Two glutathione S-transferase (GST) activities have been identified and purified from etiolated corn tissue. The first, designated GST I enzyme, is constitutively present in corn tissue, and the second, designated GST II enzyme, is present only in tissue which has been treated with chemical antidotes which protect corn against chloroacetanilide herbicides. The total activity constitutes approximately 2% of the soluble protein in these tissues. The native forms of these enzymes have molecular weights of approximately 50 000 as determined by Sephadex G-100 chromatography. On sodium dodecyl sulfate-polyacrylamide gels, GST I enzyme migrates primarily as a single band of molecular weight 29 000, and GST II enzyme migrates as primarily two bands of molecular weight 29 000 and 27 000. Both enzymes catalyze the formation of a glutathione-herbicide conjugate in vitro when the herbicide alachlor is used as a substrate. This conjugation results in elimination of the biological activity of the herbicide.
The patterns of nitrate uptake, nitrate reductase activity in the leaves, and nitrogen fixation by the nodules were investigated in field-grown soybeans (Glycine max (L.) Merr.) over the growing season.The level of nitrate-reductase activity generally paralleled the concentration of nitrate in the leaf tissue over the entire growing season. A precipitous drop in both parameters was noted within 2-3 weeks after flowering. These parameters decreased by 80-95% at mid-pod fill, a stage where ovule (seed) development was in the logarithmic growth phase, placing a heavy demand on the plant for both energy and fixed nitrogen.The activity of nitrogen fixation of soybean root nodules bore a reciprocal relationship to that of nitrate reductase. The maximum levels of nitrogen fixation were reached at early pod fill when nitrate reductase activity had dropped to 25% of maximum activity. A rapid loss of nitrogen fixation activity occurred shortly after bean fill was initiated, again at a time when the ovules were developing at maximal rates.The total protein content of soybean leaves increased over the season to a maximum level at mid-pod fill. This was followed by a 50% drop over the next 3-week period when the plants approached senescence. This drop corresponded to that found for nitrogen fixation. A similar pattern was noted for watersoluble proteins in the leaf.These studies suggest that there is a close and competitive relationship between the processes of nitrate reduction and nitrogen fixation, with the latter process dominating as the major source of fixed nitrogen after the plants have flowered and initiated pods. At this transitional stage, both soil and environmental effects could cause pertrubation in these processes that could lead to a nitrogen stress causing flower and pod abscission.The rapid decay of nitrogen fixation at the time of midpod fill also suggests a competition between roots (nodules) and pods for available photosynthate. This competition appears to lead to the breakdown of foliar proteins and senescence.
Hydroponically grown ‘Dare’ soybeans (Glycine max L.) were exposed to a number of discrete stresses at the time of flowering. Effects on initial pod set, subsequent pod abscission, and ovule growth were assessed. Lodging, when not associated with shading, did not lower pod set. Severe short‐term deficiency of calcium and nitrogen decreased pod set by one‐third. Temperatures above 40C caused severe pod abscission. Shading (63% of ambient) also caused abscission of half the pods. If plants were returned to short‐night conditions after 18 or 22 longnight cycles, flowering and initial pod set were similar to controls but ovule growth was markedly retarded. Thus, yield may be limited by high temperatures and by low light intensities, which cause pod shedding, or by inadequate night lengths which retard seed filling.
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