Effect of Photoperiod on Growth of Sugar Beet

Sugar beet grown in controlled environments were given similar daily amounts of visible radiation during three different photoperiod treatments. Plants were given (a) 115 W m−2 visible irradiance from fluorescent and tungsten lamps for 12 h; (b) 88 W m−2 of the same light for 16 h or (c) 115 W m−2 from fluorescent and tungsten lamps for 12 h extended to 16 h with low intensity (3 W m−2) incandescent light from the tungsten lamps only. Plant growth was increased similarly in both long day treatments [(b) and (c)] and dry weights were 25 per cent greater than in the 12 h photoperiod (a) after 6 weeks. Leaf area was increased by 18 per cent and net assimilation rate by 10 per cent in the 16 h photoperiod at 88 W m−2 (b). By contrast, extending the photoperiod with 4 h of incandescent light (c) triggered a photomorphogenic increase in leaf expansion which increased leaf area per plant by 47 per cent and leaf-area ratio by 12 per cent.