Effects of Gibberellic Acid on the Transport of Nitrogen from the Cotyledons of Young Pea Seedlings

Gibberellic acid increased shoot length in tall and dwarf peaseedlings, but whilst in the former it had no effect on therate of breakdown of reserve proteins nor on the transport ofnitrogen to the axis and its final distribution between theshoot and the root, it promoted these processes in the dwarfpea. The results are discussed in relation to the overall controlof the mobilization of reserves in the cotyledons during germination.     

The Effect of Gibberellic Acid on Starch Breakdown in Sprouting Tubers of Solanum tuberosum L.

The treatment of potato tubers with 150 µmol dm^–3gibberellic acid (GA₃) stimulated starch breakdown and hexoseaccumulation in tuber tissues and the transfer of dry matterto stems. These effects could not be accounted for by enhancedactivities of starch phosphorylase, amylase and acid invertase.Indeed enzyme activities either declined or remained relativelyconstant as starch degradation and hexose accumulation proceeded.Changes in the rate of starch depletion were related to changesin sink strength and sink type, the onset of tuber initiationin controls causing the rate of starch degradation to exceedthat in GA₃-treated tissues, in which tuberization was inhibited. Solanum tuberosum L., gibberellic acid, starch breakdown     

The Stability and Movement of Gibberellic Acid in Pea Seedlings

The stability and movement of gibberellic acid (GA) in intactdwarf pea seedlings growing in the light was studied by meansof both unlabelled GA and GA labelled with isotopic carbon (¹⁴C).After ¹⁴C-GA had been applied to the mature leaves of pea seedlingsmuch remained in association with the treated leaflets, but¹⁴C-GA was also extractable from the young shoots. The yieldwas approximately the same 5 to 96 hours after treatment. GApenetrated leaf surfaces only while the application solventwas moist (about 1 hour), but moved from the treated leafletsinto the shoots for at least 24 hours. Some hours after treatmentthere was an abrupt increase in the growth-rates of the plants,and crude estimates suggest that an effective dose of GA movedto the elongating tissue at about 5 cm/hr. The pattern of distributionof ¹⁴C was examined by autoradiography. The data suggest thatGA which enters the plant is redistributed from maturing leavesto immature leaves, passing through the elongating tissue, foras long as any of the substance is present. The hypothesis remainstenable that GA produces its growth effects by acting only uponexpanding tissue     

Effect of Gibberellic Acid and Ethylene on Peroxidase in Pea Internodes

Ethylene at 5–80 µl l^–1 inhibited elongationand induced swelling in internodes of light-grown normal anddwarf pea plants; GA₃ did not prevent swelling in response toethylene. GA₃ neither inhibited nor enhanced the activity of isoperoxidasesin the internodes, regardless of its effect on their elongation.Ethylene at 80 µl l^–1 enhanced peroxidase in GA₃-untreatedand treated normal and dwarf plants. At 5 µl l^–1,ethylene had only a weak effect on peroxidase activity or none.The enzyme enhancement by ethylene was not related to its effecton cell expansion and seems do be due, at least in part, tochemical injury. Electron microscopy revealed peroxidase activity in the roughER and cell walls, including intercellular spaces. Stainingof walls in ethylene-treated tissues was more pronounced thanin untreated ones. Golgi vesicles did not seem to be involvedin the assembly of the enzyme carbohydrate moiety in ethylene-treatedcells. The peroxidase fraction extracted with 20 mM phosphate buffer,pH 6, and that extracted from wall debris with 1 M NaCl accountedfor 98% of total enzyme activity. Both fractions contained thesame six cathodic isoforms which comprised 85–90% of theiractivity. Electrophoresis did not reveal differences in thequalitative isoenzyme patterns in relation to variety, age,GA₃, or ethylene. The only observed quantitative differenceswere age-dependent. Procedural artefacts during separation of protoplast and wallionically bound peroxidase fractions are discussed.     

Cyanide-insensitive Respiration in Pea Cotyledons

Mitochondria isolated by a zonal procedure from the cotyledons of germinating peas possessed a cyanide-resistant respiration. This respiration was virtually absent in mitochondria isolated during the first 24 hours of germination but thereafter increased gradually until the 6th or 7th day of seedling development. At this time between 15 and 20% of the succinate oxidation was not inhibited by cyanide. The activity of the cyanide-resistant respiration was also determined in the absence of cyanide. Relationships among mitochondrial structure, cyanide-resistant respiration, and seedling development are discussed.     

