Abstract
The purpose of this study is to measure the time variation of nitrate concentration in whole lettuce plants throughout cultivation. Lettuce plants were cultivated in growth chambers under fluorescent or light-emitting diode (LED) lamps. Four different light/dark periods and two ratios of red to blue photon flux density (R/B ratio) were prepared. The cultivation terms utilized were 1 or 2 weeks, and 289 lettuce plants were harvested. The visible-near-infrared absorption spectrum and nitrate concentration were obtained for each sample by spectrometry and the RQflex method, respectively. A regression model for the nitrate concentration was constructed from the spectra, and the correlation coefficient and standard error between the measured and estimated concentrations were 0.77126 and 1100.4 mg L−1, respectively. The spectra of the lettuce plants under the LED lamps were obtained at intervals of 20 min for 14 d. The time variations of the nitrate concentrations in the lettuce plants were obtained by using the regression model. The time variation could be approximated by a concave-down quadratic function. It was found that the nitrate concentration at harvest was lower in the cases in which the peak concentration occurred earlier.
