Interaction between atmospheric and pedospheric nitrogen nutrition in spruce (Picea abies L. Karst) seedlings

Abstract

ABSTRACTThe effect of NO2 fumigation on root N uptake and metabolism was investigated in 3‐month‐old spruce (Picea abics L. Karst) seedlings. In a first experiment, the contribution of NO2 to the plant N budget was measured during a 48 h fumigation with 100mm3m−3 NO2. Plants were pre‐treated with various nutrient solutions containing NO2 and NH4+, NO3− only or no nitrogen source for 1 week prior to the beginning of fumigation. Absence of NH4+ in the solution for 6d 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% 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 NO2 and NH4+ and were fumigated or not with 100mm3m−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 15N labelling. Root NH4+ uptake was approximately three times higher than NO3− uptake. Nitrogen dioxide uptake represented 10–15% 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% 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‐fimugated as compared with control plants. The significance of the results for the regulation of whole‐plant N utilization is discussed.