Influence of growth conditions on the value of crisphead lettuce. 1. Marketable and nutritional quality as affected by nitrogen supply, cultivar and plant age.

The marketable and nutritional quality of crisphead lettuce as affected by nitrogen supply, cultivar, and plant age at harvest was investigated in six plantings during 1989 and 1990. The optimum yield of marketable heads was obtained at a total nitrogen supply of 150 kg N per ha although only small differences were observed to the yield at 100 and 200 kg total N per ha. The total nitrogen supply included the amount of mineral nitrogen within the rhizosphere. The incidence of dry tipburn in older leaves was clearly decreased by an increased nitrogen supply, especially at late planting. The content of nitrate was increased and the content of dry matter and vitamin C decreased with increased nitrogen supply. The vitamin C content was higher for the cultivar 'Marius' than for 'Saladin'. As plants got older, the nutritional quality of crisphead lettuce decreased because the content of nutrients, especially vitamin C, decreased with increased plant age at harvest.