Interaction between atmospheric and pedospheric nitrogen nutrition in spruce (Picea abies L.Karst) seedlings
The effect of NO2 fumigation on root N uptake and metabolism was investigated in 3-month-old spruce (Picea abies L. Karst) seedlings. In a first experiment, the contribution of NO2 to the plant N budget was measured during a 48 h fumigation with 100mm3 m(-3) NO2. Plants were pretreated with various nutrient solutions containing NO3 and NH4(+) NO3 only or no nitrogen source for 1 week prior to the beginning of fumigation. Absence of NH4(+) in the solution for 6 d led to an increased capacity for NO3(-) uptake, whereas the absence of both ions caused a decrease in the plant N concentration, with no change in NO3(-) uptake. In fumigated plants, NO2 uptake accounted for 20-40 per cent of NO3(-) uptake. Root NO3(-) uptake in plants supplied with NH4(+) plus NO3(-) solutions was decreased by NO2 fumigation, whereas it was not significantly altered in the other treatments. In a second experiment, spruce seedlings were grown on a solution containing both NO3(-) and NH4(+) and were fumigated or not with 100 mm3 m(-3) NO2 for 7 weeks. Fumigated plants accumulated less dry matter, especially in the roots. Fluxes of the two N species were estimated from their accumulations in shoots and roots, xylem exudate analysis and (15)N labelling. Root NH4(+) uptake was approximately three times higher than NO3(-) uptake. Nitrogen dioxide uptake represented 10-15 per cent of the total N budget of the plants. In control plants, N assimilation occurred mainly in the roots and organic nitrogen was the main form of N transported to the shoot. Phloem transport of organic nitrogen accounted for 17 per cent of its xylem transport. In fumigated plants, neither NO3(-) nor NH4(+) accumulated in the shoot, showing that all the absorbed NO2 was assimilated. Root NO3(-) reduction was reduced whereas organic nitrogen transport in the phloem increased by a factor of 3 in NO2-fumigated as compared with control plants. The significance of the results for the regulation of whole-plant N utilization is disc ussed.