Inhibition of the alternative respiratory pathway at high temperatures leads to higher reactive oxygen species production and downregulation of the antioxidant defense system in squash leaves

In this work, the role of the alternative respiratory pathway in the heat tolerance of squashes was determined. Heat-tolerant C. moschata, heat-susceptible C. maxima and intermediate heat-tolerant 'Maxchata' plants were sprayed with an inhibitor of the alternative respiratory pathway, 2 mM salicylhydroxamic acid (SHAM), or water and were subjected to three different temperatures, i.e., 30 A degrees C (control), 37 A degrees C daytime temperature (moderate heat stress) or 42 A degrees C daytime temperature (severe heat stress), for 7 days. The results indicated that total, cytochrome and alternative respirations were high in C. moschata and 'Maxchata' at both 37 and 42 A degrees C, but decreased in C. maxima at 42 A degrees C. Moreover, the increase in reactive oxygen species production upon heat exposure was also more apparent in SHAM-treated squashes compared with water-treated ones. The transcript levels of APX1, APX2,CAT1, CAT2, CAT3, Cu/ZnSOD, FeSOD and MnSOD were determined in both the SHAM- and water-treated squash leaves that were exposed to heat stress. It was observed that CAT1 and FeSOD were downregulated in SHAM-treated C. moschata plants under heat stress conditions. In addition, the expressions of CAT2 and MnSOD were lower in SHAM-treated C. moschata and 'Maxchata' plants. These results indicate some type of interrelationship between AOX and antioxidant systems. However, the complexity of the regulatory mechanisms and their contribution to the stress tolerance of squashes requires further research.