several ecological types using DNA markers and sequencing, and flowering times of these accessions at three sowing times were recorded. †KeyResults A new allele with an insertion of a long interspersed nuclear element (LINE) at the promoter of theE1 locus (e1-re) was identified. Insertion and deletion of 36 bases in the eighth intron (E2-in andE2-dl) were observed at theE2 locus. Systematic discrimination among the alleles at theE1‐E3 loci was achieved using PCR-based markers. Allelic combinations at the E1 ‐E4 loci were found to be associated with ecological types, and about 62‐ 66 % of variation of flowering time could be attributed to these loci. †Conclusions The study advances understanding of the combined roles of theE1 ‐E4 loci in flowering and geographic adaptation, and suggests the existence of unidentified genes for flowering in soybean

Background and aims A model to predict anthesis time of a wheat plant from environmental and genetic information requires integration of current concepts in physiological and molecular biology. This paper describes the structure of an integrated model and quantifies its response mechanisms. Methods Literature was reviewed to formulate the components of the model. Detailed re-analysis of physiological observations are utilized from a previous publication by the second two authors. In this approach measurements of leaf number and leaf and primordia appearance of near isogenic lines of spring and winter wheat grown for different durations in different temperature and photoperiod conditions are used to quantify mechanisms and parameters to predict time of anthesis. Key results The model predicts the time of anthesis from the length of sequential phases: 1, embryo development; 2, dormant; 3, imbibed/emerging; 4, vegetative; 5, early reproductive; 6, pseudo-stem extension; and 7, ear development. Phase 4 ends with vernalization saturation (VS), Phase 5 with terminal spikelet (TS) and Phase 6 with flag leaf ligule appearance (FL). The durations of Phases 4 and 5 are linked to the expression of Vrn genes and are calculated in relation to change in Haun stage (HS) to account for the effects of temperature per se. Vrn1 must be expressed to sufficient levels for VS to occur. Vrn1 expression occurs at a base rate of 0·08/HS in winter 'Batten' and 0·17/HS in spring 'Batten' during Phases 1, 3 and 4. Low temperatures promote expression of Vrn1 and accelerate progress toward VS. Our hypothesis is that a repressor, Vrn4, must first be downregulated for this to occur. Rates of Vrn4 downregulation and Vrn1 upregulation have the same exponential response to temperature, but Vrn4 is quickly upregulated again at high temperatures, meaning short exposure to low temperature has no impact on the time of VS. VS occurs when Vrn1 reaches a relative expression of 0·76 and Vrn3 expression begins. However, Vrn2 represses Vrn3 expression so Vrn1 must be further upregulated to repress Vrn2 and enable Vrn3 expression. As a result, the target for Vrn1 to trigger VS was 0·76 in 8-h photoperiods (Pp) and increased at 0·026/HS under 16-h Pp as levels of Vrn2 increased. This provides a mechanism to model short-day vernalization. Vrn3 is expressed in Phase 5 (following VS), and apparent rates of Vrn3 expression increased from 0·15/HS at 8-h Pp to 0·33/HS at 16-h Pp. The final number of leaves is calculated as a function of the HS at which TS occurred (TS(HS)): 2·86 + 1·1 × TS(HS). The duration of Phase 6 is then dependent on the number of leaves left to emerge and how quickly they emerge. Conclusions The analysis integrates molecular biology and crop physiology concepts into a model framework that links different developmental genes to quantitative predictions of wheat anthesis time in different field situations.

