Consequences of nitrogen deficiency induced by low external N concentration and by patchy N supply in Picea abies and Thuja occidentalis

We examined the responses of two coniferous species Picea abies and Thuja occidentalis to decreased nitrogen availability. Plants were grown for 2 months in inorganic substrate irrigated by nutrient solution. Nitrogen availability was reduced either by lower N concentration in the nutrient solution or by a patchy supply of a high N concentration to only one side root isolated in a split-root setup where the rest of the root system received all nutrients except N. At the end of cultivation we measured rates of net photosynthetic CO₂ uptake, net nitrogen and water uptake, some structural characteristics (dry mass of fine roots, dry mass and area of needles) and the total N content of needles. For a more detailed analysis of the distribution of the newly acquired N within the shoot, ¹⁵N was administered to subsets of plants in each of the three treatments. Low N availability resulted in lower specific leaf area in Thuja but not in Picea. The decrease of net photosynthesis at lower N supply was greater in Picea than in Thuja. Photosynthetic nitrogen use efficiency, however, linearly decreased with increasing N content only in Thuja. Patchy N supply caused uneven distribution of newly acquired labeled nitrogen and total N but did not result in significantly greater heterogeneity in the rate of photosynthesis among branches both in Picea and in Thuja plants. We conclude that both examined species possess mechanisms that reduce adverse effects of patchy N supply and restricted nitrogen transport in xylem to some parts of crown on their photosynthetic carbon assimilation.