Abstract
A pot experiment conducted in a growth chamber studied the effectiveness of foliar fertilization of spinach (Spinacia oleracea L.), with different forms of nitrogen applied with the air temperatures: 5°C, 15°C and 25°C. Nitrogen was supplied three times in the form of 1% water solutions of CO(NH2)2, NH4NO3, NH4HCO3 and Ca(NO3)2 × 4 H2O, with water as the control. The obtained results showed that nitrogen in the form of Ca(NO3)2 × 4 H2O - independent of the air temperature during solution application - is the least useful in foliar fertilization of spinach leaves, whereas the effectiveness of the other forms was similar, although the best effects were given by NH4NO3. Application of all forms of nitrogen at the air temperature of 25°C had the most positive effect on the course of gas exchange (stomatal conductivity, photosynthesis, transpiration). Despite that, the highest leaf yield with the highest content of nitrogen, total chlorophyll (a + b), carotenoids and vitamin C, and the lowest content of nitrates, was given by plants with foliar application of all nitrogen forms at the temperature of 15°C.