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; )     

The Roles of the Cotyledons and Auxin in the Adventitious Root Formation of Hypocotyl Cuttings of Light-Grown Cucumber Seedlings

Adventitious root formation in encumber hypocotyl cuttings was studied. Root formation was quantitatively related to the amount of the cotyledons attached to the hypocotyl. Complete removal of the cotyledons diminished root formation entirely. Hut the removal of the apical bud had no effect. Treatment of the hypocotyl with triiodobenzoic acid resulted in the inhibition of root formation. On t he other hand, IAA promoted root formation. Promoting concentration of IAA was 1 mg/1 for the cuttings with intact cotyledons and 10 mg/1 for those with l/8th of the cotyledons. The first two or three days of treatment was most effective. The presence of auxin (IAA-like. substance) in cucumber seedlings was demonstrated by paper chromatography and the pea straight test. It is concluded that the cotyledon is necessary for root formation in cucumber hypocotyl cuttings and that auxin is at least one of the factors supplied from the cotyledons.     

The changes in the growth pattern of organs ofChenopodium rubrum photoperiodically induced to flowering

The relationship between photoperiodically changed growth of leaves, cotyledons, hypocotyl, roots and flowering has been investigated inChenopodium rubrum. It was found that all the growth characteristics recorded in leaves and cotyledons,i.e. length, area, dry weight and chlorophyll content, were inhibited during three inductive photoperiods (16 h darkness, 8 h light-SD) as compared with control plants grown under continuous illumination. Similarly, the cessation of root elongation and a decrease in root dry weight were observed. On the contrary, the elongation and dry weight of hypocotyl are stimulated by SD. The degree of the effect exerted by SD on the growth of different organs depends both on actual growth stage and the number of SD photoperiods. Increased relative rate of growth of roots and cotyledons was recorded in plants transferred after SD treatment to continuous illumination. However, this rise possesses only transitional character and the relative growth rate of treated plants equals that of control ones afterwards. The above growth changes are discussed as a possible modifying factor of floral differentiation.     

桃儿七种子解剖结构及其萌发生长期形态特征

为探明种皮和胚乳是否是限制桃儿七种子萌发的主要因素,利用组织切片和显微技术,对桃儿七种子及其不同萌发期（1、7、14、21、28 d）解剖结构和播种后一定时期内（7~210 d）的植株生长形态进行观察。桃儿七种子由种皮、胚乳和胚构成。种皮包括外种皮和内种皮,外种皮致密规整,由外至内分别为栅状石细胞和表皮层细胞,内种皮由5~6层海绵细胞组成。胚乳占种子体积的绝大部分,包括珠孔胚乳和外胚乳。胚由胚根、胚轴和子叶组成,被致密种皮、多层珠孔胚乳和外胚乳包围。萌发期1~7 d胚根和胚轴开始伸长,7~14 d两片子叶分离,14~21 d胚根突破珠孔胚乳和种皮,21~28 d胚根、胚轴和子叶继续扩张伸长。种子播种210 d后可平均形成3片功能真叶和5条不定根。致密种皮（物理休眠）和多层胚乳（机械休眠）是限制桃儿七种子萌发的两个主要因素。     

Effect of horizontal clinorotation on the root system development and on lipid breakdown in rapeseed (Brassica napus) seedlings

Seedlings of Brassica napus were cultivated on a slowly rotating clinostat (1 rpm) or in the vertical control for 5 d. The root growth, the cotyledonary reserves and the transport of 14C-labeled sucrose from cotyledons to root system were studied in both cultural conditions. The biomass (fresh weight) of the root system was 35% higher in the horizontally clinorotated seedlings than in the controls. This increase was correlated with a greater degradation of reserve lipids and faster accumulation of sucrose in the cotyledons. The activity of isocitrate lyase, one of the two enzymes necessary to conversion of lipids into glucids, was also greater in the cotyledons of clinorotated seedlings. The labeling distribution of 14C in the cotyledons, the hypocotyl and the root system after 30, 60 and 120 min of application of 14C-labeled sucrose on the cotyledons showed higher translocation of the cotyledonary sucrose to the root system of clinorotated seedlings. In addition, we studied the effects of clinorotation on the biomass of the excised root system (of 10 d old seedlings) cultivated in a medium containing 1% sucrose. The horizontally clinorotated root system grew more than that of the controls. These results showed that the horizontal clinorotation acted on the root system growth and provoked a higher sucrose translocation from source to sink, i.e. from cotyledons to root system.     

myo-Inositol Synthesis from [1-H]Glucose in Phaseolus vulgaris L. during Early Stages of Germination

