Chapter 10

Temperature Requirements for Greenhouse Crops

Cardinal Temperatures

Temperature is an important environmental factor that influences the growth of greenhouse crops. There are three important temperature points in plant growth: minimum temperature, optimum temperature, and maximum temperature. They are called cardinal temperatures. Using the minimum temperature (or base temperature), we can subjectively place crops into different temperature response categories: cold-tolerant plants, cold-temperate plants, and cold-sensitive plants. Crops do not respond in the same manner to temperatures at all stages of growth. For example, the optimum temperature for vegetative growth may not be suitable for flower development. Given adequate humidity and air movement, most crops generally grow better if the day temperature is warmer than the night.

Cardinal Temperatures for Greenhouse Crops

Every crop has specific cardinal temperatures at which growth and development are best. There are three cardinal temperatures for every crop. The minimum cardinal temperature is the lowest temperature at which crop growth can occur. The optimum cardinal temperature is the temperature when crop growth is at its greatest. Finally, the maximum cardinal temperature is the highest temperature at which crop growth can occur. Plants can tolerate temperatures above or below these cardinal temperatures, but growth and development processes are best when within the minimum and maximum range and close to the optimum cardinal temperature. Specific crops vary in their cardinal temperatures.

Minimum Temperature

The minimum temperature (or base temperature) is a species-specific temperature at which a plant stops growing. At or below the base temperature, plants no longer develop leaves, and progress towards flowering ceases. As temperature increases above the base temperature, plants develop progressively faster. The base temperature value of a crop can be experimentally estimated by growing plants at a range of temperatures, recording the time to flowering (or some other developmental event), and then extrapolating from the data the temperature at which time to flower is infinity.

Optimum Temperature

Growers should also know what the optimum temperature is for a crop. The optimum temperature is the temperature at which plant development is most rapid. The optimum temperature can be around 70 degrees F (21°C) for cool-season crops or as high as 90 degrees F (32°C) for warm-season crops. Cool-season plants also have a lower optimum temperature than do warm-season crops. Therefore, they tend to undergo heat stress symptoms at lower temperatures

Maximum Temperature

Maximum temperature stress can occur in crops when the day temperature, night temperature, or both are above some crop-specific temperature. Maximum temperatures can also reduce plant photosynthesis.

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