Tomato

Key Information

Tomatoes (Solanum lycopersicum) are self-fertile, with fruit set being similar in self- and cross-pollinated plants. Flowers are perfect (having both female and male organs), and typically consists of six stamens (♂) attached to the corolla tube, with the anthers partly fused to form a cone like structure surrounding the female pistil (see Figure 1 below). The filaments are very short, and the anther begins near the base of the stamen. The connective is prolonged beyond the anther, often by about half its length, into a sterile, terminal beak (generally of a greener colour than the anthers). The style (♀) may be shorter or longer than the tip of the connective, and can be receptive before and after the release of pollen, making cross-pollination possible. In short-style varieties however, pollen is released inwardly through vibration, and self-pollination is the norm. Tomatoes do not produce useable quantities of nectar.

Tomato flowers require vibration to release the pollen. Wind can be sufficient for pollination to occur, although it may be limited, resulting in poor fruit. In the still air of a glass or plastic house however, a pollinator such as a bumblebee is generally required. Bumblebees are exceptionally efficient pollinators of tomatoes. Bumblebees perform sonication, also known as buzz pollination. An experienced bumblebee rigidly grasps asnd bites the flower cone, then rapidly vibrates her thoracic wing muscles, but not her wings, using the resonant vibrations to dislodge the pollen from within the anther. The bumblebee leaves visible bite bruises on the flower cone, which can be used to verify that pollination has occured. Honeybees do not perform sonication, and coupled with the lack of available nectar to keep their interest, are much less efficient pollinators of buzz-pollinated plants like tomatoes.

To the right is an example of bumblebee bruising. The flower at the top of the image has not been visited by a bumblebee, while the flower at the bottom has, showing typical brown bruising.

 
The final size and weight of fruit is largely determined by the number of seeds set, which is ultimately due to the quality of pollination.
 
At high temperature and low humidity the stigmas may dry out, turn black and cease being receptive. Flowers will abort without having been fertilised. Temperatures above 30˚C may cause abnormal style elongation.
 
The optimum temperature for pollen tube growth is 21˚C. Above or below this temperature reduces the germination and growth of the pollen tube, with this being very poor below 10˚C or above 38˚C. At 34˚C, fresh mature pollen will germinate satisfactorily but tube growth is slow. Temperatures above 34˚C can also negatively affect the viability of ovules.

As with all flowering plants, the health and nutrition of the plant can affect pollen production and viability. Poor quality/quantities of pollen will have negative impacts on the bumblebee hive, and may lead to bumblebees searching for alternative pollen sources away from the crop, or to a reduction in the life of the hive. It is important to ensure that the plants are vigorous and healthy during flowering, and that their nutritional requirements are adequately met.

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