EFFECTS OF PHOTOPERIOD AND PHOTOSYNTHETIC PHOTON FLUX ON NITRATE CONTENT AND NITRATE REDUCTASE-ACTIVITY IN GREENHOUSE-GROWN LETTUCE

Lettuce plants (Lactuca saliva, cvs 'Karlo' and 'Rosana') were subjected to three light levels (0, 50, and 100 mu Lmol/m(2)/s) provided by supplementary lighting. Photoperiods extended to 16 h, 20 h, 24 h, and 24/16 h were compared with natural light in the course of five greenhouse trials. The use of supplementary lighting significantly reduced nitrate concentration in the leaves during the winter. Total nitrate reductase enzyme activity in terms of whole plants was higher owing to an increase in light levels, whereas there was no significant difference in terms of fresh weight for the various light treatments. Fluctuations:in nitrate reductase activity were observed over a 24-hour cycle. Enzyme activity was the lowest between 11:00 a.m. and 1:00 p.m. Nitrate concentration and nitrate reductase activity were the highest in the outer leaves.