A morphological study of the development of the second inflorescences in strawberry (Fragaria×ananassa Duch.)

To clarify the timing of the differentiation of the first and second inflorescences in strawberry (Fragaria × ananassa Duch.), morphological changes on shoot apices during short day and low night temperature treatments were observed by scanning electron microscopy (SEM) and optical microscopy. Axillary buds just below the first inflorescence (axillary bud 1) became visible when sepal primordia of the primary flower were differentiated. By this time, other axillary buds had already developed. Axillary bud 1 developed four leaf primordia, and then a differentiated inflorescence at its summit. The phase transition of shoot apices from the vegetative to the reproductive phase may therefore trigger the differentiation of axillary bud 1 which is destined to develop into extension crowns.     

Promotory effect of GA13 on flowering ofAmaranthus — a short day plant

Abstact The three plant types ofAmaranthus namely,A. caudatus f.albiflorus, A. caudatus f.caudatus andA. tricolor var.tristis are qualitative short day plants with critical photoperiods 16.0, 15.5 and 15.0 h, respectively. Gibberellins A₃, A₄₊₇ and A₁₃ affect extension growth, leaf differentiation and floral induction differently. Thus, in all the three plant types ofAmaranthus, whereas, GA₃ and G₄₊₇ enhanced extension growth, GA₁₃ was completely ineffective under both, 24- and 8-h photoperiods. None of the three gibberellins could affect the leaf differentiation. In all the three plant types, flowering was promoted by GA₁₃ and not by other gibberellins tried. GA₁₃ caused promotion was manifested in two manners, firstly by lowering the critical dark period requirement in each inductive cycle, and secondly by shortening the total period taken for the initiation of inflorescence primordia under inductive photoperiods. The floral induction by gibberellins inAmaranthus is contrary to the gibberellin-anthesin concept of Chailakhyan. It is suggested that gibberellins other than GA₃ may be playing an important role in floral morphogenesis of short day plants.     

Floral initiation in Rudbeckia hirta (Asteraceae) under limited inductive photoperiodic treatments

Two experiments were conducted to examine the response of Rudbeckia hirta to limited inductive photoperiodic treatments. The first examined the effects on plants grown to an thesis of the second axillary inflorescence, and the second examined the early histological events within the meristem. Plants of Rudbeckia hirta were grown to maturity under short days (SD). At maturity, half the plants were placed in long days (LD). In the first experiment, the plants remained under LD for 0, 8, 16, 24, or 32 days before being returned to SD with an additional group remaining under LD as a control. In the second experiment, the plants remained under LD for 0, 4, 8, 12, 16, 20, 24, or 28 days before being returned to SD. Meristems were sampled 0, 4, 8, or 12 days after return to SD and histologically examined. Four groups of plants receiving 32, 36, 40, or 44 LD were used as a continuous LD control. When grown to anthesis, plant height and branch number increased as the number of inductive cycles increased. Plants receiving 24 or more LD reached anthesis earlier than plants receiving fewer LD. Histological examination of plants receiving only 4 LD showed they never progressed beyond early floral initiation. After 12 LD, the meristems continued to develop even when returned to SD, indicating that enough of the floral stimulus had reached the meristem to initiate flowering. Once involucral bract primordia initiated, floral development continued whether in LD or SD conditions.     

Pre-flowering Growth and Development of the Inflorescences of Maize: I. PRIMORDIA PRODUCTION AND APICAL DOME VOLUME

