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Temperature
Temperature affects plants in many different ways. The response and growth of plants
occur over a range of three levels: (1) the minimum temperature at which no growth
occurs; (2) the optimum temperature at which the growth is greatest; and, (3) the
maximum temperature at which no growth occurs. Propagators and growers try to
maintain an optimum temperature for maximum growth.
Heat affects the complex physiological reactions of cells as well as their development and
growth. The temperature to which the plant is exposed determines the rate of reactions.
Thus, the rate of plant growth is regulated by temperature.
Plants do not respond in the same manner to temperature at all stages of growth. A seed,
for example, may still be alive after exposure to high or low temperatures for short
periods of time. Seedlings and germinating seeds exposed to the same temperatures may
suffer tissue damaged or just die. Furthermore, seedlings, cuttings, and juvenile plants
may require warmer temperatures than mature plants. Also, the optimum temperature for
vegetative growth may not be suitable for flower development. A propagator or grower
must know the optimum temperature range for each plant species as well as for each
stage of growth to ensure the highest quality crop.
Research has shown that a daytime temperature 10 to 15 0F higher than nighttime
temperature benefits crop growth and development. Seeds of most ornamental crops
germinate at an optimum air temperature range of 60 0F (15.5 0C) to 70 0F(21 0C). Some
of the temperate seeds germinate faster at slightly cooler temperatures, while some of the
tropical seeds germinate faster when exposed to a temperature range of 75 0F (24 0C) to
80 0F (26.5 0C). In addition, germination rate is faster if bottom heat is applied to the
propagation flat. The bottom heat temperature is usually maintained 5 0F above the air
temperature. When seeds are germinated under mist, bottom heat is especially important.
Supplemental heat is needed because the mist has an evaporative cooling effect that may
reduce the media temperature below the desired level.
Scientists have shown that temperature has an effect on sugar formation during
photosynthesis. If there are not limiting factors, the rate of photosynthesis increases as
the temperature raises. At 95 0F (35 0C), the rate of photosynthesis drops very quickly
and the process stops. As this temperature, protein becomes disorganized and is unable
to remain functional. Certain desert plants may withstand day temperatures of 122 0F 1310F (500C-550C) for long periods of time because of their ability to store water.
During periods of high temperature, however, very little sugar production occurs.
The lowest temperature at which photosynthesis produces sugars will vary with the plant
species. Arctic plants produce sugars near or slightly below 32 0F (0 0C), while tropical
plants are injured if the temperature drops below 50 0F (10 0C) for any extended period of
time.
Respiration a sugar reducing process and occurs in plants cells day and night.
Carbohydrates produced in daylight are used to carry on the various growth processes of
plants. Unlike photosynthesis, rate of respiration will continue to increase well beyond
95 0F (35 0C) due to rapid enzyme activity, carbon dioxide absorption, and oxygen
availability. However, if the night temperature is lower than the day temperature, the
sugars are conserved. A law formulated by the Dutch chemist, Jacobus van’t Hoff, states
that “for every 10 degree rise in temperature, the rate of respiration doubles.”
The conservation of sugars in plant tissues is an important factor in the lasting ability of
cut flowers. Flower growers can store plants after harvest without water for weeks at
temperatures of 310F (- 0.5 0C), to 33 (0.5 0C). At these temperatures, the rate of
respiration is very slow and sugars are conserved for long periods.
Transpiration and evaporation from the leaves is affected by temperature. Wilting may
occur when the rate of transpiration increases too rapidly and water cannot be supplied
fast enough to the leaves. Wilting may occur in winter after cold weather. Bright
sunlight may increase the leaf temperature 15 0F or more than the surrounding air
temperature. For those plants that experience increased transpiration rates, they will
show leaf dehydration and wilting symptoms. Watering will not correct the problem,
only shade will allow the plant to slowly recover from water loss.
The greatest activity of soil microorganisms occurs when the soil temperature is in the
range from just above freezing to light over 110 0F (44 0C). Nitrogen bacteria convert
organic nitrogen fertilizers, such as urea, cotton meal, and manure to more easily
absorbed forms of nitrogen. In addition, they are most active at temperatures above 600F
(15.5 0C). The rate of nitrogen conversion below this temperature is very slow. Thus, the
nutrients are not easily available to the plants. Organic fertilizers are not recommended
for use on greenhouse crops from late October to late April because of the effect of the
lower temperature.
Soil microorganisms also act on sources of ammonium nitrogen to convert them from
ammonia to nitrites to nitrate forms, which are easily absorbed by plants. Cold soil
temperatures also slow the rate of these conversions. If fertilizers are used in the winter,
at least 50 percent of the nitrogen must be applied in the form of nitrates.