2017 ASHS Annual Conference

Nutrient solution concentration for hydroponically grown lettuce in plant factories and greenhouses is adjusted based on electrical conductivity (EC) of the recycled solution. This is not an accurate method as EC of the solution does not indicate the concentration of individual nutrients in the supplied solution. A better approach is to adjust nutrient solution concentration using plant-based measurements. Green leaf lettuce was grown in 4-inch containers and sub-irrigated with five different levels of nitrogen in the fertilizer (83, 150, 168, 208, and 250 ppm N resulting in EC levels of 1.0, 1.8, 2.0, 2.5 and 3.0 dS/m). Reflectance from plants in the range of 400 and 700 nm was measured using a spectro-radiometer at three different times during the growth. The ratio of reflectance in the green (496 to 570 nm) to the sum of reflectance in the red (620 to 700 nm), green and blue (450 to 495 nm) wavelengths was calculated for lettuce plants in different treatments. In addition, dry weight of lettuce plants in different treatments was measured at harvest. Results indicated that dry weight of green leaf lettuce increased quadratically with increasing reflectance ratio of plants. Potential use and the physiological basis for the correlation between the reflectance ratio and dry weight will be discussed in this presentation.