Nitrate concentration of soilless lettuce plants following nutrients withdrawal from the nutrient solution: a way of reducing nitrate

摘要

An experiment was conducted to assess the effects of nutrient withdrawal from the solution on the nitrate content of lettuce. The lettuce seeds (Luctuca sativa var.Mural) were sown in the tray and then transplanted into round pots (101) filled with perlite and sand (1:1, v:v). The plants were fed with nutrient solution containing three levels of N (100, 200 and 300 mg l~(-1)). One week before harvesting, the nutrition of half of the plants was stopped and just water applied to the plants (100/0, 200/0, 300/0) while, supplying nutrients to the other half continued (100/100, 200/200, 300/300). The experiment was arranged in a completely randomized design with six replicates. As nutrients withdrawal occurred, NO_3 concentration in the outer leaves was measured. The fresh weight and chlorophyll index of the plants were measured at the end of experiment. The highest (1500 mg kg~(-1) Fwt) and lowest (600 mg kg~(-1) Fwt) nitrate concentration was observed in 300/300 and 300/0 treatments, respectively. Fresh weight of the individual plants increased with the increase of N supply. Four days after nutrient withdrawial, the concentration of NO_3 in 200/0 and 300/0 treatments was 25% and 57% less than those in 200/200 and 300/300 treatments, respectively. However, no reduction of fresh weight occurred as the feeding stopped within four days. Nutrient withdrawial significantly (P < 0.01) reduced NO_3 concentration in the stems and midribs of the leaves. Although the initial NO_3 concentration in 200/200 and 300/300 treatments tended to be higher, the reduction of NO_3 in 300/0 treatment was much higher than that of 100/0 treatment at the end of experiment. Initial high concentration of NO_3 in 300/0 treatment is most likely to compensate the reduced NO_3 in the root zone therefore, the reduction in fresh weight was negligible. There was no effect of nutrients withdrawing on the chlorophyll index.