Effect of exogenous plant growth regulators on in vitro seed growth,embryo development,and haploid production in a cross between H. vulgare (L.) and H. bulbosum (L.)

Exogenous plant growth regulators are known to increase the efficiency of interspecific and intergeneric crosses. In vitro floret culture provides a defined system for assessing the importance of various plant growth regulators on the determinants of haploid production efficiency (seed set, embryos per seeds, and plants per embryos) in Hordeum vulgare × Hordeum bulbosum crosses. The individual and combined effects of three plant growth regulators (2,4-D, GA₃ and kinetin) on in vitro seed growth, embryo development and haploid production efficiency were tested in floret culture of the cross H. vulgare, cultivar Klages × H. bulbosum. All treatments, except kinetin alone, produced larger seeds and more embryos/100 seeds than the control (no plant growth regulator). 2,4-D alone was superior to GA₃ alone in haploid production efficiency (70.6 vs. 51.5) as measured by the number of plants regenerated/100 florets pollinated. Although kinetin +2,4-D+GA₃ produced the largest seeds and embryos, no advantage over 2,4-D alone was observed in haploid production efficiency. 2,4-D alone or kinetin +2,4-D are recommended for the purpose of barley haploid production in floret culture using the bulbosum method.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - GA₃ gibberellic acid     

Simultaneous determination of different endogenetic plant growth regulators in common green seaweeds using dispersive liquid–liquid microextraction method

A simple and rapid HPLC-based method was developed for simultaneous determination of major classes of plant growth regulators (PGRs) in Monostroma and different species of Ulva. The plant growth regulators determined included gibberellic acid (GA₃), indole-3-acetic acid (IAA), abscisic acid (ABA), indole-3-butyric acid (IBA), salicylic acid and kinetin riboside (KR) and their respective elution time was 2.75, 3.3, 3.91, 4.95, 5.39 and 6.59 min. The parameters optimized for distinct separation of PGRs were mobile phase (60:40 methanol and 0.6% acetic acid in water), column temperature (35 °C) and flow rate (1 ml/min). This method presented an excellent linearity (0.2–100 μg/ml) with limit of detection (LOD) as 0.2 μg/ml for ABA, 0.5 μg/ml for KR and salicylic acid, and 1 μg/ml for IAA, IBA and GA₃. The precision and accuracy of the method was evaluated after inter and intra day analysis in triplicates. The effect of plant matrix was compensated after spiking and the resultant recoveries estimated were in the range of 80–120%. Each PGR thereby detected were further characterized by ESI-MS analysis. The method optimized in this study determined IBA along with IAA for the first time in the seaweed species investigated except Ulva linza where the former was not detected. In all the species studied, ABA level was detected to be the highest while kinetin riboside was the lowest. In comparison to earlier methods of PGR analysis, sample preparation and analysis time were substantially reduced while allowing determination of more classes of PGRs simultaneously.     

Evalution of toxicity of abcisic acid and gibberellic acid in rats: 50 days drinking water study

In the present study, the influence of subchronic effects of two plant growth regulators (PGRs) [Abcisic acid (ABA) and Gibberellic acid (GA₃)] on antioxidant defense systems [reduced glutathione (GSH), glutathione reductase (GR), superoxide dismutase (SOD), glutathione-S-transferase (GST) and catalase (CAT)] and lipid peroxidation level (malondialdehyde = MDA) in various tissues of the rat were investigated during treatment as a drinking water model. 75 ppm of ABA and GA₃ in drinking water were continuously administered orally to rats (Sprague-Dawley albino) ad libitum for 50 days. The PGRs treatments caused different effects on the antioxidant defense systems and MDA content of dosed rats compared to controls. The lipid peroxidation end product MDA significantly increased in the lungs, heart and kidney of rats treated with GA₃ without significant change in the spleen. ABA caused also a significant increase in MDA content in the spleen, lungs, heart and kidney. The GSH levels were significantly depleted in the spleen, lungs and stomach of rats treated with ABA without any change in the tissues of rats treated with GA₃ except the kidney where it increased. Antioxidant enzyme activities such as SOD significantly increased in the lungs and stomach and decreased in the spleen and heart tissues of rats treated with GA₃. Meanwhile, SOD significantly decreased in the spleen, heart and kidney and increased in the lungs of rats treated with ABA. While CAT activity significantly decreased in the lungs of rats treated with GA₃, a significant increase occurred in the heart of rats treated with both PGRs. On the other hand, the ancillary enzyme GR activity in the tissues were either significantly depleted or not changed with PGRs treatment. The drug metabolizing enzyme GST activity significantly decreased in the lungs of rats treated with ABA but increased in the stomach of rats treated with both PGRs.

