Internal anatomy of individual tomato seeds: Relationship to abscisic acid and germination physiology

Tomato seeds that have been dried, imbibed and redried (primed) develop internal free space between the embryo and endosperm. Seeds of the ABA-deficient sitiens (sit(w)) tomato mutant can exhibit internal free space at the completion of seed development even without priming. Both primed and sit(w) seeds germinate more rapidly than untreated wild-type seeds. To determine whether internal anatomy predicts germination physiology, individual sit(w) and primed wild-type seeds were sorted into three categories based upon the extent of internal free space observed nondestructively using X-radiography. Category 3 (C3, extensive free space present) sit(w) seeds completed germination more rapidly than all other seed categories and genotypes in water, in abscisic acid (ABA) or under far-red illumination. The force necessary to puncture the endosperm caps (and testa) of C3 sit(w) seeds was less, and the percentage of nuclei in C3 sit(w) radicle tips in the G₂ stage of the cell cycle was greater than for all other seed categories. Wild-type seeds exhibited free space following long-term priming, but germination was still prevented by far-red light and ABA, and endosperm cap strength and nuclear DNA contents were not altered. Endo-β-mannanase activity of individual endosperm caps was not consistently related to their resistance to puncture. While internal free space is diagnostic for primed tomato seeds and occurs in a fraction of sit(w) seeds, it is not predictive of many aspects of germination physiology.