Floral determination in the terminal and axillary buds of Nicotiana tabacum L

The terminal and axillary buds of the day-neutral plant, Nicotiana tabacum cv. Wisconsin 38, become determined for floral development during the growth of the plant. This state of determination can be demonstrated with a simple experiment: buds determined for floral development produce the same number of nodes in situ and if rooted. After several months of growth and the production of many leaves, the terminal bud became determined for floral development within a period of about 2 days. After the terminal bud became florally determined, it produced four nodes and a terminal flower. The buds located in the axils of leaves borne just below the floral branches became florally determined 5 to 9 days after the terminal bud became florally determined. Since florally-determined axillary buds were not clonally derived from a florally-determined terminal meristem, axillary buds and the terminal bud acquired the state of floral determination independently. These data indicate that a pervasive signal induced a state of floral determination in competent terminal and axillary buds.     

NODE COUNTING IN AXILLARY BUDS OF NICOTIANA TABACUM CV. WISCONSIN 38, A DAY-NEUTRAL PLANT

A mature, quiescent, primary axillary bud on the main axis of a flowering Nicotiana tabacum cv. Wisconsin 38 plant, when released from apical dominance and before forming its terminal flower, produced a number of nodes which was dependent upon its position on the main axis. Each bud produced about one more node than the next bud above it. The total number of nodes produced by an axillary bud was about 6 to 8 greater than the number of nodes present above this bud on the main axis. At anthesis of the terminal flower on the main axis, mature, quiescent, primary axillary buds had initiated 7 to 9 leaf primordia while secondary axillary buds, sometimes present in addition to the primary ones, had initiated 4 to 5 leaf primordia. When permitted to grow out independently, primary and secondary axillary buds located at the same node on the main axis produced the same number of nodes before forming their terminal flowers. In contrast, immature primary axillary buds which had produced only 5 leaf primordia and which were released from apical dominance prior to the formation of flowers on the main axis produced only as many nodes as would be produced above them on the main axis by the terminal meristem, i.e., “extra” nodes were not produced. Therefore, it is the physiological status of the plant and not the number of nodes on the bud at the time of release from apical dominance that influenced the node-counting process of a bud. When two axillary buds were permitted to develop on the same main axis, each produced the same number of nodes as single axillary buds developing at these nodes. Thus, the counting process in an axillary bud of tobacco is independent of other buds. Axillary buds on main axes of plants that had been placed horizontally produced the same number of nodes as identically-positioned axillary buds on vertical plants, indicating that gravity does not play a major role in the counting, by an axillary bud, of the nodes on the main axis.     

Determination for inflorescence development is a stable state,separable from determination for flower development in Pisum sativum L. buds

Terminal meristems of Pisum sativum (garden pea) transit from vegetative to inflorescence development, and begin producing floral axillary meristems. Determination for inflorescence development was assessed by culturing excised buds and meristems. The first node of floral initiation (NFI) for bud expiants developing in culture and for adventitious shoots forming on cultured meristems was compared with the NFI of intact control buds. When terminal buds having eight leaf primordia were excised from plants of different ages (i.e., number of unfolded leaves) and cultured on 6-benzylaminopurine and kinetin-supplemented medium, the NFI was a function of the age of the source plant. By age 3, all terminal buds were determined for inflorescence development. Determination occurred at least eight nodes before the first axillary flower was initiated. Thus, the axillary meristems contributing to the inflorescence had not formed at the time the bud was explanted. Similar results were obtained for cultured axillary buds. In addition, meristems excised without leaf primordia from axillary buds three nodes above the cotyledons of age-3 plants gave rise to adventitious buds with an NFI of 8.3 ±0.3 nodes. In contrast seed-derived plants had an NFI of 16.5 ±0.2. Thus cells within the meristem were determined for inflorescence development. These findings indicate that determination for inflorescence development in P. sativum is a stable developmental state, separable from determination for flower development, and occurring prior to initiation of the inflorescence at the level of meristems.     

