Quick and reversible inhibition of soybean root nodule growth by nitrate involves a decrease in sucrose supply to nodules.
The upper part of a nodulated soybean root hydroponically cultured in a glass bottle was monitored using a computer microscope under controlled environmental conditions, and the diameter of individual nodules was measured from 10-24 d after planting. The diameter of a root nodule attached to the primary root increased from 1 mm to 6 mm for 2 weeks under nitrogen-free conditions. The increase in diameter of the nodules was almost completely stopped after 1 d of supplying 5 mM nitrate, and was due to the cessation of nodule cell expansion. However, nodule growth quickly returned to the normal growth rate following withdrawal of nitrate from the solution. The reversible depression of nodule growth by nitrate was similar to the restriction of photoassimilate supply by continuous dark treatment for 2 d followed by normal light/dark conditions. In addition, the inhibitory effect of nitrate was partially alleviated by the addition of 3% (w/v) sucrose to the medium. Plant leaves were exposed to (11)C or (14)C-labelled carbon dioxide to investigate the effects of 5 mM nitrate on the translocation and distribution of photosynthates to nodules and roots. Supplying 5 mM nitrate stimulated the translocation rate and the distribution of labelled C in nitrate-fed parts of the roots. However, the (14)C partitioning to nodules decreased from 9% to 4% of total (14)C under conditions of 5 mM nitrate supply. These results indicate that the decrease in photoassimilate supply to nodules may be involved in the quick and reversible nitrate inhibition of soybean nodule growth.
Fujikake
Hiroyuki
H
Faculty of Agriculture, Niigata University, 2-8050 Ikarashi, Niigata, 950-2181, Japan.
Yamazaki
Akihiko
A
Ohtake
Norikuni
N
Sueyoshi
Kuni
K
Matsuhashi
Shinpei
S
Ito
Takehito
T
Mizuniwa
Chizuko
C
Kume
Tamikazu
T
Hashimoto
Shoji
S
Ishioka
Noriko-Shigeta
NS
Watanabe
Satoshi
S
Osa
Akihiko
A
Sekine
Toshiaki
T
Uchida
Hiroshi
H
Tsuji
Atsunori
A
Ohyama
Takuji
T
D001692:Biological Transport / Q000187:drug effects; D002250:Carbon Radioisotopes / Q000378:metabolism; D003624:Darkness; D008027:Light; D009566:Nitrates / Q000494:pharmacology*; D018517:Plant Roots / Q000187:drug effects / Q000254:growth & development* / Q000378:metabolism; D013025:Glycine max / Q000187:drug effects / Q000254:growth & development* / Q000378:metabolism; D013395:Sucrose / Q000494:pharmacology; D013559:Symbiosis
D002250:Carbon Radioisotopes; D009566:Nitrates; D013395:Sucrose