| Since NR is a molybdoflavoprotein, the activity of this enzyme was estimated
both in the roots as well as in the shoot (Table 3).
Higher enzyme activity in the roots was observed in the seedlings which
had 10 day stay in the nutrient solution whereas seedlings which were kept
for 15 days and 20 days in the nutrient solution showed poor enzyme activity
but no uptake as shown in Table 1 was higher.
Virtually, no relationship in NO3- absorption and the enzyme activity in
the root was noticed. However, the enzyme activity in the shoot was higher
in 15 and 20 days nutrient culture seedlings, which were also showing higher
NO3- absorption. Minimum NO3- absorption was observed in 10 days culture and
simultaneously the NR activity in these seedlings was also comparatively
less. It may be concluded that NR in the roots may not be involved as such in NO3- absorption but may possibly working in the modified form. A mutant lacking in NR in Arabidopsis was reported to absorb nitrate (DODDEMA & OTTEN, 1979). Unlike in roots, higher enzyme activity in shoots and higher nitrate uptake as noticed in some of the treatments (Table 1& 3) suggested the possibility of shoot activity and nitrate uptake. Various mechanism by which NO3- reduction in shoot controls the NO3- uptake by roots have been reported by BENZION et al. (1971) and FROST et al. (1978). They reported that nitrate arriving in the leaves undergoes a reduction, as in the root, the resulting increase in anion charge is transferred to carboxylate the K carboxylate may then return to root and may be metabolised to form bicarbonate which can be exchanged for more nitraee. BRETELER & HANISCH TENCATE (1978) concluded that Benzionis model is not universely valid. But the results obtained by KIRBY & ARMSTRONG (1980) which may be interpreted as providing direct evidence of nitrate uptake by roots regulated by nitrate uptake in the tops, the process being facilitated by the recirculation of K in the plant. In conclusion, it may be reported that though molybdenum and protein in the root system may have a direct relationship in nitrate uptake, nitrate reductase activity in shoot too has its strong influence in this aspect. The possible association of both the systems is suggested. Acknowledgement My thanks are due to Dr. G.S. SIROHI, Head of the Division of Plant Physiology for kindly providing the facilities. The author is also obliged to SHRI D.C. SAXENA and SHRI MOHINDER, Technical Assistants for their physical help. References BENZIONI, A., Y. VADDIA, & S.H. LIPS. 1970. Correlation between nitrate reduction, protein synthesis and malate accumulation. Physiol. Plant. 23 : 1039-1047. BRETELER, H. & HANISHH TENCATE, C.H. 1978. Ionic balance of root shoot nitrate transfer in dwarf bean. Physiol. Plant. 42 : 53-56. DINESH. 1980. The study of nitrate uptake and its assimilation in different wheat genotypes. M. Sc. Thesis. IARI, New Delhi-12. DODDEMA, H. & TELKEMP, G.P. 1979. Uptake of nitrate by mutants of Arabidopsis thaliana disturbed in uptake or reduction of nitrate. Physiol. Plant. 45 : 332-338. FROST, W.B., BLEVINS, D.G. & BRENNETT, N.M. 1978. Cations pretreatment effects on nitrate uptake xylem exhdate and malete levels in wheat seedlings. Plant Physiol. 61 : 323-326. HAGEMAN, R.H. & D.P. HUCKLESBY. 1971. Nitrate reductase in A. San pietro ed ; Methods in enzymology, Vol. XXIII. Academic Press, New York pp. 497-503. HOAGLAND, D.R. & D.I. ARNON. 1950. The water-cutture method for growing plants without soil. Calif. Agr. Exp. Stn. Cir. 347. JACKSON, W.A., R.J. VOLK & T.C. TUCKER. 1972. Apparent induction of nitrate uptake in nitrate depleated plants. Agronomy Journal 64 : 518-521. KIRKBY, E.A. & ARMSTONG, D.M.J. 1980. Nitrate uptake by roots as regulated by nitrate assimilation in shoot of the caster oil plants. Plant Physiol. 65 : 286-290. MINOTTI, P.L., D.C. WILLIAMS, S. & W.A. JACKSON. 1968. Nitrate uptake and reduction as affected by calcium and potassium. Soil Sci. Soc. Amer. Proc. 32 : 629-298. |
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