RESPONSE OF HYDROPONIC LETTUCE CULTIVARS TO DIFFERENT TREATMENTS OF NITROGEN: GROWTH AND FOLIAR NITRATE CONTENT
The nitrogen requirements of lettuce (Lactuca sativa L.) cultivars for obtaining plants of a high phenotypical quality and low nitrate foliar content were studied in this research. Butterhead (Divina and Prima cvs.) and loose-leaf (Grand Rapids and Brisa cvs.) lettuces were grown in a modified hydroponic floating system. Plants were subjected to four treatments differential of nitrogen and are in mg of nitrogen L-1: solution 1: 100 NO3- -N; solution 2:150 NO3--N and 50 NH4+ -N; solution 3: 200 NO3--N and 100 NH4+-N; solution 4: 250 NO3- -N and 150 NH4+ -N meanwhile as base solution was used modified Hoagland II solution. Cultivar responses to treatments were different so much in growth as foliar nitrate content. While the greatest growth was observed in‘Grand Rapids’ and ‘Brisa’ in solution 1 with the lowest nitrogen content in which NO3- -N only was supplied, the opposite was observed in ‘Divina’ and ‘Prima’ in solution 4 and where NO3-and NH4+ were supplied. These results showed that the nutritional requirements cultivars are different. Moreover, greatest plant growth was observed in cultivars where the foliar nitrate content was highest. High levels of nitrate in vegetable crops have been associated with human health problem. The four cultivars in all treatments showed foliar nitrate contents below the limits established by the European Commission Legislation. This study permit to optimize the growth under such conditions.