Salicylic acid and H2O2 function by independent pathways in the induction of freezing tolerance in potato

The effects of salicylic acid (SA) and hydrogen peroxide (H 2 O 2 ) on freezing tolerance were studied in two potato (Solanum tuberosum) cultivars (Alpha and Atlantic) that differ in cold sensitivity, Alpha being more tolerant to freezing than Atlantic. Lowest freezing survival rates were observed in 4-week-old plants. Freezing treatments consisting of exposure to 6° C for 4 h in the dark were applied 24 h after plants had been transferred from in vitro culture to soil. Catalase activity and H 2 O 2 were estimated at the following harvest points: stage (a) 4-week-old in vitro plants treated with either 0.1 mM SA or 5 mM H 2 O 2 ; stage (b) as in (a) but 24 h following transfer to soil prior to freezing treatment; stage (c) as in (b) but measured 15 days after a 4-h freezing treatment. The results show that (1) SA induced freezing tolerance in both cultivars; (2) SA inhibited ascorbate peroxidase activities in both cultivars at all harvest points but inhibited catalase activities in only at stage (a); (3) SA induced H 2 O 2 accumulation only in Atlantic at stage (a); (4) H 2 O 2 enhanced shoot catalase activities in Atlantic at stages (a) and (b) whereas this treatment had no effect on shoot catalase activities in Alpha; (5) H 2 O 2 treatment induced freezing tolerance in Atlantic, even though shoot catalase activities were lower than those of the controls following exposure to freezing temperatures. We conclude that SA does not always lead to H 2 O 2 accumulation even though catalase and ascorbate peroxidase activities are decreased as a result of the treatment. Moreover, H 2 O 2 accumulation is not always associated with the induction of freezing tolerance, for example at stage (a) where SA-induced tolerance in Alpha was not accompanied by H 2 O 2 accumulation. H 2 O 2 was able to induce freezing tolerance only in Atlantic, even though H 2 O 2 accumulated in both cultivars following this treatment.