Contribution of shoots and roots to in vivo nitrate reduction in NADH-specific nitrate reductase-deficient mutant seedlings of barley (Hordeum vulgare L.)

Abstract

Barley (Hordeum vulgare L.) has two nitrate reductase (NR) isozymes, namely NADH-specific NR and NAD(P)H-bispecific NR. To determine the effect of NADH-specific NR deficiency on in vivo nitrate reduction and distribution of reduced N to shoots and roots, the 15N-incorporation model was applied to NADH-specific NR-deficient mutant (AzI2) seedlings of barley. The N-deprived seedlings were treated with a nutrient solution containing 2.3 mM N03 - and 0.2 mM N02 -, and were labeled with different amount of 15N for 48 h under continuous illumination. In Az12, the total in vitro NR activity derived from the NAD(P)H-bispecific NR was only 10% of that of the wild type (Cv. Steptoe). In Az12, the total 15N03 - incorporation and translocation of absorbed 15N03 - to shoots were about 25% higher than those of Steptoe. Nitrate reduction in the Az12 roots was 2 times higher than that in Steptoe during the first period (0-24 h) and 1.3 times higher during the second period (24-48 h). However, nitrate reduction in the Az12 shoots was 10 to 30% lower than that in the Steptoe shoots. As a result, nitrate markedly accumulated in the Az12 shoots. Accumulation of reduced 15N in the Az12 roots was 2 times higher than that in the Steptoe roots, but 10% lower in the Az12 shoots than in the Steptoe shoots at the end of the experiment. Upward transport of reduced 15N via the xylem in Az12 was nearly 2 times more active than that of Steptoe throughout the experiment. This result suggested that the derepression of the NAD(P)H-NR isozyme in the Az12 shoots could compensate for the absence of the NADH-NR isozyme. Furthermore, increased levels of root nitrate reduction seemed to make up for the limited nitrate assimilation in the Az12 shoots.