Comparison of drought tolerance in nitrogen-fixing and inorganic nitrogen-grown common beans

In this work, we evaluated how the use of alternative N sources affects drought-stress tolerance in common beans. To this end, plants were cultivated employing either N-2 fixation or two levels of inorganic nitrogen: 1 mM NH4NO3 (limiting) or 10 mM NH4NO3 (sufficient). Drought was imposed by withholding watering at 30 days after planting (DAP) - coinciding with flowering. At 20 DAP, growth and N content were significantly higher in NH4NO3-sufficient plants than in N-2-fixing and NH4NO3-limited beans. At later times, only N-2-fixing and NH4NO3-sufficient plants continued assimilating N and growing, with the NH4NO3-sufficient plants being consistently bigger. After 10 days of stress (40 DAP), desiccation was evident, but only NH4NO3-sufficient plants suffered drought-induced senescence. After 20 days of stress (50 DAP), N content increased in NH4NO3-sufficient but not in N-2-fixing beans, despite the latter's lesser state of wilt. Pod dry weight dropped 43% in NH4NO3-sufficient beans with respect to well-watered plants, while remaining constant in N-2-fixing beans. Under drought conditions, the number of pods limited pod yield regardless of the nitrogen source used; nevertheless, the translocation of soluble matter to pods continued in both NH4NO3-sufficient and N-2-fixing beans. We conclude that common beans grown under conditions of N-2 fixation were more drought tolerant than those provided with sufficient levels of NH4NO3. The most stress-sensitive traits in these plants were the incorporation of N into their shoots and the number of pods remaining on them. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.