Translocation of Photosynthates in Tall and Dwarf Varieties of Pea, Pisum sativum

Quantitative studies of the translocation of radiocarbon from a young expanded leaf of two tall varieties (Improved Pilot and Thomas Laxton) and two dwarf varieties (Little Marvel and Meteor) of Pisum sativum showed that 40 to 45 per cent of the radiocarbon was exported from the ¹⁴CO₂ treated leaf after 24 hours in all four varieties. Although substantial export to the upper shoot always occurred it was more marked in the two tall varieties. Pre-treatment with GA did not affect total fixation but increased total export from the ¹⁴CO₂ treated leaf in cv. Meteor and decreased it in cv. Improved Pilot. GA had no effect on the translocation pattern in the tall plants but modified that of the dwarf plants to correspond to that found in the tall varieties.     

Cell length,light and14C-labelled indol-3yl-acetic acid transport inPisum satisum L. andPhaseolus vulgaris L

The putative auxin-transporting cells of the intact herbaceous dicotyledon are the young, differentiating vascular elements. The length of these cells was found to be considerably greater in dwarf (Meteor) than in tall (Alderman) varieties ofPisum sativum L., and to be greater in etiolated than in light-grown plants ofP. sativum cv Meteor andPhaseolus vulgaris L. cv Mexican Black. Under given light conditions during transport these large differences in cell length did not influence the shapes of the transport profiles or the velocity of transport of¹⁴C-labelled indol-3yl-acetic acid (IAA) applied to the apical bud. However, in both etiolated and light-grown bean and dwarf pea plants the velocity of transport in darkness was ca. 25% lower than that in light. Under the same conditions of transport velocities in bean were about twice those observed in the dwarf pea. Exposure to light during transport increased the rate of export of¹⁴C from the labelled shoot apex in green dwarf pea plants but not in etiolated plants. The light conditions to which the plants were exposed during growth and transport had little effect on the rates of uptake of IAA from the applied solutions. The results indicate that the velocity of auxin transport is independent of the frequency of cell-to-cell interfaces along the transport pathway and it is suggested that in intact plants auxin transport is entirely symplastic.     

THE EFFECT OF MALEIC HYDRAZIDE ON THE GROWTH RESPONSE OF PLANTS TO GIBBERELLIC ACID

Maleic hydrazide (MH) and gibberellic acid (GA) were applied alone and in combination at various doses to dwarf and tall varieties of garden pea, and their effect on stem extension measured. Combinations of MH and 3-indolylacetic acid (IAA) were also studied. Stern extension of dwarf peas was accelerated by GA and inhibited by MH. Their effects were not additive, since MH reduced the response to GA at all concentrations of each tested. IAA did not affect stem extension, whether applied alone or in combination with MH. Stem extensions of tall peas was not affected by GA or IAA alone. MH severely inhibited growth and this inhibition was not reduced either by GA or by IAA. At low doses MH broke apical dominance and side branches developed; extension of these was stimulated by GA and IAA and extension of the main axis correspondingly still further reduced. The results show that MH prevents the response to GA of GA-sensitive plants. It is suggested that the rapid growth of tall peas, as compared with that of dwarfs, and their lack of response to GA, are due to a greater capacity to synthesize a 'GA-like hormone'. Growth of tall peas is much more drastically inhibited by MH than that of dwarf peas and the suggestion is made that the inhibition of shoot growth induced by MH is due primarily to blocking the activity of the postulated 'GA-like hormone'.     

Auxin Physiology of Dwarfism in Pisum sativum

Similar levels of diffusible auxin are measured for the apices of both Little Marvel (dwarf) and Alaska (normal) cultivars of the pea when grown in sunlight and darkness. In sunlight, however, diffusible auxin disappears in the subtending internode of the Little Marvel plant but remains at 50 per cent of the level of the apex in the subtending internode of the Alaska plant. The enzyme preparation from the apex of the dwarf plant converts tryptophan and tryptamine to IAA more readily than that from the normal plant. Indoleacetyl aspartate synthetase activity is also higher in the dwarf plant than in the normal plant and the dwarf plant contains four times as much conjugate as the normal plant with or without treatment with gibberellic acid. Gibberellic acid (GA) does not affect the induction of the synthetase enzyme nor the enzymatic formation of indoleacetyl aspartate. The growth induced by GA is the result of an increased synthesis of auxin.     

