Effects of Low Temperature, Cooling Rate, and Moisture Content on Seed Germination of Lettuce1

Abstract

Seeds of 5 cultivars of lettuce ( Lactuca sativa L.) were adjusted to moisture contents between 5 and 22% and subjected to temperatures of 5, -18, -70, and -196°C (liquid nitrogen) for 7 days. Seeds with moisture contents up to 18% were not damaged by either -18 or -196°C; however, the -70°C treatment resulted in loss of germination even with seeds containing as little as 16% moisture. Seeds held at 5°C showed no loss in viability. The loss in germination at -70°C appeared to be related to cooling rate. High moisture (18 to 20%) seeds were cooled to -196°C at different rates from 1 to 200°C/min. Germination was reduced in seeds cooled at rates slower than 25°C/min. At 200°C/min cooling rate and 18 to 20% seed moisture both germination and root growth occurred although at reduced levels compared with control seeds held at 5°C. Prolonged storage (33 days) in liquid nitrogen of rapidly frozen high moisture lettuce seeds did not result in further loss of germination after the initial freezing to -196°C.