Each tomato inflorescence usually has between 4 and 12 flowers that are formed and mature sequentially on a raceme. Individual flowers are perfect, with six bright yellow petals that curve outward, away from the flower as the flower matures. The ovary can have anywhere from 2 (especially in cherry types) to 15 or more locules, which contain the ovules. The six stamens have compact fused anthers that form a yellow cone, 0.5 to 0.75 in (1.3 to 2 cm) long, that surrounds the pistil, with its style and stigma that usually terminates within the cone but can occasionally extend slightly beyond the tip of the cone, which has a small opening. The anthers have slit openings on the interior of the cone, and when pollen dehisces it will shower out of these pores with any kind of motion of the flowers, whether from wind or insect visitation.
As the anther cone of the flower usually points downward, the pollen will thoroughly cover the bulbous stigma, it is well within the anther cone as it is with most modern tomatoes, or the cone is exerted out of the tip of the cone as it often is with many heirlooms. The pollen, which is shed over a 2-day period, will usually pollinate its own stigma within the anther cone, supplying the pistil with plenty of pollen to fertilize a full complement of ovules.
However, the stigma is often receptive a day before pollen shed and remains receptive 2 or 3 days after the pollen from its flower has shed. This means that there are opportunities for crossing to occur, especially with the exerted stigma of the older varieties. When the style pushes the stigma out of the end of the anther cone, it is exposed to possible insect activity. While tomato flowers are not visited by a wide number of insect species, they are often visited by several types of bumblebees (Bombus spp.). Bumblebees have a unique way of clinging to the flowers upside down while vibrating their wings rapidly and shaking the pollen out of the cone onto their abdomen. If the stigma is exerted then it is possible that pollen on their abdomen from a previous flower can be transferred to the flower they are currently visiting, producing a cross-pollination. This is obviously much less likely to occur with more modern tomato varieties, which have stigmas that are well encased in the anther cone; other insect pollinators, however, will sometimes pry the flowers open and cause a cross to occur.
Climatic and Geographic Suitability
Tomatoes can have problems setting seed at temperatures that are too high or too low. At temperatures above 90°F (32°C) and below 60°F (16°C) the pollen of many varieties will be affected and fertilization of ovules will be impeded, both resulting in poor seed set. In extensive experiments with tomato pollination in the 1930s, Ora Smith of Cornell University found that the optimum temperature for pollen to germinate on the stigmatic surface is 85°F (29°C); at 100°F (38°C) or 50°F (10°C) pollen germination was virtually stopped. Smith found that even at favorable temperatures pollen tube growth is slow, taking 2 to 3 days to reach the ovules following pollination. This means that, even if temperatures are favorable at the time of pollination, any temperature swings below 60°F (16°C) or above 90°F (32°C) may severely slow or stop the growth of the pollen tube on its journey to the ovules. Therefore, even when the temperature for pollen tube growth is at or near the optimum during the day, if the temperature drops to lows at or near 50°F (10°C) during the night, any of the pollen tubes that started their journey within the last day or two can stop growing. Alternatively, in hot climates the pollen can germinate and start growing during the cooler temperatures of the morning or evening and then be stifled when hot temperatures approach or exceed 100°F (38°C) in the middle of the day. Once the pollen tube stops it usually will not resume growth. If this happens repeatedly over the course of the several days that the flower is receptive then there is a good chance that most of the embryos won’t be fertilized; hence the fruit won’t “set” and will abort.
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