Maintaining temperature in the ideal range is very important for tomato fruit set. The optimum temperatures are 60-75°F (night) and 60-90°F (day). Studies showed that exposing plants to 3-h periods of temperatures above 104°F on two successive days may caused fruit set failure. Not only is the maximal temperature critical for fruit set, maintaining night temperature in the ideal range is also essential. Effects of high temperature on fruit set are primarily on the stage of pollen development, which occurs about nine days before flowers open. High temperatures also affects flower structure by causing stigma exertion that prevents pollen from successfully landing on the stigma (Figure 1). After pollination, pollen germination can be severely reduced at temperatures above 100°F.
Preventing temperatures from reaching the extremely high level is important in high tunnel tomato production. Since biomass production and flower numbers are less likely to be affected by high temperatures compared to fruit set, maintaining ideal temperature could be overlooked until it is too late. As the temperatures continue rising in the season, it is critical for high tunnel growers to timely vent their high tunnels to prevent temperatures reaching extremely high levels.
Figure 1. Note stigma of tomato flower on the left was more extended compared to flower on the right. In addition to temperatures, genetic factors, nutritional status and light might cause stigma exertion.