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81.
Two-month-old jack pine ( Pinus banksiana Lamb.) seedlings were placed in a greenhouse where both nitrogen source and light level were varied. After 4 months, whole seedling biomass, leaf biomass and relative growth rate were greatest in seedlings grown with NH+4/NO/NO3-N and full light (FL) and least in seedlings grown with NO 3-N and low light (LL). NO 3-seedlings grown under full light and NH+4/NO3-seedlings grown under low light were approximately equal. This indicates that the extra carbon costs of assimilating only NO3-N were similar to the reduction of carbon fixation resulting from a 50% decrease in photon flux density. Percentage and total nitrogen content of needles were greater in seedlings grown under low light independent of nitrogen fertilization. Percentage and total nitrogen content of roots were higher under low light and lower when fertilized with NO3.
Nitrate reductase (NR) activity was higher in roots than in needles, while glutamine synthetase (GS) activity was higher in needles than in roots. Low light resulted in decreased NR activity (mg N)−1 in needles, but not in roots. However, no nitrate was detected in the needles in any treatment. GS activity, on the other hand, was greater under low light in both needles and roots. GS activity in needles is most likely involved with the reassimilation rather than the initial assimilation of ammonium. Some implications of these shifts in enzymatic activity for ecological phenomena in forests are discussed.  相似文献   
82.
Leaf senescence and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBP carboxylase, EC 4.1.1.39) degradation in orange [ Citrus sinensis (L.) Osbeck cv. Washington Navel] explants have been investigated. Explants consisted of a segment of stem (ca 15 cm) and 5 mature leaves. In vitro RuBP carboxylase degradation was determined by culturing the explants in water for different periods of time (3 days usually) and quantifying the two RuBP carboxylase subunits in the extracts following sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). In vitro RuBP carboxylase degradation was estimated by autodigestion of leaf extracts and SDS-PAGE. The extent of in vivo RuBP carboxylase degradation in explants cultured under 16 h light/8 h dark photoperiod varied throughout the year and showed a cyclic behaviour correlated with the growth cycle of Citrus. The highest proteolytic activity both in vivo and in vitro was found in explants made from April to August coinciding with the maximum vegetative growth period of the tree.
Leaf senescence and abscission could be retarded significantly at any time of the year by maintaining the explants continuously in the dark. Treatment of the explants in the dark with a continuous flow of ethylene enhanced both leaf abscission and rate of RuBP carboxylase degradation, proportionally to ethylene concentration (0.1-0.6 ppm). Ethylene-induced senescence of Citrus leaf explants in the dark appears to be a convenient model system to study the regulation of the proteolytic degradation of RuBP carboxylase.  相似文献   
83.
Summary The effects of CO2 enrichment on the growth, biomass partitioning, photosynthetic rates, and leaf nitrogen concentration of a grass, Bromus mollis (C3), were investigated at a favorable and a low level of nitrogen availability. Despite increases in root: shoot ratios, leaf nitrogen concentrations were decreased under CO2 enrichment at both nitrogen levels. For the low-nitrogen treatment, this resulted in lower photosynthetic rates measured at 650 l/l for the CO2-enriched plants, compared to photosynthetic rates measured at 350 l/l for the non-enriched plants. At higher nitrogen availability, photosynthetic rates of plants grown and measured at 650 l/l were greater than photosynthetic rates of the non-enriched plants measured at 350 l/l. Water use efficiency, however, was increased in enriched plants at both nitrogen levels. CO2 enrichment stimulated vegetative growth at both high and low nitrogen during most of the vegetative growth phase but, at the end of the experiment, total biomass of the high and low CO2 treatments did not differ for plants grown at low nitrogen availability. While not statistically significant, CO2 tended to stimulate seed production at high nitrogen and to decrease it at low nitrogen.  相似文献   
84.
