EFFECT OF NITROGEN FERTILIZATION AND PLANT POPULATION DURING GROWTH ON LETTUCE (Lactuca sativa L.) POSTHARVEST QUALITY
Higher relative growth rates and lower dry matter content in leaf tissues can be expected when the N availability in the soil is increased by fertilization. A high nitrate uptake leads to increased nitrate accumulation in lettuce leaves. Nitrate reduction may be limited by many factors such as light intensity and plant population.
Nitrate content in leaf tissues at harvest affects commercial and nutritional quality. It is strongly related to phenolic metabolism during storage and its accumulation tends to increase polyphenoloxidase (PPO) activity, as well as concentrations of chlorogenic acid and phenolic compounds (Leja et al., 1995). Moreover, part of the nitrate ingested by human beings may be converted into nitrite, thereby causing methaemoglobinemia or even formation of carcinogenic nitrosamine (Craddock, 1983). Since preharvest conditions such as N availability in the soil affect lettuce quality and postharvest behavior, we hypothesize that adjustments in the level of N application are required when the number of plants grown per area is increased.