Structure and histochemistry of sorghum caryopsis in relation to grain-filling

Anatomical and histochemical studies of ovary and caryopsis of sorghum reveal the importance of the chalazal complex in transporting nutrients from maternal sources to the filial diploid embryo and triploid endosperm. The presence of starch, protein, lipid, Ca, K, Mg, and Fe in various tissues at different stages of development can be revealed by a variety of histochemical techniques. Vascular supply ends at the base of the ovary and transport occurs through vascular parenchyma, pigment strand and nucellar projection where symplastic continuity is broken. Nutrients unloaded into an apoplastic placental sac then enter the endosperm and embryo through the aleurone transfer cells. The later possess characteristic wall ingrowth. The single layer of aleurone surrounding the endosperm may also help in transport during later stages of grain-filling. Grain-filling in C4 sorghum is compared with other C4 and C3 grasses showing the variety of strategies evolved to transport nutrients into filial tissues. Standardization of terminologies to describe the tissues of the crease region will help in further research and communication.

Structure of the Covering Layers of the Wild Oat (Avena fatua) Caryopsis

The anatomy of the husk (lemma and palea) and caryopsis coat (pericarp, seed coat or testa, and nucellar epidermis) of a typical mature wild oat (Avena fatua L.) caryopsis was investigated using both scanning electron microscopy and light microscopy. Both the lemma and palea consist of very thick-walled, lignified cells. In section, the lemma appears almost twice as thick as the palea. The pericarp is comprised of only one or two layers of relatively thin-walled cells and is closely appressed to the underlying seed coat over most of the grain. Both the outer seed-coat cuticle and the inner cuticle are present over almost the entire caryopsis and are continuous with the pigment strand that occurs deep in the crease region of the grain. The only discontinuities in the seed coat are at the basal end of the grain near the embryo. In the dorsal region of the caryopsis, the outer cuticle is approximately 3.5 μm in thickness, whereas the inner cuticle is less than 1 μm in thickness. Where the two pass over the embryo they are much thinner, with the inner cuticle becoming almost indistinguishable in places. The remains of the nucellar epidermis are tightly amalgamated to the seed coat and outer tangential walls of the underlying aleurone cells, which are clearly distinguishable by their large size and characteristic appearance.

Water Relations of the Developing Wheat Grain

The physiological and anatomical mechanisms underlying the reduced sensitivity of wheat grain growth to water deficits in the post anthesis period have been investigated. The water potential (Ψ) and water content of the developing wheat grain and of other tissues within the wheat spike and flag leaf were compared under controlled environment and field conditions. In the 14 days following anthesis when the amount of water in each grain was increasing, the Ψ gradient between the grain and the rest of the plant was most pronounced. This Ψ gradient disappeared when the water per grain reached its maximum level (15 days after anthesis). The apparent turgor potential (P) of the wheat grain was very small (less than 0.2 MPa) throughout the grain filling period. When water was withheld 10 and 20 days after anthesis, the grain Ψ changed little despite a large decrease in the Ψ of the glumes, rachis and flag leaf. Grain Ψ showed the same independence during a diurnal cycle of water deficit. The independence of grain Ψ under water deficit conditions may be related initially to the xylem discontinuity in the floral axis and, in longer-term water stress situations, to the deposition of lipid in the pigment strand of the grain itself.

Accumulation of an adcrusting substance in the pigment strand of a diploid and a tetraploid wheat

The pattern of accumulation of the sudanophilic substance on the wall of the cells of the pigment strand of a diploid and a tetraploid wheat was followed by high-resolution light microscopy. The cells of the pigment strand of the 4n wheat tend to elaborate and accumulate more adcrusting substance in the cells of the pigment strand than the 2n wheat. The functional significance of the accumulation of the adcrusting substance in relation to the filling mechanism of the wheat grain is discussed.

Scanning Microscope Observations of the Pigment Strand in the Caryopsis of Wheat

Structural features of the tissues of the pigment strand in the caryopsis of wheat were studied by means of the scanning electron microscope.

Aleurone Transfer Cells and other Structural Features of the Spikelet of Millet

In recent publications we have described the distribution and structure of vascular transfer cells in the floral axes of wheat (Zee and O'Brien 1971) and the presence of a xylem discontinuity composed of modified tracheary elements at the base of the peri carp (Zee and O'Brien 1970a). We have also examined the structure of the pigment strand during grain formation (Zee and O'Brien 1970b). In order to find out if these structures are present in the florets of a non-festucoid grass we have examined a member of the Paniceae, Japanese millet (Echinochloa utilis Ohwi & Yabuno).

Studies on the Ontogeny of the Pigment Strand in the Caryopsis of Wheat

The grain of wheat has a groove that extends inward nearly to the centre of the grain. At the base of this crease and extending through its length there is a strand of coloured tissue, the pigment strand. At about 3 days after anthesis the cells of this strand of tissue are similar to meristematic cells, possessing thin walls and the usual complement of organelles. At about 9 days after anthesis the cell walls thicken and lignify.