†Background and Aims The time at which plants are transferred to floral inductive conditions affects the onset of flowering and plant morphology, due to juvenility. Plants of Brunonia australis and Calandrinia sp. were used to investigate whether Australian native ephemeral species show a distinct juvenile phase that can be extended to increase vegetative growth and flowering. †Methods The juvenile phase was quantified by transferring seedlings from less inductive (short day and 30/ 208C) to inductive (vernalization or long day) conditions at six different plant ages ranging from 4 to 35 d after seed germination. An increase in days to first visible floral bud and leaf number were used to signify the end of juvenility. †Key Results Brunonia australis was receptive to floral inductive long day conditions about 18 ‐22 d after seed germination, whereas plants aged 4‐35 d appeared vernalization sensitive. Overall, transferring plants of B. australis from short to long day conditions reduced the time to anthesis compared with vernalization or constant short day conditions. Calandrinia sp. showed a facultative requirement for vernalization and an insensitive phase was not detected. Floral bud and branch production increased favourably as plant age at time of transfer to inductive conditions increased. Younger plants showed the shortest crop production time. †Conclusions Both species can perceive the vernalization floral stimulus from a very young age, whereas the photoperiodic stimulus is perceived by B. australis after a period of vegetative growth. However, extending the juvenile phase can promote foliage development and enhance flower production of both species.

† Background and Aims ERECTA has been identified as a pleiotropic regulator of developmental and physiological processes in Arabidopsis thaliana. Previous work demonstrated a role for ERECTA in the control of compensation between epidermal cell expansion and division in leaves † Methods In this work, spatial and temporal analyses of epidermal cell division and expansion were performed on successive developing vegetative leaves of Arabidopsis thaliana in both ERECTA and erecta lines, LER and Ler, respectively, to understand how the ERECTA gene regulates compensation between these two processes. † Key Results The loss of ERECTA function leads to a low cell expansion rate in all zones of a leaf and in all successive leaves of a plant. This low cell expansion rate is counterbalanced by an increase in the duration of cell division. As a consequence, the ERECTA mutation eliminates the tip to base cellular gradient generally observed in the leaf epidermis and also flattens the heteroblastic changes in epidermal cell area and number within a rosette. Ablation of floral buds eliminates the heteroblastic changes in cellular patterns in an ERECTA-dependent manner. † Conclusions The results provide a detailed description of changes in leaf growth dynamics and cellular variables in both LER and Ler. Altogether they suggest that ERECTA influences leaf cellular development in relation to whole plant ontogeny.

Background and Aims: Reproductive phase change in Arabidopsis thaliana is characterized by two transitions in phytomer identity, the differentiation of the first elongate internode (bolting transition) and of the first flower (floral transition). An evaluation of the dynamics of these transitions was sought by examining the precision of the corresponding phytomer identity changes. Methods: The length of the first elongate internode and the frequency of chimeric inflorescence structures, e. g. paraclades not subtended by a leaf (no-leaf/paraclades) and flowers subtended by a bract (bract/flowers), were measured in the Wassilewskija (Ws) accession and 47 early flowering mutants under a wide range of photo-periods. The impact of photoperiodic perturbations applied to Ws plants at different times of development was also evaluated. Key Results: In Ws, both types of characters were remarkably constant across photoperiods in spite of a high degree of interindividual variability. Bract/flowers were not normally produced in Ws, but they were observed in conditions that suggest enhanced light signalling, e. g. in response to continuous light perturbations and in mutants with reduced hypocotyl elongation. In contrast, no-leaf/paraclades were normally present in approx. 20% of Ws plants, and their frequency was increased in conditions that suggest reduced light signalling, e. g. in mutants with altered specification of long-day responses. The length of the first elongate internode was unrelated to the rate of stem elongation and to the regulation of reproductive phase change. Conclusions: Bract/flowers and no-leaf/paraclades corresponded to opposite effects on the floral transition that reflected different dynamics of progression to flowering. In contrast, the length of the first elongate internode was only indirectly related to the regulation of reproductive phase change and was mainly dependent on global morphogenetic constraints. This paper proposes that morphogenetic variability could be used to identify critical phases of development and characterize the canalization of developmental patterns.

