Abstract
Plants actively respond to changes in photoperiods, light intensities, and temperatures; an interplay of these three factors can result in increased biomass, growth, and improved plant morphology. This study aimed to investigate the effects of different photoperiods, light intensities, and day/night temperatures on the growth and quality of glasshouse-grown lettuce, as well as to determine the optimal photo-thermal conditions. Two separate experiments were conducted; in the first experiment, the effects of photoperiods (11, 14, and 17 h d(-1)) and light intensities (150, 200, 250, and 300 mu mol m(-2) s(-1) PPFD) were investigated. In the second experiment, the effects of day/night temperature combinations (15/20, 20/20, 25/20, and 30/20 degrees C) were assessed. The highest growth, biochemical, and physiological attributes of lettuce were obtained either at 14 or 17 h d(-1) + 200 and 250 mu mol m(-2) s(-1) PPFD. Lettuce plants grown at a 20/20 degrees C day/night temperature also showed promising results compared to other temperature combinations. Based on a principal component analysis and factor analysis of mixed data analyses, most of the lettuce attributes showed a negative correlation with an 11 h d(-1) photoperiod and a positive correlation with 14 and 17 h d(-1) photoperiods. The most positive responsive attributes to higher light intensity were leaf fresh weight:root fresh weight ratio, root dry, and water use efficiency. An increase in the day/night temperature significantly affected the N, P, K, root fresh weight:leaf fresh weight ratio, E, Pn, and root dry weight. Our findings suggest that the growth, biochemical, and physiological attributes of lettuce significantly increased to a certain level of light intensity (200 and 250 mu mol m(-2) s(-1) PPFD), photoperiod (14 and 17 h d(-1)), and day/night temperature (20/20 degrees C), referred to as an effective combination.