Chapter 8

Effects of Temperature on Greenhouse Crops

Temperature Influence on Plant Growth

Temperature is influences plant growth processes such as photosynthesis, respiration, transpiration, and enzyme activity. These factors govern germination, flowering, pollen viability, fruit set, rate of maturation and senescence, and harvest duration. Depending on the situation and the specific crop, ambient temperatures higher or lower than the optimum growth range will reduce growth and delay development, and subsequently decrease yield and quality.

Photosynthesis

Temperature has a direct effect on the formation of carbohydrates in the photosynthetic process. If there are no other limiting factors, such as the amounts of water, carbon dioxide, or light, the rate of photosynthesis increases as the temperature increases. At 95 degrees F (35° C) the rate of photosynthesis drops very quickly, and the process stops. The lowest temperature at which photosynthesis can take place varies with the plant species.

Respiration

In respiration, plants convert the sugars (photosynthates) back into energy for growth and other life processes (metabolic processes). The respiration rate increases as the temperature increases. This depletes the food supply needed to fuel cellular metabolism. If the night temperature is lower than the day temperature, the food manufactured by the plant during the day is conserved.

Transpiration

Plants exchange energy with the environment primarily through the evaporation of water, through the process of transpiration. Water in the roots is pulled through the plant by transpiration (loss of water vapor through the stomata of the leaves).  Transpiration uses about 90 percent of the water that enters the plant. Transpiration is a key plant process for cooling the plant, bringing nutrients in from the root system, and for the allocation of resources within the plant.

Vapor Pressure Deficit

The concept of vapor pressure deficit (VPD) can be used to establish set points for temperature and relative humidity in combination to optimize transpiration under any given light level. VPD is one of the important environmental factors influencing the growth and development of greenhouse crops, and offers a more accurate characteristic for describing water saturation of the air than relative humidity (RH) because VPD is not temperature dependent. Relative humidity refers to the amount of water vapor in the air versus what it can hold. The amount of water that air can hold varies with temperature; warmer air has a greater water-holding capacity than cooler air.

Optimum Vapor Pressure Deficit Values

Since the principles of VPD can be used to control the transpiration rate, there is a range of optimum VPDs corresponding to optimum transpiration rates for maximum sustained yield. For example, growers should aim to have fairly low VPD, for example 0.3 kPa, when rooting cuttings in greenhouses.

Adjusting Vapor Pressure Deficit

Once a grower knows the VPD level in the greenhouse, adjustments can be made accordingly to create an ideal growing environment. For example, if the greenhouse isn’t humid enough, and VPD is too high, water vapor can be increased in the air through misting, fogging or steam injection.

Temperature Influence of Plant Quality

There is often a trade-off between high quality crops and crop timing. Cooler temperatures typically produce higher quality plants (especially cold-tolerant crops), but they take longer to reach maturity, and energy consumption per crop can be higher. Whereas crops grown at warm temperatures develop faster and thus have shorter crops times and require less energy for heating, but the quality of plants is often not as high.

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