THE GROWTH OF WINTER GLASSHOUSE LETTUCE WITH ARTIFICIAL LIGHT
Work at the University of Nottingham has been concerned with defining critical levels of light and temperature under controlled conditions in growth rooms for satisfactory growth and heading of the cultivar, Cheshunt 5B. Light intensities ranged from 65 to 650 lm ft-2 in 16 hour days, i.e. approximately 3.5 to 35 cal cm-2 day-1 visible radiation from fluorescent daylight and warm white lamps. Some experiments were carried out in short (9 hour) days. The temperatures used were 13°C, 16°C and 20°C.
Net assimilation rate values in the early stages before Leaf Area Index (LAI) equalled 0.5 have been used for calculating the efficiency of energy utilisation, which was found to be higher at lower light intensities. The habit of the plant, however, was undesirable and growth was slow in very restricted amounts of light. There was some evidence that energy conversion was more efficient at 13°C than at 16°C or 20°C.
The compensation point was not reached at the lowest level of light and, by extrapolation, a value of 2.0 cal cm-2day-1 or less is postulated.
The possibilities of growing the crop entirely in artificial light are discussed, but it is concluded that only during the early propagation stage is the use of artificial light likely to be economic. The high efficiency of energy conversion at low light intensities, however, suggests that greater responses may be obtained from lower levels of supplementary light than might have been thought worthwhile.