‘GL 659’ and ‘Vanguard’ lettuce seed lots germinated almost completely at 30°C when treated with ethephon (100 mg/liter). Similar treatment at 35°C failed to release seeds from dormancy. Kinetin treatment (10 mg/liter) at 35°C was only moderately effective in breaking heat dormancy, but when ethephon and kinetin were combined at this temperature, the interaction was synergistic and germination was almost complete. The preincubation time requirement at 25°C before maximum germination could occur after transferring to 35°C was nearly eliminated by treating seeds with the ethephon-kinetin mixture. ‘Calmar’ seeds, which had a lower heat tolerance, could not be induced to germinate completely at high temperature by any treatment.

Lettuce (Lactuca sativa L.) seed coatings containing a granular activated carbon layer adjacent to the seed and overcoated with standard coating materials gave faster, more complete and more uniform seedling emergence than did simple coatings with standard materials. Evidence suggests the granular carbon layer acts to speed O2 diffusion into and adsorb endogenous growth inhibitors excreted from the germinating seeds. Variability of seedling emergence was found to be inversely related to the mean emergence rate.

Lettuce (Lactuca sativa L.) seed weight and size were unrelated to rate of germination and germination percentage at 25°C. In 4 cultivars tested, all seed physical parameters, except density index were highly significant predictors of radicle elongation (weight>thickness>iength = width>density index). However, more than 62% of the growth variation remained unexplained. Seed weight did not influence the uniformity of 7-day-old seedlings.