Take Heed With Temperature

Temperature is a key factor in controlling plant development and crop timing. Knowing the basics of temperature control, and the science behind temperature and its impact on other plant growth factors, is necessary to achieve desired crop results. Growers who master the ability to grow plants at the right temperatures are ultimately the masters of controlling their crops.

Temperature affects every aspect of the plant growth process,” says Ryan Warner, an associate professor of horticulture at Michigan State University.
“Plant growth increases as temperature increases, up to a point, and then declines if temperature continues to increase, while respiration continues,” Warner explains.

DIF

“With temperature, growers can control the rate at which plants grow. One of the main ways to do this is with DIF,” Warner says.

“Plant height is impacted by the relative day and night temperatures for most crops, the old DIF concept,” he says. “If the day is warmer than the night, plants will generally be taller than if they are grown under a constant temperature or in a temperature regime in which night temperatures are warmer than day temperatures.”

The key to using DIF is moving from a positive DIF (warm days, cool nights) to zero DIF (same average daily temperature, 24 hours a day) or negative DIF (cool days, warm nights), which significantly impacts plant height. Plants respond more to DIF under short days (late fall/winter) than under long days (spring, summer). However, crop time increases as growers lower temperatures, which can cause an economic impact if greenhouses are tied up longer.

Heat And Cold Stress

Every plant species has an optimum temperature range, and most can maintain growth between 50°F to 77°F. But when extreme temperatures set in and plant temperature drops below or goes above the ideal, plants become distressed.

“Heat stress can damage the photosynthetic machinery, which prevents the plant from being able to fix carbon and thus, grow,” Warner says. “Heat stress can reduce growth for several days after exposure. Some symptoms may be very subtle, for example just a lack of growth. Under more extreme heat stress, plants become chlorotic and floral initiation and development may be inhibited, as flowers are more heat-sensitive than vegetative growth.”

Under more severe heat and drought conditions, pores on the leaf (stomata) will close, which can ultimately cause plant death,” Warner says.“When stomata are closed, plants can’t take up carbon dioxide and leaf temperatures will increase. Water evaporated through the stomata (transpiration) helps to cool the plant and this process also closes down. Plants can die under prolonged heat/drought stress, as they can’t fix carbon but continue to metabolize the carbon that has already been used for growth.”

Cold temperatures affect plants differently than warm temperatures, as cold slows down the plant growth process. Plant roots grow poorly, respiration slows and plants become nutrient deficient.

“Mild cold stress results in slow or no growth, so plants look stunted,” Warner says. “If plants are wet, cold temperatures will keep plants and soil wet longer, which promotes disease. Extended cold stress can damage plant membranes, depending on the severity of the stress, which may result in water-soaked lesions appearing on the plant.”

Botrytis, Pythium and Thielaviopsis are common in some crops exposed to cold stress. While slowly dropping temperatures cause water to move out of cells before they freeze, and plants can sometimes recover if caught in time, sudden freezing temperatures cause plant cells to freeze and die.

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