Bulb Formation in Onions as Affected by Photoperiod and Spectral Quality of Light

In glasshouse experiments bulb formation occurred in 5 north-temperate onion cultivars in photoperiods of 14–15 h and longer. Some response was obtained by giving 14 long days, but a greater response was obtained when the plants were kept in continuous long days. Three cultivars in the Rijnsburger group all differed from each other in the extent of their bulb development in all of several photoperiodic regimes tried, but there was no cultivar × photoperiod interaction. Thus it is possible that cultivars could usefully be compared in the glasshouse for earliness by providing any one of a wide range of photoperiodic treatments.

When grown in controlled environments with artificial light, little or no bulb formation occurred in cv. Rijnsburger with warm-white fluorescent lamps, but with de-luxe warm-white lamps it occurred when the photoperiods were 17 or more hours long. These lamps appeared to be elfective because they emitted a greater proportion of far-red light than the warm-white lamps, although not as much as sunlight. Because it is possible that the sensitivity of cultivars to the spectral composition of light may vary, it is concluded that attention should be given to this in comparative studies of the photoperiodic requirements of different cultivars.