Effect of Intercalated Long Days and Light Interruption of Dark Period on Flowering, Extension Growth and Senescence of impatiens balsamina

Plants of Impatiens balsamina L. grown under long days were divided into 5 lots to receive 1, 2, 4, 8 and 16 consecutive short day (SD) cycles respectively. Each lot was divided into 5 groups to receive 1, 2, 4, 8 and 16 long day (LD) cycles subsequent to SD regime and the cycles were repeated till the end. Observations on the number, position and time of emergence of floral buds, flowers and extension growth were recorded. The floral buds are initiated and these develop into flowers even when Individual SDs are intercalated with 16 LD cycles, showing that the sub-threshold stimulus is not wiped off but becomes effectively summated through a long non-inductive period. The floral bud initiation in lots receiving less than 4 and flowering in those receiving less than 8 consecutive SD cycles are delayed with decreasing number of consecutive SDs and increasing number of intercalating LDs. This progressive delay is probably due to the delay that is caused by these treatments in the completion of requisite number of SD cycles. The first node to show floral bud initiation is shifted up with increasing intercalated LDs only in plants receiving less than 4 SD cycles and not in those receiving more. Some of the lower floral buds in plants receiving less than 8 consecutive SD cycles either abort or revert to vegetative growth. The first node to flower is, therefore, shifted up. The number of such buds increases either with a decrease in the number of consecutive SDs or an increase in the number of intercalated LDs. The number of floral buds produced in plants receiving 2 or more and flowers in those receiving 4 or more consecutive SD cycles does not differ much with the number of intercalated LDs, but decreases in those receiving less number of SDs. Some nodes bear more than one floral bud and flower. Such nodes are observed in plants receiving individual SD cycles only when intercalated with individual LDs but in all groups in plants receiving 16 consecutive SD cycles. The rate of extension growth increases with an increase in the number of consecutive SDs. The rate in plants receiving individual SDs closely resembles that of plants grown under continuous LDs and that of consecutive 16 SDs with that of control SD plants. The attainment of maximum and the consequent steep fall preceding senescence is successively delayed with an increase in the number of intercalated LDs in plants receiving 16 consecutive SD cycles. Light interruption of the dark period inhibits both the initiation of floral buds and their development Into flowers. showing that in this plant. short days are necessary both for the initiation of floral buds and their development into flowers.     

Induction and floral determination in the terminal bud of Nicotiana tabacum L. cv. Maryland Mammoth,a short-day plant

Floral determination in the terminal bud of the short-day plant Nicotiana tabacum cv. Maryland Mammoth has been investigated. Plants grown continuously in short days flowered after producing 31.4±1.6 (SD) nodes while plants grown continuously in long days did not flower and produced 172.5±9.5 nodes after one year. At various ages, expressed as number of leaves that were at least 1.0 cm in length above the most basal 10-cm leaf, one of three treatments was performed on plants grown from seed in short days: 1) whole plants were shifted from short days to long days, 2) the terminal bud was removed and then rooted and grown in long days, and 3) the terminal bud was removed and then rooted and grown in short days. Whole plants flowered only when shifted from short days to long days at age 15 or later. Only rooted terminal buds from plants at age 15 or older produced plants that flowered when grown in long days. Only terminal buds from plants at age 15 or older that were rooted and grown in short days produced the same number of nodes as they would have produced in their original locations while buds from younger plants produced more nodes than they would have in their original locations. Thus, determination for floral development in the terminal bud, as assayed by rooting, is simultaneous with the commitment to flowering as assayed by shifting whole plants to non-inductive conditions.Abbreviations LD long day(s) - SD short day(s) - DN dayneutral     

The maryland mammoth allele and rooting both perturb the fate of florally determined apices in Nicotiana tabacum.