Effects of Gibberellic Acid on Growth and Parthenocarpy in the Dwarf Telephone Pea

Growth in height and production of seedless pods wore studied in Dwarf Telephone pea seedlings treated with relatively large amounts of the potassium salt of gibberellic acid applied in solution to leaflets. Such treatment produced a saturation in growth in height with 2 mg or more per plant when the plants were allowed to grow to maturity. Larger amounts caused no inhibitory effects on growth. Maximum growth with 1 mg per plant was attained only if the treated leaflets were left on the plant for about 14 days, at least, but increasing numbers of weekly 10-mg applications produced no additional growth affects beyond two applications. Increasing age of plants from 10 to 59 days of age at the time of single 10-mg applications per plant resulted in decreasing final heights. Gibberellic acid caused the formation of seedless pods a number of weeks after application, and deformed flower petals, elongated peduncles, and conspicuously inflated pods were associated with seedlessness. GA was most effective in inducing parthenocarpy when applied to the first true leaves of young seedlings and became progressively less effective with age. A complete absence of seeds in pods formed on the main plant axis was produced by one application of 10 mg GA per plant, while six applications were required for the suppression of seed formation on side branches.     

Effect of Gibberellic Acid on Rate of Extension and Maturation of Pea Internodes

Gibberellic acid (GA) does not delay maturation of pea internodes;on the con-trary, maturation (i.e. cessation of extension) takesplace slightly earlier. Thus the increased length of internodesresulting from GA treatment is due entirely to increased rateof extension. In this experiment, GA treatment of plants accelerated the visibleproduction of the first flower bud by about 4 days: the nodebearing the first flower was not altered. The total number offlower buds produced by the end of the experiment was increasedas a result of GA treatment, but many of those first formedon plants receiving high doses (I-IOµg.) withered beforeopening.     

The Relation between Cell-wall and Protein Synthesis in Dwarf Pea Plants treated with Gibberellic Acid

The amount of protein and soluble nitrogen present in expandinginternodes of intact dwarf pea seedlings, was investigated atdifferent times after treatment of plants with gibberellic acid(GA). There was a marked increase in rate of protein synthesisfollowing GA treatment, but the rate of synthesis did not keeppace with internode expansion, so that the amount of proteinper unit length fell. The rate of cell-wall synthesis was alsoincreased, and, in contrast to protein, the amount of cell wallper unit length remained approximately constant during internodeexpansion. It is suggested that the increased rate of cell-wallsynthesis which follows GA treatment is mediated by a changein protein metabolism. The amount of soluble nitrogen presentin expanding internodes was also increased. There was littleeffect of GA upon the protein content of internodes which werealmost fully expanded at the time of treatment.     

Respiratory Activity in Pea Cotyledons during Seed Development

Three phases were recognized in the course of the respirationrate of pea (Pisum sativum L.) cotyledons during seed development.(1) The respiration rate per cotyledon initially increased alongwith the mitochondrial activity. (2) During the second phase,the respiration rate increased further until a constant levelwas reached and then decreased. The mitochondria now startedto lose their capacity to oxidize malate, followed by a decreasingcapacity to oxidize succinate. (3) During the maturation phasethe respiration rate decreased further. The rate of ascorbateoxidation started to decline at this time. Ascorbate oxidationwas increasingly stimulated by cytochrome c. The changes inrespiration rate are considered in relation to changes in growthand maintenance respiration. When the water content of the seeds was maintained by storingthem at high humidity, the respiration rate of cotyledons ofearly harvested seeds decreased sharply whereas that of laterharvested seeds hardly changed. This change in response wasused to mark the transition between the second and third phase.During humid storage changes in the functional integrity ofthe mitochondria still occurred. The results are discussed in relation to the ability of peaseeds to withstand desiccation. Key words: Pisum sativum, Seed development, Respiration, Mitochondrial activity     

Stimulation of Endomitotic DNA Synthesis and Cell Elongation by Gibberellic Acid in Epicotyls Grown from Gamma-irradiated Pea Seeds

Large doses of γ-irradiation, given to air-dried pea seeds, inhibit the endomitotic DNA synthesis in pea epicotyls during germination in darkness. The cortex cells of the etiolated epicotyls reach only the 4 C DNA level, whereas cortex cells of unirradiated seeds reach the 8 C DNA level. Epicotyl elongation and cell elongation are also reduced.     

Activation and Inactivation of Alcohol Dehydrogenase in Germinating Pea Cotyledons

Alcohol dehydrogenase (E.C.1.1.1.1.) activity increases markedly in the germinating pea cotyledon in the first 2 days. The activity was not suppressed by the administration of actinomycin D, 6-methylpurine, DL-p-fluorophenylalanine, and D-chloramphenicol. The compounds rather depressed the decrease of alcohol dehydrogenase activity in cotyledons after 3 days of germination. The alcohol dehydrogenase activity in ungerminated pea seeds was activated by treatment with sodium lauryl sulphate, sodium dioctyl sulfosuccinate, dithiothreitol, 2-mercaptoethanol and NADH. The inhibitory effect caused by the extract from 7 day-old cotyledons was diminished markedly in the presence of dithiothreitol and 2-mercaptoethanol, as well as by addition of bovine serum albumin. If dithiothreitol was added to the extraction medium, the enzyme activity from older cotyledons was greatly enhanced.     

