Interrelations between photoperiod, frost hardiness and sulfhydryl groups in cabbage.

Hardening of SD (8 and 12 hrs) and LD (18- and 24-hr photoperiods) cabbage plants in stages at temperatures starting with + 5 degrees and ending with - 3 degrees led to the following changes:Soluble protein plus nonprotein N showed a net increase only in the SD plants. In both SD and LD plants, it decreased to a minimum toward the end of the first stage of hardening, increased to a maximum in the second stage. The degree of this change was proportional to the photoperiod. These changes were mainly due to the proteins.As in previous investigations, SH content rose during the first 1 to 2 weeks, but only in the SD plants. This rise was primarily due to the protein fraction. After the first 1 to 2 weeks both SD and LD plants showed a decrease in SH content, most pronouncedly during the second stage of hardening.Nonprotein SH content was very low and decreased during hardening in both SD and LD plants. Nonprotein SS increased during the first stage of hardening. Total nonprotein SH+2SS rose to a maximum during the first stage of hardening, paralleling both osmotic potential and hardiness. During the second stage the total decreased, in spite of the rise in hardiness. These results held true for both SD and LD plants.These results are all explainable on the basis of the SHright harpoon over left harpoonSS theory of frost resistance.