THE FATE OF THE DWARF SHOOT APEX IN BRISTLECONE PINE (PINUS LONGAEVA)

The fate of the pine dwarf shoot (DS) apex after needle initiation has been controversial. Dwarf shoot primordia of Pinus longaeva were examined to determine the developmental basis for DS with and without interfoliar buds. Interfoliar buds are microscopic buds derived from the original terminal apex of the DS. In October, all the DS primordia are similar in size and appearance. However, as the needles elongate in the following June the apices of more proximal DS decrease in size, such that by July there is a clear diminishing size gradient of apical domes in going from the most distal to the most proximal positions. The distal DSs start to form bud scales in July and have fully formed interfoliar buds by mid-August. In contrast, those DS apices lacking protective bud scales at needle maturity become suberized and can never proliferate into long shoots. The distal placement of interfoliar buds may be due to a group effect, where each developing DS inhibits the more proximal DSs in the long shoot terminal bud.     

Long-day induction of flowering in Lolium temulentum involves sequential increases in specific gibberellins at the shoot apex

One challenge for plant biology has been to identify floral stimuli at the shoot apex. Using sensitive and specific gas chromatography-mass spectrometry techniques, we have followed changes in gibberellins (GAs) at the shoot apex during long day (LD)-regulated induction of flowering in the grass Lolium temulentum. Two separate roles of GAs in flowering are indicated. First, within 8 h of an inductive LD, i.e. at the time of floral evocation, the GA(5) content of the shoot apex doubled to about 120 ng g(-1) dry weight. The concentration of applied GA(5) required for floral induction of excised apices (R.W. King, C. Blundell, L.T. Evans [1993] Aust J Plant Physiol 20: 337-348) was similar to that in the shoot apex. Leaf-applied [(2)H(4)] GA(5) was transported intact from the leaf to the shoot apex, flowering being proportional to the amount of GA(5) imported. Thus, GA(5) could be part of the LD stimulus for floral evocation of L. temulentum or, alternatively, its increase at the shoot apex could follow import of a primary floral stimulus. Later, during inflorescence differentiation and especially after exposure to additional LD, a second GA action was apparent. The content of GA(1) and GA(4) in the apex increased greatly, whereas GA(5) decreased by up to 75%. GA(4) applied during inflorescence differentiation strongly promoted flowering and stem elongation, whereas it was ineffective for earlier floral evocation although it caused stem growth at all times of application. Thus, we conclude that GA(1) and GA(4) are secondary, late-acting LD stimuli for inflorescence differentiation in L. temulentum.     

Synergism between Gibberellin and Auxin in the Growth of Intact Tomato

Gibberellin A_4&7 was more effective than gibberellic acid in increasing shoot elongation when applied to the apex of intact Lycopersicum esculentum seedlings of Tiny Tim, a dwarf cultivar, and Winsall, a tall cultivar. After 14 days, gibberellic acid and gibberellin A_4&7 stimulated growth of the dwarf more than the tall tomato. In tall tomato the application of indole-3-acetic acid alone (6.1 μg/plant) showed an inhibitory growth effect, but when applied with 17.5 μg per plant of gibberellic acid, it had a synergistic effect at 7 days but not at 14 days. When the auxin concentration was reduced to 0.61 μg per plant a synergistic effect was observed on tall plants at 7 and 14 days between indole-3-acetic acid and gibberellic acid. Application of gibberellin A_4&7 with auxin did not give a synergistic response in tall or dwarf tomato.     

Role of Assimilates and Phosphates in the Control of Internode Elongation in Tall and Dwarf Indica Rice Varieties

Six indica rice varieties belonging to three duration groups,each group containing one tall and one dwarf variety, were grownunder irrigated field conditions during the wet season of 1986.At the time of culm elongation, the distal 5–6 internodesof a variety elongated. The pattern of extension was hierarchical,each successive internode dominating the one immediately belowit and hence the final internode lengths increased in acropetalfashion from the bottom to the top. Between the tall and dwarfvarieties, the lengths of the individual intemodes did not differmuch in the apical and basal positions, but significant differenceswere found between the lengths of the middle internodes. Withincrease in length, the internodes became visibly thinner anddry matter content per unit length decreased. Analyses of solublecarbohydrates, amino acids, and phosphate contents of the internodesindicated that the concentration of the metabolites increasedin acropetal succession from the base to the apex. In general,the dwarf varieties contained at least the same, if not a higherconcentration of the metabolites as the tall varieties. Thepossible role of the metabolites in internode extension is discussed. Key words: Rice, internode extension, assimilates, phosphates, sink     

Cytokinin production by Asparagus shoot apex cultured in vitro

The cytokinin producing capacity of asparagus (Asparagus officinalis L.) shoot apex was examined by means of shoot apex culture in vitro, where adventitious roots were never formed. The cultured shoot apices continued to diffuse a small but constant amount of cytokinin into the medium throughout five passages of subculture. The cytokinin content in the apices at the end of the subculture was not different from that at the beginning of the subculture. These results indicate a production of cytokinin by the apices. However, the finding is not in conflict with the hypothesis that the root tip is the major source of cytokinin supply, because the root tip of asparagus produced more cytokinin than the shoot apex and the decline of shoot growth observed during the subculture was partially restored by an application of zeatin into the medium.     

