The effect of nitrogen form and air temperature during foliar fertilization on gas exchange, the yield and nutritive value of spinach (Spinacia oleracea L.)

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.