Effects of different light intensities on chlorophyll fluorescence characteristics and yield in lettuce

The worldwide demand for Romaine lettuce (Lactuca sativa L.) is increasing. Regulation measures of supplementary or shading light are often used in the production of lettuce in some regions. However, inconsistent results on light saturation point of lettuce from previous studies did not facilitate the regulation of light intensity. In the present study, the effects of different light intensities on chlorophyll fluorescence characteristics and yield in lettuce were investigated. The results show that in the 100 and 800 μmol m−2 s−1 treatments, the values of qP, ΦPSII, and ETR were low, resulting in low light use efficiency and plant yield in lettuce. In the 200, 400, and 600 μmol m−2 s−1 treatments, the values of qP, ΦPSII, and ETR were high, resulting in high light use efficiency and plant yield in lettuce. Light-use efficiency in the 200 μmol m−2 s−1 treatment was the highest, whereas plant yield in the 600 μmol m−2 s−1 treatment was the highest. No significant difference in plant yield was observed between the 400 and 600 μmol m−2 s−1 treatment. In addition, judging by the values of Fv/Fm, the lettuce samples were under rather serious stress in the 800 μmol m−2 s−1 treatment, whereas they were under mild stress in the 600 μmol m−2 s−1 treatment. Based on these results, an optimal regulation strategy of light intensity that can maximize economic benefits for lettuce growers in some regions of the world is proposed. That is the range of 400–600 μmol m−2 s−1 is a recommendable light intensity for production of certain types of lettuce, and light intensity of 400 μmol m−2 s−1 can be an optimal value of supplementary light for winter greenhouse production of certain types of lettuce in higher latitudes, while light intensity of 600 μmol m−2 s−1 can be an optimal value of shading light for late spring and early autumn production of certain types of lettuce in lower latitudes.