Seed dimorphism in Salicornia europaea: Nutrient reserves   总被引:1,自引:0,他引:1  
Median and lateral seeds of Salicornia europaea L. were separately analysed for their sizes and nutrient reserves. The mean air-dry weight of a single median and lateral seed was 0.31 and 0.25 mg, respectively. The composition as well as the concentration of the nutrient reserves were similar in both seed types. The bulk of the cations was derived from K+, followed by Mg2+, Na+ and Ca2+. The chloride content was somewhat higher than the sodium content, and phosphate was equalled by acid soluble Ca2+ and Mg2+. Starchy compounds and sucrose were present in equal amounts, each of them accounted for about 50% of the carbohydrates. Glucose and fructose were less than 1%. Protein-nitrogen (ethanol-insoluble N) was about 34 g (kg dry seeds)−1. About 7 g (kg dry seeds)−1 was ethanol-soluble nitrogen, of which 10% was derived from amino acids. The total lipid content was more than 290 g (kg dry seeds)−1, 65% were calculated to be glycerides. More than 90% of the fatty acids consisted of linoleic and oleic acids, the majority (72%) of which was linoleic acid.  相似文献   
85.
Six Argentinian wheat ( Triticum aestivum L.) cultivars grown in nutrient solutions in controlled environment were compared for their nitrate uptake rates on a root dry weight basis. Up to 3-fold differences were observed among the cultivars at 16, 20 and 24 days from germination, either when measured by depletion from the nutrient solution in short-term experiments, or by total N accumulation in the tissue during 8 days.
No differences in total N concentration in root or shoots were found among cultivars. Although the different cultivars showed significant differences in shoot/root ratio and nitrate reductase activity (EC 1.6.6.1) in the roots, none of these parameters was correlated with the nitrate uptake rate. However, nitrate uptake was found to be positively correlated (r = 0.99) with the shoot relative growth rate of the cultivars. The three cultivars with the highest nitrate uptake rates and relative growth rates showed a positive correlation between root nitrate concentration and uptake. However, this correlation was not found in the cultivars with the lowest growth and uptake rates.
Our results indicate that the difference in nitrate uptake rate among these cultivars may only be a consequence of their differences in growth rate, and it is suggested that at least two mechanisms regulate nitrate uptake, one working when plant demand is low and another when plant demand is high.  相似文献   
86.
Carbon and nitrogen partitioning was examined in a wild-type and a nitrate reductase-deficient mutant (A317) of Pisum sativum L. (ev. Juneau), effectively inoculated with two strains of Rhizobium leguminosarum (128C23 and 128C54) and grown hydroponically in medium without nitrogen for 21 days, followed by a further 7 days in medium without and with 5 mM NH4NO3. In wild-type symbioses the application of NH4NO3 significantly reduced nodule growth, nitrogenase (EC 1.7.99.2) activity, nodule carbohydrates (soluble sugars and starch) and allocation of [14C]-labelled (NO3, NH4+, amino acids) in roots. In nodules, there was a decline in amino acids together with an increase in inorganic nitrogen concentration. In contrast, symbioses involving A317 exhibited no change in nitrogenase activity or nodule carbohydrates, and the concentrations of all nitrogenous solutes measured (including asparagine) in roots and nodules were enhanced. Photosynthate allocation to the nodule was reduced in the 128C23 symbiosis. Nitrite accumulation was not detected in any case. These data cannot be wholly explained by either the carbohydrate deprivation hypothesis or the nitrite hypothesis for the inhibition of symbiotic nitrogen fixation by combined nitrogen. Our result with A317 also provided evidence against the hypothesis that NO3 and NH4+ or its assimilation products exert a direct effect on nitrogenase activity. It is concluded that more than one legume host and Rhizobium strain must be studied before generalizations about Rhizobium /legume interactions are made.  相似文献   
87.