Radiolabeled d-[1-³H]glucose was fed by imbibition under sterile conditions to bean (Phaseolus vulgaris L.) seeds. After 72 and 96 hours of feeding, the ³H was located in uronic acid and pentose residues as well as hexose residues of cell wall polysaccharides in growing hypocotyl and root. Free myo-inositol present in cotyledons, hypocotyl, and root also contained ³H, showing that de novo synthesis of myo-inositol from [1-³H]glucose did occur during the first 72 hours of germination. More than 90% of the labeled, free myo-inositol was present in the cotyledons. The ³H percentage in trifluoroacetic acid-soluble arabinose residues of cell wall polysaccharides from 72-hour-old bean hypocotyls was only half of their mole percentage. On the other hand, ³H percentages in hexose residues were higher than their mole percentages. The results suggest that myo-inositol is synthesized from reserve sugars during the very early stages of germination, and that the newly synthesized myo-inositol, as well as that stored in cotyledons, can be used for the construction of new hypocotyl and root cell wall polysaccharides after conversion into uronic acids and pentoses via the myo-inositol oxidation pathway.     

Effect of benzyladenine on development of derooted watermelon seedlings

We have examined the effects of benzyladenine (BA) on derooted watermelon ( Citrullus vulgaris Schrad., cv. Fairfax) seedlings with special attention to the cotyledons. The growth regulator was supplied either as a droplet of solution between the cotyledons (application from above) or through the hypocotyl stump (application from below). Application of BA from above stimulates several developmental parameters of cotyledons [growth, lipid breakdown, chlorophyll and carotenoid accumulation, hydroxypyruvate reductase (EC 1.1.1.81.) activity]. The stimulation is much smaller with application from below. Also the distribution of labelled BA changes according to the method of application. The bulk of the label remains in the cotyledons when BA is supplied from above and in the hypocotyl when it is supplied from below. The absolute amount of radioactivity found in the cotyledons after 24 h of treatment is approximately the same in both cases. This seems to indicate that the different effects of the two treatments depend on concentration ratios of BA (or metabolic derivates) between hypocotyl and cotyledons rather than on absolute growth regulator levels.     

Promotion of Hypocotyl Elongation in Watermelc Seedlings by 6-Benzyladenine

Hypocotyl elongation under white fluorescent light was aboutdoubled in dwarf watermelon (Citrullus lanatus0 (Thunb.) Matsu.and Nakai) seedlings treated with 0.1 to 0.3 µg apicaland 3 x 10–6 to 10.3 M root applications of 6-benzyladenine(BA). BA-enhancement of growth occurred primarily during thefirst 48 h after treatment. Increased hypocotyl length in BA-treatedseedlings was attributed more to longer cells than to an increasein cell number. Early hypocotyl growth of normal seedlings wasalso significantly enhanced by BA although final hypocotyl lengthwas not substantially affected. Benzyladenine caused expansion of cotyledons and, at higherdoses, lateral expansion of hypocotyls. BA-induced increasesin fresh weight of cotyledons and hypocotyls were accompaniedby an increase in dry weight of hypocotyls at the expense ofroots which had less dry matter than untreated seedlings.     

Stimulation of cytokinins and chlorophyll synthesis in cucumber cotyledons by triadimefon

Triadimefon [1-(4-chlorophenoxy)-3,3-dimethyl-1-(1,2,4-triazol-1-yl)-2-butanone] changes the morphology and partitioning of dry matter in cucumber ( Cucumis sativus L. cv. National Pickling) seedlings. The dry weights, potassium and cytokinin levels in the cotyledons and roots of the treated seedlings were higher, whereas the hypocotyl weights were lower than the controls. When etiolated intact seedlings or cotyledons excised from triadimefon-pretreated dark-grown seedlings were exposed to light, chlorophyll synthesis in the pretreated cotyledons was stimulated. Triadimefon does not have cytokinin-like activity in the cucumber cotyledon greening bioassay, but appears to induce the plants to produce more cytokinims, probably by stimulating root growth. Hence it is proposed that the stimulation of chlorophyll production by triadimefon in cucumber cotyledons is mediated by maintaining high levels of potassium and cytokinins in the cotyledons.     

Nitric oxide,hydrogen cyanide and ethylene are required in the control of germination and undisturbed development of young apple seedlings

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are emerging as important regulators of plant development (germination, flowering, senescence), acting as secondary messengers in cooperation with classical phytohormones. Apple seeds are dormant, unless they undergo a 3 month long cold stratification. Deep dormancy of isolated apple embryos can also be broken by short pre-treatment with HCN or NO with the effect associated with enhanced ethylene synthesis. Non-dormant embryos germinate well and young seedlings grown from non-dormant embryos do not exhibit any morphological anomalies, such as asymmetric growth and greening of cotyledons. One of the aims of this work was to investigate the correlation between RNS- mediated (HCN- and NO-dependent) dormancy removal and ROS (H₂O₂ and O₂^−•) accumulation in the embryos. The beneficial effect of NO and HCN on germination of dormant apple embryos has been associated with marked increases in H₂O₂ and O₂^−• concentration in the embryos at early germination stages. We also analyzed growth of young seedlings developed from embryos pre-treatment with HCN or NO or exposed to ethylene (ethephone) and its precursor 1-aminocyclopropane-1-carboxylic acid (ACC). ACC and ethephone removed all morphological anomalies of the seedlings (asymmetric growth and greening of cotyledons) but the radicle growth was rather slight. We propose that accumulation of ROS provoked by HCN and NO pre-treatment is required for embryo germination “sensu stricto”, while ethylene is required for post-germination seedling growth.     