The growth and development of the main stem and the two uppermostaxillary apices of maize were studied during the period fromemergence until flowering. Plants were grown in the field undervarying levels of applied nitrogen fertilizer at two times ofsowing. The effects of daylength were isolated from those oftemperature by making comparisons of growth and developmenton a thermal time basis. The growth and development of the terminal (male) apex and thetwo uppermost axillary (female) apices followed the same patterns,with apex volumes increasing curvilinearly with increase innumber of leaf or husk primordia. The RGR(relative growth rateof volume) of the terminal apex was, however, only approximatelyone-tenth of the axillary apices. There was no difference ingrowth and development between the first and second axillaryapices before flowering: other factors, such as accumulationof dry weight, rather than primordia production, must be responsiblefor an axillary apex's potential to bear grain. Applied N, andto a lesser extent short days, increased the rates of growthand development of all the apices. For example, applied N increasedthe RGR (volume) of the apical domes, and the rate of productionof spikelet primordia, by about 25%. All axillary apices and treatments showed a single relationshipbetween number of spikelets and surface area of the ear: a favourableenvironment (e.g. high N) simply accelerated the progressionof spikelet production and area expansion along this singlepath. We conclude that this path is probably determined geneticallyand that N and time of sowing influence potential yield of maizethrough effects on the surface area of the ear but not on thedensity of spikelets formed. Key words: Maize, inflorescence, N application, daylength, temperature, apex volume     

Organographic and ontogenetic studies on the inflorescence ofLespedeza cuneata (Dum.-Cours.) G. Don (Leguminosae)

Structure of inflorescence and its variation were organographically and ontogenetically studied inLespedeza cuneata (Dum.-Cours.) G. Don. An axillary inflorescence of the species forms a compound inflorescence which is composed of three or four component inflorescences. Each component inflorescence bears four (rarely six), three, two, or one flowers. Based on the arrangement of inflorescence phyllomes, the component inflorescence with four flowers is interpreted as a pseudoraceme bearing two shortened lateral shoots (partial inflorescences) each of which has two flowers. The component inflorescence with one flower appears to be terminated by the flower and to compose the cyme. Organographic observations revealed that the terminally located flower is not truly terminal, but axillary in origin. Ontogenetic observations showed that the apices of component inflorescence and partial inflorescence exist in early developmental stages in spite of variation in the form of component inflorescence. The terminally located flower in the cyme-like inflorescence was thus demonstrated to be laterally borne on the partial inflorescence axis. The component inflorescence composing the cyme-like one inL. cuneata is a reduced form in the number of partial inflorescences and of flowers from the pseudoraceme. The cyme-like inflorescence inL. cuneata resembles the inflorescence ofKummerowia.     

The induction of flowering in vitro in stem segments of Plumbago indica L.

Summary Internode segments excised from vegetative Plumbago indica plants are responsive to photoperiodic treatments in vitro. Under long days, they produce vegetative buds; under short days, they develop inflorescences. These inflorescences can remain devoid of flowers (vegetative inflorescences), or produce normal flowers which open in the test tubes. The minimum duration of the short-day treatment capable of inducing flowering is of the order of 4 weeks.The production of inflorescences under short days is affected by various factors. An adequate level of sucrose is necessary. Sucrose can be replaced by maltose and, to a small extent, by cellobiose, but not by mannitol or lactose. Auxins and gibberellins inhibit the production of flower buds, whereas cytokinins and adenine do not. Guanine, thymine, cytosine or uracil alone are ineffective, but thymine or its precursor, orotic acid, enhance the production of floral buds when adenine and kinetin are also present in the medium. Several amino acids, as well as glutamine and asparagine, tend to reduce inflorescence formation at 3×10^-4 M or above; urea increases it slightly at the same concentrations. Both the cis- and the trans-isomer of abscisin II enhance inflorescence formation under short days, but have no such effect under long days.High concentrations of adenine re-established the red coloration of the petals which is typical of the clone used. Otherwise, the color of the flowers grown in vitro was pink, presumably because of the depressing effect of kinetin on anthocyanin synthesis.The techniques used have been described in the preceding article (Nitsch and Nitsch, 1966). All segments were excised from internodes of stock plants kept in completely vegetative conditions in a greenhouse by means of 16-hour photoperiods.     

