Environmental, Chemical, and Genetic Reduction of Ethylene Sensitivity in Crop Plants

Ethylene is an endogenously synthesized plant hormone that dissipates quickly in field conditions and seldom exceeds five nmol mol-1. Ethylene can accumulate to 1000 times this level in closed environments. The best-known effects of ethylene are its impacts on fruit ripening and senescence, yet ethylene influences growth and development throughout the plant life cycle. At low, continuous concentrations (20 to 50 nmol mol-1), ethylene reduces yield of many plants. Clean-air treatment during critical stages of floral development, silver thiosulfate (STS), and 1-methylcyclopropene (1- MCP) may delay flower senescence and reduce the detrimental effects of ethylene on peas and tomatoes grown in continuous ethylene. There is evidence of species differences in ethylene sensitivity, but limited information on cultivar differences. To address these issues, ethylene sensitivity of two dwarf tomato cultivars, Micro-Tom and Micro-Tina, and one dwarf pea cultivar, Earligreen, was examined. Ethylene by temperature interactions were examined in tomatoes at 0, 20, and 40 nmol mol-1 ethylene and 22 and 28°C. Three-day-long clean-air treatments were applied to tomatoes from germination through fruit set to identify the most ethylene-sensitive stage of floral development. The actions and toxicities of STS and 1-MCP were compared. Ethylene sensitivities of the two closely related dwarf tomato cultivars were examined. At 22°C, the 20 and 40 nmol mol-1 red fruit yields were 51 and 11 % of control. At 28°C, yields were 37 and 4% of control. Vegetative growth at 20 and 40 nmol mol-1 was 96 and 91 % of control, at both temperatures. Three-day-long clean-air treatments from days 22 to 33 (axillary flower opening) improved fruit set and final yield. Floral bud abortion in elevated ethylene occurred primarily at or before microsporogenesis. Floral bud initiation and vegetative development were not significantly affected. Tomato plants grown in continuous 70 nmol mol-1 ethylene conditions retained only 3% of the total number of floral buds initiated. STS-treated plants retained 50 to 54% of their floral buds. Leaf area of plants subjected to 100 nmol mol-1 ethylene was 26% of control, and plants subjected to 200 nmol mol-1 ethylene were 21 % of control. When plants were treated daily for 10 hours with 35 nmol mol-1 l-MCP, leaf area improved to 81 and 64% of control. Manipulating temperature had neither a statistically nor a biologically significant effect on ethylene sensitivity. Ethylene reduced yield primarily by arresting floral bud development and causing early floral bud senescence. Both STS and 1-MCP looked promising for improving yield in high ethylene environments, but concentrations and durations of application need to be further refined. Yield of Micro-Tom was significantly less sensitive to ethylene than Micro-Tina. These results indicate that solving ethylene sensitivity issues in controlled environments may be accommodated by cultivar choice as well as timely control of environmental ethylene, chemical inhibitors, and genetic manipulation