Ethylene production under high temperature stress causes premature leaf senescence in soybean

Leaf senescence in soybean (Glycine max L. Merr.) occurs during the later stages of reproductive development and can be triggered or enhanced by high temperature (HT) stress. Ethylene production can trigger premature leaf senescence, but it is unclear whether HT stress produces ethylene and the subsequent influence on physiology and yield of soybean is also uncertain. We hypothesised that ethylene produced under HT stress is involved in premature leaf senescence and that use of an ethylene perception inhibitor would influence physiology and yield. Objectives of this study were to (1) quantify HT-stress-induced ethylene production; (2) quantify effects of HT stress and application of an ethylene perception inhibitor (1-methylcyclopropene; 1-MCP) on source strength traits such as photosynthetic rate, oxidant production, membrane damage and sugar accumulation; and (3) evaluate efficacy of 1-MCP on minimising HT-stress-induced effects on physiological and yield traits. Soybean plants were exposed to HT (38/28°C) or optimum temperature (OT, 28/18°C) for 14 days at the beginning of pod set. Plants at each temperature were treated with 1μg L⁻¹ 1-MCP or left untreated (control). HT stress enhanced ethylene production rates in leaves and pods by 3.2- and 2.1-fold over OT. HT stress decreased photochemical efficiency (5.8%), photosynthetic rate (12.7%), sucrose content (21.5%), superoxide dismutase (13.3%), catalase (44.6%) and peroxidase (42.9%) enzymes activity and increased superoxide radical (63%) and hydrogen peroxide (70.4%) content and membrane damage (54.7%) compared with OT. Application of 1-MCP decreased ethylene production rate and premature leaf senescence traits by enhancing the antioxidant defence system. HT stress decreased seed set percentage (18.6%), seed size (64.5%) and seed yield plant⁻¹ (71.4%) compared with OT, however, foliar spray of 1-MCP increased the seed set percent and seed size, which resulted in a higher yield than the unsprayed control. The present study showed HT stress increased ethylene production rate, which triggered premature leaf senescence, whereas 1-MCP application reduced or postponed premature leaf senescence traits by inhibiting ethylene production.