Seeds of the Smooth-leafed Elm (Ulmus carpinifolia) and of the tropical forest tree Terb (Terminalia brassii) were stored hermetically and sampled at intervals for periods of up to two years. Both species possess ‘orthodox’ seed (increasing longevity is observed as either moisture content or temperature are reduced) within the temperature ranges from — 13 to 52°C (Elm) and from —4 to 42°C (Terb) and within the moisture content ranges from 3 to 19 per cent (Elm) and from 5 to 14 per cent (Terb) on a fresh weight basis. Elm seed stored at —75°C showed the expected relationship between longevity and moisture content, but did not differ significantly in longevity from seed kept at — 13°C when moisture contents were held constant. Probit analysis of the relationship between germination percentage and time was performed for each storage environment, yielding a slope from which the standard deviation of the distribution of seed deaths over time (σ) was calculated. Standard deviation values were used in turn to determine the values of constants in a viability equation which had previously been applied to seed of barley, chickpea, cowpea and soybean. The equation, which gave a good fit to the results obtained, can be used to predict viability for seed in storage over a wide range of environmental conditions.
Some limitations to the applicability of the viability equation were defined. At 22 per cent and higher moisture contents Elm seed survived longer than predicted. Furthermore, all Elm and Terb seed was killed quickly on placing in —75°C at 22 and 20 per cent moisture content respectively, but high viability was retained for several days at 19 and 17 per cent respectively. Practical implications of the results are discussed.