Changes in sensitivity of Amaranthus retroflexus L seeds to ethylene during preincubation. II. Effects of alternating temperature and burial in soil

The effects of diurnally alternating temperatures and of prolonged burial in the soil on germination response of redroot pigweed (Amaranthus retroflexus L.) seeds to ethylene were investigated. Percentage germination in a 12 h/12 h, 23° C/35° C temperature regime roughly equalled that observed at constant 35° C, and greatly exceeded that observed at 30°C. Preincubation for 61 d in alternating temperatures, which were gradually increased to simulate soil warming in spring, caused little germination in the absence of ethylene, but considerably enhanced sensitivity to ethylene. Seeds kept in soil in the same temperature regime failed to show the response to ethylene, and the soil itself removed ethylene from the soil atmosphere. After burial in a field plot either over winter or during the summer, seeds had a very low ethylene response threshold (0.01−0.05 cm3 m−3) and strong response to ethylene (70–95% germination at 51 cm3 m−3 compared to 1–20% without ethylene). Germinability of seeds buried overwinter declined between 10 May (85%) and 24 May (7%), and 90% of those recovered on or after 24 May had a visible rupture in the seed coat. Apparently, germination had begun during burial, but was arrested by unknown causes in an early phase and was followed by seed deterioration. Although the role of ethylene in germination of buried seeds remains uncertain, the greatly enhanced sensitivity to ethylene observed in pigweed seeds after burial deserves further investigation.