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 NH 4NO 3 (limiting) or 10 mM NH 4NO 3 (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 NH 4NO 3-sufficient plants than in N 2-fixing and NH 4NO 3-limited beans. At later times, only N 2-fixing and NH 4NO 3-sufficient plants continued assimilating N and growing, with the NH 4NO 3-sufficient plants being consistently bigger. After 10 days of stress (40 DAP), desiccation was evident, but only NH 4NO 3-sufficient plants suffered drought-induced senescence. After 20 days of stress (50 DAP), N content increased in NH 4NO 3-sufficient but not in N 2-fixing beans, despite the latter’s lesser state of wilt. Pod dry weight dropped 43% in NH 4NO 3-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 NH 4NO 3-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 NH 4NO 3. The most stress-sensitive traits in these plants were the incorporation of N into their shoots and the number of pods remaining on them.