Abstract:

Under low growth-irradiance and/or excessive nitrogen (N) fertilization, the roots of leafy vegetables could take up nitrate (NO₃^-) faster than the plant can convert it to organic nitrogen compounds. NO₃^- is suspected to have carcinogenic effect in human when eaten in high quantity. In this study, lettuce plants were first grown in an aeroponics system with full nutrients (full NO₃^–) under full sunlight. Six weeks after transplanting, plants were subjected to 7 days of full sunlight and shade with full, 1/2 and 0 NO₃^– respectively. Shoot NO₃^– concentration was higher under shade thanunder full sunlight regardless of NO₃^– availability after 7 days of treatments. The higher shoot NO₃^– concentration of shade plants was derived from the high NO₃^– accumulated in their roots during the 6 weeks of growth prior to treatments. There were no significant differences in NO₃^– concentrations of shoot and root after re-exposing all plants to full sunlight and full NO₃^– for another 7 days. Total shoot reduced N concentrations were similar among all plants regardless of treatments. These results indicate that reduction or withdrawing NO₃^– from nutrient solution did not affect N metabolism. Low productivity and photosynthesis under shade condition did not result from NO₃^– availability but they were directly caused by low growth irradiance. Thus, to prevent high accumulation of NO₃^– in the shoot, it may be a good practice to withdraw NO₃^– from nutrient solution during cloudy days or to extend the plant growth period a few more days under full sunlight before harvest.