Form of inorganic nitrogen influences mycorrhizal colonisation and photosynthesis of cucumber

The effects of arbuscular mycorrhizal (AM) colonisation by Glomus mosseae (Nicolson and Gerdemann) Gerdemann and Trappe on growth and photosynthesis of cucumber (Cucumis sativus L. cv. Telegraph Improved) were studied using plants supplied with low phosphorus (0.13 mM P) nutrient solutions containing nitrate (NO3-), ammonium (NH4+) or a NO3-/NH4+ mixture, with the same total (4 mM) N concentration. Mycorrhizal colonisation varied with N-form; plants supplied with NO3- only had the highest total AM colonisation and highest proportion of arbuscules, with no differences between the other two N treatments. Root systems of NH4+-fed AM plants had the lowest number of vesicles per unit root length compared with AM plants grown in NO3- or NO3-/NH4+. In the presence of NH4+ either on its own or in combination with NO3-, both AM and uninoculated plants had higher leaf N concentrations than NO3--fed plants. Tissue P concentrations were less dependent on the form of N supplied, although lower concentrations of P and higher N:P ratios were generally found in the shoot tissues of uninoculated plants in the presence of NH4+. Both AM and uninoculated plants had a similar maximum rate of photosynthesis (P-m), when supplied with NO3-. Leaves of AM plants grown in the presence of NH4+, had higher P-m values than uninoculated plants, although this was unrelated to leaf N concentration. They were, however, associated with significantly lower N:P ratios, suggesting that NH4+ supply may cause an alteration between N and P metabolism, which can be moderated by AM colonisation. The higher AM-associated values for P-m were not related to calculated rubisco activities, electron transport capacity or stomatal conductance but may be due, in part, to differences in internal diffusion of CO2 within the leaf. Despite the lower values for P-m in uninoculated plants in the presence of NH4+ and NO3-/NH4+ supply, there were no differences in biomass production and the possible reasons for this are discussed. (C) 2002 Elsevier Science B.V All rights reserved.