Oxidative stress tolerance and salicylic acid levels in early-flowering populations derived from two cultivars of annual flax (Linum usitatissimum L.)

The accumulation of reactive oxygen species (ROS) causes oxidative stress. Long-lived organisms should exhibit greater oxidative stress tolerance than short-lived organisms. For annual plants, such as flax (Linum usitatissimum L.), flowering time and lifespan are positively correlated. On this basis, early-flowering populations of two flax cultivars (Royal [R] and Stormont Cirrus [L]) were predicted to exhibit lower oxidative stress tolerance than normal-flowering controls. Oxidative stress tolerance was assessed by growing plants in water or 30 mM H₂O₂ and measuring i) mitochondrial uncoupling, via measurements of intact leaf respiration in the presence and absence of an uncoupling agent, ii) catalase activity, and iii) peroxide-induced cell membrane damage, via an electrolyte leakage assay. Endogenous salicylic acid (SA) levels were also measured since SA regulates both oxidative stress tolerance and flowering time. Early- and normal-flowering populations did not differ significantly for any of these parameters, suggesting that L. usitatissimum has evolved such low oxidative stress tolerance (as an annual species) that it cannot be further reduced. Differences were found between the two cultivars. Mitochondrial uncoupling was < 100% in R plants but > 100% in L plants, suggesting that oxidative phosphorylation was inhibited by the uncoupling agent in the latter but uncoupled in the former; catalase activity was higher in L plants than R plants, especially in early-flowering populations grown in H₂O₂, suggesting L plants eliminate ROS more rapidly; and peroxide-induced cell membrane damage was higher in L plants than R plants, suggesting that R plants experience less oxidative damage to their membrane phospholipids. SA may play some role in these cultivar-specific responses. As R and L cultivars are bred for seed oil and fibres, respectively, their differences may reflect trade-offs between oxidative stress tolerance and trait selection.