Effects of nh4+–n/no3−–n ratios on photosynthetic characteristics, dry matter yield and nitrate concentration of spinach

Most plants prefer nitrate (NO₃⁻–N) to ammonium (NH₄⁺–N). However, high NO₃⁻–N in soil and water systems is a cause of concern for human health and the environment. Replacing NO₃⁻–N in plant nutrition regimes with an appropriate amount of NH₄⁺–N may alleviate these concerns. The purpose of this study was to evaluate the effects of different NH₄⁺–N/NO₃⁻–N ratios on chlorophyll content, stomatal conductance, Rubisco activity, net photosynthetic rate, dry matter yield and NO₃⁻–N accumulation in spinach grown hydroponically. The NH₄⁺–N/NO₃⁻–N percentage ratios were 0:100 (control), 25:75, 50:50, 75:25 and 100:0. Chlorophyll a and b, total chlorophyll, stomatal conductance, initial activity and activation state of Rubisco and net photosynthetic rate in spinach leaves were all reduced by increased NH₄⁺–N/NO₃⁻–N ratios. Significant correlation existed between these measurements. However, no statistical differences in dry matter yield were revealed between the 0:100 and 25:75 treatments. Leaf nitrate concentrations were reduced by 38% at the 25:75 treatment relative to the 0:100 treatment. These findings suggest that lowering the relative proportion of NO₃⁻–N in fertilizer could effectively reduce NO₃⁻–N contents in leafy vegetables without decreasing their yields.