Effect of incubation temperature on the variation of imbibition in northern pine (Pinus sylvestris L.) seeds

To improve the quality of conifer seed lots by removing unproductive seeds, the IDS method (incubation - desiccation - separation) is used in forest nurseries. This method is based on the theory that live and dead seeds imbibe water similarly, but that they have different water-binding capacities when subjected to similar drying conditions. The combination of moisture content, temperature, and duration of incubation is chosen on the basis of the germination indices of the seed lot. The IDS treatment, however, has not always resulted in optimal separation. The purpose of the present work was to assess the significance of incubation temperatures on the variation of imbibition and outcome of the IDS treatment. The results indicated that an apparently uniform pine (Pinus sylvestris L.) seed lot with good germination indices had differences in imbibition between individual seeds, but incubation at 10 deg C served to even these differences. The best separated fractions contained only live, readily germinable seeds, which were found to be fully imbibed when examined radiographically. The live seeds in the fraction of castoff seeds, in turn, appeared to be incompletely imbibed and not markedly different in density from dead seeds. Due to the great proportion of these live seeds, the germination percentage of the castoff seeds was equally high as that of non-treated seeds. The length of incubation needed to diminish the moisture content differences between individual seeds also had an adverse effect, as was shown by the reduced 7-day germination particularly of the seeds incubated at 5 deg C. Apart from the floating test, radiography was a useful tool for timing the different stages of the treatment. The good result at 10 deg C was thought to be due to the adaptation of the northern mother trees of the seed orchard to their native environment. The length and temperature of incubation should be adjusted so as not to sort out those seeds with slow imbibition