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  • Electronic Resource  (5)
  • steroidogenesis  (3)
  • Belize  (2)
  • 1
    Electronic Resource
    Electronic Resource
    Nitrogen distribution in hybrid and local corn varieties and its possible relationship to a declining soil nitrogen pool under shifting agriculture at Indian Church, Belize (1982)
    Springer
    Plant and soil 67 (1982), S. 119-127 
    Details
    ISSN: 1573-5036
    Keywords: Belize ; Corn ; N-allocation ; N-cycling ; P-allocation ; Traditional agriculture
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Description / Table of Contents: Resumen Se determinaron los niveles de nitrógeno en maiz local, híbrido y malezas durante tres años. Las pérdidas de nitrógeno por cosecha del grano fueron 45% del nitrógeno total asimilado por la milpa. Los porcentajes de N y P en el híbrido y en el maiz negro local fueron también determinados observándose tasas de absorción sigmoidales en las raices y hojas de ambas variedades hasta alcanzar el periodo reproductivo. Luego el porcentaje de estos nutrimentos bajó rápidamente en las hojas. Los niveles en las semillas maduras en el híbrido bajaron a 78% y 52% de los valores iniciales de N y P respectivamente. mientras que en la variedad local éstos bajaron a 58% y 50% respectivamente. El maiz híbrido no se consideró apropiado para los campesinos locales debido al alto porcentaje de nitrógeno total extraido por la cosecha además de la suceptibilidad del grano cosechado a las plagas durante el almacenamiento.
    Notes: Abstract Nitrogen levels at harvest in hybrid corn and weeds were determined over a three year period. Nitrogen losses due to grain removal accounted for 45% of all assimilated-N in the milpa. Percent N and percent P levels in the hybrid and a local black corn were also determined. Uptake of nitrogen and phosphorus in roots and leaves of both varieties were sigmoid until reproductive growth was initiated, at which time the percentage of both nutrients in the leaves declined rapidly. Levels in the seed at maturity in the hybrid variety accounted for 78% N and 52% P; levels were 58% N and 50% P in the local corn. Hybrid corn was not considered suitable for local farmers because a high percentage of total-N was removed at harvest and the hybrid had a high susceptibility to weevil attack.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02182760
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  • 2
    Electronic Resource
    Electronic Resource
    Nitrogen cycling in the seasonally dry forest zone of Belize, Central America (1982)
    Springer
    Plant and soil 67 (1982), S. 333-342 
    Details
    ISSN: 1573-5036
    Keywords: Agriculture ; Belize ; Cohune palm ; Dry forest ; Fallow ; Hardwood ; Litter ; N-cycling
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Description / Table of Contents: Resumen Dos asociaciones boscosas (Palma Cohune y Matorral Alto) se estudiaron en relación a sus reservas y flujos de nutrimentos y sus posibilidades de aprovechamiento agricola. Se siguió la evolución del nitrógeno en el suelo y en la caida de hojarasca durante un año obteniéndose asi información sobre los procesos pedogenéticos en el barbecho del bosque. A través de muestreos destructivos se encontró que en la biomasa del bosque de Palma Cohune habían 690 kg N ha−1 y en el Matorral Alto solo 203 kg N ha−1. La biomasa de hojarasca era excepcionalmente alta en Palma Cohune alcanzando un valor de 497 kg materia seca ha−1; en el bosque de Matorral Alto la hojarasca era de 65 kg materia seca ha−1. Esto probablemente se deba a la baja velocidad de descomposición en el caso de la palma. Las reservas sustanciales encontradas en ambos barbechos para el nitrógeno podrían ser parcialmente utilizadas para la producción agrícola campesina.
