Yield in grass seed crops is decreased by seed shattering, which is generally caused by abscission . Abscission layer development in perennial ryegrass, Lolium perenne L., was investigated to determine its histogenesis in relation to the morphological stages of the inflorescence . Spikelets were fixed with weekly intervals from heading to harvest ripeness . Florets were dissected and microtome sectioned .Abscission layers were located in the rachilla just below each floret . They were already present at the heading stage . Only slight histological changes were observed in the cells of the abscission layers during flowering and seed development . Breaking of the abscission layers occurred four to five weeks after anthesis, starting at the epidermis . No degradation of the abscission layer cells was detected, indicating that abscission took place mechanically . No histological differences in abscission mechanism were found between groups of genotypes with contrasting seed retention, suggesting that in our material differences in seed retention were probably caused by morphological factors other than abscission layer development . Selection for high seed retention is briefly discussed .

SUMMARY The nucellus of immature crassinucellate ovules of Spinacia cannot be penetrated by pollen tubes, which cluster together in and around the micropyle. After maturation of the ovule, pollination activates the embryo sac, i.e. the synergids, to excrete substances which dissolve the middle lamellae of the nucellus tissue in front of the synergids. Eventually the nucellar cuticle near the micropyle becomes interrupted. At that developmental stage pollen tubes can penetrate into the nucellus, initially growing intercellularly, and subsequently following various pathways. More than one tube can reach the embryo sac, but only one pollen tube penetrates the degenerated synergid via the filiform apparatus. Sperm cells, vegetative nucleus and tube cytoplasm with many amyloplasts are discharged into the degenerated synergid. The vegetative nucleus and the sperm cells are pressed to the chalazal part of the synergid, of which the plasma membrane is degenerated previously. The discharge and the fusion processes of the sperm cells with the egg cell and central cell (7–9 hrs after pollination) are described and discussed. Apparently no or very few organelles of the sperm cells enter the female gametes.

The ultrastructure and histochemistry of the developing spinach ovule have been examined. The development and differentiation of the integuments, nucellus and female gametophyte results in an ortho-amphitropous organisation of the ovule. In the nucellus four parts can be distinguished: the conductive part, the original chalazal part, the chalazal proliferating part and the laterial part. The cells ofthe various parts have common features as wellas distinguishing characteristics related totheir position and function. The inner and outer integuments show different features during their development. In the outer integument 3-5 cell layers develop, but the inner integument shows a development of two differing cell layers. Contact by plasmodesmata between the two cell layers diminishes and stops at maturity. The localization of different reserve substances (starch, other polysaccharides, proteins, lipids) have been studied in the developing ovules to determine the nutritional supply of the embryo sac and embryo. Extensive changes in the amount of reserve substances have been observed in the ovule right up to maturity of the embryo sac. In the outer integument storage of starch increases much up to maturity and after fertilization a gradual decrease occurs. The storage and transfer function of some cell types in relation to the nutrition of the embryo sac and embryo is discussed. The presence and location of polysaccharides during megasporogenesis and megagametogenesis is considered. Finally, the possible pathway of metabolites in spinach during different phases of the developing ovule is discussed.

Sweet potato (Ipomoea batatas) is one of the ten most important staple crops and provides a livelihood for many people around the globe. To adapt to ever-changing circumstances farmers and breeders need to have access to a broad diversity of germplasm. This study focuses on the development of a cryopreservation protocol that allows the long term storage of different sweet potato cultivars. For this, a droplet vitrification protocol was optimized, comparing several parameters; preculture method (0.3 M sucrose vs no preculture); meristem position (axillary vs apical); plant age (3 to 9 weeks); regeneration medium (MS + 2.22 µM BA, Hirai and MS); and length of loading solution treatment (20 to 360 min). Two months after cryopreservation, the regeneration rates of the meristems were compared, which resulted in significant differences for the preculture method, meristem position and loading solution. With these new insights an optimized droplet vitrification protocol was developed with the following parameters: use of 3–9 week old axillary meristems, no preculture phase, 20 min LS treatment, 30 min PVS2 treatment, exposure to liquid nitrogen by droplet vitrification, warming treatment in RS for 15 min, 1 day 0.3 M sucrose recuperation culture, 1 month MS + 2.22 µM BA followed by 1 month of MS cultures. This protocol was subsequently tested on 10 representative accessions resulting in a post cryopreservation regeneration rate of more than 40% for 70% of the tested cultivars, showing that this protocol could be implemented for a large portion of existing sweet potato collections.

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