Bolting Susceptibility of Sugar Beet (Beta vulgaris) in Relation to Contents of Sulfhydryls and Disulfides and to Protein Composition of Membranes

The aim of the investigation was to correlate the sensitivity to low temperature that leads to bolting of sugar beet with a property of seedlings or seeds, analyzable without cold treatment. Populations or inbred lines were investigated, the bolting percentages of which had been determined in field trials. Sulfhydryls (—SH) and disulfide sulphur (‐S‐) were analyzed by amperometric titration with silver nitrate. Disulfides were broken with sulphite, and proteins were unfolded with urea. Negative correlations were found between the bolting percentage and (‐SH + ‐S‐) per leaf fresh or dry weight unit and per leaf protein nitrogen, with urea at the titrations. Positive correlations were obtained between the bolting percentage and the ratio of (‐SH + ‐S‐) titratable in the absence of urea to that titratable in the presence of urea, when leaf homogenates or leaf or seed protein precipitates were examined. Centrifugation experiments showed that membrane proteins were responsible for the correlation. By polyacrylamide gel electrophoresis of the membrane proteins it was found that the proportion between some of these proteins was correlated with the bolting percentage. From these results and such from analyses of beet plants during and after cold treatment a hypothesis was constructed: Low bolting susceptibility, i.e., low sensitivity to low temperature, is caused by a high proportion of a hydrophobic membrane protein, rich in ‐SH or ‐S‐. This protein interferes with the reactivity of the plants to gibberellins resulting from low temperatures or long days.