Dinitrogen fixation in pea beans (Phaseolus vulgaris) as affected by growth stage and temperature regime

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Abstract
Tolerance of beans (Phaseolus vulgaris L.) to low temperatures is essential for growth under Canadian growing conditions. At air temperatures typical of a southern Alberta growing season (10–23 °C), 'Aurora' fixed 433 mg N per plant, more than 'Sanilac' and 'Comtesse,' which averaged 307 mg N per plant, and 'Kentwood,' which fixed the least: 214 mg N per plant. At a higher temperature regime (15–23 °C), typical of a southern Ontario growing season, 'Aurora' again fixed the most N2 (456 mg N per plant), while the other three cultivars fixed an average of 370 mg N per plant. The amount of N2 fixed increased as the time to reach maximum acetylene (C2H2) reducing activity lengthened. This suggested that it may be advantageous to select beans that have a longer vegetative stage to prolong the time of active N2 fixation within the limits of acceptable season length to ensure maturity. 'Aurora' was not affected by the lower temperature regime, and although these temperatures delayed the initiation of N2 fixation by 21 days, 'Aurora' compensated by producing 26% greater nodule dry weight and 40% higher maximum C2H2-reducing activity than that observed at the higher temperature regime. Growth and N2 fixation of the other three cultivars were adversely affected by the lower temperature regime, not because of lower nodule numbers or mass but because of lower nodule activity. The good root growth of 'Aurora' at low temperatures may explain its superiority for N2 fixation at lower temperatures. C2H2 reduction and nodule dry weight were acceptable indicators of N2 fixation only during early plant growth when all nodules were new and uniformly active. The ideal time to select the bean cultivars for N2-fixing ability was just before the peak of maximum C2H2-reducing activity and corresponded to the stage from buds-near-breaking to anthesis for both temperature regimes. Molar ratios of C2H2 reduced to N2 fixed suggested that the relative efficiency of N2 fixation varied with temperature and growth stage but not with bean cultivar.