Handbook of Vanilla Science and Technology

Fig. 6.2 Ti mecoursechangein cell sizeofvanilla pod during on-the-vinedevelopment (left)and associated changes in relative abundance of green and cortical white tissue in the outer cell wall tissue (right). Reproduced with permission from Havkin-Frenkel, D., French, J.C., Graft, N.M., Pak, F.E., Frenkel, C. and Joel, D.M. (2004) Interrelation of curing and botany in vanilla (Vanilla planifolia) bean. Acta Horf. (ISHS), 629, 93-102.

6.2.3 Fruit anatomy

The epidermis, a cell layer enveloping the fruit, contains iso-diametric ground epidermal cells, which lack prominent chloroplasts. Each epidermal cell contains a rhomboidal crystal of calcium oxalate and is bounded by thickened, pitted cell walls. Stomata are widely spaced. In some varieties, dozens of extra floral nectaries occur on the fruit. In other varieties, these extra floral nectaries are entirely absent. The outer green fruit wall region contains a ring of about 15 vascular bundles. The vascular bundles are unbranched, and each contains a strand of xylem and phloem with a sclerotic bundle sheath. The xylem consists of annular to helical and reticulate elements. Tissue outside the ring of vascular bundles is composed of thin-walled parenchyma cells several times longer than wide. Each ground parenchyma cell in the cortex of the outer fruit wall contains chloroplasts and occasional rhomboidal calcium oxalate crystals. Needle-shaped crystal (raphide) “vessels” are abundant in the outer fruit wall and, when the fruit is cut, these cells release mucilage-containing raphides, which are highly irritating when coming into contact with skin. No attempt has been made to determine the development or structure of these large, complex cells, which are many times the length of ground epidermal cells and contain tightly packed bundles of raphides if undisturbed. Compared with the outer fruit wall region, the wall tissue inside the ring of vascular bundles contains larger cells with somewhat less abundant and smaller chloroplasts, and is much less green in freshly cut beans.

6.2.4 Pollination initiates ovary and fruit development

The inferior ovary of the non-pollinated vanilla flower has three weakly developed parietal placentae separated from each other by the smooth inner epidermis of the ovary. Pollination triggers the placenta to begin extensive branching, followed by ovule development. Perhaps more important for the vanilla industry are unusual glandular hair cells that begin to develop quickly in the regions between the placentae. Each hair cell is unbranched and soon reaches a length of about 300 micrometers. Following pollination, large numbers of pollen tubes progress down the ovary moving in three groups, each located in a narrow pocket at one side of each of the three placentae, flanked by the hairs. The hairs become cemented together during their development, and later break down, releasing their contents into the surrounding locule. The developing hair cells have abundant endoplasmic reticulum, ribosomal structures, enlarged plastids containing lipid globules, and other features that are the hallmarks of metabolically active cells.

6.2.5 Mature fruit

As the fruit develops, the inter-placental hairs develop thickened walls and a complex cytoplasm. Because of their size, number, and thick walls, the hairs are easily observed in transverse sections of vanilla pod, as three lustrous white bands. Many seeds become appressed into the hairs in the mature fruits. The three panels of hairs extend the full length of the fruit. The cells contain abundant lipids, which are released onto the locule and coat the seeds when the hairs senesce later in ripening. The hairs develop complex cell walls, which cement the hairs together in mature beans. Swamy (1947) suggested that vanillin is produced in these hairy cells. This suggestion has been confirmed by our work (Joel et al. 2003), showing that vanillin and related intermediates in the vanillin biosynthetic pathway accumulate in the inner white tissue of a developing vanilla pod, around the plancental hairs (Figure 6.3). This information may be important for understanding of the curing protocol, as outlined in Sections 6.3 and 6.4.