Effects of night lighting and critical illuminance on the growth and bolting of spinach cultivars

Abstract

The problem that night lighting of the street lamp caused bolting of spinach grown under a plastic greenhouse have happened. Therefore, effects of night lighting and critical illuminance (light intensity under which over 80% of the spinach plants are marketable) on the growth and bolting of spinach cultivars grown under a plastic greenhouse were investigated and evaluated. 1. The growth and flower stalk development of plants grown under night lighting differed among cultivars and between sowing dates. 2. The natural photoperiod and the average temperature in greenhouse during the November experiment were about 11-hour, 13 °C and during the May experiment were about 15-hour, 23 °C. When plants were grown under night lighting of high illuminance, such as 25 lx (0.31 μmol · m-2 · s-1), plants height and the number of leaves per plant increased but leaf size remained small. May-sown seedings did not grow as tall as those sown in November. May-sown plants grown under night lighting of 2 (0.02) to 3 lx (0.03) had more leaves but of shorter length. 3. In plants exposed to night lighting of 1 (0.01) to 25 lx in November, the day to flower stalk development and flowering were promoted at the higher intensities; the converse was true in the May-sown plants. Thus, in cultivars such as 'Okame' sown in May, flower stalks appeared at relatively low light intensities, whereas November-sown plants bolted under high light intensities. 4. Likewise, the critical illuminance of cultivars, such as 'Parade' sown in November ranged from 3 to 5 lx (0.05), whereas in May-sowing plants, 50-99% of the plants at 1 lx bolted. Critical illuminance for 'Okame' and 'Tonick' sown in November, it was 2 to 3 lx and 13 lx (0.16) respectively.