Growth and mineral nutrition of non‐mycorrhizal and mycorrhizal Norway spruce (Picea abies) seedlings grown in semi‐hydroponic sand culture

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Abstract
Summary: In seedlings of Norway spruce(Picea abies(L.) Karst.), grown in semi‐hydroponic sand culture, mycorrhizal infection decreased growth (Eltrop & Marschner, 1996). Possible reasons for this growth depression were investigated in the present study by comparing the plant and fungal biomass distribution and carbon partitioning between non‐mycorrhizal and mycorrhizal(Pisolitlius tinctorius)plants supplied with ammonium or nitrate as source of N.Despite the high mycorrhizal infection rate (55–71% of total root tips), the amount of fungal biomass in roots and extramatrical mycelium of mycorrhizal plants accounted for less than 3% of plant dry matter and was significantly lower in nitrate‐ than in ammonium‐supplied plants.The CO2assimilation rates were higher in mycorrhizal than in non‐mycorrhizal plants supplied with ammonium as well as those supplied with nitrate. High light intensity considerably increased CO2assimilation rates. The respiration rates of the intact root systems were significantly increased in ammonium‐supplied mycorrhizal plants compared with non‐mycorrhizal plants. The amount of CO2lost in root respiration as a percentage of the amount of CO2gained in photosynthesis (respiratory quotient), ranged from 49.3% at 30°C root zone temperature and low light intensity (290 μmol m−2S−1) to 11.7% at 10 °C and high light intensity (990 μmol m−2s−1). In ammonium‐supplied plants grown at 22 °C and low light intensity, the proportion of carbon lost by root respiration was significantly higher in mycorrhizal than in non‐mycorrhizal plants. This increase in root respiration was of the same order of magnitude (7.4 %) as the decrease in dry matter production by mycorrhizal plants (10.1 %). In nitrate‐supplied plants, no significant difference in the respiratory quotient was found between non‐mycorrhizal and mycorrhizal plants. The respiration rate per unit d. wt of the fungal mycelium was estimated to account for 31.3 % (ammonium supply) and 8.3 % (nitrate supply) of that of the total mycorrhizal root system although the dry weight accounted for only 4. % (ammonium supply) and 2.5 % (nitrate supply). Accordingly, the respiration rate was calculated to be 11.1 times higher with ammonium supply, and 3.4 times higher with nitrate supply, than that of the root tissue. The carbohydrate concentrations in shoots and roots were not consistently different between non‐mycorrhizal and mycorrhizal plants.It was concluded that increased root respiration was mainly responsible for the growth reduction in mycorrhizal compared with non‐mycorrhizal plants, whereas the production of fungal biomass in the extramatrical mycelium of mycorrhizal plants was of minor importance.