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.