Background and aims Ecotypic differentiation has been explored in numerous plant species, but has been largely ignored in the Orchidaceae. Applying a specific germination protocol for widespread seed sources may be unreliable due to inherent physiological or genetic differences in localized populations. It is crucial to determine whether ecotypic differentiation exists for restoration and conservation programmes. Calopogon tuberosus var. tuberosus, a widespread terrestrial orchid of eastern North America, is a model species to explore ecotypic differences in germination requirements, as this species occupies diverse habitats spanning a wide geographical range. Methods Mature seeds were collected from south Florida, north central Florida, three locations in South Carolina, and the upper Michigan peninsula. Effects of three photoperiods (8/16, 12/12, 16/8 h L/D) were examined on asymbiotic in vitro seed germination and seedling development of C. tuberosus. Germination and early development was monitored for 8 weeks, while advanced development was monitored for an additional 8 weeks. In an additional experiment, asymbiotic seed germination and development was monitored for 8 weeks on six culture media (BM-1 terrestrial orchid medium, Knudson C, Malmgrem, half-strength MS, P723, and Vacin and Went). A tetrazolium test for embryo viability was performed. Key results Short days promoted the highest germination among Florida populations, but few differences among photoperiods in other seed sources existed. Different media had little effect on the germination of Michigan and Florida populations, but germination of South Carolina seeds was higher on media with higher calcium and magnesium. Tetrazolium testing confirmed that South Carolina seeds exhibited low viability while viability was higher in Florida seeds. Seed germination and corm formation was rapid in Michigan seeds across all treatments. Michigan seedlings allocated more biomass to corms compared with other seed sources. Conclusions Rapid germination and corm formation may be a survival mechanism in response to a compressed growing season in northern populations. Ecotypic differentiation may be occurring based on seed germination and corm formation data.

BACKGROUND AND AIMS: Accurately representing development is essential for applying crop simulations to investigate the effects of climate, genotypes or crop management. Development in wheat (Triticum aestivum, T. durum) is primarily driven by temperature, but affected by vernalization and photoperiod, and is often simulated by reducing thermal-time accumulation using vernalization or photoperiod factors or limiting accumulation when a lower optimum temperature (T(optl)) is exceeded. In this study T(optl) and methods for representing effects of vernalization and photoperiod on anthesis were examined using a range of planting dates and genotypes. METHODS: An examination was made of T(optl) values of 15, 20, 25 and 50 degrees C, and either the most limiting or the multiplicative value of the vernalization and photoperiod development rate factors for simulating anthesis. Field data were from replicated trials at Ludhiana, Punjab, India with July through to December planting dates and seven cultivars varying in vernalization response. KEY RESULTS: Simulations of anthesis were similar for T(optl) values of 20, 25 and 50 degrees C, but a T(optl) of 15 degrees C resulted in a consistent bias towards predicting anthesis late for early planting dates. Results for T(optl) above 15 degrees C may have occurred because mean temperatures rarely exceeded 20 degrees C before anthesis for many planting dates. For cultivars having a strong vernalization response, anthesis was more accurately simulated when vernalization and photoperiod factors were multiplied rather than using the most limiting of the two factors. CONCLUSIONS: Setting T(optl) to a high value (30 degrees C) and multiplying the vernalization and photoperiod factors resulted in accurately simulating anthesis for a wide range of planting dates and genotypes. However, for environments where average temperatures exceed 20 degrees C for much of the pre-anthesis period, a lower T(optl) (23 degrees C) might be appropriate. These results highlight the value of testing a model over a wide range of environments.

Background and aims The Madagascar periwinkle (Catharanthus roseus) produces the monoterpenoid alkaloid vindoline, which requires a specialized cell organization present only in the aerial tissues. Vindoline content can be affected by photoperiod and this effect seems to be associated with the morphogenetic capacity of branches; this association formed the basis of the study reported here. Methods Vindoline-producing in vitro shoot cultures were exposed either to continuous light or a 16-h photoperiod regime. New plantlet formation and alkaloid biosynthesis were analysed throughout a culture cycle. Key results In cultures under the photoperiod, the formation of new plantlets occurred in a more synchronized fashion as compared to those under continuous light. The accumulation of vindoline in cultures under the photoperiod occurred in co-ordination with plantlet formation, in contrast to cultures under continuous light, and coincided with a peak of activity of deacetylvindoline acetyl CoA acetyltransferase (DAT), the enzyme that catalyses the last step in vindoline biosynthesis. When new plantlet formation was blocked in cultures under the photoperiod by treatment with phytoregulators, vindoline synthesis was also reduced via an effect on DAT activity. No association between plantlet formation and other biosynthetic enzymes, such as tryptophan decarboxylase (TDC) and deacetoxyvindoline 4-hydroxylase (D4H), was found. Effects of light treatment on vindoline synthesis were not mediated by ORCA-3 proteins (which are involved in the induction of alkaloid synthesis in response to elicitation), suggesting that the presence of a different set of regulatory proteins. Conclusions The data suggest that vindoline biosynthesis is associated with morphogenesis in shoot cultures of C. roseus.

