Tomato plants have certain light and temperature requirements for optimum growth. In late spring and into summer, tomato plants thrive in the garden under full sun in temperatures ranging from 60 degrees F to 90 degrees. If night temperatures fall below a certain level or day temperatures top 90 to 100 degrees, your tomato plants will likely suffer and fail to produce further, according to University of Illinois Extension.
A lack of warmth has a negative effect on tomato plants. If soil temperatures are still cool, as in early spring when nighttime temperatures drop, the macronutrient phosphorous is unavailable for plant-root uptake. This results in a nutrient deficiency. The leaves lose their green color and droop. The roots fail to develop and the plant dies.
Plant tomato seedlings after the nighttime temperatures are consistently above 55 degrees. The soil then is warm and the daytime temperatures are conducive to tomato plant growth. Tomato plants require full sun with day time temperatures ranging from approximately 70 to 90 degrees. Established tomato plants are more likely to tolerate the higher temperatures.
When tomato plants flower, they are preparing to set fruit, that is, to produce tomatoes. The daytime temperature during flowering impacts the plant's ability to complete the process and set fruit. Should temperatures rise over 90 degrees during flowering, the flowers are likely to brown and fall off the stem. This disrupts the process and the plant fails to produce fruit.
The plant may complete the process and have green tomatoes formed on the vine. Higher temperatures, though, may result in sunscald and poor color. Sunscald is identified by a large, whitish area, papery in appearance on one side of the fruit. Fruit unprotected by plant foliage is susceptible to sunscald even within optimal temperature ranges due to direct exposure to the sun's heat.
Fruit may also fail to ripen properly in higher temperatures. The excessive heat disrupts the synthesizing of ethylene, the chemical that allows the fruit to mature.
Tomato plants experience heat stress at temperatures over 95 degrees. The plants are disrupted at the cellular level, adversely affecting their ability to produce the female hormones necessary for pollination. Should the higher temperatures set in before the plant flowers, it will fail to flower and in turn fail to set fruit. Heat stress also affects tomato plants' ability to properly produce and process carbohydrates, resulting in nutrient deficiency. The lack of carbohydrates causes tomato plants to wither and die.