Dissecting the heat stress-induced alterations in the leaf ultrastructure and some antioxidant network components in interspecific (Cucurbita maxima x Cucurbita moschata) inbred line of squash 'Maxchata' as to its parents possessing variable heat tolerance

We previously found 'C. moschata' to be heat-tolerant', 'C. maxima' to be heat-susceptible, and their interspecific (Cucurbita maxima x Cucurbita moschata) inbred line 'Maxchata' to exhibit intermediary thermotolerance through its photosynthetic response. In this work, we investigated the ultramorphological, biochemical, and transcriptional changes induced in the leaves of 'Maxchata' and its parents upon exposure to moderate (37 A degrees C) and severe (42 A degrees C) heat stress to obtain insights into heat-tolerance indicators. The results showed that heat stress caused a relatively lower degree of membrane damage with lower malondialdehyde level and electrolyte leakage and higher proline contents in 'C. moschata' and 'Maxchata'. The electron microscopy highlighted the maximum degradation of the leaf ultrastructure of 'C. maxima' upon heat exposure. In contrast, 'C. moschata' and 'Maxchata' exhibited lower degree of subcellular injury. The enzymatic activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and guaiacol peroxidase (POD) were found to be highest in 'C. moschata', moderate in 'Maxchata', and lowest in 'C. maxima'. In addition, the gene expressions of the CAT (CAT1, CAT2, and CAT3), SOD (Cu/ZnSOD, FeSOD, and MnSOD), and APX (APX1 and APX2) isozymes were unique in each of the studied squashes with increase in temperature indicating their differential contribution to heat tolerance. Although the selected genes were genetically similar in 'C. maxima' and 'Maxchata', most of the genes were more highly induced at elevated temperatures in 'Maxchata' compared with 'C. maxima'. Hence, the heat stress-induced gene expression in 'Maxchata' improved the efficiency of its antioxidant network and provided it some degree of thermotolerance.