Histopathology of Seed-Borne Infections - Applied Research Center ...

Reproductive Structures and Seed Formation 17body is formed in the region of entrance in response to penetration by the incompatibletube. Pollen tube growth ceases after formation of the reaction body.Recently, Atkinson et al. (1993) and Anderson et al. (1996) detected proteinaseinhibitors in Nicotiana alata stigmas, and they believe that these may be involvedin protecting the sexual tissues against potential predators and pathogens. Andersonet al. (1996) have found that the most abundant defense-related molecules in thestigma of N. alata are a series of serine proteinase inhibitors.2.4 NECTARIESThe nectaries occur on flowers (floral nectaries) and on vegetative parts (extrafloralnectaries). The floral nectaries are found in most insect- and bird-pollinated flowers.Fahn (1953) has classified nectaries on the basis of their distribution on floral partsinto perigonial (perianth), toral (torus or receptacle), staminal, ovarial (Figure 2.5C), and stylar nectaries. A nectary is usually composed of small cells with thin walls,relatively large nuclei, dense cytoplasm, and small vacuoles. The tissue containsbranches of vascular bundles with a high proportion of phloem elements (Frei, 1955;Frey-Wyssling, 1955). According to Agthe (1951), the type of vascular tissue presentin the nectariferous tissue is correlated with the sugar concentration of the nectar.If the sugar concentration in the nectar is high, the ends of vascular branches consistonly of phloem. When nectaries produce nectar with low sugar concentration, thebranch endings comprise equal amounts of xylem and phloem.The exudation of nectar from the nectary depends on the structure of the tissuesecreting nectar. If the nectar is secreted from the epidermal cells, which lack anobservable cuticle, nectar diffuses through the wall. In case the secretory epidermalcells are covered by a cuticle, exudation takes place through the pores in the cuticle,by its rupture or by the cuticle being permeable (Frey-Wyssling, 1933). When thesecretion comes from parenchymatous cells, the nectar is collected in the intercellularspaces from where it is exuded through stomata, which remain open as the guardcells are not able to close the opening.2.5 OVULEThe ovule developing from the placenta in the ovary is the site for the formation ofan embryo sac (female gametophyte) and is the forerunner of seed (Figure 2.6A).The primordia for ovules are two- or three-zonate. The three-zonate primordiumgenerally gives rise to a large-size ovule as found in Brassicaceae, Cucurbitaceae,Malvaceae, Euphorbiaceae, Apiaceae, Solanaceae, Poaceae, etc., and the two-zonateprimordium forms primarily small-size ovules (Kordyum, 1968; Bouman, 1984).2.5.1 STRUCTURE AND TYPES OF OVULESA normal ovule has the funiculus, chalaza, nucellus, and integument (Figure 2.7A).The size of the funicle is variable and when absent, the ovule is sessile. The part ofthe funicle that gets adnate to the body of the ovule is called the raphe. The openingin the integument forming a passage above the nucellus is called the micropyle. The