The inflorescence of vanilla is a raceme, meaning individual flowers are borne on stalks arranged around a central axis. An individual plant normally produces 8–15 racemes or more (Anonymous, 1998, 2004; Ranadive, 2005), depending on the age, the quantity of flowering and rooting shoots, and the prevailing environmental conditions. From the initial formation of the inflorescence to the time the first flower opens is a period of 45–60 days.
Each raceme develops 10–20 floral buds, which open sequentially, starting from the base of the raceme. Normally, only one flower per raceme opens per day. Sometimes two flowers open simultaneously and sometimes no flowers open when there is rain or when temperatures are low. The flowers are fully open in the early morning and last 6–8 h before closing in the heat of the afternoon.
Vanilla-producing countries in the northern hemisphere (e.g., Mexico, India) experience flowering in March–May, with a peak in April. Southern hemisphere countries (e.g., Madagascar, Indonesia) have a flowering season from September–December.
Mexico is one of the few countries where it is possible to obtain vanilla beans through natural pollination, although it happens rarely, accounting for only about 1% of all fruits. The identity of the natural pollinator(s) of vanilla is unclear, and for a long time it has been said that bees (Melipona beechii), hummingbirds (Cynniris sp.), and bats pollinate vanilla. The preponderance of evidence favors the hypothesis that the most common pollinator is the shiny green orchid bee Euglossa viridissima (Soto, 1999a, 2003; Lubinsky et al., 2006). These bees have been documented visiting vanilla flowers but their visits are irregular and their potential for effecting pollination even smaller, perhaps only just 1 fruit per 100 or 1000 flowers (Soto, 1999a, 1999b; Hagsater et al., 2005).
With natural pollination, only one fruit is observed per raceme. Sometimes up to four are observed, all of which are usually larger and heavier than vanilla beans obtained through hand pollination because more resources are allocated to their development.
Other orchid bees, namely, individuals of Eulaema sp. ( jicotes) frequently visit the flowers of Vanilla pompona (Figure 6.3) in northern Veracruz, Mexico. On rare occasions, they also effect pollination of the flowers (5%) while looking for nectar inside and at the base of the labellum.
FIGURE 6.3 Eulaema sp. (jicote) bees, probable natural pollinator of V. pompona.
The mechanism by which the aforementioned bees actually pollinate flowers of vanilla is yet to be documented.
Inside the labellum of the vanilla flower, the part which attaches to and wraps around the column, is a tissue that flaps down from the column called the rostellum. The rostellum hangs exactly in between the stigma (female organ) and the anther sac (male organ), and is considered to be a product of evolution selected to prevent self-fertilization. In hand pollination, pollen is manually moved from the anther sac to the stigma, bypassing the rostellum.
Hand pollination is performed with a small, thin stick roughly the size and shape of a toothpick, but can be made from bamboo, bone, spines, or other materials. The relevant parts and reproductive organs of the vanilla flower are shown in Figure 6.4.
FIGURE 6.4 Floral structures of V. planifolia. (a) Complete flower, (b) column (side view), (c) column (front view), and (d) rostellum and anther sac lifted revealing the stigma.
Hand pollination was discovered by Charles Morren in 1836 and the first to put into practice on the island of Reunion was by Edmond Albius in 1841 (Lecomte, 1901; Childers and Cibes, 1948). This method of hand pollination is the same one that is in use today (Figure 6.5) and consists of the following steps:
1. Use a toothpick or similar tool to make a longitudinal slit in the labellum on the side opposite of the column to reveal the reproductive structures.
2. With the same end of the toothpick, lift underneath the rostellum and flip vertical so that the anther sac can hang down unimpeded over the stigma lobes.
3. Gently press the anther to the stigma until the two stick together and remove the toothpick.
FIGURE 6.5 Method for hand pollination of V. planifolia.
Pollen of V. planifolia has been found to be viable for a period that begins 23 h before anthesis and ending 16 h after the flower closes. Likewise, the stigma is receptive 41 h before flower opening and remains so until 17 h after the flower has closed (Shadakshari et al., 2003). For practical reasons, hand pollination is performed from 7 a.m.—noon, or a little bit later when it is overcast, but never when the flowers have already closed or withered.
Hand pollination should be conducted by able and experienced people. Women are more commonly involved in the task. An experienced person pollinates 1000–1500 flowers per 5–7 h period (ca. 4 flowers/min), assuming that the plants are more or less in the same area. The first flowers in the raceme that are pollinated yield longer and straighter fruits while the last flowers to open characteristically produce smaller and curved fruits that have less value.
Hand pollination is a daily task for a period of three months. Per hectare, 300– 600 days of work is required to carry out pollination, depending on the abundance of flowers, their location, efficacy of the pollinator, and distance between plants.
Fertilized/pollinated flowers do not separate and fall from their pedicel (in the case of vanilla, the pedicel is also the inferior ovary). On occasion, the column may abscise from the fertilized ovary, but the ideal is for the column and petals to stay attached to the developing fruit as they serve to keep the fruit hydrated and diminish the colonization of pests or fungal disease.
Flowers that are not pollinated, pollinated incorrectly, or have been damaged by rain or high temperatures abscise or abort 2–3 days following pollination. On rainy days, pollinated flowers abort up to 50% of the time because the pollen humidifies, loses its adhesive quality, and falls away from the stigma.
In general, 6–8 flowers per raceme are pollinated to ensure a minimum yield of 4–5 fruits per raceme of acceptable quality. Obtaining 100–120 fruits per plant requires 8–15 flowers per raceme to be pollinated (Anonymous, 1998). These approximations are rough since much depends on environmental conditions, the position and vigor of the plants, as well as the biological characteristics of the clone or cultivar. Vanilla growers determine the amount of flowers to be pollinated by considering pricing as well. Overpollination leads to an abundance of many smaller fruits of lesser value that increase the cost of pollination and exert a heavy cost on the plants. Overpollination is also associated with major fluctuations in production volume from year to year.