Ultrastructure of developing hard and soft red winter wheats after air- and freeze-drying and its relationship to endosperm texture

A transmission electron microscopic study was conducted on air- and freeze-dried developing wheat to determine the effects of drying on the structure of the starchy endosperm. Field-grown hard red winter wheat (Karl) and soft red winter wheat (Clark) were harvested at 15, 18, 21, 23, 25, 28, and 35 days after flowering (DAF). Wheat was dried by either air-drying in the spike at 28 degrees C or freeze-drying following freezing in liquid nitrogen. Dried wheat was prepared for microscopy. Fresh samples of Karl and Clark were also harvested on the same days and prepared immediately for microscopy. The method of drying greatly affected cellular ultrastructure. The most pronounced change upon air-drying of developing samples was disappearance of individual protein bodies and conversion of the cytoplasm into a matrix-like material similar in appearance to storage protein matrix found in mature wheat endosperm. Freeze-dried wheats maintained nearly natural ultrastructure but exhibited various amounts of freeze damage. Conversion of protein bodies to a matrix was not observed in freeze-dried samples. The results suggest that hardness develops as a result of endosperm senescence rather than accumulation of particular grain components. Senescence may cause changes in the starch granule surface such that surrounding components bind tightly in hard wheats, whereas the binding is weaker in soft wheats. Therefore, the surface of starch granules might be more important than components the starch granules bind to in determining hardness.