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 affecte d by nitrogen supply, cultivar, and plant age at harvest was investiga ted in six plantings during 1989 and 1990. The optimum yield of market able 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 o f mineral nitrogen within the rhizosphere. The incidence of dry tipbur n in older leaves was clearly decreased by an increased nitrogen suppl y, especially at late planting. The content of nitrate was increased a nd the content of dry matter and vitamin C decreased with increased ni trogen supply. The vitamin C content was higher for the cultivar 'Mari us' than for 'Saladin'. As plants got older, the nutritional quality o f crisphead lettuce decreased because the content of nutrients, especi ally vitamin C, decreased with increased plant age at harvest.