Enzymatic antioxidants in response to methyl jasmonate and salicylic acid and their effect on chilling tolerance in lemon fruit [Citrus limon (L.) Burm. F.]

- Chilling injury symptoms were associated with the source of fruit, storage temperature and storage duration.
- Treatment with 10 μM MJ plus 2 mM SA suppressed ROS accumulation in fruit.
- Treatment with 10 μM MJ plus 2 mM SA reduced CI symptoms in fruit.
- Treatment with 10 μM MJ plus 2 mM SA enhanced CAT, APX, and GR activity.
- Treatment with 10 μM MJ plus 2 mM SA enhanced heat shock protein expression.

To prevent excessive accumulation of reactive oxygen species (ROS), as typically occurs during cold storage, fruit have evolved antioxidant defence mechanisms, such as the production of heat shock proteins (HSPs), that reduce chilling injury (CI). It was hypothesised that treatments with methyl jasmonate (MJ) and salicylic acid (SA) may enhance chilling tolerance in lemon fruit by inducing the production of enzymatic antioxidants. Fully mature lemon fruit were sourced from three production sites with varying climate: moderate subtropical (New Venture Farm), warm temperate (Tala Valley Citrus Estate) and cool subtropical conditions (Sun Valley Estate). The fruit were treated with MJ and SA, waxed and stored at −0.5, 2, or 4.5 °C for 28 days plus 7 days at 23 °C. The manifestation of CI and changes in ROS, as well as in enzymatic antioxidant systems, such as catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR), as well as HSPs were investigated in lemon flavedo. Symptoms of CI were more severe in control fruit stored at 4.5 °C than at 2 or −0.5 °C. Fruit sourced from the moderate subtropical and warm temperate locations had higher enzymatic antioxidant activities (CAT, APX and GR), a higher HSPs expression and a lower accumulation of ROS than those sourced from the cool subtropical location. Treatment with 10 μM MJ plus 2 mM SA reduced CI, suppressed ROS production, increased CAT, APX and GR activity, and enhanced the accumulation of HSPs, suggesting that enzymatic antioxidants and HSPs are involved in conveying chilling tolerance to MJ- and SA-treated lemon fruit. The increase in activity of these antioxidant enzymes, together with HSPs, could be part of the mode of action by which MJ and SA convey chilling tolerance to lemon fruit.