Physiological and nutritional responses by Lactuca Sativa L. to nitrogen sources and mycorrhizal fungi under drought conditions

We measured the growth, nutrition, and N assimilation of arbuscular-mycorrhizal and non-mycorrhizal lettuce (Lactuca sativa L.) as affected by forms of N and drought. Moisture was maintained at 80% water-holding capacity, and N was applied as NO _inf3 ^sup- , NH _inf4 ^sup+ , or NO _inf3 ^sup- /NH _inf4 ^sup+ (3:1, 1:1, or 1:3). The growth of Glomus fasciculatum-colonized plants was comparable to that of uncolonized P-supplemented plants when N was provided as NH _inf4 ^sup+ or combined NO _inf3 ^sup- /NH _inf4 ^sup+ . When N was supplied solely as NO _inf3 ^sup- , G. fasciculatum-colonized plants produced a higher yield than P-fertilized plants, suggesting that the uptake and/or assimilation of NO _inf3 ^sup- was particularly affected by mycorrhizal status in this water-limited situation. Nutrient availability, except Ca, was less limited for mycorrhizal plants than for P-fertilized plants. P fertilization increased the growth, glutamine synthetase activity, and protein content of lettuce to the same extent that G. fasciculatum colonization did when N was applied as NH _inf4 ^sup+ . With NO _inf3 ^sup- -fertilization, G. fasciculatum-colonized plants showed increased growth, nitrate reductase activity, and protein content compared to P-fertilizer treatment. Plants colonized by G. mosseae showed increased photosynthetic activity and proline acumulation, and these mechanisms may be important in adaptation by the plant to drought conditions. The present results confirmed that under drought conditions, the uptake or metabolism of N forms is particularly affected in mycorrhizal fungi-colonized plants, depending on the mycorrhizal endophyte and the N source added. Thus the significance of arbuscular-mycorrhizal fungus selection for plant growth in drought conditions is a consideration for management strategy.