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Role of Placental Sac in Endosperm Carbohydrate Import in Sorghum Caryopses¹

An anatomical study of tissues involved with carbohydrate import into developing caryopsis endosperm in three cultivars of sorghum [Sorghum bicolor (L.) Moench] was complemented by determination of the assimilate concentration of placental sac fluids to investigate the role of the placental sac. Individual panicles were tagged daily at anthesis. Caryopses were harvested from anthesis to maturity and from 10 to 20 days postanthesis at 5-day intervals for microscopic examination and placental sac fluid carbohydrate analysis. Formation of endosperm tissue by 5 days postanthesis coincided with differentiation of transfer cells within the outer surface of endosperm tissue adjacent to the placental region. All caryopsis tissues thought to be associated with carbohydrate movement to the endosperm (vascular tissue, chalazal tissue, remnant nucellar tissue, the placental sac, and aleurone transfer cells) were present in the cultivars by 10 days postanthesis. The role of the placental sac as an apoplastic sink for hexose import into the endosperm was supported by anatomical location and high glucose and fructose concentrations relative to sucrose concentrations in placental sac fluids of all cultivars at 10,15, and 20 days postanthesis. A model for hexose import into the endosperm following vascular unloading in sorghum was developed based on prior findings for maize (Zea mays L.). Results indicate that the placental sac provides an intermediate apoplastic sink for assimilate accumulation from placental tissues and subsequent hexose import by transfer cells of the endosperm.

Key Words: Sorghum bicolor (L.) Moench • Transfer cells • Placental region • Translocation • High performance liquid chromatography

² Graduate research assistant (now graduate research assistant, Dep. of Agronomy, Oklahoma State Univ., Stillwater, OK 74078) and professor, Soil and Crop Sciences Dep., Texas A&M Univ., College Station, Texas 77843.

Received for publication December 23, 1985.

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