USING A PUNCTURE TEST TO IDENTIFY THE ROLE OF SEED COVERINGS ON THERMOTOLERANT LETTUCE SEED-GERMINATION

Temperature is an important environmental factor that affects lettuce (Lactuca sativa L.) germination, The present research tvas conducted t o determine the role of seed coverings on lettuce seed germination at high temperature. Five lettuce genotypes were primed in order to bypas s thermoinhibitional effects on germination. During germination of pri med and nonprimed seeds, imbibition followed a normal triphasic patter n. Primed seeds had higher final water content, a decreased imbibition al phase II, and germinated at 36 degrees C compared to nonprimed seed s of thermosensitive genotypes, which did not germinate at 36 degrees C. Puncture tests were conducted to determine the force required to pe netrate the whole seed or endosperm of the five genotypes at 24 and 33 degrees C. 'Dark Green Boston', a thermosensitive genotype, had the h ighest mean resistance (0.207 N) and PI 251245, a thermotolerant genot ype, had the lowest (0.139 N). Resistance to penetration of the endops perm of the five genotypes was different at both temperatures. However , three thermotolerant genotypes had lower endosperm resistance than t wo thermosensitive types. At 36 degrees C, the penetration force for p rimed and nonprimed seeds was compared after the first hour of imbibit ion and 1 hour before radicle protrusion. The force required to penetr ate the seed was affected by genotype, seed priming, and duration of i mbibition. Puncture force decreased as imbibition time at 36 degrees C increased in primed and nonprimed seed of each thermotolerant genotyp e but not in the thermosensitive genotypes. Priming reduced the initia l force necessary to penetrate the seed and endosperm in all genotypes . Thus, for radicle protrusion to occur, there must first be a decreas e in the resistance of the endosperm layer as evidenced by priming or thermotolerant genotype, Then, the pericarp and integument are suffici ently weakened so that tissue resistance is lower than the turgor pres sure of the expanding embryo, allowing germination to be completed.