Effect of temperature and humidity on the longevity of Orobanche seeds

The effects of moisture and temperature on seed longevity were studied over periods of up to 426 days in three species of Orobanche. Saturated salt so lutions were used to achieve equilibrium relative humidities (e.r.h.) betwe en 11% and .85% at temperatures of 20-60 degrees C. The viability equation describing loss of viability in these conditions applied to the three speci es such that the lower the temperature and lower the equilibrium relative h umidity the greater the longevity. The initial viability differed between s pecies with the seed lot of Orobanche crenata having the lowest viability. However, the relative effect of temperature on longevity and the relative s ensitivity of seed longevity to changes in equilibrium relative humidity we re the same in all species. Longevity, estimated by the standard deviation of the seed survival curves was greater in Orobanche aegyptiaca and O. cren ata than in Orobanche minor, being for example about 100, 103 and 49 days, respectively, at 40 degrees C, 50% e.r.h. Accurate prediction of longevity is necessary to understand and quantify the population dynamics of weeds an d will help the farmer to optimize control measures such as solarization. T aken in conjunction with published data on loss of viability of imbibed see ds, the annual loss of viability on a typical Eritrean farm is predicted to be about 38%.