† Background and Aims Long photoperiods from flowering to maturity have been found to delay reproductive development in soybean (Glycine max) and to increase the number of seeds per unit land area. This study was aimed to evaluate whether sensitivity to photoperiod after flowering (a) is quantitatively related to the length of exposure to long days and (b) persists throughout the whole pod-setting period. It was also evaluated whether seed number was related to changes in the duration of post-flowering phenophases. † Methods Two field experiments were conducted with an indeterminate cultivar of soybean of maturity group V. In expt 1, photoperiods 2 h longer than natural daylength were applied during different numbers of days from the beginning pod stage (R3) onwards, while in expt 2 these photoperiod extensions were imposed during 9 consecutive days starting at different times between R3 and R6 (full seed) stages. † Key Results There was a quantitative response of development to the number of cycles with a long photoperiod. The exposure to long photoperiods from R3 to R5 (beginning of seed growth) increased the duration of R3‐R6 regardless of the timing of exposure. The stages of development comprised in the R3‐R6 phase were delayed by current as well as by previous exposure to long days. A positive relationship was found between seed number and the duration of R3‐R6, irrespective of the timing and length of exposure to the long photoperiod. † Conclusions Sensitivity to photoperiod remained high during the reproductive period and was highly and positively coupled with the processes of generation of yield.

International audience; Background and Aims: Plant aerial development is well known to be affected by day length in terms of the timing and developmental stage of floral transition. Arabidopsis thaliana is a ‘long day’ plant in which the time to flower is delayed by short days and leaf number is increased. The aim of the work presented here was to determine the effects of different day lengths on individual leaf area expansion. The effect of flower emergence per se on the regulation of leaf expansion was also tested in this study. Methods: Care was taken to ensure that day length was the only source of micro-meteorological variation. The dynamics of individual leaf expansion were analysed in Ler and Col-0 plants grown under five day lengths in five independent experiments. Responses at cellular level were analysed in Ler plants grown under various day lengths and treatments to alter the onset of flowering. Key Results: When the same leaf position was compared, the final leaf area and both the relative and absolute rates of leaf expansion were decreased by short days, whereas the duration of leaf expansion was increased. Epidermal cell number and cell area were also altered by day-length treatments and some of these responses could be mimicked by manipulating the date of flowering. Conclusions: Both the dynamics and cellular bases of leaf development are altered by differences in day length even when visible phenotypes are absent. To some extent, cell area and its response to day length are controlled by whole plant control mechanisms associated with the onset of flowering.

International audience; Background and aims: Many angiosperms exhibit cleistogamy, the production of both cleistogamous flowers (CL), which remain closed and obligately self-pollinated, and chasmogamous flowers (CH), which are potentially open-pollinated. The CH proportion can be plastic. Plasticity is adaptive if environmental changes can be reliably assessed and responded to with an appropriate phenotype and if plastic genotypes have higher fitness in variable environments than non-plastic ones.Methods: We studied the plastic response of four natural populations from northern and southern France of an annual cleistogamous plant, Lamium amplexicaule, to predictable seasonal variation. Plants were grown in a semi-controlled environment in spring and in autumn. We assessed the variation in flower number, phenology and cleistogamy-related traits, which were all plastic with respect to season. The CH proportion was higher in spring than in autumn in all four populations.Key results: We showed significant stabilizing selection for cleistogamy traits, with higher optimal CH proportions and more pronounced stabilizing selection in spring than in autumn. Observed CH proportions were close to the predicted optimal CH proportions in each season except in autumn for southern populations, which do not experience the autumnal growing season in nature.Conclusions: These results are consistent with adaptive plasticity across seasons of cleistogamy in L. amplexicaule.We propose that adaptive plasticity of cleistogamy could be driven by pollination environment variation, with CL flowers providing reproductive assurance when pollinators are scarce and CH flowers reducing the inbreeding depression in offspring when pollinators are abundant.