Nucleic Acid, Protein, and Starch Synthesis in Developing Cotyledons of Pisum arvense L.

During the cell-division period of cotyledon development inPisum arvense L. cell volume increases slightly but nuclearvolume shows little variation and the DNA content remains atthe 2C to 4C level. During the main period of cell expansionthere is a close correlation between cell volume, nuclear volume,and nuclear DNA content, the nuclei of the largest storage cellsfinally attaining the 64C level. The rate of RNA synthesis increasesseveral days after the increase in DNA has begun and at thesame time accumulation of reserve protein and starch begins.RNA and starch synthesis apparently cease some time before maturationbut protein synthesis continues until the seeds are ripe. Cotyledondevelopment was found to comprise two distinct phases: an initialphase of cell division and differentiation during which DNA,RNA, and protein per unit volume of cell decline; and a phaseof reserve accumulation in which DNA per unit volume of cellremains constant but RNA and protein per unit volume increase,starch synthesis is initiated, and all the cotyledon cells assumethe properties of storage cells.     

Changes in the Level of Peptidase Activities in Pea Ovaries during Senescence and Fruit Set Induced by Gibberellic Acid

The activities and changes in the levels of exopeptidase and endopeptidase activities were characterized in unpollinated ovaries of Pisum sativum L. cv Alaska during senescence and early fruit development induced by gibberellic acid (GA₃). Two aminopeptidases and one iminopeptidase were electrophoretically separated. These peptidases were sensitive to inhibitors of sulfhydryl proteases. Carboxypeptidase activity was inhibited by phenylmethyl sulfonyl fluoride. An azocasein-degrading endopeptidase, sensitive to thiol protease inhibitors, was also found. An increase in the specific activity of aminopeptidase during both fruit development and ovary senescence was observed. In contrast, the specific activity of carboxypeptidase and endopeptidase increased only during senescence of the ovary. Changes in exopeptidase activity in senescing ovaries could be mainly the consequence of a greater stability to proteolysis while the rise in endopeptidase activity appeared to be due to new or increased synthesis of the enzyme. These results suggest that endopeptidase, and not amino or carboxypeptidase, plays a key role in the senescence of pea ovaries and that the changes in unpollinated ovaries leading to ovary senescence or fruit development can be controlled by gibberellins.     

Complementary action of Gibberellic acid and auxins in Pea internode extension

Gibberellic acid (GA) induced extension of green pea-stem sectionsin light only if an auxin was also present. Of the auxins tested3-indolylacetic acid, 2-methyl-4-chloro-phenoxyacetic acid,2: 4-dichloro-phenoxyacetic acid and I-naphthylacetic acid wereeffective in increasing extension of sections and in elicit-inga response to GA. Excised internodes from plants pre-treatedwith GA extended appreciably faster in vitro than those fromuntreated plants only if an auxin was supplied in the incubationmedium. This and other evidence suggests that in the intactplant GA elicits a growth-response only in the presence of auxin.By comparing growth-rates of excised internodes in vitro andof intact internodes in vivo under comparable conditions, usinguntreated plants and plants pre-treated with GA, evidence hasbeen obtained that in untreated plants growth-rate is somehowlimited to a level below that made potentially possible by theendogenous auxin supply; treatment with GA appears to releasethe plant from this state of inhibition. Growth of intact peainternodes is considered to be regulated by a three-factor system,consisting of auxin, an inhibitory system, and a hormone withphysiological properties similar to those of GA.     

Influence of Cotyledons upon alpha-Amylase Activity in Pea Embryonic Axes

α-Amylase activity remained relatively low in the axes of intact etiolated pea seedlings; the activity was predominantly confined to the epicotyl. Starch accumulated slightly. When the cotyledons were removed and the axes cultured on medium containing no carbon source, the starch reserve in the axes disappeared within a few days. This was accompanied by a 10- to 15-fold increase in α-amylase activity, in the absence of additional epicotyl growth. The phenonemon was observed for axes throughout early growth, although the relative accumulation of α-amylase activity in cultured axes was less for older seedlings. This change was attributed to a reduced response by nongrowing tissues. There was no corresponding change in β-amylase activity. These observations, described for several varieties of peas, demonstrate the control of cotyledons upon the utilization of stored reserves within the axis, with α-amylase as a key enzyme.