Effects of nitrogen on accumulation and partitioning of dry matter and nitrogen of vegetables .3. Spinach

Six experiments were carried out with different amounts and different dates of application of nitrogen to analyse the dynamics of dry matter and nitrogen accumulation in spinach. Frequent measurements were carried out on dry matter and nitrogen accumulation in leaf blades, petioles and stems. The total accumulation of dry matter and nitrogen differed largely among and within experiments. More nitrogen increased yield of dry matter and nitrogen, whereas splitting nitrogen application had much smaller effects. However, the partitioning of dry matter or nitrogen in spinach proved insensitive to nitrogen treatments. Harvest indices for dry matter (about 0.67) or nitrogen (about 0.74) of crops in a marketable stage were fairly constant over treatments and experiments. Increasing or splitting the nitrogen application affected the nitrogen accumulation more than the dry matter production resulting in large effects on N concentrations. The lack of variation in response to N for different N regimes facilitates the development of N application techniques aiming at high yield, high quality and reduced emission. The organic nitrogen concentration of leaf blades and petioles decreased with leaf age, although in most experiments this decrease was smaller at higher leaf numbers. The nitrate nitrogen concentration decreased with increasing leaf number at any sampling date. It was higher when nitrogen was abundant. High yields in autumn crops were associated with high nitrate concentrations but also with potentially high losses of nitrogen.