As a conclusion, the rats resisted oxidative stress via the antioxidant mechanism. But the antioxidant mechanism could not prevent the increases in lipid peroxidation in rat's tissues. This data, along with changes, suggests that PGRs produced substantial systemic organ toxicity in the spleen, lungs, stomach, heart and kidney during a 50-day period of subchronic exposure.     

Application of gibberellic acid to the surface of soybean seed (t Glycine max (L.) Merr.) and symbiotic nodulation,plant development,final grain and protein yield under short season conditions

In short-season soybean production areas, low soil temperature is the major factor limiting soybean establishment, nodulation and nitrogen fixation. Gibberellic acid (GA) pretreatment of crop seeds can overcome low soil temperature inhibition of seed germination and seedling development. However, previous studies have found that the application of GAs decreased legume nodulation and nitrogen fixation under optimal growth conditions. A field experiment was conducted under short season conditions in eastern Canada to determine whether the application of GA₃ to soybean seed could accelerate germination, and increase plant nodulation and nitrogen fixation. The results indicated that GA₃ application accelerated seedling emergence but decreased plant nodulation and nitrogen accumulation at early plant growth stages. However, these initial negative effects were overcome as the plants developed. Gibberellic acid applied to soybean seed at the time of planting did not influence final grain and protein yield.     

Rapid and repetitive plant regeneration in sweetpotato via somatic embryogenesis

An efficient in vitro plant regeneration system characterized by rapid and continuous production of somatic embryos using leaf and petiole expiants has been developed in sweetpotato [Ipomoea batatas L. (Lam.)]. The optimal somatic embryogenic response was obtained in the genotype PI 318846-3 with a two-step protocol: (1) stage I-incubation of expiants in the dark for 2 weeks on Murashige Skoog (MS) medium containing 2,4-dichlorophenoxyacetic acid (2,4-D) (2.5 mg/l) and 6-benzylaminopurine (0.25 mg/l) and, (2) stage II-culture in the light on MS medium with abscisic acid (ABA) (2.5 mg/l). The addition of ABA was critical for enhanced production of somatic embryos. Secondary somatic embryos were produced from the primary embryos cultured on MS medium with 2,4-D at 0.2 mg/l. The somatic embryos were converted into normal plantlets when cultured on basal MS medium. Upon transfer to soil, plants grew well and appeared normal with no mortality. The system of somatic embryogenesis described here will facilitate tissue culture, germplasm conservation and gene transfer research of sweetpotato due to its rapidity (6 to 10 weeks), prolific plant production by direct embryogenesis, ease of secondary somatic embryo production and reproducibility.Abbreviations ABA abscisic acid - BAP 6-benzylaminopurine, 2,4-D-2,4-dichlorophenoxyacetic acid - GA₃ gibberellic acid - KIN kinetin - MS medium of Murashige and Skoog (1962) - NAA 1-naph-thaleneacetic acid - PIC picolinic acid - TDZ thidiazuron     

Role of some growth regulators on cytogenetic activity of barley under salt stress

