Seed Viability Constants for Lettuce

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Abstract
Seeds of two lettuce cultivars (Lactuca sativa L., cv. Meikoningin and cv. Grand Rapids) were hermetically stored with constant moisture contents ranging between 3.6 and 17.9 per cent (fresh weight basis) at constant temperatures ranging between 5 and 75 °C. The decline with time in percentage germination and percentage normal seedlings was determined for each storage treatment. The data were fitted to an equation which contains the constants: K1, the probit of the initial percentage germination or normal seedlings; KE, a species constant; CW, the constant of a logarithmic moisture term; CH, the constant of a linear temperature term and CQ, the constant of a quadratic temperature term. Regression analysis of data from storage periods up to 5.5 years at temperatures of 5–75 °C and seed moisture contents of 3.6–13.6 per cent yielded the following values: KE= 8.218, CW=4.797±0.163, CH=0.0489±0.0050 and CQ=0.000365±0.000056. Although this equation consistently provided a better fit, simplified equations, assuming either a log-linear relationship between seed longevity and temperature, or a log-linear relationship between seed longevity and both moisture content and temperature, accounted for more than 94 per cent of the variation at the restricted temperature range of 5–40 °C. Longevity of the same seed lots at sub-zero temperatures (−5, −10 and −20 °C) was studied in separate tests. Freezing damage, resulting in abnormal seedlings in the germination test, occurred at −20 °C when the moisture content of the seeds exceeded 12 per cent. No decline in percentage normal seedlings was observed after a storage period of 18 months or longer at −20 °C, provided the seed moisture content did not exceed 9.5 per cent. For seeds stored at −5 and −10 °C with 9.6–12.5 per cent moisture content, the observed rate of decline of percentage normal seedlings was adequately predicted by the viability equation, using the above values for the constants. This suggests that for low moisture contents the viability equation can be applied to estimate longevity at sub-zero temperatures.