Abstract: Experiments were conducted modeling changes in S. faberi seed moisture loss, (drydown) to elucidate the roles water partitioning between seeds and environment might play in germination. Changes in moisture content during drydown were estimated by modeling the behavior of 2 seed structures: the exterior hull (palea, lemma) and the interior caryopsis. Water saturates the hull by adhesion and the formation of surface films. In contrast, water typically enters and hydrates the gas and water-tight caryopsis only by passing through the placental pore. By analyzing the rate of water loss during a drying period, it was possible to determine the initial moisture content of the 2 anatomical structures and to reveal how the loss of water from the hull (partition 2) and caryopsis (partition 3) varied with moisture availability and thermocycle conditions in the local environment (partition 1). The drying procedure removed water in 3 distinct phases. In the first, water was lost quickly from the hull surface and more slowly from the caryopsis via the narrow placental pore. In the second, water continued to be lost from the caryopsis alone. After some time the interior water was tightly bound in the seed, the final phase. The moisture incubation experiments demonstrated that S. faberi seeds did not absorb water in the manner expected if it was a homogeneous material. Hydration of dry S. faberi seeds began with preferential partitioning of water to the interior seed caryopsis and the embryo. With additional local moisture availability the hull and caryopsis compartments absorbed moisture in a similar manner until the moisture content becomes sufficiently high to saturate the caryopsis. The caryopsis saturated at a lower local water availability content than did the seed hull. With saturation of the caryopsis, the hull preferentially absorbed the excess moisture. Hull water was spread uniformly over the seed surface forming a water film (partition 2). It is speculated that the water film changes the manner in which, the seed interfaces with the external soil microsite environment (partition 1), the locus of oxygen exchange. The delivery rate of oxygen to the embryo is a function of its phase, partial pressure gradient and diffusion rate to the embryo (partition 3). The thickness of the seed surface water film is therefore a powerful mechanism regulating the pool of dissolved oxygen pool (partition 2) directly available to the seed embryo (partition 3), hence, its crucial role in determining subsequent seed behaviors. Evidence that hull surface water quantity, in the physical form of boundary layer film thickness, controls germination was provided by the observation that maximum seed germination occurs in conjunction with intermediate levels of moisture availability in the soil environment. Soil moisture levels above and below this optimal hydration level resulted in lower germination. Consistent with the theory of oxygen-water control of S. faberi germination, these observations support the concept that optimal water film thicknesses on the seed surface are those that maximize the trade-off between hull surface area available for oxygen diffusion and water availability to support and sustain germination metabolism. These experiments indicate a robust, dynamic system of germination control by the transduction of external moisture and thermocycle signals from the soil to the seed embryo as modulated by the characteristics of the seed hull surface and the placental pore. The seed in unable to control its temperature, but the morphology of its exterior surface provides a means by which, the seed is able to regulate the other 2 requisites for germinative growth, moisture and oxygen.
Jack Dekker and Edward C. Luschei , 2009. Water Partitioning Between Environment and Setaria faberi Seed Exterior-Interior Compartments. Agricultural Journal, 4: 66-76.