The effects of gibberellic acid (GA₃), kinetin (KIN), benzyladenine and ethylene (E) on mitotic activity and chromosomal aberrations in root tips of barley seeds (Hordeum vulgare L. cv. “Bülbül 89”) germinated under salt stress were investigated. It was determined that all of these plant growth regulators (PGRs) decreased mitotic index in root tips of barley seeds germinated at 20 °C and in distilled water. Furthermore, some of the PGRs studied increased significantly the frequency of chromosomal aberrations. The frequency of chromosomal aberrations in seeds treated with E and KIN was considerably higher than in the seeds germinated under nonstress conditions. The inhibitory effect of salt stress on mitotic index increased with increasing salt concentration (0.30, 0.35, 0.40 and 0.45 molal, m). GA₃ and KIN pretreatments showed a successful performance in ameliorating the negative effects of increasing salinity on mitotic activity. The number of chromosomal aberrations also increased with increasing NaCl concentration. However, most of the PGR pretreatments studied alleviated the detrimental effects of increasing salinity on chromosomal aberrations. KIN pretreatment at 0.30 and 0.35 m salinity could not rescued the cytogenetic activity of salt stress on this parameter.     

The effect of plant growth regulators on growth, morphology and condensed tannin accumulation in transformed root cultures of Lotus corniculatus

This study concerns the effects of four different classes of plant growth regulators on root morphology, patterns of growth and condensed tannin accumulation in transgenic root cultures of Lotus corniculatus L. (Bird's-foot trefoil). Growth of transformed roots in 2,4-dichlorophenoxyacetic acid (2,4-D) resulted in decreased tannin levels relative to controls at concentrations of 10^-6 M and above, while gibberellic acid (GA₃) inhibited tannin accumulation at concentrations of 10^-7 M and above. Benzyladenine (BA) had little effect at low concentrations (10^-7 M and below) but resulted in an increase in tannin levels at 10^-6 M. Abscisic acid had little effect on levels of condensed tannins at any of the concentrations used. Experiments involving growth regulator addition and medium transfer demonstrated that 2,4-D inhibition of tannin accumulation could be reversed by GA₃ and BA, while GA₃ downregulation could only be reversed by the addition of 2,4-D. Although 2,4-D inhibited tannin accumulation, addition of 2,4-D to root cultures grown for 14 or 28 days in the absence of plant growth regulators stimulated both growth and tannin biosynthesis. Characteristic alterations in root morphologies accompanied growth regulator-mediated modulation of tannin biosynthesis. Growth in 2,4-D resulted in partially de-differentiated root cultures while growth in GA₃ produced roots with an elongated phenotype. Restoration of tannin biosynthesis in 2,4-D-treated roots was accompanied by root re-differentiation and the production of new lateral roots.Abbreviations ABA abscisic acid - BA benzyladenine - 2,4-d 2,4-dichlorophenoxyacetic acid - GA₃ gibberellic acid 3 - FW fresh weight     

Control of protein synthesis in barley aleurone layers by the plant hormones gibberellic acid and abscisic acid

The patterns of protein synthesis in barley aleurone layers treated with gibberellic acid (GA₃) and abscisic acid (ABA) are compared with the patterns observed in wheat germ in vitro translation assays directed by RNA isolated from similarly treated layers. When used alone, GA₃ and ABA both induce the formation of new translatable mRNAs and cause new proteins to be synthesized. The effects of GA₃ are more dramatic than those of ABA. In GA₃-treated tissues, overall protein synthesis is redirected to produce large quantities of α-amylase and a few other GA₃-induced proteins, while other protein synthesis is reduced or stopped. Large amounts of new translatable mRNA for α-amylase are also induced such that the dominant in vitro translation product is α-amylase. These changes are blocked by the simultaneous addition of ABA to the tissue. In GA₃ plus ABA-treated layers, few changes in protein synthesis in vivo are observed when compared to protein synthesis in untreated tissue, although the induction of mRNA for α-amylase and the other GA₃-induced mRNAs does occur. This indicates that ABA does not interfere with GA₃ induction of translatable mRNAs but prevents the translation of these mRNAs in vivo. Thus ABA and potentially GA₃ regulate the translation of proteins in vivo in barley aleurone layers.     

