Heat Stress and Heat Stressed Tomatoes

Published: August 1, 2011

Wrapping up July, 2011, I realized that I have never heard complaints about summer being not hot this year. Channel 17 claimed this summer one of the top ten hottest summers in record. So, "Is the excessive heat enough to 'cook' tomatoes"? The answer probably could be a "yes".

Warm season vegetables, such as cucumber, tomato, eggplant, pepper and green beans, grow best at temperatures of 68-86°F. Growth slows down significantly beyond 86°F and stops roughly above 104°F (Please note head index does not apply to plants). Temperatures above 86°F will result in heat stress to warm season vegetables (not for heat tolerant species such as Watermelon). The growth, development, biomass accumulation, and yield will all be adversely affected by heat stress although the damage depends on the crop's ability to withstand, acclimate, or recover from the stress. Heat stress is also closely associated with drought stress. The combination of heat/drought stresses kills or will kill a plant quickly.

For a tomato plant, when sufficient water is available in soil, visual symptoms of heat stress include reduced plant size, low number of leaves, small and curling leaves, and dry flowers. Fruit set is poor as pollen and stigma viability, anthesis, pollination, pollen tube growth, fertilization, and early embryo development are all highly susceptible to heat stress. Typically we would see a gap of fruit set along the plant (picture 1). Plants with some heat tolerance (picture 2) may set fruit, but fruit will be small and ripen early-cell expansion is inhibited but more plant hormone ethylene (responsible for fruit ripening) is released.

Picture 1: symptoms of heat stress. Note the fruit set change.

Picture 2: a heat tolerant cultivar with small fruit.

There are no good ways to fight high temperatures in field. Over head sprinkling would help to some extent to reduce air/leaf temperatures. This, however, has limited help under continuous and excessive heat condition, and may cause disease problems.

Shade cloth to cover high tunnels will ease the case. For plants to survive, water plants more frequently, about 2-3 times more. Growers can flood between rows in a raised bed system. Mowing weeds between rows instead of Rounding-up to keep moisture in soil. There are some tomato cultivars will survive better in high temperatures because of the built-in heat-tolerant genes, for example, some Florida and BHN series (Picture 2). Information on heat tolerant tomato varieties will be summarized later.