Effects of nitrogen source on growth, nutrition, photosynthetic rate and nitrogen metabolism of mycorrhizal and phosphorus‐fertilized plants of Lactuca sativa L.

Abstract

summaryPlants of Lactuca sativa L. cv. Romana were inoculated with two vesicular‐arbuscular (VA) mycorrhizal fungi, Glomus mosseae (Nicol. & Gerd.) Gerd. & Trappe and Glomus fasciculatum (Thaxter sensu Gerd.) Gerd. & Trappe, or provided with phosphorus. These three treatments plus a control were supplemented with 3.0 mM nitrogen as nitrate or ammonium in 1 of 5 NO3−/NH4+ ratios. Leaf and root weights, N, P, K, Ca and Mg absorption, as well as the percentage of root length colonized, were evaluated. Physiological parameters such as leaf area, photosynthetic rate, glutamine synthetase (EC 6.3.1.2) and nitrate reductase (EC 1.6.6.1) activity and leaf proline accumulation were also determined. Mycorrhizal lettuce and P‐fertilized plants derived significantly more benefit from N fertilization regardless of the N‐source than did controls. Ammonium was a better N‐source for non‐mycorrhizal plants and nitrate for mycorrhizal plants. P‐fertilized lettuce reached maximum growth when N was supplied as 50% NO3−/ 50% NH4+ so that mycorrhizal status particularly affected the plant response to the form of N provided. Growth of mycorrhizal plants was comparable to non‐mycorrhizal P‐fertilized plants when N was provided as 100% NH4+ or 25 % NO3−/75 % NH4+. When N applications were as 50% NO3−7 50% NH4+ or 75% NO3−/25% NH4+, growth of P‐fertilized plants was greater than mycorrhizal ones. Lettuce colonized by G. mosseae grew fastest when fed nitrate only. Separation of NO3− and NH4+assimilation into shoot and root compartments may account for the differential effects of such ions into symbiotic and non‐symbiotic plants. Uptake or metabolism of nitrogenous forms was particularly affected in G. mosseae‐colonized plants, independent of P nutrition. P fertilization increased activities of nitrate reductase (NR) by 29 % and glutamine synthetase (GS) by 24% compared with activities in control plants. In the case of G. fasciculatum‐colonized plants, NR and GS were increased 54% and 117 %, respectively, over control activities. In general, concentration of minerals, except Ca, were increased in mycorrhizal and P‐fertilized plants compared with controls. Assimilation of CO2 and accumulation of proline were increased in G. fasciculatum‐colonized lettuce under all N regimes.