Maize (Zea mays L.) and ricebean (Vigna umbellata [Thumb.] Ohwi and Ohashi) were grown in intercrop and monoculture on Tropaqualf soils under rainfed conditions in Northern Thailand yearly from 1983 to 1986. De Wit's replacement design was used to compare intercrops and monocultures with a constant plant density equivalent to 80 000 maize or 160 000 ricebean plants ha−1. Combined nitrogen was applied at varying levels to 200 kg N ha−1. In the final two seasons the intercrop ratio of maize: ricebean was also varied. At the time of maize maturity intercrops yielded upt 49 kg ha−1 more N in the above ground plant parts than the best monoculture. Dry matter, grain and nitrogen yield of maize and ricebean in intercrop relative to their monoculture yields (RY, relative yield) were significantly greater than their respective share of the plant population. Relative yield totals (RYT) for grain, dry matter and nitrogen were always greater than 1. Nitrogen uptake per maize plant increased with progressive replacement of maize by ricebean plants. This increase was similar to that obtained by applying combined N. Available soil nitrogen tended to decrease with increasing maize:ricebean ratio. Increasing the maize:ricebean ratio increased the % of nitrogen derived from fixation in ricebean, the increase being equivalent to that obtained by decreasing combined nitrogen application. Approximately the same amount of fertilizer and soil nitrogen was taken up by maize plus ricebean in intercrop as the maize monoculture. The results suggest that the improved nitrogen economy of the intercrop resulted from the strong competitiveness of maize in the use of mineral nitrogen and the enhancement of nitrogen fixation in intercropped ricebean which made it less dependent on the depleted pool of soil nitrogen.  相似文献   
88.
Soon  Y. K. 《Plant and Soil》1988,109(2):171-179
A field study with barley was conducted in 1984 and 1985 to provide data on uptake rates of N, P, K and Mg and their variation as the growing season progressed. Two varieties were grown: Galt in 1984 and Otal in 1985. Soil fertility was maintained at or near optimum conditions. Samples were obtained approximately every 10 days for shoot dry weight, nutrient content and root length measurements. The approximate method (Williams, 1948) traditionally used for calculating uptake rates was found to be invalid for most of the nutrients studied. The method used for measuring uptake rates was the functional approach proposed by Hunt (1973). Inflow,i.e. uptake rate per unit root length, of plant nutrients, decreased with time. However, maximum uptake rates measured in kg ha–1d–1 occurred at about 50 days from sowing because of increasing root length density with time. Inflow or uptake rates were low in 1985 because of moisture deficiency, and grain yield (0.89 t ha–1) was severely depressed. This study demonstrated that Hunt's method is superior and more advantageous than the traditional, approximate method.  相似文献   
89.
The effects of soil acidity on the growth and N2-fixing activity of white clover in seven acid topsoils and subsoils of New Zealand were investigated using a glasshouse experiment.The application of phosphate (Ca(H2PO4)2) to the soils resulted in very large increases in white clover growth on all soils. The application of phosphate, as well as increasing P supply, also decreased 0.02M CaCl2-extractable Al levels, but had little effect on exchangeable Al levels.Where adequate phosphate was applied, increasing rates of lime (CaCO3) resulted in increased plant growth on most soils. N2[C2H2]-fixing activity was increased by the first level of lime for one soil, but generally remained approximately constant or declined slightly at higher rates of lime. Up to the point of maximum yield, white clover top weight was more highly correlated with 0.02M CaCl2-extractable soil Al than with exchangeable Al or pH. At pH values greater than 5.5, plant yield declined on some soils, apparently because of Zn deficiency. The data suggest that white clover is unlikely to be affected by Al toxicity at 0.02M CaCl2-extractable Al levels of less than about 3.3 g g–1. However, there were differences between soils in apparent plant tolerance to 0.02M CaCl2-extractable Al, which appeared to be caused by differing C levels in the 0.02M CaCl2 extracts.  相似文献   
90.
The low gas permeability of a diffusion barrier in the cortex of soybean nodules plays a significant role in the protection of nitrogenase from oxygen inactivation. It may also set an upper limit on nodule respiration and nitrogen fixation rates. Two methods which have been used to quantify the gas permeability of leguminous nodules are reviewed and found to be unreliable. A new assay technique for determining both the nodule activity and gas permeability is developed and tested. This ‘lag-phase’ assay is based on the time nodules require to reach steady-state ethylene production after being exposed to acetylene. The technique is rapid, insensitive to errors in biochemical parameters associated with nitrogenase, and is non-destructive. The method was tested with intact aeroponically grown soybean plants for which the mean nodule gas permeability was found to be 13.3×10−3 mms−1. This corresponds to a layer of cells approximately 35 um thick and is consistent with previously reported values.  相似文献   
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