| Seeds of different cultivars of Glycine max L. Merr (soybean) have strikingly different rates of water imbibition. Seeds that readily imbibe water are termed "soft-seeded", while those that remain non-permeable, even after several days in water, are referred to as "hard", "stone" or "impermeable" seeds. It has been widely accepted that soybean seed imbibition is regulated by the extra-hilar region of soybean seed coats. However, no satisfactory results have been reported as to what chemicals determine this difference. The objective of this research is to address the chemical basis for the difference in water permeability between different soybean cultivars. A bulk chemical analysis of cell wall modification components in the extra-hilar region of soybean seed coats, including lignin, cutin and cutin associated waxes, showed that it is the outermost cutin polymer that determines the water permeability of the seeds. The outermost cutin of the seed coat was shown to have an unusual chemical composition, lacking mid-chain-hydroxylated fatty acids but being relatively rich in 2-hydroxy- and o-hydroxy fatty acids. The cutin of the impermeable cultivar contained a disproportionately high amount of hydroxylated fatty acids relative to that of the permeable ones. Moreover, a brief treatment with hot alkali to the natural impermeable seeds rendered them permeable by removing part of the cutin monomer. In this study, it was also found that some of the outermost cutin monomers are covalently linked to cell wall carbohydrates, with a larger amount in the impermeable cultivar than the permeable one. It was also shown that the developmental deposition of seed coat cutin is not synchronous with seed growth, especially at the late stage, when the rate for cutin deposition is zero while the rate of seed growth is the largest. The integrity of seed coat cutin may be retained or broken at this stage of development depending on the composition of cutin in different cultivars, which directly correlates with the water permeability.;Key words. Conjugate of aliphatics and carbohydrate, cuticle, cutin, cutin development, lignin, seed coat, seed imbibition, soybean (Glycine max L. Merr), stone seeds, water permeability, wax. |
