Cool Nightlife Bad for Tomatoes
Plant physiologist Don Ort will insert the leaf of a tomato plant exposed to cool night temperatures into the airtight sample chamber of a device designed and constructed in his laboratory. Light guides and hoses lead to instruments that simultaneously measure photosynthetic activities in living plants.
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Imagine a footrace in which the runner’s feet moved in opposing directions. Mission impossible?
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Scientists in the ARS Photosynthesis Research Unit at Urbana, Illinois, have discovered a drop in the overnight temperature below 50°F can create a biochemical version of mission impossible for some crops like tomatoes, soybeans, and corn. The result is less efficient photosynthesis, reduced yields, and an explanation for the geographic limits imposed on these plants because of their temperature sensitivity.
Don Ort, a plant physiologist at Urbana, says the warm-weather evolutionary origins of plants like tomatoes and corn make them more sensitive to changes in temperature during the growing season.
"Plants have an inborn timekeeping mechanism—a circadian rhythm played out over 24 hours—during which specific chemical reactions take place," Ort says.
The circadian rhythms are important because they regulate the timing of processes within the plant, he adds. “There are specific reactions that are timed to occur at a given period of day or night.” If allowed to occur simultaneously, they would compete and stall photosynthesis—just as competing foot movements would paralyze a runner.
In plants such as tomatoes, low temperature disrupts the circadian clock. "The mistiming of the expression of certain genes upsets photosynthetic metabolism, giving rise to the characteristic chilling sensitivity of these crops," says Ort.
Low night temperatures inhibit daytime photosynthesis in these types of plants by effectively delaying until after dawn those reactions and processes that would normally take place at night.
For example, in tomatoes, if the nighttime temperature were to drop below 50°F at 10 p.m. and not warm up until 8 a.m. the next day, the plant would behave as if it were still night and continue nighttime activities during daylight hours. At the same time, the plant would initiate daytime processes that compete with such ongoing nighttime processes as the breakdown of starch into sugars.
The regulation of phosphoprotein phosphatase gene transcription gives rise to the circadian pattern in activity of sucrose phosphate synthase and nitrate reductase. It is the effect of low temperature on the transcription of this gene that causes delay in the circadian activity pattern of these two key enzymes. Ort says it is very likely that what differentiates a chilling-sensitive plant from a chilling-tolerant one has to do with expression of phosphatase genes.
"What makes it doubly intriguing is, if you look at the same things in a native plant, you don't see this effect," he says.
Using this information, scientists hope to narrow the focus of their research to a specific realm of the photosynthetic process and to use molecular engineering to override low-temperature sensitivity.
"If we're successful, it could have a significant impact on several economically important crops," says Ort. "For instance, an improvement of even one or two degrees Fahrenheit in temperature tolerance would significantly expand the geographic range of these crops to new regions, as well as dramatically improve the year-to-year consistency of yields where the crops are currently grown." -- By Dawn Lyons-Johnson, ARS.
Donald Ort is in the USDA-ARS Photosynthesis Research Unit, 190 ERML, 1201 W. Gregory Drive, Urbana, IL 61801; phone (217) 333-2093.
"Cool Nightlife Bad for Tomatoes" was published in the October 1996 issue of Agricultural Research magazine.
Freezing and chilling injury in tomato and pepper plants
Although frost occurs, by definition, when the temperature drops to 0° C at 1.5 meters above the ground, this may or may not result in freeze damage to crops. The actual temperature at which freezing will occur depends on such factors as plant species and variety, plant vigor, soil conditions, surface cover, duration of the freezing temperature, thawing conditions, cloud cover, and wind conditions.
In tomato, freezing causes a darkening of the leaf or stem tissues. Damaged areas later wilt and turn brown. It may be difficult, initially, to determine whether the growing point has been killed and damage may become more evident on the day after the frost. Peppers are more sensitive than tomatoes to freezing temperatures and may be injured or killed by a light frost.
Tomato plants are also susceptible to chilling injury at temperatures between 0 and 5° C. Chilling can cause stunted growth, wilting, surface pitting or necrosis of foliage, and increased susceptibility to disease. Low soil temperatures also stunt plant growth and prevent root development. Temperatures below 10° C during flowering can interfere with pollination and result in catfacing of fruit.
Pepper plants experience chilling injury with prolonged temps of 0-10° C (32-50° F). Injury may show up as puckering of the leaves and stunting of the plant.
The effect of temperature on flowering in tomatoes and peppers
It is well known that flowering, pollination, and fruit set of tomatoes and peppers can be adversely affected by temperature extremes. The effect of various temperatures during flowering and fruit set of peppers and tomatoes is shown in Tables 1 and 2.
Table 1: The effect of temperature during flowering and fruit set of tomato
Temperature
Effect on flowering, pollination, fruit set
Greater than 35° C (95° F)
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Reduced fruit set
18.5 - 26.5° C (65-80° F)
Optimum for fruit set
Less than 13° C (55° F)
Misshapen or catfaced fruit may result
Less than 10° C (50° F)
Poor fruit set
Table 2: The effect of temperature during flowering and fruit set of pepper
Temperature
Effect on flowering, pollination, fruit set
Greater than 32° C (90° ) day temp.
Pollen sterility occurs, flowers may drop
16° C (61 ° F)
Optimum for flowering and fruit set
Less than 15.5° C (60° F) or greater than 24° C (75° F) night temp.
Poor fruit set
What you may not think about when you see blossoms and fruit developing, is that low temperatures experienced by the plant weeks before flower buds were visible, can also affect flowering and fruit set.
A tomato plant which experiences temperatures below 15.5° C (60° F) for extended periods of time will begin to flower profusely. These flowers may remain open on the plant for several weeks, without fruit being formed. Larger flowers and increased branching of clusters can show up as a result of low temperatures experienced by the plant weeks before flower buds are visible.
Believe it or not…
Daytime temperatures of 15.5°C (60°F) with night-time temperatures of 10°C (50°F), four to five weeks before a tomato flower cluster blooms, may result in misshapen or catfaced fruit.
Fact…
Night temperatures of 7-10°C (45-50°F) during pepper flower development can cause the fruit to be smaller than normal, or somewhat misshapen.
Chilling and freezing injury of tomato and pepper fruit
The fruit of warm season crops like tomato and pepper can be injured by low temperatures. Chilling injury occurs in tomato fruit if they experience temperatures of 10° C for longer than 14 days or temperatures of 5° C for more than 6 to 8 days. Tomato fruit exposed to a shorter duration of low temperatures may still be prone to storage problems, even if obvious injury did not occur in the field. Pepper fruit can be injured by prolonged temperatures below 8° C.
Frost injury is more severe than chilling injury. Tomato and pepper fruit are usually damaged between 0 and -1 ° C.
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