SEED DORMANCY AND TISSUE-CULTURE RESPONSE IN WHEAT

High levels of seed dormancy in wheat (Triticum aestivum L. em Thell) effect tolerance to pre-harvest sprouting conditions. An inverse relat ionship between the level of seed dormancy at maturity and growth of c alli derived from immature embryo explants was previously observed amo ng a small group of wheat genotypes. In this study, 24 genotypes repre senting a diverse genetic base were used to examine the relationship b etween seed dormancy and tissue culture response in wheat. Immature em bryo explants were cultured on a modified Murashige and Skoog medium. Increase in callus fresh weight and subjective callus rating were dete rmined after 30 and 60 d. Cumulative germination curves, percentage ge rmination, a modified Promptness Index, and time to reach 50% germinat ion were evaluated as methods of characterizing the relative level of dormancy among the genotypes. Visual assessment of cumulative germinat ion curves categorized the genotypes into four dormancy groups. Numeri cal reduction of these curves Indicated that genotypes differed most i n percent germination and modified Promptness Index after 6 d (PG6 and MPI6, respectively). PG6 and MPI6 were highly correlated (r = 0.98). Median germination produced a similar ranking of genotypes for seed do rmancy with predicted population midpoints of 2.3 to 56 d. After 60 d in culture, calli ranged from an average of 0.29 to 1.00 g, and watery , loose and translucent to friable, compact and yellow-white in appear ance. Callus ratings and weights were all significantly correlated. Wh eat genotypes with good tissue culture response exhibited a wide range in mature seed dormancy and no correlation between the two biological phenomena was evident. Therefore, the level of mature seed dormancy a nd tissue culture response in wheat are largely independent traits, an d wheat breeding programs may use tissue culture to develop sprouting- resistant cultivars.