Fig. 6-13 Visualization of the effect of an ion channel inhibitor, using the positron imaging method Radioactivity in a barley shoot supplied with 13NO3- was observed using PETIS for 30 min. The decrease of 13NO3- transportation to the shoot of a barley plant to which a channel blocker has been applied shows up clearly in the PETIS images.

Fig. 6-14 Nitrate ion uptake vs. time in the relative level of radioactivity in a barley shoot as measured by PETIS over a 30 min time interval The pattern of arrival of 13N-labeled nitrate ions in the shoots was the same in the barley with ion channel blocker applied as in the reference barley plant. Only the amount transported was decreased through application of the channel blocker.

Fig. 6-15 Imaging of radioactivity in whole barley seedlings In the barley with channel blocker applied, transportation of 13N-labeled nitrate ions was smaller than that in the reference barley plant, but the accumulation of 13N-labeled nitrate ions in the roots of the barley with channel blocker applied was the same as that in the roots of the reference barley plant. The images suggest that the channel blocker does not affect the uptake and accumulation of nitrogen ions in the roots, but affects the transportation of nitrate ions from the roots to the shoot.

Nitrate ions taken up from the soil are a major nitrogen nutrient for plants. The nitrate ions taken up by the roots accumulate in root cells and are transferred to the xylem for long-distance transportation. Nitrate ions have to cross many cell membranes in being transferred from the outside cells of roots to the xylem vessel. Loading of nitrate ions into the xylem vessel in roots is considered to be a key step in the control of xylem nitrate ion transportation in plants. It is believed that the ion channel in the cell membrane is the pathway for nitrate ions. Thus, we have used a nitrate ion channel blocker, which inhibits the penetration of nitrate ions through cell membranes, in order to understand the relation between the activity of ion channels and nitrate uptake and transportation. In these experiments, we have applied an anion-channel blocker to young barley plants, and have observed the absorption and transportation of 13NO3- (nitrate ions which have been labeled with the positron emitting nuclide 13N) from the roots up into young leaves, using a positron emitting tracer imaging system (PETIS). We at JAERI have constructed the PETIS, allowing us to observe the transportation of substances in living plants (Fig. 6-13). The results obtained show that the transportation of 13NO3- in a barley to which channel blocker has been applied was smaller than that in the normal barley plant (Fig. 6-14). However, accumulation of 13NO3- in the roots of the barley with channel blocker applied was found to be at the same level as that in a normal barley plant (Fig. 6-15). Through the observation of nitrate ion transportation in living plants using the PETIS, we have succeeded in showing that the channel blocker affects only the transportation of nitrate ions through the cell membranes, but has no effect on the uptake of nitrate ions by the roots.

Reference T. Kawachi et al., Effects of Anion Channel Blockers on Xylem Nitrate Transport in Barley Seedlings, Soil Sci. Plant Nutr., 48(2), 271 (2002).

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