Nicotine synthesis in Nicotiana tabacum L. induced by mechanical wounding is regulated by auxin

The effects of different kinds of mechanical wounding on nicotine production in tobacco plants were compared, with sand or hydroponics culture under controlled conditions. Both removal of the shoot apex and damage of the youngest unfolded leaves nos 1 and 2 by a comb-like brusher with 720 punctures caused an increase in nicotine concentration in whole plants at day 3, and reached its highest level at day 6. The nicotine concentration induced by excision of the shoot apex was much higher than that induced by leaf wounding. Both treatments also caused an increase in jasmonic acid (JA) concentration within 90 min in the shoot, followed by an increase in the roots (210 min), in which the JA concentration induced by leaf wounding was significantly higher than that induced by excision of the shoot apex. The increase in nicotine concentration occurred throughout the whole plant, especially in the shoot, while the increase in JA concentration in the shoot was restricted to the damaged tissues, and was not observed in the adjacent tissues. Removal of the lateral buds that emerged after excision of the shoot apex caused a further increase in nicotine concentrations in the plant tissues. Removal of mature leaves, however, did not cause any changes in nicotine concentration in the plant, even though the degree of wounding in this case was comparable with that occurring with apex removal. The results suggest that the nicotine production in tobacco plants was not correlated with the degree of wounding (cut-surface or punctures), but was highly dependent on the removal of apical meristems and hence on the major sources of auxin in the plant. Furthermore, immediate application of 1-naphthylacetic acid (NAA) on the cut surface after removing the shoot apex completely inhibited the increase both in nicotine in whole plants and in JA in the damaged stem segment and roots. Application of an auxin transport inhibitor around the stem directly under the shoot apex of intact plants also caused an increase in nicotine concentration in the whole plant. The results strongly suggest that auxin serves as a negative signal to regulate nicotine synthesis in roots of tobacco plants.     

Xyloglucan endotransglycosylase activity in pea internodes. Effects of applied gibberellic acid.

Xyloglucan endotransglycosylase (XET) activity extractable from internodes of tall and dwarf varieties of pea (Pisum sativum L.) was assayed radiochemically using tamarind seed xyloglucan as donor substrate and an oligosaccharidyl-[3H]alditol as acceptor substrate. Internodes I and II showed little elongation during the period 15 to 21 d after sowing; XET activity remained relatively constant and was unaffected by exogenous gibberellic acid (GA3). A single application of GA3 to the dwarf genotype resulted in a small enhancement of elongation in internode III between d 17 and 21 and caused a small increase in XET activity in internode III. Repeated applications of GA3 caused internode V to elongate between d 20 and 26, to the same extent as in the tall variety, and concomitantly led to greatly elevated XET activity (expressed per unit fresh weight, per unit of extractable protein, and per internode). Thus, XET activity correlated with GA3-enhanced length in pea internodes; the possibility that this represents a causal relationship is discussed.     

Surface-viewed shoot apex ofAngiopteris lygodiifolia Ros. (Marattiaceae)

Marattian ferns are thought to be an exception to the rule that a single apical cell is always present in the shoot apex of ferns; the occurrence of plural apical initials has been generally accepted for these ferns. However, a contradicting conclusion was reached in this study which examined the apical organization of the shoot ofAngiopteris lygodiifolia Ros., using fresh materials which had not been fixed. Shoot apices were hand-sectioned transversely into thin sections, including the surface layer of the shoot apex, which were observed by differential interference contrast microscopy without staining. In contrast with the generally accepted view, the shoot apex ofA. lygodiifolia was found to usually possess a single apical cell with three cutting faces. The segments cut off from the apical cell are regularly arranged in a helical sequence. The apical cell seems to actually function as an initial cell of the whole shoot apex. The shoot apices, particularly those of plants cultivated in a greenhouse, sometimes show somewhat irregular organization. In extreme cases, no apical cell is recognizable. However, even in these exceptional cases of such apparently irregular shoot apices, plural apical initials are not found.     