Carbonic Anhydrase,Photosynthesis, and Seed Yield in Mustard Plants Treated with Phytohormones

The leaves of 30-d-old plants of Brassica juncea Czern & Coss cv. Varuna were sprayed with 10^–6 M aqueous solutions of indole-3-yl-acetic acid (IAA), gibberellic acid (GA₃), kinetin (KIN), and abscisic acid (ABA) or 10^–8 M of 28-homobrassinolide (HBR). All the phytohormones, except ABA, improved the vegetative growth and seed yield at harvest, compared with those sprayed with deionised water (control). HBR was most prominent in its effect, generating 32, 30, 36, 70, 25, and 29 % higher values for dry mass, chlorophyll content, carbonic anhydrase (E.C. 4.2.1.1) activity, and net photosynthetic rate in 60-d-old plants, pods per plant, and seed yield at harvest, over the control, respectively. The order of response to various hormones was HBR > GA₃ > IAA > KIN > control > ABA.     

Interrelationship between ethylene and growth regulators in the senescence of lettuce leaf discs

The interrelationship between ethylene and growth regulators in the senescence of romaine lettuce (Lactuca sativa L.) leaves was studied. Gibberellic acid (GA₃), kinetin, and 3-indoleacetic acid (IAA) retarded chlorophyll loss from leaf discs which were floated on hormone solutions. Abscisic acid (ABA) and ethephon enhanced chlorophyll loss and antagonized the senescence-retarding effect of GA₃ and kinetin. A high concentration of IAA (10^–4 M) caused accelerated chlorophyll loss, whereas a similar concentration of kinetin neither retarded nor promoted chlorophyll loss. The ineffectiveness of IAA and kinetin at their supraoptimal concentrations in retarding leaf senescence was related to increased production of ethylene induced in the treated leaf discs. GA₃ was the most effective in retarding chlorophyll loss and did not stimulate ethylene production at all. The senescence-enhancing effect of ABA was not mediated by ethylene. However, the moderately increased production of ethylene, induced by relatively high concentrations of ABA, could act synergistically with the latter to accelerate chlorophyll loss. It is proposed that the effectiveness of exogenously applied hormones, both in enhancing and retarding senescence, is greatly affected by the endogenous ethylene concentration of the treated plant tissue.Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel, No. 2571-E, 1988 series.     

The effects of timing of N application and plant growth regulators on morphogenesis and yield formation in wheat

The influence of timing of N application and PGRs on wheat morphogenesisand yield formation was studied under non-lodging condition. N treatmentsconsisted of three different proportions applied at sowing, 3-leaf stage, stemelongation stage and booting stage, respectively. PGR treatments were 30ppm uniconazole (S₃₃₀₇), 30 ppmgibberellicacid (GA₃) and a mixture of S₃₃₀₇ and GA₃.S₃₃₀₇ increased tillering, but had no obvious effect on the finalnumber of ears per plant. The combined application of S₃₃₀₇ andGA₃increased ears per plant. S₃₃₀₇ significantly reducedplant height and the length of two basal internodes, the area of two topmostleaves, number of florets per spike, shoot and grain weight per plant in all Ntreatments, particularly in N treatment 6040 (see below), but increased dryweight per unit length of two basal internodes. It is suggested thatapplicationof plant growth retardant such as S₃₃₀₇ may reduce grain yield byinhibiting formation and development of both source (leaf area) and sink(floret) under little or no lodging condition. When applied together,GA₃ modified some inhibition of growth by S₃₃₀₇. Thetiming of N application had significant influence on most of the charactersstudied. The N treatment 6040, in which 60% of total N fertiliser wasapplied at sowing and 40% at stem elongation, produced the greatest earnumber per plant, florets and grains per ear, the largest area of the twotopmost leaves, and the highest grain yield, indicating that more N applicationat stem elongation was favourable to high yield formation.     