FLOWER DEVELOPMENT IN DIOECIOUS SPINACIA OLERACEA (CHENOPODIACEAE)

Spinacia oleracea (Chenopodiaceae) is a potential model system for studies of mechanisms of sex expression and environmental influences on gender in dioecious species. Development of the male and female flowers and inflorescences of spinach were studied to determine when the two sex types can be distinguished. We found that female inflorescence apices are significantly larger than those of the male. Flower primordia are similar in size prior to perianth initiation, but the male primordia develop at a faster rate. Another distinguishing feature at this early stage is the larger bract subtending the female primordium. The two flower types become readily distinguishable when the perianth initiates. Male flowers produce four sepals and four stamens in a spiral pattern in close succession. Female flowers produce two alternate perianth parts that enlarge somewhat before the gynoecium becomes visible. There are no traces of gynoecia in male flowers or of stamens in female flowers. We propose that plant sex type is determined before inflorescence development, prior to or at evocation.     

Inheritance of Factors Affecting Floral Primordia Initiation in Cestrum; Hybrids of C. elegans and C. nocturnum

Photoperiod patterns of hybrids of Cestrum elegans (Brongn.) Schlect., a day neutral plant, and C. nocturnum L., a long-short day and long day plant, were investigated. Plants of the F₁ generation, F₂ generation, and backcrosses to each parent were tested on short day, long day, continuous light, long-short day and short-long day for floral primordia initiation. The data recorded suggest 2 independent genes or gene groups controlling floral primordia initiation in C. nocturnum, a single dominant gene that is activated by long-short day treatment and a recessive gene or genes responding to long day treatment. Further, these data suggest that the day neutral condition in C. elegans is the result of the series of independent genes or gene groups that respond to various photoperiods, the combination of these genes resulting in floral primordia initiation on all photoperiods.     

Inflorescence and floral development in Astragalus lagopoides Lam. (Leguminosae: Papilionoideae: Galegeae)

Inflorescence and floral organogenesis and development of the bushy perennial legume Astragalus lagopoides of the section Hymenostegis were studied by means of epi-illumination light microscopy. Based on our observations, the primordia of lanceolate racemose inflorescences are born in the axils of leaves. Each inflorescence apex initiates acropetally bracts and floral apices for some time and then eventually ceases meristematic activity and forms an oblong-shaped terminal structure. The formation of such atypical terminal protrusion on the inflorescence meristem is judged to be a diagnostic feature for well-organized cessation of meristem morphogenesis. Pentamerous perfect flowers of the plant show strong zygomorphy and marked overlap in time of initiation among different organ primordia. Unexpectedly, sepal initiation is bidirectional starting from the lateral sides of the floral apex. Other significant developmental feature includes the existence of two types of common primordia, which are formed successively. From the primary common primordia there are produced antesepalous stamens and secondary common primordia. In comparison, the five secondary common primordia subdivide into a petal and an antepetalous stamen primordia. Initiation of two different types of common primordia is possibly the result of rising overlap in time of initiation of organs and demonstrates an advanced developmental style in the genus Astragalus.     

HETEROMORPHIC FLOWER DEVELOPMENT IN NEPTUNIA PUBESCENS,A MIMOSOID LEGUME

The characteristic of heteromorphic inflorescences in some mimosoid legumes such as Neptunia is a puzzling one which can be approached developmentally. Each spicate inflorescence of Neptunia pubescens includes three types of flowers: perfect in the upper half, functionally male just below the middle, and sterile or neuter at the base. Developmental studies of the inflorescence show that order of initiation of bracts on the inflorescence is acropetal, but that order of subsequent development of flowers is both acropetal and basipetal on the axis. Bract growth and initiation of the axillary floral apices at the base are inhibited or retarded, while those in the middle and upper levels continue development without interruption. The three types of floral primordia are similar during initiatory stages of organ formation and through early development. At mid-development, differences arise in floral symmetry, petal form, stamen form, and size and shape of the carpel. The functionally male flowers become strongly dorsiventral and zygomorphic while the other two morphs remain actinomorphic or nearly so. Heteromorphy arises from a combination of early suppression of organogeny plus mid-stage innovations of zygomorphy and lateral expansion of stamen primordia. These divergent developmental pathways in one inflorescence can be interpreted in part using Gould's concept of heterochrony: changes in timing of developmental events to produce different structures. Other changes in Neptunia cannot be explained by this concept, however; such changes as omission of processes (i.e., meiosis) in some organs, or addition of processes not normally present (i.e., blade formation in stamen primordia which become staminodia). It is becoming evident from work on this and other legume flowers that actual loss of organs is rare, compared to initiation followed by suppression or modification.     