    Notes: Abstract Two forest associations, cohune palm (Cohune Ridge) and mixed tropical hardwood (High Bush), were assessed on the basis of nutrient movement and storage for their suitability for agriculture. Continuous monitoring of soil nitrogen and leaf litterfall over a one-year period provided information on soil building processes in the forest fallow. Destructive cuts revealed the storage of 690 kg N ha−1 in the standing biomass of the Cohune forest versus 203 kg N ha−1 in the High Bush. Litter biomass was exceptionally high in the Cohune Ridge (497 kg ha−1 dry matter) as compared to the High Bush (65 kg ha−1 dry matter) and other tropical forests. This is probably because of a low rate of decomposition in the Cohune Ridge palm forest. A substantial reserve of nitrogen is present in both forests' fallows, and this can in part be harvested by the small farmer for crop production.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02182780
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  • 3
    Electronic Resource
    Electronic Resource
    Gonadal steroidogenesis and the possible role of steroid glucuronides as sex pheromones in two species of teleosts (1986)
    Springer
    Fish physiology and biochemistry 2 (1986), S. 101-107 
    Details
    ISSN: 1573-5168
    Keywords: Brachydanio rerio ; Clarias gariepinus ; teleosts ; ovary ; testis ; seminal vesicles ; steroidogenesis ; steroid glucuronides ; sex pheromones
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract In general, female zebrafish,Brachydanio rerio, ovulate only in the presence of males. The stimulant must be pheromonal as even male holding water is capable of inducing ovulation. After ovulation the mating phase begins. During this phase the male follows the female and oviposition as well as fertilization takes place. Both the ovulation and the mating are controlled by pheromones synthesized by the gonads. Ovulation can be induced by testicular homogenates. After the lipid material has been extracted from the testicular homogenates, the remaining aqueous phase can still induce ovulation. However, when the aqueous phase is treated with the enzymeβ-glucuronidase, it loses the ability to induce ovulation. This is an indication that glucuronides, probably steroid glucuronides, are the compounds responsible. During the mating phase, ovulated female zebrafish become attractive to males. It was found that, after ovulation, ovarian extracts contain the compounds responsible for attracting males. The attractant consists of a mixture of steroid glucuronides. After incubation of the gonads with3H-precursors seven steroid glucuronides have been identified in the testis and five in the ovary. Under fish culture conditions the African catfish,Clarias gariepinus, can produce postivitellogenic oocytes throughout the year. However, in capitivity neither males nor females spawn. In female catfish maturation and ovulation can be induced by treatment with gonadotropins. It might be possible that, analogous to the zebrafish, some reproductive processes in the catfish have to be induced by pheromones. It has been demonstrated that pheromonal compounds released by the seminal vesicles are involved in the attraction of female conspecifics. The steroid glucuronide synthesizing capability of the testes and the seminal vesicles of the male catfish are examined, as well as that of the ovary before and after ovulation of the female catfish. Both testes and seminal vesicles appear to be capable of steroid biosynthesis but only the latter synthesizes steroid glucuronides. Six of these conjugates have been isolated and identified. In the female catfish the ovaries are capable of synthesizing seven steroid glucuronides, but only after ovulation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02264077
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  • 4
    Electronic Resource
    Electronic Resource
    Pubertal development of male African catfish,Clarias gariepinus. In vitro steroidogenesis by testis and interrenal tissue and plasma levels of sexual steroids (1997)
    Springer
    Fish physiology and biochemistry 16 (1997), S. 129-138 
    Details
    ISSN: 1573-5168
    Keywords: Clarias gariepinus ; African catfish ; teleost fish ; puberty ; spermatogenesis ; steroidogenesis ; testis ; interrenal tissue ; androgen blood levels