The stability of the florally determined state in terminal and axillary buds of two tobacco cultivars was studied. We used Hicks and Hicks Maryland Mammoth, near-isogenic cultivars of Nicotiana tabacum differing at the recessive maryland mammoth locus which confers short-day behavior. The experimental design consisted of growing plants in short-day conditions and subjecting them to three bioassays in long-day conditions: in vitro culture of apices consisting of meristems and three to four leaf primordia; rooting of buds consisting of meristems and 8 to 12 leaves, leaf primordia, and internodes; and release from apical dominance of axillary buds in situ. Cultured terminal and axillary apices expressed floral determination, indicating that meristems can be florally determined. Two lines of evidence indicate that rooting destabilizes an already acquired florally determined state: cultured apices from both axillary and terminal buds produced fewer nodes after excision than homologous buds which were rooted; and a lower percentage of rooted axillary buds from Hicks Maryland Mammoth plants expressed floral determination than did homologous axillary buds grown out in situ in noninductive conditions. Rooted buds from the two genotypes expressed floral determination at different frequencies, but produced abnormal inflorescences at similar frequencies, indicating that roots and the maryland mammoth allele influence common as well as unique processes associated with floral determination.     

Flowering response of Ipomoea batatas scions grafted onto Pharbitis nil stocks

Young seedlings of Ipomoea batatas (L.) Lam. cv. Big One did not form floral buds, but were induced to flower when grafted onto Pharbitis nil Chois. cv. Violet with its cotyledons exposed to a 16 h dark period (SD). Four SD were required to induce flowering in I. batatas scions when the grafted plants were first grown under an 8 h dark period (LD) for 18 days and then exposed to SD. Transmission of the flowering stimulus across the graft union required 4 days. It was also slow in the graft combination of P. nil and P. nil , but increased greatly when the graft union was established more completely. These results suggest that the flowering stimulus of P. nil may move symplastically and its life may be between 4 and 6 days. Although the leaves of I. batatas inhibited flowering, the flowering response of P. nil grafted onto I. batatas suggested that the involvement of a transmissible flowering-inhibitor was unlikely.     

Axillary bud flowering after apical decapitation in Pharbitis in relation to photoinduction

The flowering response of axillary buds of seedlings of Pharbitis nil Choisy, cv. Violet, was examined in relation to the timing of apical bud removal (plumule including the first leaf or second leaf) before or after a flower-inductive 16-h dark period. When the apical bud was removed well before the dark period, flower buds formed on the axillary shoots that subsequently developed, but when removed just before, or after, the dark period, different results were observed depending on the timing of the apical bud removal and plant age. In the case of 8-day-old seedlings, fewer flower buds formed on the axillary shoots developing from the cotyledonary node when plumules were removed 20 to 0 h before the dark period. When the apical bud was removed after the dark period, no flower buds formed. Using 14-day-old seedlings a similar reduction of flowering response was observed on the axillary shoots developing from the first leaf node when the apical bud was removed just after the dark period. To further elucidate the relationship between apical dominance and flowering, kinetin or IAA was applied to axillary buds or the cut site where the apical bud was located. Both chemicals influenced flowering, probably by modulating apical dominance which normally forces axillary buds to be dormant.     

The Maryland mammoth allele reduces floral stimulus activity in stem piece explants of Nicotiana tabacum (Solanaceae)

The response of axillary buds to floral stimulus activity in stem pieces was examined in two near-isogenic cultivars of tobacco that differ in the recessive maryland mammoth (mm) allele, which confers short-day behavior. All axillary buds from day-neutral plants assayed on six-internode stem pieces made few nodes (less than 20) before flowering, while axillary buds from plants homozygous for mm assayed on six-internode stem pieces either did not flower in noninductive conditions or made many nodes before flowering in inductive conditions. About 80% of day-neutral axillary buds grafted onto day-neutral stem pieces did not respond to floral stimulus in stem pieces, indicating that the floral stimulus in stem pieces is ephemeral. In other graft combinations, the proportion of axillary buds that did respond to floral stimulus in stem pieces was substantially reduced from the 20% of day-neutral buds on day-neutral stem pieces that responded. These results indicate that the mm allele probably reduces both the amount of floral stimulus activity in stem pieces and the competence of axillary buds to respond.     

