PHENOTYPIC CHARACTERIZATION OF THE DWARF-4 MUTANT OF LETTUCE

The single recessive mutant dwarf-4 (d-4) of lettuce (Lactuca sativa L.) was selected as a potential genetic source of resistance to premature stem elongation (bolting). This study was undertaken to define the mutant phenotype and to characterize its reaction to applied gibberellin (GA). Scanning electron micrographs of mutant and wild-type achenes revealed surface and subsurface aberrations in d-4, suggesting abnormal development. Observed mutant irregularities included (i) abnormal achene formation causing irregularly shaped seeds, (ii) delayed and irregular germination, (iii) delayed but normal leaf growth and leaf area, (iv) 66% reduction in stem height, (v) 50% reduction in shoot dry weight, (vi) 0.45% viable pollen formation, and (vii) 98.9% self-sterility. Applications of GA, did not affect the delay in growth, floral sterility, or abnormal achene shape but did increase stem height and shoot dry weight to wild-type levels at maturity. Hand pollinations were able to decrease self-sterility to 93.6%. Component analysis of mutant and wild-type achenes revealed d-4 had 32% lower fatty acid levels. There were no differences in carbohydrates, cell wall constituents, and moisture levels, although higher levels of protein and inorganic compounds were found. The observed d-4 phenotype suggests that its genetic lesion is not directly tied to irregularities of gibberellin biosynthesis but has some other genetic basis.