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract In fish, sex steroids initiate and/or accelerate the maturation of the brain-pituitary-gonad axis. In order to obtain information on the steroid milieu during the pubertal development of male African catfish, we have monitored the conversion of [3H]-pregnenolone and [3H]-androstenedione by testis and [3H]-pregnenolone by interrenal tissue fragmentsin vitro. Pubertal development occurs between two and six months of age. Testicular development proceeds through four main stages that are characterised histologically by the presence of spermatogonia (stage I), spermatogonia and spermatocytes (stage II), spermatogonia, spermatocytes and spermatids (stage III), and all germ cells including spermatozoa (stage IV). 11β-Hydroxyandrostenedione and cortisol were the main products of testes and interrenal tissue, respectively, in all stages of the pubertal development; a change in the steroidogenic pattern was not observed during this period. The interrenal tissue displayed no significant conversion of [3H]-pregnenolone to 11-oxygenated androgens. Blood plasma was analyzed for the presence of five androgens; testosterone, 11β-hydroxytestosterone, 11β-hydroxyandrostenedione, androstenetrione, and 11-ketotestosterone. 11-Ketotestosterone was the quantitatively dominating androgen in the circulation at all stages of development, which was more pronounced in stages III and IV. The obvious differences between thein vitro andin vivo results, namely 11β-hydroxyandrostenedione being the main testicular productvs. 11-ketotestosterone dominating in the blood, may indicate that 11β-hydroxyandrostenedione is converted to 11-ketotestosterone at extratesticular sites.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00004670
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  • 5
    Electronic Resource
    Electronic Resource
    Steroidogenesis during induced oocyte maturation and ovulation in the African catfish,Clarias gariepinus (1989)
    Springer
    Fish physiology and biochemistry 6 (1989), S. 61-78 
    Details
    ISSN: 1573-5168
    Keywords: oocyte maturation ; ovulation ; steroidogenesis ; teleosts
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Changes in ovarian steroidogenesis accompanying oocyte maturation and ovulation were studied in the African catfish,Clarias gariepinus. Laboratory-reared females with postvitellogenic ovaries were treated with pimozide and LHRH-analogue. The plasma gonadotropin levels were determined by means of a homologous radioimmunoassay, the condition of the ovaries was studied by histological examination of the follicles, and the steroidogenetic capacity of the ovaries was analyzed byin vitro incubation of tissue fragments for 3 h with [3H]-pregnenolone and [3H]androstenedione as precursors. Data were collected at regular intervals between 0 and 16 h after pimozide-LHRH analogue administration. Until 4 h after the beginning of the experiments the plasma gonadotropin levels did not rise above 25 ng/ml, the ovaries remained in the stage of postvitellogenesis, and testosterone was the main end product of steroidogenesis. Four hours later the gonadotropin concentration in the blood had risen to more than 150 ng/ml, and the ovaries had entered the stage of germinal vesicle migration. At the same time steroidogenesis shifted towards the production of 17α,20β-dihydroxy-4-pregnen-3-one, 5β-pregnane-3α, 17α-diol-20-one, 5β-pregnane-3α,6α,17α-triol-20-one, 5β-pregnane-3α,17α,20β-triol and 5β-pregnane-3α,6α,17α,20β-tetrol. During the subsequent stage of germinal vesicle breakdown the plasma gonadotropin level remained high, and the synthesis of the C21-steroids showed a further increase. Simultaneously, the production of some C19-steroid glucuronides was enhanced. The preovulation and especially the postovulation stages were accompanied by a gradual decrease in steroidogenic capacity of the ovaries, even though the plasma gonadotropin level remained high. It is concluded that the prematuration surge of gonadotropin influences the activity of enzymes involved in steroidogenesis, leading to a reduced C17,20-lyase and to an augmented activity of the enzymes 20β-hydroxysteroid dehydrogenase (HSD), 5β-reductase, 3α-HSD, 6α-hydroxylase and UDP-glucuronosyltransferase. During ovulation the activity of all steroidogenic enzymes, including such key enzymes as 3β-HSD and 17α-hydroxylase, gradually decreases. Not only 17α,20β-dihydroxy-4-pregnen-3-one, but also the 5β-reduced pregnanes may be involved in inducing oocyte maturation and/or ovulation. The very polar triol and tetrol products may function, together with the steroid glucuronides as sex pheromones.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01875480
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