Nitrate Reductase Activity and Polyribosomal Content of Corn ( Zea mays L.) Having Low Leaf Water Potentials

244

The roles that leaf nitrate content and nitrate flux play in regulating the levels of nitrate reductase activity (NRA) In certain experiments, root systems were rapidly (2 min) cooled while shoots remained at growth temperatures. Hoagland solution (8 C) was poured through the Vermiculite until the desired root temperature was reached. The cold-treated pots were then placed into copper coils through which water (3 C) was circulated. Temperature was regulated at 13.5 ± 0.5 C in the root medium by controlling the flow of chilled water using a thermoregulator placed in one of the pots. Control roots were kept at 29 ± 0.5 C throughout the day, but they cooled to 21 + 0.5 C at night as the growth chamber temperature changed.When experiments were done with excised shoots, the plants were placed in darkness for 20 min prior to excision. Whole plants were pulled out of the Vermiculite and cut twice between the roots and the blade of the first leaf under freshly degassed water. The shoots were then placed in appropriate solutions under growth conditions.For uptake experiments with excised shoots, all solutions contained 5 mm K-phosphate at pH 6 and various concentrations of KNO3. When ABA was used, it was obtained from a fresh stock solution of 1 mm ABA prepared by adding 96 ml of H20 to 25 mg of ABA dissolved in 1 ml of absolute ethanol. The pH was adjusted to 6, and the solution was stored in darkness at 4 C.Leaf water potential (NV,,) was measured by thermocouple psychrometry using the isopiestic technique as described by Boyer (5).Extraction and Assay of NRA and Leaf Nitrate Content. Tissue samples (1 g) from all the leaves of several plants were ground for 35 sec with a Polytron (Brinkman Instruments, New York) in 6 ml of extraction medium (15). The homogenate was centrifuged at 27,000g for 15 min, and the supernatant was used to assay both NRA (20) and leaf nitrate content (12). All assays 499 www.plantphysiol.org on May 9, 2018 -Published by Downloaded from

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confidence: 99%

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confidence: 99%

Nitrate Reductase Activity in Maize (Zea mays L.) Leaves

Shaner

Boyer²

1976

244

“…Rapid changes in polyribosome levels are also known to occur in mosses (3,6). In other studies with higher plants, a number of workers have shown that polyribosome levels (4, 12,13) and protein synthesis (7,12,13) are reduced following stress; but in most cases, specific efforts were not made to determine how quickly these changes could occur. The study reported here was initiated to determine if polyribosome levels of several higher plants could be rapidly and consistently reduced by stress and to gain a better understanding of the possible relationships existing among polyribosome changes, cellular water status, and growth rates.…”

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confidence: 99%

Water Stress, Rapid Polyribosome Reductions and Growth

Rhodes

Matsuda²

1976

Measurements of the water status of various plat tissues exposed to differing levels of salts for 1 hour were made using the recently developed Campbell J-14 press (Logan, Utah). Values obtaied with the press were found to correlate well with estimates of relative water content, and experiments with 3-day-old pumpkin seedlings showed that detectable changes in press values of cotyledon tissues could be obtained within 5 minutes folowing salt-or desiccation-induced stress.Pobyiosome levels were measured in tissues from various plant species following short duration water stress. A small reduction in

“…Among the documented effects of water stress are: decreased tissue protein content (5,10), decreased incorporation of amino acids into protein (3,9), decreased nitrate reductase activity (19), increased ribonuclease activity (8,12), dissociation of polyribosomes (13,14), and altered mRNA transcription and translation (16). Changes in the pattern of protein synthesis during water stress have been observed in double-labeling experiments (6, 7) indicating that the effect is selective.…”

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confidence: 97%

“…Morilla et al (19) have shown that nitrate reductase activity in maize is severely inhibited by mild water stress; however, no data are available for nonnodulated hydroponically grown soybeans. The possibility that ovules might be able to reduce nitrate is precluded by the work of Thompson et al (23).…”

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confidence: 99%

Maintenance of Normal or Supranormal Protein Accumulation in Developing Ovules of Glycine max L. Merr. during PEG-Induced Water Stress

Pikaard

Cherry²

1984

Water stress has been shown to alter protein metabolism in vegetative tissues of many plant species (2,15). Among the documented effects of water stress are: decreased tissue protein content (5, 10), decreased incorporation of amino acids into protein (3, 9), decreased nitrate reductase activity (19), increased ribonuclease activity (8, 12), dissociation of polyribosomes (13,14), and altered mRNA transcription and translation (16). Changes in the pattern of protein synthesis during water stress have been observed in double-labeling experiments (6, 7) indicating that the effect is selective. This raises the possibility that water stress may alter the nutritional quality of high protein legume crops such as soybean, the mature seeds of which are 40 to 45% protein on a dry weight basis. However, little is known about the effect of water stress on storage protein synthesis and accumulation in developing legume seeds.In this paper we report that protein accumulation in developing ovules of two soybean varieties is not prevented and can actually be increased by moderate or severe (leaves senesced) water stress. We also present evidence for stress-induced foliar nitrogen mobilization which appears to account for the maintenance of normal or supranormal seed protein accumulation. Greensboro, NC) at a concentration of 12.5 mg/l. KNO3 was the major N source in this solution. Young plants were grown in half-strength nutrient solution for the first 2 weeks after transfer into the hydroponic system and full strength solution thereafter. The pH of the solution was maintained between 6.2 and 6.5 and the solution was replaced twice a week. Aeration was provided by bubbling compressed air through the nutrient solution. Newly opened flowers were tagged daily with color-coded thread for approximately 1 month to provide a precise record of seed age at harvest.Administration of Water Stress. Stress was applied stepwise by addition ofPEG-6000 (Carbowax 6000, Union Carbide Corp.) to the nutrient solution in 2.5% (w/v) increments every 2 d to a final level of 12.5% PEG (about -9.2 bars). The PEG concentration was held at 12.5% for 10 to 13 d. For the severe stress experiment in the summer of 1981, the plants were grown in 2.5-L pots, and transpired solution was replaced with deionized H20 every evening. During the day the solution level was allowed to drop due to transpiration thereby decreasing the water potential to -15 to -20 bars. For the moderate stress experiment in the summer and fall of 1982, the plants were grown in 5-L pots such that transpiration had little effect on the solution level and the water potential remained between -9 and -12 bars. At the end of the stress period, pods were harvested, sorted by age, shelled, and the seeds frozen in liquid N and stored at -30°C.