Growth, nitrogen uptake and mineral nutrient concentrations in the plant tissues were determined in non–mycorrhizal and mycorrhizal Norway spruce (Picea abies (L.) Karst.) seedlings grown under controlled conditions in a semi-hydroponic culture system with quartz sand as substrate and a percolating nutrient solution. The culture system allowed the determination of nutrient uptake rates in mycorrhizal root systems with an intact extramatrical mycelium. The rate of infection of the roots by the mycorrhizal fungi Pisolithus tinctorius and Laccaria laccata was high but the rate of infection by Paxillus involutus was low.
When supplied with ammonium nitrate, the d. wt of the roots and particularly of the shoots was significantly lower in mycorrhizal than in non–mycorrhizal plants. Despite the lower root d. wt, the number of root tips and the root branching ratio (number of root tips per unit root length) were significantly higher in mycorrhizal plants infected with L. laccata and P. tinctorius than in non–mycorrhizal plants. The depletion of ammonium in the external solution was faster than the depletion of nitrate. Nitrate uptake rates increased at ammonium concentrations below 400μM. The maximal N uptake rates (Vmax), calculated after Lineweaver–Burk, were significantly higher for ammonium than for nitrate. The N uptake rates did not differ significantly between non–mycorrhizal and mycorrhizai plants. The concentrations of N, P, K, Ca and Mg tended to be higher in the smaller mycorrhizal than in the larger non–mycorrhizal plants. A significant increase in mineral nutrient concentration in mycorrhizal compared with non–mycorrhizal plants was found only for N concentrations in the needles of mycorrhizal plants infected with P. tinctorius.
When they were supplied with ammonium ((NH4), SO4) as source of N, but not when they were supplied with nitrate (KNO3), the d. wt was lower in mycorrhizal plants infected with P. tinctorius than it was in non-mycorrhizal plants. Therefore, N uptake rates were increased in mycorrhizai plants with P. tinctorius only when they were supplied with ammonium but not with nitrate.
The insignificant differences in uptake rates of N, P, K, Ca and Mg between non–mycorrhizal and mycorrhizal plants indicate that at unlimited spatial nutrient availability the contribution of the extramatrical mycelium to nutrient uptake by mycorrhizal plants was small. It is suggested that the decreased growth of mycorrhizal plants is due to the demand of the mycorrhizal fungus for photosynthates, i.e. source limitation.