Erratum

The number of nodes produced by a bud meristem before differentiation into a flower is defined as its developmental potential. Decapitation, rooting, and grafting experiments were used to measure the developmental potential of the vegetative axillary bud meristems on Nicotiana tabacum. Decapitation experiments measure the in situ developmental potential while rooting and grafting experiments measure developmental potential in isolation and at a new location on the organism, respectively. A rooted or grafted bud exhibits one of two patterns of development: (1) It develops like an in situ bud or (2) It develops according to its new environment. For example, second axillary buds below the inflorescence produced 18.8 ± 0.8 nodes in situ, 17.9 ± 0.9 or 39.8 ± 1.1 nodes when rooted, and 22.2 ± 0.6 or 34.2 ± 0.7 nodes when grafted to the base of the plant. These results suggest that the buds which develop like in situ buds are developmentally determined while buds that develop according to their new environment are undetermined. On an individual plant, determined and undetermined buds are separated by one internode and only first, second, and third buds below the inflorescence exhibit determination.     

Winter bud content according to position in 3-year-old branching systems of 'Granny Smith' apple

An investigation was made of the number of preformed organs in winter buds of 3-year-old reiterated complexes of the 'Granny Smith' cultivar. Winter bud content was studied with respect to bud position: terminal buds were compared on both long shoots and spurs according to branching order and shoot age, while axillary buds were compared between three zones (distal, median and proximal) along 1-year-old annual shoots in order 1. The percentage of winter buds that differentiated into inflorescences was determined and the flowers in each bud were counted for each bud category. The other organ categories considered were scales and leaf primordia. The results confirmed that a certain number of organs must be initiated before floral differentiation occurred. The minimum limit was estimated at about 15 organs on average, including scales. Total number of lateral organs formed was shown to vary with both bud position and meristem age, increasing from newly formed meristems to 1- and 2-year-old meristems on different shoot types. These differences in bud organogenesis depending on bud position, were consistent with the morphogenetic gradients observed in apple tree architecture. Axillary buds did not contain more than 15 organs on average and this low organogenetic activity of the meristems was related to a low number of flowers per bud. In contrast, the other bud categories contained more than 15 differentiated organs on average and a trade-off was observed between leaf and flower primordia. The ratio between the number of leaf and flower primordia per bud varied with shoot type. When the terminal buds on long shoots and spurs were compared, those on long shoots showed more flowers and a higher ratio of leaf to flower primordia.     

[14C]-Assimilate partitioning in photoperiodically induced seedlings of Pharbitis nil. The effect of benzyladenine

Partitioning of [¹⁴C]-labeled assimilates was studied in relation to photoperiodic floral induction and evocation in one-week-old Pharbitis nil Choisy cv. 'Violet' seedlings. In plants kept under 16 h photoperiods, one 15 h night induced 100% axillary flowering whereas a 24 h night induced both terminal and axillary flowering. A 15 min night break of red light given 8 h after the beginning of the dark period inhibited flowering. Total [¹⁴C]-assimilate distribution among major sinks (plumules + epicotyl and roots + hypocotyl) from a single source cotyledon was unchanged by one inductive night; however, import of [¹⁴C]-assimilates into shoot apices was increased in induced plants compared to vegegative controls. This increase was several-fold in plants subjected to a 24 h night. N⁶-Benzyladenine (BA) application to cotyledons or plumules under non-saturating night lengths increased the number of floral buds per plant without affecting the position of the first floral bud (i.e. the speed of induction). The same treatment caused increased label accumulation in induced apices, while it only slightly affected non-induced ones. The mode of action of BA on flowering through growth stimulation and resulting assimilate mobilization is discussed.     

Flowering Habit and Vegetative Behaviour in Tea (Camellia sinensis L) Seed Trees in North-East India

