Primary dormancy in tomato (Lycopersicon esculentum cv. Moneymaker): Studies with the sitiens mutant

Intact wild-type tomato (Lycopersicon esculentum cv. Moneymaker) seeds do not complete germination to the same percentage or at the same speed as intact ABA-deficient si tiens (sit^w) mutant seeds when seeds of both genotypes are imbibed on polyethylene glycol (PEG) solutions of -0.3 to -1.5 MPa osmotic potential. However, if the thicker testas of wild-type seeds are removed (stripped) from the micropyle without damaging the endosperm, both the percentage and speed of germination at low external water potential are similar to that of sit^w mutant seeds. Removing the micropylar end of the testa from sit^w seeds did not enhance either the speed or percentage of germination on PEG solution. Despite similar germination percentage and speed between stripped wild-type seeds and either stripped or intact sit^w seeds under osmotic stress, some differences in seed metabolism are evident between genotypes. The activity of endo-β-mannanase was greater in the endosperm of sit^w mutant seeds compared to the endosperm of wild-type seeds when seeds were exposed to osmotic stress. Although a-galactosidase activities were similar between the two genotypes, monosaccharide, sucrose and raffinose contents were greater in the sit^w mutant embryos than the wild-type embryos when tomato seeds were exposed to -0.3 MPa external water potentials. Neutral sugar contents actually increased above the amounts of these sugars in 1 h imbibed seeds during germination on -0.3 MPa PEG solution. However, neither greater endo-β-mannanase activity nor greater amounts of sugar in the sit^w seeds result in significant differences in germination parameters when stripped wild-type and stripped or intact sit^w seeds were germinated on PEG solutions of -0.3 MPa. The ability of intact, ABA-deficient sit^w mutant tomato seeds to complete germination at much lower external osmotic conditions relative to the intact wild-type Moneymaker cultivar was solely dependent on a structural alteration in the mutant testa, making it more delicate and lesssening its resistance to penetration by the radicle.