Nitrate Accumulation, Productivity and Photosynthesis of Temperate Butter Head Lettuce under Different Nitrate Availabilities and Growth Irradiances

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 than under 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.