Differential synthesis in vitro of barley aleurone and starchy endosperm proteins

To widen the selection of proteins for gene expression studies in barley seeds, experiments were performed to identify proteins whose synthesis is differentially regulated in developing and germinating seed tissues. The in vitro synthesis of nine distinct barley proteins was compared using mRNAs from isolated endosperm and aleurone tissues (developing and mature grain) and from cultured (germinating) aleurone layers treated with abscisic acid (ABA) and GA₃. B and C hordein polypeptides and the salt-soluble proteins β-amylase, protein Z, protein C, the chymotrypsin inhibitors (CI-1 and 2), the α-amylase/subtilisin inhibitor (ASI) and the inhibitor of animal cell-free protein synthesis systems (PSI) were synthesized with mRNA from developing starchy endosperm tissue. Of these proteins, β-amylase, protein Z, and CI- 1 and 2 were also synthesized with mRNA from developing aleurone cells, but ASI, PSI, and protein C were not. CI-1 and also a probable amylase/protease inhibitor (PAPI) were synthesized at high levels with mRNAs from late developing and mature aleurone. These results show that mRNAs encoding PAPI and CI-1 survive seed dessication and are long-lived in aleurone cells. Thus, expression of genes encoding ASI, PSI, protein C, and PAPI is tissue and stage-specific during seed development. Only ASI, CI-1, and PAPI were synthesized in significant amounts with mRNA from cultured aleurone layers. The levels of synthesis of PAPI and CI-1 were independent of hormone treatment. In contrast, synthesis of α-amylase (included as control) and of ASI showed antagonistic hormonal control: while GA promotes and ABA reduces accumulation of mRNA for α-amylase, these hormones have the opposite effect on ASI mRNA levels.     

Plant growth regulators promote lipid and carotenoid accumulation in <Emphasis Type="Italic">Chlorella vulgaris</Emphasis>

Microalgae are photosynthetic organisms with the ability to produce a variety of high-value compounds such as polyunsaturated fatty acids (PUFAs), proteins, pigments, and lipids. The high cost of microalgae production is one of the biggest obstacles for their commercialization. Plant growth regulators might be an ideal choice since they could potentially induce microalgae to produce lipids and other high-value secondary metabolites thereby reducing production cost. This study investigated the effects of eight plant growth regulators (PGRs), namely, salicylic acid (SA); 1-naphthaleneacetic acid (NAA); gibberellin (GA₃); 6-benzylaminopurine (6-BA); 2,4-epibrassinolide (EBR); abscisic acid (ABA); ethephon (ETH); and spermidine (SPD) on the induction of lipids, proteins, carotenoids, and unsaturated fatty acids (UFAs) in Chlorella vulgaris. Moreover, the expression profiles of seven fatty acid biosynthethis genes were studied in the PGR-treated biomass. All PGRs used in the study caused significant increases in total lipid contents in non-dose-dependent manners when compared to control. However, lipid productivities were increased due to four of the eight PGRs (ABA, 6-BA, NAA, and ETH). Similar to lipids, total carotenoid contents were significantly higher in all of the PGR-treated microalgal biomass except ABA. However, soluble protein contents were not affected by the PGR treatments except SA at 10 mg L^?1. Furthermore, 6-BA, NAA, ABA, and ETH treatments resulted in significant increases in UFAs especially DHA, linolenic acid, arachidonic acid, and EPA which were confirmed by the upregulation of fatty acid biosynthesis genes including stearoyl-ACP-desaturase, ω-3 fatty acid desaturase, biotin carboxylase, and acyl-acyl carrier protein. Our findings, therefore, indicate that the treatment with PGR used in this study could be a useful tool to produce biodiesel and other high-value metabolites from microalgal biomass.     

Somatic embryogenesis from mature embryo-derived leaflets of peanut (Arachis Hypogaea L.)

Summary Embryogenic masses were obtained from immature leaves of peanut (Arachis hypogaea L.) cultured on a medium containing 20 mg/l 2,4-D. Somatic embryos developed from these masses following transfer to a medium containing 3 mg/l 2,4-D. The embryo morphology was quite variable. Following transfer to hormone-free medium, these embryos germinated. Shoot elongation was obtained in 25% of the embryos following transfer to a medium supplemented with 0.5 mg/l each of BAP and Kn. The plants grown in vitro by this method survived in sand:soil mixture and were grown to maturity.Abbreviations ABA abscisic acid - BAP 6-benzyl amino purine - 2,4-D 2,4 dichlorophenoxyacetic acid - GA₃ gibberellic acid - Kn kinetin - NAA 1-naphthaleneacetic acid - 2,4,5-T 2,4,5-trichlorophenoxyacetic acid - Z zeatin     

