Tomato Pollination and Excessive Heat

Jerry Brust, IPM Vegetable Specialist, University of Maryland;

The extreme heat we had will play havoc on tomato fruit that was just flowering or ripening, causing problems in fruit development due to poor pollination. Constant exposure of a tomato plant to high temperatures (day/night temperatures of 95/80°F) significantly reduces the number of pollen grains produced and released per flower and decreases the pollen’s viability. Most pollen is shed between 10:00 a.m. and 4:00 p.m. and 3-hours or more at 103oF on two consecutive days can cause fruit set failure. Temperatures at night may play a more important role in determining whether or not pollination takes place than day time temperatures. This is because ideal fruit set occurs within a very narrow range of night temperatures (60°-70° F). If tomato plants experience night temperatures above 75°F, interference with the growth of pollen tubes can occur preventing normal fertilization and causing blossom drop (Fig. 1). Prolonged high humidity (>80%) also will hinder good fruit set as the pollen either will not shed freely or the pollen grains may bind together, resulting in poor pollination. Poor pollination may result in under-size fruit that looks ‘normal’ but is just a great deal smaller. Other problems include poor development of the gel inside the fruit. This causes the fruit to appear angular and soft when squeezed (Fig. 2). When this type of fruit is cut in half, open cavities can be seen between the seed gel and the outer wall (Fig. 2). High temperatures during the ripening period additionally can cause ‘internal whitening’ in tomato fruit (Fig. 2). This white tissue only is noticeable when the fruit is cut. The hard, white areas tend to be in the vascular tissues in the outer and center walls of the fruit. Low potassium levels are also associated with ‘internal whitening’. There is not a great deal that can be done about any of the environmental problems other than to be sure to water enough and do not over fertilize during these extreme conditions. Although growth regulating chemicals can be used sometimes to help fruit set under cooler than ideal conditions there is no growth regulator that will induce normal fruit development under high temperature conditions.

Blossom drop (arrows) in tomato due to high night temperatures

Figure 1. Blossom drop (arrows) in tomato due to high night temperatures

internal whitening on tomato Angular sides of fruit due to poor pollination.

Figure 2. Angular sides of fruit due to poor pollination. When cut open you can see the lack of gel resulting in pockets inside the fruit as well as ‘internal whitening’–spots in the outer wall.

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