� 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 [9h short day (SD) vs. 16h long day (LD)] and temperatures (16/10, 22/16 and 28/22 � C day/night) in four ecotypes originating in arid, semi-arid and mesic habitats (110, 276 and 810mm 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 � 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.

Background and Aims Lotus (Nelumbo nucifera) has been cultivated as an ornamental and food plant in Japan for more than 1000 years. As large areas are required for its cultivation (approximately 2m 2 per plant), physiological research, such as into the effect of environmental factors on dormancy, has not been well studied until recently. In this paper, seedlings were used to examine environmental factors affecting dormancy induction. Methods In a first experiment, seeds were sown from 6 April to 6 October at 2-month intervals, and cultivated for 2 months in an unheated greenhouse. In a second experiment, seeds were prepared for germination on 16 November and 16 May and the seedlings were grown at 25 or 30 C under natural daylength in phytotron growth rooms. After 1 month, the seedlings were cultivated at 20, 25 or 30 C for a further month. The number of leaves and rhizome branches on the main stem were counted, and growth of rhizomes on the main stem was calculated using a rhizome enlargement index (= maximum internode diameter/internode length) after 2 months of culture in both experiments. Key Results Rhizomes elongated without enlargement when the seeds were sown in April and June. Sowing the seeds in August and October resulted in rhizome enlargement from the tenth and fifth internodes, respectively. Rhizomes enlarged in the November-sowing but elongated in the May-sowing irrespective of temperature treatments under natural daylength in the phytotron rooms. The seedlings cultivated from May at 25‐30 C for 2 months had more leaves, and more rhizome branches and nodes than those cultivated from November. Conclusions Short days led to induced dormancy in lotus.

A model has been developed that can be used to determine the phases of sensitivity to photoperiod for seedlings subjected to reciprocal transfers at regular intervals between long (LD) and short day (SD) conditions. The novel feature of this approach is that it enables the simultaneous analysis of the time to flower and number of leaves below the inflorescence. A range of antirrhinum cultivars were grown, all of which were shown to be quantitative long‐day plants. Seedlings were effectively insensitive to photoperiod when very young (juvenile). However, after the end of the juvenile phase, SD delayed flowering and increased the number of leaves below the inflorescence. Plants transferred from LD to SD showed a sudden hastening of flowering and a decrease in leaf number once sufficient LD had been received for flower commitment. Photoperiod had little effect on the rate of flower development. The analysis clearly identified major cultivar differences in the length of the juvenile phase and the photoperiod‐sensitive inductive phase in both LD and SD.

The aim of this work was to establish the role of factors that may trigger elongation growth in the dehardening response, namely temperature during daylight, photoperiod and vernalization. Fully cold-acclimated seedlings of winter (with incomplete vernalization) and spring oilseed rape were subjected to deacclimation under temperatures of 2/12, 12/2, 12/12, 12/20, 20/12 and 20/20 degrees C (day/night) and a 12 h photoperiod. Plants were also deacclimated under photoperiods of 8 and 16 h at constant temperatures of 12 and 20 degrees C. After deacclimation, plants were subjected to reacclimation. Results suggest that the level of growth activity induced during deacclimation affects both the deacclimation rate and the capacity for reacclimation. Deacclimation is fully reversible if it is not accompanied by induction of elongation growth. In such cases the rate of the decrease in freezing tolerance depends on the mean temperature of deacclimation. Deacclimation becomes partially or completely irreversible when it is connected with promotion of elongation growth. The stimuli triggering elongation growth during deacclimation may be the growth-promoting temperature (20 degrees C) during the day and the lack of vernalization blockage of elongation growth. When elongation growth was stimulated by other factors such as long-day treatments, rehardening was also disturbed.

