Basic study on material production in plant factory. (1). Effects of photoperiod, light intensity, and CO2 concentration on photosynthesis of lettuce.

To collect basic data for a plant factory, effects of photoperiod, light intensity, and CO2 concentration on the photosynthesis of lettuce were studied throughout the growing period under the conditions of 12-24 h photoperiod, 400-3200 ppm CO2 concentration, and 137-316μE/m2/s light intensity. The results obtained are: (1) The net photosynthetic rate per plant (NPR) of lettuce decreases with increasing photoperiod when either the CO2 concentration or the light intensity was increased; (2) At a photoperiod of 24 h, the effect of CO2-enrichment on the increases in the fresh weight and the dry weight is greater at a lower light intensity of 137 μE/m2/s; (3) At a light intensity of 237 μE/m2/s, both the stomatal resistance and the CO2 compensation point of lettuce are increased by CO2-enrichment, the extent of these increases being greater at a photoperiod of 24 h.Thus, in the case of lettuce, the stomatal resistance and the CO2 concentration in leaves increase when the integral amount of light energy illuminated per day is increased, which suggest that the CO2 uptake rate is decreased. It may be concluded that either the continuous illumination of low light intensity or the intermittent illumination of high light intensity is useful for improving the cost effectiveness of cultivating lettuce in plant factory.