The response of short day plantChenopodium rubrum L. to abscisic acid and gibberellic acid treatment applied at two levels of photoperiodic induction

Abscisic acid (ABA) (5 x 10^-4M and 5 x 10^-5M) and gibberellic acid (1 x 10^-4M) was applied to the plumula ofChenopodium plants with partly (one dark period) or completely (three dark periods) fulfilled photoperiodic requirements for flowering. Morphological and cytoogical criteria were used to investigate the time-course of the differentiation of the treated shoot apices. Both substances were ineffective in increasing the mitotic activity of the shoot apex at the suboptimal level of induction. The degree of branching was temporarily stimulated by ABA and GA treatment under these conditions. Moreover, GA caused the elongation of the shoot apex. With the completely induced plants ABA hastened flowering and the rise in branching was observed in all the treatment 48 h following the application of growth substances.     

Growth Regulators in Populus tremula IV. Apical Dominance and Suckering in Young Plants

Experiments with small plants of Populus tremula L. growing in solution culture indicate that polarly transported auxin is an important factor in the control of axillary bud growth. If the auxin supply from the growing apex is eliminated, the number of buds released is influenced by factors translocated in the transpiration stream from the roots. Suckers may be induced to develop from aspen roots, the age of which is six weeks or more. Removal of the growing apex and the axillary buds or stoppage of shoot growth by short day treatment were effective in inducing abundant suckering in small aspen plants. Some mature leaves had to be maintained, indicating the dependence of sucker formation on carbohydrate supply. These treatments are known to decrease auxin production in the shoots. Extraction and biological assay showed a decrease in the content of auxin in the roots as a consequence of removal of growing shoot parts. The results indicate that suckering in roots of intact aspen plants is prevented by auxin transported into the roots from growing shoot parts.     

A florigenic effect of sucrose in Fuchsia hybrida is blocked by gibberellin-induced assimilate competition

The use of gas chromatography-mass spectrometry-selected ion monitoring along with a (13)C internal standard has allowed sensitive measurements of the sucrose (Suc) content of individual shoot apices of Fuchsia hybrida. With intact plants, as the photosynthetic irradiance increased, so did shoot apex Suc content, reaching saturation at about 500 micromol m(-2) s(-1). These same plants flowered at the higher irradiances, remaining vegetative in 10-h short days at an irradiance of 230 micromol m(-2) s(-1). The strong correlation (r = 0.93) in these studies between flowering and shoot apex Suc content indicates a role for Suc as a stimulus to flowering in this species. However, Suc is not the long-day (LD) "florigen" of F. hybrida because 2 to 4 LD given as a 14-h low-irradiance photoperiod extension (10-15 micromol m(-2) s(-1)) induced flowering but without increase in shoot apex Suc content. Flowering induced by either pathway, the LD- or the Suc-mediated one, was inhibited by applying gibberellin (GA) to the shoot tip. Such inhibition of flowering by GA, at least for the LD pathway, was associated with a reduced apex Suc content, enhanced elongation of subapical stem tissue, and a reduced import into the shoot apex of leaf-sourced assimilate. Thus, our findings show how GA inhibits flowering of F. hybrida and confirm the importance of nutrient diversion in regulating flowering.     

Surface Changes on Fresh Carnation Shoot Apices During Scanning Electron Microscopy

The general morphology of fresh intact shoot apices of carnation,Dianthus caryophyllus L. cv. Improved White Sim was studiedwith a scanning microscope. The changes in the surface withtime under the electron beam were: (1) individual cells beingvisible, (2) a smooth apex and (3) wrinkling until the specimenbecame unsatisfactory for study. As indicated by time coursestudies, the initial individual cell observations representthe true features of the intact apex. The observation of a smoothapex was related to a ballooning of the surface layer resultingfrom water vaporization. Incisions in the apex to prevent pressurebuild-up under the surface layer revealed the smooth characteristicof the apex surface was due to lifting of the cuticle from underlyingcells. Areas adjacent to a micropuncture in the apical domedid not exhibit ballooning.     

The effect of cotyledons on hypocotyl elongation in light-grown bean seedlings: Comparison between dwarf and tall varieties

Hypocotyl sections with and without the cotyledons were cutfrom bean seedlings and incubated under white light of 6000lux. The cotyledons had an inhibitory effect as well as a promotiveeffect on hypocotyl growth. The former effect was more strikingin the dwarf variety, and the latter in the tall variety. Whenthe hypocotyl units were exposed to light for shorter times(6 hr or less) or incubated under weaker light (1600 and 50lux), the inhibitory effect of the cotyledons decreased greatly,and in the tall variety the presence of cotyledons producedno inhibition, but a promotion of hypocotyl growth. GA treatmentenhanced hypocotyl growth and counteracted the growth inhibitioncaused by the cotyledons. On the whole, the GA effect was moremarked in the tall variety than in the dwarf. The elongation of bean hypocotyls may be controlled by a balancebetween the inhibitory and promotive effects of cotyledons,and the predominance of the former over the latter may be oneof the causes for expressing dwarfing. (Received November 13, 1976; )     

