Response of sugar beet (Beta vulgaris L.) to photoperiod before and after bolting and flowering

This research was carried at The University of Reading in 1999. The aim was to estimate the effects of 18 hd^-1 (long-day, LD) and 12 hd^-1 (short-day, SD) light exposure duration of plants before bolting or flowering on time to bolt and flower in two bolting susceptible (BSG, IC1) and bolting resistant (BRG, SAXON) genotypes of sugar beet. So, two experiments were conducted. The experiments were carried in a glasshouse containing two compartments. The photoperiod in compartments one and two was adjusted to 18 hd^-1 and 12 hd^-1,respectively. In the experiment one, the aim was to determine duration from root replanting to bolting and flowering. So, plants were exchanged from LD to SD and vice versa at 0, 8, 21, 33, 40, and 47 days after root replanting.  In the experiment two, the aim was to determine the duration from bolting to flowering. So, at first step we let all the plants bolt in the LD and then they were reciprocally exchanged from LD to SD and vice versa at 0, 8, 16, 24, 40, and 30 days after bolting. Results showed that BSG responded to LD from root replanting whereas BRG had a longer photoperiod-insensitive pre-inductive phase. The resistant genotype plants did not bolt even after 180 days. When BSG plants were exposed to SD after bolting all of them  eventually flowered. This results indicated that by the time of bolting, photoperiod requirement for flowering was also fulfilled, whereas BRG needed further inductive photoperiod after bolting to flower. Therefore, bolting and flowering are two independent phenomena and they require different LD cycles. BSG is a quantitative LD plant in which bolting and flowering occur sooner in LD than in SD. However, very resistant genotypes (e.g. Saxon) showed a more or less obligate requirement for LD and did not bolt in 12 h photoperiod within 180 days after root planting. So, it is suggested that sensitivity to day-length seems to be a good selection tool to reduce the tendency for bolting in plant populations.