Leaflet photosynthesis rate and carbon metabolite accumulation patterns in nitrogen-limited, vegetative soybean plants

Prolonged inorganic nitrogen (NO₃ ^–+NH₄ ⁺) limitation of non-N₂-fixing soybean plants affected leaflet photosynthesis rates, photosynthate accumulation rates and levels, and anaplerotic carbon metabolite levels. Leaflets of nitrogen-limited (N-Lim), 27–31-day-old plants displayed 15 to 23% lower photosynthesis rates than leaflets of nitrogen-sufficient (N-Suff) plants. In contrast, N-Lim plant leaflets displayed higher sucrose and starch levels and rates of accumulation, as well as higher levels of carbon metabolites associated with sucrose and starch synthesis, e. g., glycerate-3-phosphate and glucose phosphates, than N-Suff plant leaflets. Concurrently, levels of soluble protein, chlorophyll, and anaplerotic metabolites, e.g., malate and phosphoenolpyruvate, were lower in leaflets of N-Lim plants than N-Suff plants, suggesting that the enzymes of the anaplerotic carbon metabolite pathway were lower in activity in N-Lim plant leaflets. Malate net accumulation rates in the earliest part of the illumination period were lower in N-Lim than in N-Suff plant leaflets; however, by the midday period, malate accumulation rate in N-Lim plant leaflets exceeded that in leaflets of N-Suff plants. Further, soluble protein accumulation rates in leaflets of N-Suff and N-Lim plants were similar, and the rate of dark respiration, measured in the early part of the dark period, was higher in N-Lim plant leaflets than in N-Suff plant leaflets. It was concluded that during prolonged N-limitation, foliar metabolite conditions favored the channelling of a large proportion of the carbon assimilate into sucrose and starch, while assimilate flow through the anaplerotic pathway was diminished. However, in some daytime periods, there was a normal level of carbon assimilate channelled through the anaplerotic pathway for ultimate use in amino acid and protein synthesis.Abbreviations ADPG-PPiase ADPglucose pyrophosphorylase - Ce CO₂ in the leaf photosynthesis measuring cuvette - Ci leaf internal CO₂ during photosynthesis measurement - Chl chlorophyll - DHAP dihydroxyacetone phosphate - GAP glyceraldehyde-3-phosphate - G_sw stomatal conductance with units as mmol H₂O m^–2 s^–1 - G1P glucose-1-phosphate - G6P glucose-6-phosphate - F6P fructose-6-phosphate - FBP fructose-1,6-bisphosphate - FBPase-pH 8.1 chloroplastic fructose-1,6-bisP (C-1) phosphatase (pH 8.1) - MAL malate - N inorganic nitrogen, i.e. NO₃ ^–+NH₄ ⁺ (at levels and molar ratios indicated) - PE post-emergence - PEP phosphoenolpyruvate - PEPCase phosphoenolpyruvate carboxylase - PGA 3-phosphoglycerate - PYR pyruvate - PYR kinase pyruvate kinase - Pn net CO₂ photoassimilation in leaves - PPFD photosynthetic photon flux density - PPRC pentose phosphate reductive cycle - RuBP ribulose-1,5-bisphosphate; rubisco-ribulose-1,5-bisphosphate carboxylase/oxygenase - SLW specific leaf mass - SPS sucrose-6-phosphate synthase - TCA cycle tricarboxylic acid cycle; triose-P-DAP+GAP