Journal of Taiwan Agricultural Research

Author:Chiao-Ling Hsiao, Chia-Hsun Ho, and Chwen-Ming Yang*

Abstract:

Accumulation of nitrate content in leafy vegetables is affected by not only genetic factors but also nitrogen (N) fertilization and lighting conditions during plant growth. Pot experiments were conducted under a protected structure covered with plastics in an experimental farm located in the Taiwan Agricultural Research Institute. Four amounts of N fertilizers were applied during growing period of lettuce (Lactuca sativa cv. ‘Ming Feng No. 3’). Supplemental lighting was provided by different artificial light sources at 4 or 6 h before harvest. A total of 6 supplemental lighting treatments using light-emitting diode (LED) and energy-saving light bulb (ELB; a compact fluorescent lamp) were used as light sources and plants treated with no light as a control. Results showed that plants treated with N fertilizer at a rate of 30 and 203 kg N．ha^-1 had comparatively lower nitrate contents than those of higher N inputs. Increasing the amount of N fertilizer promoted plant growth. Plants after being applied with higher N rates (346 and 376 kg N．ha^-1) showed better growth, in terms of traits of plant height, aboveground fresh weight and leaf number, however, higher N generated a greater nitrate accumulation. The contents of nitrate were compared in lettuce plants after being treated with 6 h of supplemental lighting of blue light LED (LED_B), red light LED (LED_R), 3 blue/1 red mix LED (LED_3B:1R), 3 red/1 blue mix LED (LED_3R:1B), or ELB before harvest. The results indicated that a lower nitrate content was found in plants treated with supplemental lighting prior to harvest in all 4 nitrogen application rates, compared to that of plants without light treatment. At the highest nitrogen rate of 376 kg N．ha^-1, the nitrate content reduced by 14% and 23% with the treatments of LED_B and LED_3B:1R, respectively. No difference was found among plants treated with 4 h and 6 h of supplemental lighting of either LED_B or ELB in plants applied with 376 kg N．ha^-1. The relationship between photosynthetic rate (i.e., net CO₂ flux) and nitrate content was curvilinear, in which nitrate content was decreased curvilinearly with the increasing of photosynthetic rates in leaves of lettuce. At photosynthetic rate greater than 4.5 μmol CO₂ m^-2 s^-1, nitrate content was maintained in less than 3,000 mg kg^-1 (3,000 ppm). Accordingly, this study demonstrated the effects of nitrogen fertilization on nitrate content and growth performance in lettuce. To further reduce nitrate content at time of harvesting, a supplemental lighting by artificial light sources for 4 h prior to harvest would be as effective as that of 6 h to save energy cost.

Key words:Nitrate, Lettuce (Lactuca sativa), Light environment, Light-emitting diodes (LEDs), Nitrogen fertilization

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