INFLORESCENCE AND FLOWER DEVELOPMENT IN THE PIPERACEAE. II. INFLORESCENCE DEVELOPMENT OF PIPER

The inflorescence development of three species of Piper (P. aduncum, P. amalago, and P. marginatum), representing Sections Artanthe and Ottonia, was studied. The spicate inflorescences contain hundreds or even thousands of flowers, depending on the species. Each flower has a tricarpellate syncarpous gynoecium and 4 to 6 free stamens, in the species studied. No sepals or petals are present. In P. marginatum the apical meristem of the inflorescence is zonate in configuration and is unusually elongate: up to 1,170 μm high and up to 480 μm wide during the most active period of organogenesis. Toward the time of apical cessation both height and diameter gradually diminish, leaving an apical residuum which may become an attenuate spine or may be cut off by an abscission zone just below the meristem. The active apex produces bract primordia; when each is 40–55 μm high, a floral apex is initiated in its axil. Both bract and floral apex are initiated by periclinal divisions in cells of the subsurface layer. The bracts undergo differentiation rather early, while the floral apices are still developing. The last-produced bracts near the tip of the inflorescence tend to be sterile.     

ONTOGENY AND CORRELATIVE RELATIONSHIPS OF THE PRIMARY THICKENING MERISTEM IN FOUR-O'CLOCK PLANTS (NYCTAGINACEAE) MAINTAINED UNDER LONG AND SHORT PHOTOPERIODS

The developmental anatomy of Mirabilis jalapa was investigated during the first 90 days of growth. The primary thickening meristem (PTM) initially differentiates in the pericycle at the top of the cotyledonary node 18 days after germination, then basipetally in the pericycle through the hypocotyl. The PTM differentiates acropetally into the stem and in the pericycle of the primaiy root, commencing 22 days after germination. Endodermis is easily identifiable in hypocotyls as well as in primary roots because of Casparian thickenings in its cells. It has not been definitely identified in stems. There are three rings of primary vascular bundles in the stem. The PTM differentiates as segments of cambium in a layer of cells (probably in the pericycle) on an arc between vascular bundles of the outer bundle ring. Later, arcs of PTM differentiate externally to the phloem of each bundle. Each arc forms a connection between original segments of PTM lying on either side of each vascular bundle. Thus, the PTM becomes a continuous cylinder. The PTM differentiates in the pericycle outside vascular tissue in the hypocotyl and root. Differentiation of the PTM and the mode of secondary thickening is similar in plants exposed to short (8-hr) and to long (18-hr) photoperiods, but some differences were observed. The PTM differentiates closer to the stem apex in all plants over 18 clays of age growing vegetatively under long photoperiods. That is, the diffuse lateral meristem, in whose cells the PTM differentiates in young intemodes, is shorter in nearly all investigated plants growing in long photoperiods. The hypocotyl and base of the primary root of 40-day-old plants in short photoperiods were more enlarged than those of the same age plants in long photoperiods; but, at the end of 64 days, the hypocotyl and primaiy root base were larger in plants growing under short photoperiods. Thirty-four days after seed germination, flower initiation occurs in plants exposed to short photoperiods. One hundred fifty days after seed germination, flowers differentiate on plants exposed to long photoperiods.     

Effects of Photoperiod, Nitrogen Nutrition and Temperature on Inflorescence Initiation and Development in Onion (Allium cepa L.)