Apical growth of a tea shoot occurs by a succession of flushesseparated by short periods of rest. This paper describes theexternal morphology of flowering, fruiting, and abscission ofleaves of the tea plant in north-east India in relation to thephasic activity of shoot apices. All shoots on a tree make leafy growth when a new cycle of growthbegins in the spring, but terminal buds apparently become dormantas the season advances. Apparently dormant terminal buds shedbud scales, leaving on the stem a considerable number of scars,representing leafless cataphyllary flushes. These cataphyllaryflushes are produced at the same time as the leafy flushes onother shoots. A flower is formed only in the axil of a bud scale. Flowerswhich appear to develop in leaf axils are in fact inserted inthe axils of bud scales of the axillary buds. A distal leafy flush is without flowers. Flowers appear in itsleaf axils only when the terminal bud starts growth for thenext higher flush. A distal floriferous cataphyllary flush appearsas a terminal cluster of flowers. Thus, there is an acropetalsuccession of flowers, flush by flush on a caulome, determinedby the phasic activity of the apical bud. The main crop of flowers exposes anthers from the end of thethird flush (late September to early October) until the endof the winter period of growth (late January to early February).In some plants a second, minor crop of flowers appears in thespring between the end of the first and beginning of the secondflushes. In spite of considerable time lag between anthesis,the fruits produced by these two crops of flowers mature anddehisce at the same time during October to November. Abscission of leaves is also dependent upon the phasic activityof the apical buds. Only the top two flushes of a shoot possessleaves. Resumption of apical growth for a third flush, leafyor cataphyllary, causes the abscission of leaves on the lowermostof the three flushes. Two cataphyllary flushes therefore resultin the loss of all leaves on a shoot.     

Inhibitory Effect of Methyl Jasmonate on Flowering and Elongation Growth in Pharbitis nil

The effect of methyl jasmonate (JA-Me) on the floral bud formation and elongation growth in the short-day plant Pharbitis nil was investigated. The placing of 4-day-old seedlings of P. nil in a solution of JA-Me for a period of 24 h before an inductive (16 h or 14 h of darkness) night led to a dramatic reduction in the number of flower buds formed by the plant. Plants treated with JA-Me also totally lost their capacity to form a generative terminal bud. JA-Me applied after photoinduction does not inhibit flowering. Gibberellic acid (GA3) partly reverses the inhibitory effect of JA-Me. Plants treated simultaneously with JA-Me and GA3 formed about 3 flower buds more than plants treated with JA-Me only. JA-Me at a concentration of 10-7 M stimulates slightly, but at higher concentrations it inhibits root growth and shoot growth. A distinct lack of correlation between the effect of JA-Me on inhibition of flowering and shoot and root growth was noted. This indicates the independent action of JA-Me in controlling both processes.     

Inhibitory Effect of Methyl Jasmonate on Flowering and Elongation Growth in Pharbitis nil

The effect of methyl jasmonate (JA-Me) on the floral bud formation and elongation growth in the short-day plant Pharbitis nil was investigated. The placing of 4-day-old seedlings of P. nil in a solution of JA-Me for a period of 24 h before an inductive (16 h or 14 h of darkness) night led to a dramatic reduction in the number of flower buds formed by the plant. Plants treated with JA-Me also totally lost their capacity to form a generative terminal bud. JA-Me applied after photoinduction does not inhibit flowering. Gibberellic acid (GA3) partly reverses the inhibitory effect of JA-Me. Plants treated simultaneously with JA-Me and GA3 formed about 3 flower buds more than plants treated with JA-Me only. JA-Me at a concentration of 10-7 M stimulates slightly, but at higher concentrations it inhibits root growth and shoot growth. A distinct lack of correlation between the effect of JA-Me on inhibition of flowering and shoot and root growth was noted. This indicates the independent action of JA-Me in controlling both processes.     

Effect of Gibberellic Acid and Phosfon on the Critical Dark Period Reqnirement for Flowering of Impatiens balsamina cv. Rose

The critical dark period requirement for flowering of Impatiens balsamina L. cv. Rose, an obligate short day plant, is about 8.5 hours. While GA₃ completely substituted for the dark period requirement, Phosfon prolonged it to 9.5 hours. GA₃ hastened and Phosfon delayed the initiation of floral buds under all photoperiods. Floral buds opened into flowers only during 8 and 14 hour photoperiods in control and Phosfon-treated plants but during all photoperiods in GA₃-treated ones. The delay in floral bud initiation and flowering was correlated with shifting up of the node bearing the first floral bud and flower respectively. While GA₃ increased the numher of floral buds and flowers in all photoperiods except 8-hour, Phosfon increased their number in the 14-hour photoperiod only. The number of flowering plants decreased with increasing photoperiod regardless of GA₃ and Phosfon application. The effect of Phosfon was completely or partially overcome, depending upon the photoperiod, by simultaneous application of GA₃.     

