| Budget Amount *help |
¥5,300,000 (Direct Cost: ¥5,300,000)
Fiscal Year 1989: ¥1,000,000 (Direct Cost: ¥1,000,000)
Fiscal Year 1988: ¥4,300,000 (Direct Cost: ¥4,300,000)
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| Research Abstract |
In vivo nitrate reduction in roots and shoots and exchange of reduced N between organs were quantitatively estimated in intact 13-day-old barley seedlings using the ^<15>NO^-_-incorporation model. N-depleted seedlings were exposed to media containing both nitrate (1.8 mM) and nitrite (0.2 mM) under a light-dark cycle of 12:12 h at 20ﾟC; the media contained different amounts of ^<15>N labeling. Experiments were started either immediately after the beginning or immediately prior to the end of the light period, and plants were sampled subsequently at each light-dark transition throughout 36 h. We also assayed the activities of NAD(P)H nitrate reductase and NADH nitrate reductase in roots and shoots at each 4 h interval throughout the same experimental period.
In vivo nitrate reduction in roots was higher in the dark than in the light, and the reverse was the case in shoots. Roots were responsible for most nitrate reduction during the initial nitrate utilization, particularly in darkness. H
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owever, thereafter the shoots were generally the main organ for reducing nitrate, particularly in the light. When in vivo nitrate reduction in the root decreased, shoot nitrate reduction increased, with downward translocation of reduced N by way of phloem increasing. Xylem translocation of nitrate drastically decreased in the dark, but that of reduced N rather increased, with phloem translocation of reduced in decreasing. In root tissues, NAD(P)H nitrate reductase was present as much as NADH nitrate reductase. On the other hand, almost all of the nitrate reductases was NADH-specific enzyme in shoot tissues. NAD(P)H enzyme also functioned well in root tissues.
We conclude that the cycling of reduced N through the plant is important for N feeding of each organ, and that the transport system of reduced N by way of xylem and phloem, as well as nitrate reduction by root and shoot, can be modulated in response to the relative magnitude of reduced-N demands by the root and shoot, with the one or the other predominating under different circumstances. We also confirm that root nitrate reduction plays an important role in transporting reduced N to shoot when nitrate reduction in shoots is low. Less
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