The effect of nitrogen fertilisation and elevated atmospheric CO₂ concentration on the oxalate content of spinach (Spinacia oleracea L.) leaves : A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Science at Lincoln University

Oxalate is an anti-nutrient and consuming plants such as spinach that contain high levels of soluble oxalate can have a major effect on calcium bioavailability, leading to the formation of stones in the urinary tract in human beings. This thesis focuses on the effects of nitrogen concentration and form (nitrate, NO₃- and ammonium, NH₄⁺), and elevated atmospheric CO₂ concentration on growth and oxalate levels of spinach leaves grown under greenhouse and controlled environmental conditions. Under greenhouse conditions, the biomass of spinach leaves increased from 1.5 ± 0.3 to 9.2 ± 0.4 g/plant fresh weight (FW) with little change in % dry matter (DM) (27.4 ± 0.2 - 31.4 ± 0.3). The total oxalate levels in spinach leaves decreased from 9.2 ± 0.1 to 3.3 ± 0.1 g/100 g DM with an increase in NO₃- concentrations from 1-10 mM. Biomass of spinach leaves increased from 1.2 ± 0.2 to 2.9 ± 0.3 g/plant FW with little change in % DM (25.9 ± 0.2 – 27.7 ± 0.2) with an increase in the NH₄NO₃ supply from 1-3 mM. The total oxalate content of spinach leaves decreased from 3.6 ± 1.1 to 2.1 ± 0.3 g/100 g DM with an increase in NH₄NO₃ levels from 1-3 mM. Under controlled environment conditions at ambient CO₂, biomass of spinach leaves increased from 2.6 ± 0.4 to 13.7 ± 0.7 g/plant FW and % DM increased from 23.9 ± 1.3 - 36.6 ± 1.7 with increased concentration of NO₃- from 1-10 mM. The total oxalate levels reduced from (10.9 ± 0.1 to 5.1 ± 0.3 g/100 g DM) with increased concentration of NO₃- supply from 1- 10 mM. The biomass of spinach leaves increased from 2.1 ± 0.3 to 3.8 ± 0.4 with little change in % DM (28.2 ± 0.2 to 32.3 ± 0.2) with increased NH₄NO₃ supply from 1-3 mM. The total oxalate levels reduced from 7.6 ± 0.3 to 5.4 ± 0.4 g/100 g DM with increased concentration of NH₄NO₃ from 1-3 mM. biomass of spinach leaves increased from 7.3 ± 0.3 to 9.7 ± 1.0 g/plant FW with little change in % DM (28.8 ± 0.4 to 30.4 ± 0.6) with increased NH₄NO₃ supply from 1-3 mM. The total oxalate levels in spinach leaves decreased from 5.0 ± 0.1 to 2.8 ± 0.1 with increased supply of NH₄NO₃ from 1-3 mM At elevated atmospheric CO₂, the biomass of spinach leaves increased from 8.1 ± 0.6 to 19.8 ± 1.7 g/plant FW and % DM increased from 29.6 ± 1.7 - 35.7 ± 1.2 with increasing concentrations of NO₃- from 1-10mM. The total oxalate levels in spinach leaves reduced from 6.8 ± 0.1 to 2.2 ± 0.3 g/100 g DM with increasing concentrations of NO₃- from 1-10 mM. The In conclusion, across experiments, growth increased with increased N supply (NO₃- and NH₄NO₃) and further increased with elevated CO₂ at all N levels. The total and soluble oxalate levels in spinach leaves were negatively correlated with growth with different nitrogen and CO₂ supply across the experiments.