Correlative Inhibition in the Shoot of Agropyron repens ( L.) Beauv

Correlative inhibition was investigated in plants of Agropyronrepens at two temperatures. Reciprocal inhibition ocrurred betweenthe main shoot apex and the outgrowing axillary shoots, withthe balance of inhibition varying with temperature. Apical dominancewas stronger at 10 °C than at 20 °C , but even at 10°C release of apical dominance by decapitation had onlyminor effects on the timing of outgrowth, growth pattern andrate of dry weight aocumulation of the axillary shoots. Dominanceof the main shoot apex by the axillary shoots was stronger at20 °C than at 10 °C. Removal of axillary buds preventeddecline in size and activity of the main shoot apex ard resultedin increased rates of primordium initiation, leaf emergenceand dry weight accumulation in the main shoot. It is suggestedthat a system of reciprocal dominance provides a mechanism formaintaining the characteristic habit of the grass plant andlimits growth in height of vegetative shoots. Agropyron repens (L.) Beauv, couch grass, correlative inhibition, apical dominance, shoot, apex     

The involvement of gibberellin signalling in the effect of soil resistance to root penetration on leaf elongation and tiller number in wheat

Background and aims

The concept of root-sourced chemical signals that affect shoot growth in response to drought is widely reported; in particular the role of ABA in regulating stomatal conductance has received much attention. ABA, alone, does not fully explain all the effects of abiotic stresses in the root zone on shoot architecture. An increase in mechanical impedance, which can occur on even relatively modest soil drying, results in reduced root and shoot growth, processes that are potentially regulated by gibberellins (GAs).

Methods

In this study we explored the role of mechanical impedance and exogenous gibberellin (GA₃) on root and shoot architecture in wheat seedlings containing the Rht-B1a (tall), Rht-B1b (semi-dwarf) or Rht-B1c (dwarf) alleles in the April-Bearded or Mercia backgrounds. Our experiments were based on the use of the sand culture system which allows the mechanical impedance of the root growth environment to remain constant and independent of water and nutrient availability. We investigated the effects of the application of exogenous GA₃ to the root system.

Results

We found that impeding soil reduced leaf elongation in the tall and semi-dwarf lines, confirming the stunting effect of mechanical impedance which is widely reported. However, leaf elongation in the dwarf lines was not affected by root impedance. Application of GA₃ to the roots restored leaf elongation in the tall and semi-dwarf lines growing in impeding soil, with some growth response even in the dwarf line, the longest leaves being obtained when GA was applied to impeded roots of a tall line. Both exogenous GA and root impedance reduced the number of tillers, but there was no interaction with the Rht genotype. The genetic background did not affect the results.

Conclusion

We suggest that the GA signalling pathway has an unidentified role in the leaf elongation response to mechanical impedance to root growth.     

The Metabolism of Gibberellin A20 to Gibberellin A1 by Tall and Dwarf Mutants of Oryza sativa and Arabidopsis thaliana

The purpose of this study was to demonstrate the metabolism of gibberellin A20 (GA20) to gibberellin A1 (GA1) by tall and mutant shoots of rice (Oryza sativa L.) and Arabidopsis thaliana (L.) Heynh. The data show that the tall and dx mutant of rice and the tall and ga5 mutant of Arabidopsis metabolize GA20 to GA1. The data also show that the dy mutant of rice and the ga4 mutant of Arabidopsis block the metabolism of GA20 to GA1. [17-13C,3H]GA20 was fed to tall and the dwarf mutants, dx and dy, of rice and tall and the dwarf mutants, ga5 and ga4, of Arabidopsis. The metabolites were analyzed by high-performance liquid chromatography and full-scan gas chromatography-mass spectrometry together with Kovats retention index data. For rice, the metabolite [13C]GA, was identified from tall and dx seedlings; [13C]GA1 was not identified from the dy seedlings. [13C]GA29 was identified from tall, dx, and dy seedlings. For Arabidopsis, the metabolite [13C]GA1 was identified from tall, ga5, and ga4 plants. The amount of [13C]GA1 from ga4 plants was less than 15% of that obtained from tall and ga5 plants. [13C]GA29 was identified from tall, ga5, and ga4 plants. [13C]GA5 and [13C]GA3 were not identified from any of the six types of plant material.