The effects of photoperiod, nitrogen nutrition and temperatureon inflorescence initiation and development in onion cv. Rijnsburgerand cv. Senshyu Semi-globe Yellow were studied in controlledenvironments. Rates of inflorescence initiation were estimatedusing the data for leaf numbers formed prior to flower formationand the rates of leaf initiation. At 9 °C inflorescenceinitiation was accelerated by long photoperiods particularlyfor cv. Rijnsburger where the average time for initiation was86 days in 8 h and 38 days in 20 h photoperiods. Initiationwas as rapid at 12 °C as at 9 °C but was slower at 6°C. A reduction in the nitrate concentration in the nutrientsolution from 0.012 to 0.0018 M greatly accelerated inflorescenceinitiation particularly in photoperiods and temperatures notconducive to rapid initiation. Cv. Senshyu initiated more slowlythan cv. Rijnsburger and was less sensitive to photoperiod andnitrogen level. The development rate of inflorescences afterinitiation was accelerated by long photoperiods and increasesin temperature from 6 to 12 °C but was retarded by the lowernitrogen level. Allium cepa L., onion, flower initiation, inflorescence development, photoperiod, nitrogen nutrition, temperature, vernalization     

Environmental control of flowering and sex expression in Carex flava

The environmental control of flowering and sex expression has been studied under controlled environment conditions in three populations of the sedge Carex flava L. A dual floral induction requirement was demonstrated in all populations. Low temperature (< 12°C) was obligatory for, and short photoperiods strongly enhanced, primary induction and inflorescence initiation. Stem elongation and inflorescence development were promoted by long photoperiods, although most plants developed stunted flower stems also under short day (SD) conditions. Growth vigour, abundance of flowering and primary induction requirements varied widely among the populations, with critical exposure times for full flowering varying from less than 9 to about 12 weeks in SD at 9°C, and from about 9 to more than 15 weeks in long days (LD). Sex expression in the normally male terminal spike was shifted towards femaleness by marginal or incomplete primary induction. Primary induction in LD resulted in a complete change to entirely female inflorescences, whereas marginal induction in SD resulted in a similar sex reversal in some plants. The results are discussed in relation to environmental and hormonal factors known to modify sex expression in flowering plants and the significance of the results to Carex systematics and classification.     

Photocontrol of germination in Amaranthus caudatus

Summary Germination of Amaranthus caudatus is inhibited by light, far-red being the most effective part of the spectrum. At temperatures of 25° and below there is a low final germination percentage under continuous far-red whereas above 25° there is only a delaying effect. In the presence of a saturating concentration of gibberellic acid (GA₃) at 25° seeds germinate under continuous far-red although they are delayed. At 25° seeds exposed to 48 hr far-red fail to germinate when transferred to darkness. This induced dormancy can be broken by a single short exposure to red light given at any time after the far-red illumination. This effect of short red can be reversed by a subsequent short period of far-red indicating that the seeds are phytochrome controlled. Although most seeds have escaped from the reversing effect of short far-red after an intervening dark period of 5 hours, germination is greatly reduced by continuous far-red at this time. Results of exposing seeds to varying periods of far-red before and after dark imbibition are interpreted in terms of a continual production of phytochrome in its active P _fr form and a requirement for P _fr action over a long period of time. Effects of intermittent and continuous low intensity far-red on the inhibition of germination provides further evidence for a low energy photoreaction involving phytochrome. Effects on Germination Index of continuous illumination with various light sources maintaining different P _fr /P _total ratios have been investigated. The results suggest that the proportion of phytochrome in the P _fr form is the most important factor in the regulation of germination. A scheme for the phytochrome control of germination in Amaranthus caudatus is presented and possible explanations for the dependence on P _fr /P _total ratio are discussed.Holder of a Science Research Council Studentship.     

Enzyme activity and electrophoretic pattern of isoenzymes of peroxidase,esterase and alkaline and acid phosphatase in relation to flowering inAmaranthus viridis L. - a quantitative SD plant

Amaranthus viridis is a quantitative SD plant in which inflorescence primordia are initiated under both 24- and 8-h photoperiods after 12 and 10 days, when 8 and 7 leaves are differentiated, respectively. Photoperiod plays a non-determinate role, whereas the maturity of plants linked with the attainment of minimum leaf number is significant and of primary importance in floral induction. This is further confirmed by the more or less identical nature of changes in the total enzyme activity and isoenzyme patterns of peroxidase, esterase and alkaline and acid phosphatase under the two photoperiods. These changes occur once the minimum vegetative growth has been achieved prior to the reproductive transformation, irrespective of the photoperiod, pointing to the activation of a general common pathway of events leading to floral induction.     