Gametophytes of Onoclea sensiblis L. were grown under various light and media-ion conditions to gain a better understanding of the source/sink relationships between photosynthetic and ion-absorbing cells. There was a clear interdependency between green cell and rhizoid functions, such that the growth and development of the rhizoids was completely dependent on the internal delivery of photosynthates from green cells, and conversion of the one-dimensional filament into the two-dimensional prothallus required monovalent cations that could only be provided by rhizoid uptake. The need for monovalent cations was related to osmotic demands of dividing and expanding cells; prothallial development was blocked by monovalent cation deficiency, and the system resorted to Na + uptake to support cell expansion when K + was absent. Surgical excisions of filament cells further demonstrated the high degree of coordinated growth between the light-absorbing and ion-absorbing regions. It was also learned that excised sub-apical cells of the protonemata, like the intensively studied apical cell, were capable of remodelling remnants of the filament into a normal prothallus. a 2002 Annals of Botany Company

Potatoes (Solanum tuberosum L.) are highly productive in mid- to high-latitude areas where photoperiods change significantly throughout the growing season. To study the effects of changes in photoperiod on growth and tuber development of potato cv. Denali, plants were grown for 112 d with 400 micromol m-2 s-1 photosynthetic photon flux (PPF) under a 12-h photoperiod (short days, SD), a 24-h photoperiod (long days, LD), and combinations where plants were moved between the two photoperiods 28, 56, or 84 d after planting. Plants given LD throughout growth received the greatest total daily PPF and produced the greatest tuber yields. At similar levels of total PPF, plants given SD followed by LD yielded greater tuber dry mass (DM) than plants given LD followed by SD. Stem DM per plant, leaf DM, and total plant DM all increased with an increasing proportion of LD and increasing daily PPF, regardless of the daylength sequence. When studies were repeated, but at an enriched (1000 micromol mol-1) CO2 concentration, overall growth trends were similar, with high CO2 resulting in greater stem length, stem DM, leaf DM, and total plant DM; but high CO2 did not increase tuber DM.

The flowering and fruit-set of a bambara groundnut selection from Ankpa, Nigeria, were studied in greenhouses at constant exposure to photoperiods of 10, 12, 12·5, 13, 14 and 16 h. The development of embryos was determined in ovaries from plants under photoperiods of 11·5 h and ⩾ 14 h. The beginning of flowering, recorded as the number of days from sowing to the first open flower, was delayed by lengthening the photoperiod. It started 7 d later under 16 h than under 10 h. This difference increased during the production of the next nine open flowers. Lengthening the photoperiod also caused a delay in the beginning of fruit development. Under 13 h it was delayed by more than 40 d compared with fruit development under 10 h. Some plants under 14 and 16 h even failed to produce pods. After the beginning of fruit development dry matter partitioning to pods was substantially less under 14 and 16 h photoperiods than under photoperiods of 13 h or less; this was reflected in a strong reduction of pod growth rates. Under an 11·5 h photoperiod two groups of ovaries could be distinguished. In both, embryo development was identical up to 17 d after anthesis, but then the embryos in the first group continued to develop until they were full-grown at about 41 d after anthesis, whereas the growth of the embryos in the second group stopped. Embryo development under a photoperiod of ⩾ 14 h was similar to that in the ovaries with discontinued embryo growth under the 11·5 photoperiod. Healthy-looking embryos were found in ovaries up to 32 d after anthesis under a photoperiod ⩾ 14 h. From then onwards embryos started to shrivel and degenerate. Finally, the ovaries aborted.

