Gas exchange and antioxidant response of sweet pepper to foliar urea spray as affected by ambient temperature

This paper analyses the effect of different air temperatures (10, 20 and 30 °C) on the response of sweet pepper plants (Capsicum annuum L. cv. Herminio) to foliar urea applications after growing plants for 20 day with and without nitrogen (N) applied to the growing substrate. Leaf CO2 assimilation, chlorophyll fluorescence, root respiration, lipid peroxidation and antioxidative enzymes were analysed. Spraying plants with urea increased leaf CO2 assimilation of N-deficient plants when applied at 20 or 30 °C, compared with non-sprayed plants. When plants were sprayed with urea at 10 °C chlorophyll fluorescence of leaves was similar to that of plants that were supplied with full N in the nutrient solution. Root respiration was not affected by urea sprays whilst leaf NO3− concentration was increased by urea but only when it was sprayed at 10 or 20 °C. Lipid peroxidation and ascorbate peroxidase in N-deficient plants were reduced significantly by urea sprays, especially when plants were sprayed at 20 °C with N-limitation in the growing substrate. This study shows that N-limitation in the growing substrate induces a temperature-dependant increase in the activities of antioxidant enzymes in leaves of pepper and applications of foliar urea can be optimised, when applied at the appropriate temperature, to partly replace the N supplied to the roots of sweet pepper.

Research highlights▶ Response of pepper leaves to foliar urea changed with temperature at which leaves were sprayed. ▶ Spraying N-deficient plants with urea at 20 °C, increased leaf CO2 assimilation and leaf NO3− concentration whilst reducing the ascorbate peroxidase activity and lipid peroxidation, compared to N-deficient plants that were not sprayed. ▶ The temperature at which foliar urea is used is a key factor in crop nutrient management practices, when N supply to the roots is limited.