Vigna Radiata Seed Germination under Salinity

Salinity reduced mung bean (Vigna radiata Wilczek) radicle and root elongation, delayed and inhibited hypocotyl elongation and mobilization of reserves from the cotyledons to the embryo axis. Fresh and dry masses and water content of the embryo axes were reduced. Under salinity, a net leakage of K to the media increased with time and increasing NaCl concentrations. Sugars present in the cotyledons of seeds were of primary importance for growth of the embryo axis upto 18 h after sowing whereas breakdown of starch by amylase contributed later, the contribution being delayed and reduced with increasing NaCl concentration. Even when amylase activity in the cotyledons was progressively reduced with increasing NaCl concentration, the increasing contents of soluble sugars in the cotyledons indicated that sugars were not limiting for mung bean seedling growth under salinity.     

A Comparative Study of the Role of the Cotyledon in Seedling Development

Seedling development was studied in four species in which thecotyledons had different capacities for expansion and CO²-flxation. The highest growth-rates were shown by the two species in whichthe cotyledons showed the most expansion and also the greatest¹⁴CO²-fixation capacity. Cotyledons of these species becamevery leaf-like and retained a high proportion of the radiocarbonfixation products during their growth in contrast to the hypogealrunner-bean cotyledon which had a high percentage export valuealthough total fixation was low. Seedlings with leaf-like cotyledons which had the dual roleof storage and subsequent provision of photosynthetic organshad delayed leaf development with early growth concentratedin cotyledon, hypocotyl, and root whereas species with cotyledonsless well adapted for photosynthesis produced leaves at an earlierstage. Thus, the pattern of early seedling development was closelyrelated to the degree of photosynthetic adaptation shown bythe cotyledons. The evidence suggests that in the species studied, cotyledonsadapted for the dual role appear to provide a more efficientsystem for early seedling development.     

Growth Suppression of Canola through Wheat Stubble II. Investigating Impacts of Hypocotyl Elongation using Simulated Stubble

In south-eastern Australia, surface-retained wheat stubble can reduce the growth and yield of canola as a result of reductions to the quality and quantity of light under the stubble and the associated elongation of the hypocotyl. This paper reports a series of pot experiments that examined the effect of hypocotyl elongation on the leaf area development of canola, the allocation of dry mass, and the absolute and relative growth rates compared to non-etiolated seedlings. The primary aim was to determine the magnitude of the growth reductions caused by hypocotyl elongation in canola seedlings under controlled conditions. Seedling hypocotyl elongation was induced by growing canola seedlings in narrow poly-pipe tubing of different lengths placed over the seedlings as they began to emerge through the soil, and removing the shade cloth covering the top of the tube when the cotyledons had reached it, to mimic the plant reaching and overtopping a stubble layer. Plants with longer hypocotyls had smaller root systems, less leaf area and less leaf and root biomass. These plants had lower relative growth rates than plants that allocated fewer resources to hypocotyls and more to roots and leaves. The magnitude of the growth responses observed in these experiments was similar to those of plants with long hypocotyls growing through stubble layers in previously reported field studies. This suggests that a significant portion of the effect of stubble observed in the field under stubble retention is due to the re-allocation of resources to the production and growth of the hypocotyls, rather than other biochemical effects of the stubble.     

Natural growth inhibitors associated with the cotyledon effect in light-grown dwarf and tall beans

The contents of non-acidic, acidic and bound growth inhibitorsin hypocotyls and cotyledons were compared between dark- andlight-grown dwarf and tall beans by means of thin-layer chromatographyand bioassay. In the non-acidic fraction, one major inhibitoryactivity appeared on the chromatogram, but its Rf zone was differentbetween hypocotyls and cotyledons. In both the acidic and boundinhibitor fractions, one major inhibitory activity appearedat the Rf zone corresponding to ABA. The ABAIike substance whichwas the major inhibitor in the hypocotyl was more abundant inlight-gorwn than dark-grown ones, especially in the dwarf variety,but light irradiation did not cause its transport from cotyledonsto the hypocotyl. A larger amount of bound ABA-like substance,which was the major inhibitor in the cotyledon, was presentin the dwarf than the tall variety regardless of the light condition.Cotyldon-enhanced photoinhibition of hypocotyl growth couldnot be explained by the levels of the xanthoxinand ABA-likeinhibitors, or the transport of these free inhibitors from cotyledons. (Received September 8, 1977; )     

Characterization of the activity of fatty-acid epoxide hydrolase in seeds of castor bean (Ricinus communis L.)

