Nitrate assimilation and nitrogen circulation in Austrian pine

Abstract

Nitrate uptake, reduction and translocation were examined in 5‐week‐old Austrian pines ( Pinus nigra Arnold var. nigricans Host.) during exposure to 5 m M NaNO 3 . The rate of nitrate uptake was linear during the 7 h light period. 15 N‐NO 3 − was detected in all parts of the pine except in the needles. By the 7th hour, 43% of the absorbed nitrate had been reduced, and this increased to 64% by the 24th hour. The major part of the total reduction occurred in the roots at this growth stage. Accumulation of 15 N in reduced soluble and insoluble fractions was more prevalent in roots than in shoots. In the needles, the translocated nitrogen was mainly incorporated into the insoluble fraction. It is likely that most of the nitrogen from nitrate was transported from the roots to the aerial organs as organic nitrogen; however part of the upward nitrogen flux took place as nitrate ions. An experiment in which an exposure for 24 h to 5 m M Na 15 NO 3 was followed by 13 days exposure to Na 14 NO 3 (pulse chase experiment) revealed a half time of about 1 day for depletion of root nitrate. A large part of this depletion was due to the loss of 15 N‐NO 3 − to the nutrient solution. The remaining pool of 15 N‐nitrate was partitioned between a metabolically inactive and an active pool. During the chase period, the simultaneous decrease of 15 N‐incorporation in the soluble N fraction and increase in the insoluble N fraction in different pine parts, particularly in the needles, suggested that protein synthesis occurred mostly in young tissues of the shoot and was the major sink of the newly absorbed 15 N‐NO 3 .