| Botanically, the seed of lettuce is an achene. For germination to occur, the embryonic axis must penetrate the endosperm, integument, and pericarp. At high temperature, germination of lettuce seed is inhibited. The objective of this research was to investigate how lettuce seeds bypass thermoinhibition in order to germinate at supraoptimal temperatures.;Germination tests were conducted on the thermogradient bar in order to identify the role of temperature during seed development on the subsequent alteration of the level of thermotolerance during germination. 'Dark Green Boston' and 'Valmaine' were found to be thermosensitive and 'Floricos 83', 'Everglades', and PI 251245 thermotolerant. Seeds of the five genotypes were matured at four growth temperatures. Seeds matured at 30;A puncture test was used to determine the role of the seed coverings in restricting seed germination at high temperatures. Primed seeds or seeds of thermotolerant genotypes required less force to penetrate the seed coverings, especially the endosperm, at high temperatures as germination proceeded toward radicle protrusion.;For seeds that overcame thermoinhibition, the structural alterations of endosperm in the micropylar area during radicle emergence at 36;Thermotolerance of seed was increased by manipulating temperature during seed production. The endosperm was identified as the physical barrier on seed germination at supraoptimal temperatures. Genotype, seed maturation temperature, and seed priming could release thermoinhibition on germination. In all cases, a reduction of physical resistance by endosperm cell wall weakening and dissolution of stored reserves in endosperm cells led to germination at 36... |