Early inflorescence and floral development in Cocos nucifera L. (Arecaceae: Arecoideae)

Palms are generally characterized by a large structure with a massive crown that creates difficulties in anatomical studies. The flowering behaviour of palm species may be a useful indicator of phylogenetic relationships and therefore evolutionary events. This paper presents a detailed histological study of reproductive development in coconut (Cocos nucifera L.), from initiation up to maturation of staminate and pistillate flowers. Reproductive development in coconut consists of a sequence of individual events that span more than two years. Floral morphogenesis is the longest event, taking about one year, while sex determination is a rapid process that occurs within one month. The inflorescence consists of different ultimate floral structural components. Pistillate flowers are borne in floral triads that are flanked by two functional staminate flowers. The staminate flowers are born in floral diads towards the base of the rachilla followed by solitary flowers in the middle to top of the rachilla. Three primary phases were identified in reproductive development, namely, transition of axillary bud into inflorescence bud, formation of floral buds, and sexualisation of individual flower buds. All developmental events with respect to stage or time of occurrence were determined.     

Correlation between development of female flower buds and expression of the CS-ACS2 gene in cucumber plants

Ethylene plays a key role in sex determination of cucumber flowers. Gynoecious cucumber shoots produce more ethylene than monoecious shoots. Because monoecious cucumbers produce both male and female flower buds in the shoot apex and because the relative proportions of male and female flowers vary due to growing conditions, the question arises as to whether the regulation of ethylene biosynthesis in each flower bud determines the sex of the flower. Therefore, the expression of a 1-aminocyclopropane-1-carboxylic acid synthase gene, CS-ACS2, was examined in cucumber flower buds at different stages of development. The results revealed that CS-ACS2 mRNA began to accumulate just beneath the pistil primordia of flower buds at the bisexual stage, but was not detected prior to the formation of the pistil primordia. In buds determined to develop as female flowers, CS-ACS2 mRNA continued to accumulate in the central region of the developing ovary where ovules and placenta form. In gynoecious cucumber plants that produce only female flowers, accumulation of CS-ACS2 mRNA was detected in all flower buds at the bisexual stage and at later developmental stages. In monoecious cucumber, flower buds situated on some nodes accumulated CS-ACS2 mRNA, but others did not. The proportion of male and female flowers in monoecious cucumbers varied depending on the growth conditions, but was correlated with changes in accumulation of CS-ACS2 mRNA in flower buds. These results demonstrate that CS-ACS2-mediated biosynthesis of ethylene in individual flower buds is associated with the differentiation and development of female flowers.     

Changes in organ growth ofChenopodium rubrum due to suboptimal and multiple photoperiodic cycles with and without flowering effect

The growth changes of cotyledons, leaves, hypocotyls and roots due to photoperiodic induction in short day plantChenopodium rubrum were investigated in relation to flowering. Six-day old plants were induced by photoperiods with a different number of dark hours. We found that the degree of inhibition which occurred during induction in the growth of leaves, cotyledons and roots similarly as the stimulation of hypocotyl is proportional to the length of dark period. The photoperiods with 12, 16 and 20 dark hours bring about marked inhibition of growth and at the same time induce flowering in terminal and axillary meristems. The inhibitory effect of critical period for flowering,i.e. 8 dark hours, is not apparent in all criteria used and even the flower differentiation is retarded. The photoperiods of 4 and 6 dark hours did not affect growth and were ineffective in inducing flowering even if their number has been increased. The experiments with inductive photoperiod interrupted by light break have clearly shown that growth pattern characteristic for induced plants can be evoked in purely vegetative ones. Such statement did not exclude the possible importance of growth inhibition as a modifying factor of flower differentiation. We demonstrated that the early events of flower bud differentiation are accompanied by stimulation of leaf growth. The evaluation of growth and development of axillary buds at different nodes of insertion enabled us to quantify the photoperiodic effect and to detect the effects due to differences in dark period length not exceeding 2 hours.