† Background and Aims Oil palm flowering and fruit production show seasonal maxima whose causes are unknown. Drought periods confound these rhythms, making it difficult to analyse or predict dynamics of production. The present work aims to analyse phenological and growth responses of adult oil palms to seasonal and inter-annual climatic variability. † Methods Two oil palm genotypes planted in a replicated design at two sites in Indonesia underwent monthly observations during 22 months in 2006‐2008. Measurements included growth of vegetative and reproductive organs, morphology and phenology. Drought was estimated from climatic water balance (rainfall ‐ potential evapotranspiration) and simulated fraction of transpirable soil water. Production history of the same plants for 2001‐ 2005 was used for inter-annual analyses. † Key Results Drought was absent at the equatorial Kandista site (0855 0 N) but the Batu Mulia site (3812 0 S) had a dry season with variable severity. Vegetative growth and leaf appearance rate fluctuated with drought level. Yield of fruit, a function of the number of female inflorescences produced, was negatively correlated with photoperiod at Kandista. Dual annual maxima were observed supporting a recent theory of circadian control. The photoperiodsensitive phases were estimated at 9 (or 9 þ 12 � n) months before bunch maturity for a given phytomer. The main sensitive phase for drought effects was estimated at 29 months before bunch maturity, presumably associated with inflorescence sex determination. † Conclusion It is assumed that seasonal peaks of flowering in oil palm are controlled even near the equator by photoperiod response within a phytomer. These patterns are confounded with drought effects that affect flowering (yield) with long time-lag. Resulting dynamics are complex, but if the present results are confirmed it will be possible to predict them with models.

†Background In arabidopsis (Arabidopsis thaliana), FLOWERING LOCUS T (FT) and FLOWERING LOCUS C (FLC) play key roles in regulating seasonal flowering-responses to synchronize flowering with optimal conditions. FT is a promoter of flowering activated by long days and by warm conditions. FLC represses FT to delay flowering until plants experience winter. †Scope The identification of genes controlling flowering in cereals allows comparison of the molecular pathways controlling seasonal flowering-responses in cereals with those of arabidopsis. The role of FT has been conserved between arabidopsis and cereals; FT-like genes trigger flowering in response to short days in rice or long days in temperate cereals, such as wheat (Triticum aestivum) and barley (Hordeum vulgare). Many varieties of wheat and barley require vernalization to flower but FLC-like genes have not been identified in cereals. Instead, VERNALIZATION2 (VRN2) inhibits long-day induction of FT-like1 (FT1) prior to winter. VERNALIZATION1 (VRN1) is activated by low-temperatures during winter to repress VRN2 and to allow the long-day response to occur in spring. In rice (Oryza sativa )a VRN2-like gene Ghd7, which influences grain number, plant height and heading date, represses the FT-like gene Heading date 3a (Hd3a) in long days, suggesting a broader role for VRN2-like genes in regulating day-length responses in cereals. Other genes, including Early heading date (Ehd1), Oryza sativa MADS51 (OsMADS51) and INDETERMINATE1 (OsID1) up-regulate Hd3a in short days. These genes might account for the different day-length response of rice compared with the temperate cereals. No genes homologous to VRN2, Ehd1, Ehd2 or OsMADS51 occur in arabidopsis. †Conclusions It seems that different genes regulate FT orthologues to elicit seasonal flowering-responses in arabidopsis and the cereals. This highlights the need for more detailed study into the molecular basis of seasonal flowering-responses in cereal crops or in closely related model plants such as Brachypodium distachyon.

Un protocole est developpe afin de favoriser la croissance secondaire sur des morceaux de tiges a une feuille de la pomme de terre, apres tuberisation. Ce type de protocole, utilisant des variations de photoperiode, de temperature et la decapitation ou le retrait des feuilles, est rapide et reproduit de facon comparable le phenomene physiologique survenant naturellement

