Chapter 10

Temperature Requirements for Greenhouse Crops

Day/Night Temperature Differential (DIF)

The growth habit and flowering time of many ornamental and greenhouse crop plants can be shaped by alternating day and night temperatures, commonly referred to as DIF. A positive DIF (+DIF) occurs when the day is warmer than the night while a negative DIF (–DIF) is a cooler day than night. Day-night temperature differential refers to the difference between daytime and nighttime temperatures. Day-night temperature differential affects internode elongation, plant height, leaf orientation, shoot orientation, chlorophyll content, lateral branching, and petiole and flower stalk elongation. Plant response to day-night temperature relationships varies depending on the species and cultivar. This relationship between day and night temperatures can be expressed as the difference in the day and night temperature, abbreviated DIF:

DIF’s Influence on Plant Height

The principle of DIF can be applied in the greenhouse to control plant height and reduce the need for chemical growth retardants. The primary effect of DIF is to influence internode elongation. Plant height can be decreased by decreasing the day temperature or increasing the night temperature or both by achieving a close to zero or negative DIF. Conversely, to increase plant height, increase the day temperature or decrease the night temperature or both by achieving a close to zero or positive DIF.

Side Effects of Negative DIF

There are some side effects from the use of a negative DIF. These include reduced leaf area, change of leaf orientation (plants exposed to positive DIF tend to have upright leaf growth while plants exposed to negative DIF tend to have horizontal leaf growth), downward curling of leaves, shoot orientation, reduced chlorophyll content, leaf nutrient composition, lateral branching, and carbon partitioning.

Limitations of Applying DIF

The primary limitation of using a close to zero or negative DIF for height control is the climatic effects on the greenhouse environment during the summer months. During hot weather, it is very difficult, if not impossible, to maintain greenhouse temperatures during the day that are cool enough to implement a close to zero or negative DIF. For example, if DT is 92 degrees F (33°C), NT would have to be at least that same temperature to achieve height control (in this case, 0 DIF). Such a night temperature would be excessively hot and could cause serious damage to the plants. In general, night temperatures above 75 degrees F (24°C) should be avoided because many crops are adversely affected by high temperatures. For example, some floriculture crops will not bloom at all and the plants may be stunted

Computer Control of DIF

DIF control can be managed using thermostats and manual settings but this can be difficult to control if managing a number of crop zones requiring different DIF values. Under this scenario the best option is to rely on greenhouse computer software that includes DIF control that can calculate the average daily temperature needed for controlling the date of crop maturity.

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