Role of fixing nitrogen in common bean growth under water deficit conditions

In common bean (Phaseolus vulgaris L), limited information is available if potential of nodulation and symbiotic nitrogen fixation is associated with the expression of traits that confer better adaptation to drought conditions. Adaptive traits were therefore measured in N‐fixing bean lines BAT 477, DOR 364, and PAN 185 and in nonnodulating lines BAT 477‐NN and DOR 364‐NN. Drought affected growth in all bean lines and decreased chlorophyll content but more in DOR 364‐NN and BAT 477‐NN, and also reduced nodule size with highest reduction in DOR 364 (21.6%). Loss of nitrogen fixation under drought conditions was further associated with lower water use efficiency, measured as carbon isotope ratio of 13C/12C, and lower %N in shoots when compared to capability to fix nitrogen. Loss of symbiotic nitrogen fixation in mutant bean lines DOR 364‐NN and BAT 477‐NN negatively affected root architectural traits under drought. Under drought conditions, line BAT 477‐NN had about 50% lower values for all root architectural traits, such as root angle, strongly emphasizing an association of capability to fix nitrogen with root development. An association between capability to fix nitrogen and maintaining a better harvest index and seed yield was also found in the study. Under well‐watered conditions, all N‐fixing lines had a higher harvest index and seed yield (2.6–2.8 t of seed yield/ha) in comparison with the two nonfixing bean lines (1.4 t/ha). Although the harvest index and seed yield declined due to drought in all tested lines, this decline was only significant for line DOR 364‐NN (harvest index) and lines DOR 364‐NN and BAT 477‐NN (seed yield). Our study has overall demonstrated an important association between the capability of maintaining atmospheric N‐fixation and expression of traits conferring better adaptation to drought conditions with any change in nitrogen fixation affecting these traits