The interactions of plant growth regulators and H<Subscript>2</Subscript>O<Subscript>2</Subscript> during germination improvement of sweet corn seed through spermidine application

Low seed vigor was the main constraint on the production of sweet corn in China. Spermidine (Spd) was proved to enhance sweet corn seed germination. However, little was known about the metabolisms and interactions of plant growth regulators (PGRs) and H₂O₂ in the enhancement of Spd upon sweet corn seed germination. Spd, GA, C₂H₄ and H₂O₂ soaking treatments significantly enhanced seed vigor; while their respective biosynthesis inhibitors and ABA significantly declined seed vigor. Besides, as compared with control, seed vigor showed no significant difference in Spd+ProG (prohexadione-calcium, the inhibitor of GA), however it decreased significantly in Spd+ABA. The seed vigor treated by Spd+AVG (aviglycine hydrochloride, the inhibitor of C₂H₄) and Spd+NAC (n-acetyl-l-cysteine, a scavenger of H₂O₂) were significantly lower than those soaked in Spd solution, but still significantly higher than the control. Spd+NAC with significantly lower H₂O₂ content still up-regulated GA and C₂H₄ contents and down-regulated ABA content during seed germination. The results suggested that it was Spd rather than H₂O₂ (produced through Spd) made a direct effect on PGRs metabolism regulation in seed germination enhancement by Spd. The metabolism of GA and ABA played crucial rolesas compared with C₂H₄ and H₂O₂. Besides, complicated PGRs interactions and crosstalk between H₂O₂ and PGRs existed during sweet corn seed germination after Spd soaking, and ABA might be a key hormone in this process.     

Profile of Plant Hormones and their Metabolites in Germinated and Ungerminated Canola (Brassica napus) Seeds Imbibed at 8°C in either GA4+7, ABA, or a Saline Solution

Abscisic acid (ABA) and gibberellins (GAs) are two major phytohormones that regulate seed germination in response to internal and external factors. In this study we used HPLC-ESI/MS/MS to investigate hormone profiles in canola (Brassica napus) seeds that were 25, 50, and 75% germinated and their ungerminated counterparts imbibed at 8°C in either water, 25 μM GA₄₊₇, a 80 mM saline solution, or 50 μM ABA, respectively. During germination, ABA levels declined while GA₄ levels increased. Higher ABA levels appeared in ungerminated seeds compared to germinated seeds. GA₄ levels were lower in seeds imbibed in the saline solution compared to seeds imbibed in water. Ungerminated seeds imbibed in ABA had lower GA₄ levels compared to ungerminated seeds imbibed in water; however, the levels of GA₄ were similar for germinated seeds imbibed in either water or ABA. The ABA metabolites PA and DPA increased in seeds imbibed in either water, the saline solution, or ABA, but decreased in GA₄₊₇-imbibed seeds. In addition, ABA inhibited GA₄ accumulation, whereas GA had no effect on ABA accumulation but altered the ABA catabolism pathway. Information from our studies strongly supports the concept that the balance of ABA and GA is a major factor controlling germination.     

Hormones in the grains and roots in relation to post-anthesis development of inferior and superior spikelets in japonica/indica hybrid rice

Grain filling is usually not adequate in later-flowering inferior spikelets in japonica/indica (J/I) hybrid rice (Oryza sativa) although it shows stronger hybrid vigor than indica/indica (I/I) hybrid. This study investigated the potential causes by examining changes in zeatin (Z) + zeatin riboside (ZR), indole-3-acetic acid (IAA), gibberellins (GAs, GA₁ + GA₄), and abscisic acid (ABA) in spikelets and roots during the grain filling period. The inferior spikelets of J/I hybrid exhibited low rate of endosperm cell division and slow grain filling. During the early grain filing period, they had less Z + ZR, IAA, and ABA, but more GAs, than the earlier-flowering superior spikelets. If compared to the inferior spikelets of the I/I hybrid, the J/I inferior spikelets also had less Z + ZR, IAA, and ABA. Rates of endosperm cell division and grain filling were positively and significantly correlated with Z + ZR and ABA contents in both grains and roots or IAA in grains, whereas not significantly correlated with GAs either in grains or roots or IAA in roots. Applications of kinetin, IAA, or ABA to spikelets, or kinetin and ABA to roots, enhanced cell division and grain filling in the inferior spikelets. Results suggest that low contents of cytokinins and ABA in both grains and roots and low contents of IAA in grains may result in the poor filling of inferior spikelets in the J/I hybrid.     

