Effects of nitrate/ammonium nitrogen ratio or deficient nitrogen nutrition on photosynthesis and oxalate parameters of hydroponically grown pigweed (Amaranthus hybridus)

Background and Aims : The nitrogen (N) form absorbed and assimilated by plants affects many aspects of their physiology including soluble oxalate and calcium oxalate (CaOx) crystals. Based on the above relationship, attempts have been made to reduce the oxalate content of plants used for human consumption or animal feed by controlling nutrition. Methods The effects of N form in hydroponically grown pigweed ( Amaranthus hybridus ) plants using different NO ₃ ⁻ /NH ₄ ⁺ ratios (1:0, 1:1 or 1:3; always summing to 7.5 m M in nutrient solution) or deficient levels of N (0.5 m M NO ₃ ⁻ ) were studied. The changes in oxalate physiology including CaOx crystals’ number, size and chemical composition, leaf and root soluble and insoluble oxalate content, and root oxalate exudation were examined. Also, the effects of N nutrition on photosynthesis were studied considering its association with both N and CaOx crystals, the latter particularly in photosynthesis under stress, also known as alarm photosynthesis. Results Increase of NH ₄ ⁺ supply resulted in a dramatic decrease in insoluble and, much more in, soluble oxalate. Photosynthetic rate showed a reducing trend while, in contrast, JIP-test chlorophyll fluorescence parameters were improved. Deficient supply of NO ₃ ⁻ -N resulted in considerable negative effects in photosynthesis while oxalate parameters were not affected to a notable extend. Conclusion Pigweed is not impaired by N nutrition indicating its facultative calcicole physiotype and wide ecological adaptation, compatible with its cosmopolitan distribution. A direct interaction between oxalate physiology and N nutrition is evident, while those with photosynthesis are much more puzzling, yielding future research questions.