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The Path of Carbon Flow during NO(3)-Induced Photosynthetic Suppression in N-Limited Selenastrum minutum 总被引:4,自引:4,他引:0 下载免费PDF全文
Nitrate addition to nitrate-limited cultures of Selenastrum minutum Naeg. Collins (Chlorophyta) resulted in a 70% suppression of photosynthetic carbon fixation. In 14CO2 pulse/chase experiments nitrate resupply increased radiolabel incorporation into amino and organic acids and decreased radiolabel incorporation into insoluble material. Nitrate resupply increased the concentration of phosphoenolpyruvate and increased the radiolabeling of phosphoenolpyruvate, pyruvate and tricarboxylic acid cycle intermediates, notably citrate, fumarate, and malate. Furthermore, nitrate also increased the pool sizes and radiolabeling of most amino acids, with alanine, aspartate, glutamate, and glutamine showing the largest changes. Nitrate resupply increased the proportion of radiolabel in the C-4 position of malate and increased the ratios of radiolabel in aspartate to phosphoenolpyruvate and in pyruvate to phosphoenolpyruvate, indicative of increased phosphoenolpyruvate carboxylase and pyruvate kinase activities. Analysis of these data showed that the rate of carbon flow through glutamate (10.6 μmoles glutamate per milligram chlorophyll per hour) and the rate of net glutamate production (7.9 μmoles glutamate per milligram chlorophyll per hour) were both greater than the maximum rate of carbon export from the Calvin cycle which could be maintained during steady state photosynthesis. These results are consistent with the hypothesis that nitrogen resupply to nitrogen-limited microalgae results in a transient suppression of photosynthetic carbon fixation due, in part, to the severity of competition for carbon skeletons between the Calvin cycle and nitrogen assimilation (IR Elrifi, DH Turpin 1986 Plant Physiol 81: 273-279). 相似文献
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Active Uptake of Amino Acids by Leaves of an Epiphytic Vascular Plant, Tillandsia paucifolia (Bromeliaceae) 总被引:1,自引:0,他引:1 下载免费PDF全文
Specialized epidermal trichomes on the leaves of the epiphyte, Tillandsia paucifolia (Bromeliaceae) accumulate amino acids from solution. Simultaneous net uptake of 17 amino acids was determined using high performance liquid chromatography. Uptake occurs against concentration gradients at least as high as 104. 相似文献
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Chlorophyll a Fluorescence Predicts Total Photosynthetic Electron Flow to CO(2) or NO(3)/NO(2) under Transient Conditions 下载免费PDF全文
A model which predicts total photosynthetic electron flow from a linear regression of the relationship between corrected steady-state quantum yield and nonphotochemical quenching (E Weis, JA Berry [1987] Biochem Biophys Acta 894: 198-208) was formulated for N-limited cells of the green alga Selenastrum minutum. Unlike other models based on net CO2 fixation, our model is based on total photosynthetic electron flow measured as gross O2 evolution. This allowed for the prediction of total photosynthetic electron flow from water to both CO2 fixation and NO3−/NO2− reduction. The linear regression equation predicting electron flow is of the form: J = I · Qq[0.4777-0.3282 QNP] (where J = gross photosynthetic electron flow, I = incident PAR, Qq = photochemical quenching, QNP = nonphotochemical quenching). During steady-state photosynthesis, over a range of irradiance, the model predicted a photosynthetic light saturation curve which was well correlated with that observed. Although developed under steady-state conditions, the model was tested during nonsteady-state photosynthesis induced by transient nitrogen assimilation. The model predicted transient rates of gross O2 evolution which were in excellent agreement with the rates observed under a variety of conditions regardless of whether CO2 or NO3−/NO2− served as the physiological electron acceptor. The fluorescence transients resulting from ammonium and nitrate assimilation are discussed with respect to metabolic demands for reductant and ATP. 相似文献
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Gonadotropin releasing hormone (GnRH) treatment was examined as a means of improving the efficacy of embryo collection in the sheep following intrauterine insemination of frozen-thawed semen. In summary, treatment consistently improved fertilization rates and the number of fertilized ova collected per ewe was enhanced compared with untreated ewes. The yield of fertilized ova in ewes treated with follicle stimulating hormone (FSH) was maximized by administering GnRH 36 h after progestagen treatment; 24 h was the preferred time in ewes treated with pregnant mare serum gonadotropin (PMSG). There was a significant (P < 0.001) increase in the percentage of unfertilized ova in the former treatment when GnRH was given at 24 h. An examination of the time of insemination (0, 6, 12 and 18 h before the median time of ovulation) indicated that fertilization rates were highest when insemination occurred at 6 h in both GnRH-treated ewes and in untreated ewes. In GnRH-treated ewes, the recovery of ova was lowest when insemination occurred at the time of ovulation. The number of motile frozen-thawed spermatozoa required for fertilization following treatment was estimated to be approximately 20 x 10(6) per uterine horn. GnRH-treatment also improved the yield of fertilized ova in sheep that were naturally mated, although this yield was lower than that obtained with intrauterine insemination of frozen-thawed semen. It is concluded that fertilization failure, a major problem in sheep embryo collection, can be eliminated through judicious use of GnRH treatment and properly timed intrauterine insemination. 相似文献
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In a study of the time of ovulation following synchronization of estrus in the ewe, the effect of time of treatment with GnRH (24 vs 36 h after pessary removal) and dosage (6.25 to 100 ug per ewe) were examined. All treatments synchronized the time of ovulation irrespective of when untreated ewes commenced to ovulate. As part of an evaluation of GnRH treatment in artificial insemination programs, an assessment was made of the quality of eggs obtained from control ewes and ewes treated with GnRH at either 24 or 36 h after pessary removal. Treatment at 24 h increased the number of retarded embryos (P < 0.01) and unfertilized ova (P < 0.01) collected per ewe, reduced the number of embryos collected per ewe (P < 0.01), and reduced (P < 0.05) the percentage of pregnant ewes compared with other groups. However, there were no differences between control ewes and ewes treated with GnRH at 36 h. GnRH treatment at 36 h was consequently examined as a means of improving conception rates following the intrauterine insemination of frozen-thawed semen. Insemination of GnRH-treated ewes 8 to 12 h before the median time of ovulation resulted in a nonsignificant increase (range 5.7 to 7.3%) in the percentage of ewes of mature age which became pregnant. Insemination 0 to 4 h before the median time of ovulation resulted in a nonsignificant decrease in the percentage of pregnant ewes. GnRH treatment did not influence the number of fetuses per ewe. Reasons for the failure of this treatment to significantly improve ewe fertility are discussed. 相似文献
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