Interspecific Competition for Soil N and its Interaction with N2 Fixation, Leaf Expansion and Crop Growth in Pea–Barley Intercrops

Field experiments were carried out during three successive years to study through a dynamic approach the competition for soil N and its interaction with N₂ fixation, leaf expansion and crop growth in pea–barley intercrops. The intensity of competition for soil N varied between experiments according to soil N supply and plant densities. This study demonstrates the key role of competition for soil N which occurs early in the crop cycle and greatly influences the subsequent growth and final performance of both species. Relative yield values for grain yield and N accumulation increased with the intensity of competition for soil N. Barley competed strongly for soil N in the intercrop. Its competitive ability increased steadily during the vegetative phase and remained constant after the beginning of pea flowering. The period of strong competition for soil N (500–800 degree-days after sowing) also corresponded to the period of rapid growth in leaf area for both species and therefore an increasing N demand. For each species, the leaf area per plant at the beginning of pea flowering was well correlated with crop nitrogen status. Barley may meet its N needs more easily in intercrops (IC) and has greater leaf area per plant than in sole crops (SC). Barley having a greater soil N supply results in an even higher crop N status and greater competitive ability relative to pea in intercrop. Competition by barley for soil N increased the proportion of pea N derived from fixation. The nitrogen nutrition index (NNI) values of pea were close to 1 whatever the soil N availability in contrast to barley. However N₂ fixation started later than soil N uptake of pea and barley and was low when barley was very competitive for soil N. Due to the time necessary for the progressive development and activity of nodules, N₂ fixation could not completely satisfy N demand at the beginning of the crop cycle. The amount of N₂ fixed per plant in intercrops was not only a response to soil N availability but was largely determined by pea growth and was greatly affected when barley was too competitive.     

Somatic embryogenesis in Cucurbitaceae

Strategies based on the application of biotechnologies to crop improvement programmes generally require regeneration of whole plants from cells or tissues cultivated in vitro. In Cucurbitaceae, regeneration can occur either through a caulogenic or an embryogenic developmental pathway. Reports of somatic embryogenesis have dealt with the main cultivated crops, i.e. cucumber, melon, squash and watermelon. Somatic embryogenesis and plant recovery are obtained from numerous sources including protoplasts, but the best results are observed with explants coming from seedlings, especially cotyledons and hypocotyls. The genetic constitution of mother plants also seems to play a key role in the success of embryogenesis, but few systematic studies on genotype effect have been published. Somatic embryos can exhibit developmental abnormalities, particularly when they arise from protoplast-derived cultures. Generally, data concerning embryo yield, rate of germination and plant development and characteristics of regenerated plants and their progeny, has not been provided in previous reports. The potential use of somatic embryogenesis in cucurbit breeding programmes is stressed in this review.Abbreviations ABA abscisic acid - BAP 6-benzylaminopurine - B₅ Gamborg et al. (1968) - CH casein hydrolysate - CW coconut water - 2,4-d 2,4-dichlorophenoxy acetic acid - GA₃ gibberellin A₃ - GA₄ gibberellin A₄ - H Heller (1953) - IAA indole 3 acetic acid - IBA indole 3 butyric acid - 2ip 2 isopentenyladenine - KIN kinetin - MS Murashige & Skoog (1962) - N Nitsch (1951) - N6 Chu et al. (1975) - NAA 1 naphthalene acetic acid - TDZ thidiazuron - 2,4,5-T 2,4,5-trichlorophenoxyacetic acid