The photoperiodic regulation of bulbing in onions (Allium cepaL.). I. Effects of irradiance

Abstract

SummaryA high level of photosynthetically active radiation combined with a long photoperiod accelerated both bulbing and final bulb size in onion. Plants grown under a short photoperiod with a low irradiance level produced bulbs earlier when transferred to a long photoperiod with high irradiance than plants which had received a short photoperiod with a high irradiance level and were then moved to long photoperiod with low irradiance. Therefore, immediate photosynthesis seems to be more important than stored assimilates for bulbing. The radiation prior to the transfer to the inductive conditions was shown to affect the time to bulbing. Only 6 weeks of a long photoperiod with a high irradiance gave a similar bulb size as plants which received 17 weeks of a long photoperiod with low irradiance. Although scale leaf differentiation is controlled by long photoperiod their numbers were affected greatly by irradiance level.