• Background and Aims Nitrate is the major nitrogen source for many plants. The first step of the nitrate assimilation pathway is the reduction of nitrate to nitrite, catalysed by nitrate reductase (NR). Circadian oscillations in expression and activity of NR have been demonstrated in many plant species. The pathway by which this circadian behaviour is regulated remains to be elucidated. In this study, based on recent experimental observations, a mathematical model is proposed to explain the origin of diurnal and circadian oscillations in NR gene expression and enzyme activity. • Methods The dynamic model is based on the feedback interconnections between NR and its substrate, nitrate. In the model, NR activity is regulated at the transcriptional level, in response to the balance between nitrate influx and reduction, and at the post-translational levels in response to signals from carbon assimilation. Conditions for the model system to generate self-sustained circadian oscillations are investigated by numerical simulations. • Key Results and Conclusions Under light/dark cycles, the simulation results are in agreement with the observed diurnal pattern of changes in leaf nitrate concentration, NR transcript level and NR activity. Within a range of kinetic parameter values, circadian oscillation behaviour persists even under constant light, with periods of approx. 24 h. These simulation results suggest that sustained circadian oscillations can originate from the feedback interactions between NR and its substrate, nitrate, without the need to postulate the existence of an endogenous ‘circadian clock’.

� Background and Aims Exposure to low temperatures (LT) produces innumerable changes in morphological, biochemical and physiological characteristics of plants, with the result that it has been difficult to separate cause and effect adjustments to LT. Phenotypic studies have shown that the LT-induced protective mechanisms in cereals are developmentally regulated and involve an acclimation process that can be stopped, reversed and restarted. The present study was initiated to separate the developmental factors determining duration from those responsible for rate of acclimation, to provide the opportunity for a more in depth analysis of the critical mechanisms that regulate LT tolerance in wheat (Triticum aestivum). � Methods The non-hardy spring wheat cultivar ‘Manitou’ and the very cold-hardy winter wheat cultivar ‘Norstar’ were used to produce reciprocal near-isogenic lines (NILs) in which the vrn-A1 (winter) alleles of ‘Norstar’ were inserted into the non-hardy ‘Manitou’ genetic background and the Vrn-A1 (spring) alleles of ‘Manitou’ were inserted in the hardy ‘Norstar’ genetic background so that the effects of duration and rate of LT acclimation could be quantified. � Key Results Comparison of the acclimation curves of the NILs and their parents grown at 2, 6 and 10 � C established that the full expression of LT-induced genetic systems was revealed only under genotypically dependent optimum combinations of time and temperature. Both duration and rate of acclimation were found to contribute significantly to the 13� 8 � C difference in lowest survival temperature between ‘Norstar’ and ‘Manitou’. � ConclusionsDuration of LT acclimation was dependent upon the rate of phenological development, which, in turn, was determined by acclimation temperatures and vernalization requirements. Rate of acclimation was faster for genotypes with the ‘Norstar’ genetic background but the ability to sustain a high rate of acclimation was dependent upon the length of the vegetative stage. Complex time/temperature relationships and unexplained genetic interactions indicated that detailed functional genomic or phenomic analyses of natural allelic variation will be required to identify the critical genetic components of a highly integrated system, which is regulated by environmentally responsive, complex pathways. a 2004 Annals of Botany Company

Bulbing was studied in shallot plants cultured in vitro. Bulbing occurred under a 16 h photoperiod with fluorescent + incandescent light and 30-50 g 1(-1) sucrose in the culture medium. Exogenous gibberellin (10 microM GA3) inhibited leaf and root growth and bulbing. When added to the medium at a concentration of 10 microM, three inhibitors of gibberellin biosynthesis (ancymidol, flurprimidol and paclobutrazol) promoted bulb formation and the percentage of bulbing. When ancymidol was used in combination with GA3, it did not reverse the effect of GA3 applied alone. Under treatments with 30-70 g l(-1) sucrose, bulbing ratios greater than those found in control plants were achieved by addition of ancymidol, and bulb fresh weight was increased in the same way. Ancymidol caused a 66% decrease in sucrose content in leaf bases but greatly increased the glucose, fructose and fructan contents. The increase in fructan content by ancymidol could result from the three-fold rise in total [14C]sucrose uptake per plant from the culture medium associated with a marked increase in leaf base labelling at the expense of root labelling. The possible role of ancymidol is discussed and evidence supports a major regulatory role for gibberellins in bulbing.