Restricted cell/cell communication in the shoot apex ofSilene coeli-rosa during the transition to flowering is associated with a high mitotic index rather than with evocation

Summary Plants ofSilene coeli-rosa given 5 or more long days (LDs) flowered, even when the LDs were followed by 48 hours of darkness before their return to short days (SDs). The mitotic indices of shoot apices from induced plants shortly after induction were significantly higher than the indices of shoot apices from vegetative plants. Two major mitotic peaks were observed in the shoot apices of plants given 7 long days (LDs) on day 8. One coincided with that reported byFrancis andLyndon (1979).Cell to cell movement was tested in the shoot apices of vegetative and LD treated plants using probes with a molecular size of 749 daltons (fluorescein-hexaglycine) and 847 daltons (fluorescein-leucyl diglutamyl leucine). These probes showed some movement in the shoot apices of both short day (SD) and LD treated plants, but fluorescein-leucyl diglutamyl leucine was immobile in the induced apices of 7 LD plants on day 8 at time intervals which coincided with major mitotic activity in the shoot apex. Symplasmic restriction in the shoot apex was also observed in plants given 8 LDs (i.e., plants not returned to SDs on day 7).In plants that were placed in 48 hours of darkness after the 7 LD treatment or in plants given 5 LDs, there was no strong peak in the mitotic index, even though all these LD treatments resulted in 100% flowering. In such plants no symplasmic restriction was found in the shoot. Thus the symplasmic restriction on day 8 of 7 LD plants is associated with the high mitotic index, but neither of these phenomena is an essential part of the evocation process.Abbreviations F(Glu)₂ L-glutamylglutamic acid conjugated to fluorescein isothiocyanate isomer I (F-) - F(Gly)₆ F-hexaglycine - FLGGL F-leucyl-diglutamyl-leucine - F(PPG)₅ F-the pentamer (propyl-propyl glycine) - LD long day - LDs long days - SD short day - SDs short days     

SYMMETRY AND DEVELOPMENT OF BUTOMUS UMBELLATUS (BUTOMACEAE) AND LIMNOCHARIS FLAVA (LIMNOCHARITACEAE)

The prostrate rhizome of Butomus umbellatus produces branch primordia of two sorts, inflorescence primordia and nonprecocious vegetative lateral buds. The inflorescence primordia form precociously by the bifurcation of the apical meristem of the rhizome, whereas the non-precocious vegetative buds are formed away from the apical meristem. The rhizome normally produces a branch in the axial of each foliage leaf. However, it is unclear whether the rhizome is a monopodial or a sympodial structure. Lateral buds are produced on the inflorescence of B. umbellatus either by the bifurcation or trifurcation of apical meristems. The inflorescence consists of monochasial units as well as units of greater complexity, and certain of the flower buds lack subtending bracts. The upright vegetative axis of Limnocharis flava has sympodial growth and produces evicted branch primordia solely by meristematic bifurcation. Only certain leaves of the axis are associated with evicted branch primordia and each such primordium gives rise to an inflorescence. The flowers of L. flava are borne in a cincinnus and, although the inflorescence is simpler than that of Butomus umbellatus, the two inflorescences appear to conform to a fundamental body plan. The ultimate bud on the inflorescence of Limnocharis flava always forms a vegetative shoot, and the inflorescence may also produce supernumerary vegetative buds. Butomus umbellatus and Limnocharis flava exhibit a high degree of mirror image symmetry.     

Gibberellic-acid causes flowering in the short-day plants Panicum miliaceum L., P. miliare lamk., and Setaria italica (L.) P. Beauv.

Treatment with gibberellic acid (GA₃) causes formation of flowers in Panicum ciliaceum and Panicum miliare, two short-day plants, under long days (continuous light), and hastens the emergence of ears in Setaria italica, a quantitative short-day plant, under both inductive and non-inductive photoperiods. The GA₃-induced inflorescences, however, remain short and bear only few spikelets; in the two Panicum species, the spikelets also remain sterile.