Superoxide dismutase activity in arbuscular mycorrhizal Lactuca sativa plants subjected to drought stress

Abstract

Total superoxide dismutase (SOD; EC 1.15.1.1) activity, SOD specific activity and changes in the SOD isoenzymatic pattern were determined in roots and shoots of plants of Lactuca sativa L. cv. Romana colonized by the arbuscular mycorrhiza (AM) fungi Glomus mosseae (Nicol. & Gerd.) Gerd. & Trappe or Glomus deserticola (Trappe, Bloss & Menge), and in non-mycorrhizal phosphorus-fertilized controls. Lettuce plants were grown under well watered or drought conditions and were harvested 5 wk or 8 wk after planting. Previous studies have reported that mechanisms that decrease oxidative stress can play an important role in drought tolerance of plants. However, there is little information on either the role of superoxide dismutase in the symbiosis between AM fungi and plants, or about the response at biochemical level of the mycorrhizal symbiosis to drought stress. In our experimental conditions the protein concentration in shoots decreased in all plants as a result of drought stress (except in plants colonized by Glomus mosseae after 8 wk of growth). In general, shoots of mycorrhizal plants had more protein than did shoots of P-fertilized plants, G. deserticola being the treatment with the highest protein concentration. However, in roots the protein concentrations did not vary as clearly as in shoots. The SOD specific activity of 5-wk-old plants under stress conditions was considerably higher in shoots and roots of mycorrhizal than in those of P-fertilized plants, showing increases which ranged from 99 % in G. mosseae-mycorrhizas to 150 % in G. deserticola-mycorrhizas. Under well watered conditions shoots showed no significant differences in SOD specific activity between treatments, whereas in roots mycorrhizal colonization increased it by 16-18 %. Drought stress increased SOD activity of shoots of control plants by 17%, and that of shoots of plants colonized by G. mosseae and G. deserticola by 93 % and 128 %, respectively. Similar results were also found for roots, regardless of the plant age. The increased SOD activity in mycorrhizal plants was not related to the nutritional status of the plants. Instead, it could be ascribed to a direct mechanism of AM association in response to drought exposure. However, no changes in the SOD isozyme pattern as a consequence of mycorrhizal association or water treatment were observed.