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 sitiens (sit") 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" mutant seeds. Removing the micropylar end of the testa from s/ f 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" seeds under osmotic stress, some differences in seed metabolism are evident between genotypes. The activity of endo-/7-mannanase was greater in the endosperm of s/ f 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* 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-^-mannanase activity nor greater amounts of sugar in the s/'f" seeds result in significant differences in germination parameters when stripped wild-type and stripped or intact sit" seeds were germinated on PEG solutions of —0.3 MPa. The ability of intact, ABA-deficient sit" mutant tomato seeds to complete germination at much lower external osmotic conditions relative to the intact wildtype 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.