Handbook of Vanilla Science and Technology

1.12 FLOWERING AND POLLINATION

In general, the first flowering, or “rehearsal” (“ensayo”), happens 3 years following planting. When Citrus sp. are used as supports, or when vanilla is cultivated in shade houses, flowering initiates in the second year since the plants tend to grow more vigorously as a result of more consistent shade and management.

The physiological cue to flower is promoted by climatic or mechanical stress. The principal stress in Mexico that induces flowering are the low temperatures of Autumn and Winter, when cool air masses known as “nortes” blow down unimpeded from the Arctic Circle, dropping temperatures to below 10°C; the lower the temperature, the greater the expectation of a good flowering year. The cool temperatures “burn'' the apical tip, killing it, and break the apical dominance of the plant while stimulating lateral floral buds to develop. The flowering season is March to May, with peak flowering occurring in April.

1.12.1 Percent of flowering plants

The percentage of plants that flower varies each year. The first flowering usually involves a low percentage (27.19%) of plants, but by the third year of flowering (fourth or fifth year after planting) this amount reaches 97.07%. After the third flowering, the percentage of plants that flower may increase or decrease. Heavy flowering in one year is generally followed by reduced flowering the following year, due principally to the low number of developed flowering shoots. There are also numerous other mitigating factors, such as the amount of light filtering through to the vanilla plants, the health of the plants, etc.

1.12.2 Natural pollination

Mexico is one of 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-Arenas 1999a, 2003; Hagsater et al. 2005; 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 1,000 flowers (Soto-Arenas 1999a,b).

Other orchid bees, namely, individuals of Eulaema sp. (“jicotes”), frequently visit the flowers of V. pompona in northern Veracruz, Mexico (Figure 1.3). On rare occasions, they also effect pollination of the flowers (5%) while looking for nectar inside and at the base of the labellum. The mechanism by which these bees actually pollinate vanilla flowers is yet to be documented.

Fig. 1.3 An Eulaema sp. bee on a flower of V. pompona.

1.12.3 Hand pollination

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 (Figure 1.4). The method of hand pollination consists of:

Fig. 1.4 Hand pollination of a vanilla flower.

I 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.

II With the same end of the toothpick, lift underneath the rostellum and flip vertically so that the anther sac can hang down unimpeded over the stigma lobes.

III Gently press the anther to the stigma until the two stick together and then remove the toothpick.

Hand pollination is performed from 7 am to 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 1,000 to 1,500 flowers per 5 to 7 hour period (ca. 4 flowers/minute), assuming that the plants are 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.