Epoxide hydrolase (EC 3.3.2.3) activity was measured with [1-¹⁴C]cis-9,10-epoxystearic acid as the substrate. Homogenates were prepared from the endosperm tissue of germinating seeds of castor bean (Ricinus communis L. zanzibariensis). The activity of fatty-acid epoxide hydrolase was characterized with respect to dependence on time, amount of protein, pH and temperature. Analyses of enzyme distribution in endosperm, cotyledons, root and hypocotyl showed the highest total activity in the endosperm, less in the cotyledons and low activity in the root and hypocotyl. The specific activity was similar for cotyledons and endosperm. Analysis of the temporal expression of the enzyme in the endosperm during germination revealed high activity already in the imbibed seed. Activity was maximal between days four to six and then decreased at the end of one week. Subcellular fractionation of endosperm revealed a dual distribution of activity between the glyoxysomal and the cytosolic fractions.     

Comparative effects of gibberellic acid and N6-benzyladenine on dry matter partitioning and osmotic and water potentials in seedling organs of dwarf watermelon

Apical applications of 0.2 μg N⁶-benzyladenine (BA), a synthetic cytokinin, or 5 μg of gibberellic acid (GA₃) significantly enhanced hypocotyl elongation in intact dwarf watermelon seedlings over a 48-h period. Accompanying the increase in hypocotyl length was marked expansion of cotyledons in BA-treated seedlings and inhibition of root growth by both compounds. A study on dry matter partitioning indicated that both growth regulators caused a preferential accumulation of dry matter in hypocotyls at the expense of the roots; however, GA₃ elicited a more rapid and greater change than did BA. In comparison to untreated seedlings, BA decreased total translocation of metabolites out of the cotyledons. Water potentials of cotyledons and hypocotyls were determined by allowing organs to equilibrate for 2 h in serial concentrations of polyethylene glycol 4000. Osmotic potentials were determined by thermocouple psychrometry. During periods of rapid growth in cotyledons and hypocotyls of BA-treated seedlings and in hypocotyls of GA-treated seedlings, the osmotic potential increased and the turgor pressure decreased in relation to untreated seedlings, indicating that cell wall extensibility was being increased. Osmotic potentials were lower in hypocotyls of GA-treated than in those of BA-treated seedlings, even though growth rates were higher in GA-treated seedlings, indicating that the latter treatment was generating more osmotically active solutes in hypocotyls.     

Relationship of IAA-oxidase Activity to Gibberellinand IAA-induced Elongation of Light-grown Cucumber Seedlings

The growth and IAA-oxidase activity of light-grown cucumber seedlings (cv. Aonagajibae) were investigated in response to GA₃ and IAA. Both GA₃ and IAA induced significant elongation of the hypocotyl. The fresh and dry weights of the hypocotyl increased due to GA₃ or IAA treatment, whereas no significant change was observed in the cotyledons of GA₃-treated seedlings as compared with the controls. The fresh and dry weights of IAA-treated cotyledons were both lower than those of controls. Treatment with GA₃ or IAA resulted in retardation of IAA-oxidase activity in the hypocotyl and cotyledons. The degree of retardation was less in the cotyledons than in the hypocotyl. An inverse relationship was recognized between GA₃- or IAA-induced elongation and IAA-oxidase activity in the hypocotyl. The auxin-mediated mechanism for gibberellin action was discussed.     

Pathways of infection of Brassica napus roots by Leptosphaeria maculans

Infection of Brassica napus cotyledons and leaves by germinating ascospores of Leptosphaeria maculans leads to production of leaf lesions followed by stem cankers (blackleg). Leptosphaeria maculans also causes root rot but the pathway of infection has not been described. An L. maculans isolate expressing green fluorescent protein (GFP) was applied to the petiole of B. napus plants. Hyphal growth was followed by fluorescence microscopy and by culturing of sections of plant tissue on growth media. Leptosphaeria maculans grew within stem and hypocotyl tissue during the vegetative stages of plant growth, and proliferated into the roots within xylem vessels at the onset of flowering. Hyphae grew in all tissues in the stem and hypocotyl, but were restricted mainly to xylem tissue in the root. Leptosphaeria maculans also infected intact roots when inoculum was applied directly to them and hyphae entered at sites of lateral root emergence. Hyphal entry may occur at other sites but the mechanism is uncertain as penetration structures were not observed. Infection of B. napus roots by L. maculans can occur via above- and below-ground sources of inoculum, but the relative importance of the infection pathways under field conditions is unknown.