Combined nitrogen [nitrate (NO 3 -), ammonium (NH 4 +), and urea] will inhibit all components of symbiotic nitrogen (N 2 ) fixation if present in sufficient concentrations. It is generally accepted that nitrate is particularly inhibitory to nodule growth and nitrogenase activity, and somewhat less inhibitory to the infection process. This project examined whether providing low (0.1 -0.5 mM) static concentrations of NO 3 -to pea (Pisum sativum L. cv. Express), seedlings could avoid the period of N hunger experienced prior to the establishment of N 2 fixation, without delaying or reducing the symbiotic N 2 fixation. All concentrations of NO 3 -tested significantly inhibited all measured components of N 2 fixation. The nodulation process as measured by nodule number was inhibited to a similar degree as the other parameters. A concentration dependent response was evident, with 0.1 mM NO 3 -causing less inhibition than the 0.2 or 0.5 mM concentrations. Our results indicate the within the concentrations of 0.1 mM and 0.5 mM NO 3 -, it is not possible to stimulate the growth of pea plants without inhibiting nodulation and N 2 fixation.

The mechanisms involved in the inhibition of nitrogenase activity in legume nodules by nitrate is unclear. This paper reviews and evaluates proposed mechanisms of this inhibition. Emphasis is placed on recent developments, which suggest that nitrate causes an O2 limitation of nitrogenase activity. Several mechanisms that involve a nitrate‐induced increase in resistance to O3 diffusion in the nodule cortex are discussed.

Background and objectives Free asparagine (ASN) is the precursor to the formation of acrylamide, which is a probable neurotoxic and carcinogenic compound formed during high‐temperature (>120°C) processing of starchy foods, such as cereal‐based products. Controlling the acrylamide concentration of cereal‐based products, for example, bread, within the allowable levels established by the European Commission, is necessary for food safety purposes. One effective measure recommended by the European Commission to mitigate acrylamide in cereal‐based products is to reduce free ASN levels in raw ingredients, for example, wheat. Therefore, knowledge of free ASN levels in Canadian commercial wheat and the strategies to reduce its formation is necessary for the Canadian wheat industry to secure global market access for Canadian wheat. The objective of this study was to understand the effects of genotype, growing environment, and fertilization on free ASN concentration of whole‐wheat flour from Western Canadian wheat varieties. Findings The free ASN concentration of whole‐wheat flours in this study ranged from 281 to 1,014 µg/g (dry basis). The variation in free ASN levels in wheat was mainly influenced by the growing environment (44%), followed by genotype (31%) and the interaction between genotype and environment (18%). Although being significant, the effects of fertilization and interactions involving fertilization on free ASN concentration in whole‐wheat flour were minimal (0.7%–2.2%). Conclusion Growing wheat genotypes in suitable environments, along with the selection of wheat genotypes with lower potential for free ASN formation, are the most effective strategies to control free ASN levels in Canadian wheat. Significance and novelty Limited knowledge in regard to free ASN concentration and the strategies to reduce its formation in Canadian wheat may have a serious impact on Canadian wheat access to global markets, especially the European market. This study provides an understanding of the effects of growing environment, genotype, and fertilization on free ASN concentration of select Western Canadian commercial wheat varieties.

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Although the inhibitory effects of high concentrations of mineral N (> 1.0 mM) on nodule development and function have often been studied, the effects of low, static concentrations of NH4+ (< 1.0 mM) on nodulation are unknown. In the present experiments we examine the effects of static concentrations of NH4+ at 0, 0.1 and 0.5 mM in flowing, hydroponic culture on nodule establishment and nitrogenase activity in field peas [Pisum sativum L. cv. Express (Svalöf AB)] for the initial 28 days after planting (DAP). Peas grown in the presence of low concentrations of NH4+ had significantly greater nodule numbers (up to 4-fold) than plants grown without NH4+. Nodule dry weight per plant was significantly higher at 14, 21 and 28 DAP in plants grown in the presence of NH4+, but individual nodule mass was lower than in plants grown without NH4+. The nodulation pattern of the plants supplied with NH4+ was similar to that often reported for supernodulating mutants, however the plants did not express other growth habits associated with supernodulation. Estimates of N2 fixation indicate that the plus-NH4+ peas fixed as much or more N2 than the plants supplied with minus-NH4+ nutrient solution. There were no significant differences in nodule numbers, nodule mass or NH4+ uptake between the plants grown at the two concentrations of NH4+. Nodulation appeared to autoregulate by 14 DAP in the minus-NH4+ treatment. Plant growth and N accumulation in the minus-NH4+ plants lagged behind those of the plus-NH4+ treatments prior to N2 fixation becoming well established in the final week of the experiment. The plus-NH4+ treatments appeared not to elicit autoregulation and plants continued to initiate nodules throughout the experiment.