The response of living organisms to regular changes in temperatures, either day/night or seasonal, is known as thermoperiodism. Thermoperiod-ism exerts effects on the seasonal biology of insects and the growth and development of plants. Effects on insects include rates of growth and development, determination of diapause and dormancy, and acclimatization to low temperatures. Effects on growth and development of plants vary from one species to another. Crops such as soybean, maize, tomato, potato, eucalyptus, and mango are classified as thermoperiodic, while wheat, oats, peas, and cucumber are classified as nonthermoperiodic.

In soybean, a cool day/night temperature combination of 18/14°C disrupts floral development, leading to physically malformed parts. Normal floral initiation and pod development occurs at 30/18°C and 30/22°C, respectively, while the greatest number of pods per plant is obtained at 26/14°C (Judith and Raper, 1981). Tomatoes grow faster when the temperature is 26°C by day and 17°C by night than at the constant temperature of 26°C or any intermediate temperature. For this reason, tomatoes do not grow well in warm countries except in those locations where the temperature falls appreciably at night.

Some maize cultivars respond to a daily temperature fluctuation. In experiments with three lines differing in earliness (early, mid, and late) grown in a phytotron from two-week-old seedlings raised in a 16 h photoperiod under three different day/night temperature regimes, there was a direct corre lation between chlorophyll content and grain yield. It proved best for yield and energy savings to raise seedlings of early lines at a high diurnal temperature (day/night = 24/15°C), and midseason or late lines at a constant day and night temperature of 21°C (Stolyarenko et al., 1992).

Potato plants grown under the fluctuating temperature treatment develop normally, develop tubers, and have a fivefold or greater total dry weight compared to those under constant temperature. This suggests a thermo-period could allow normal plant growth and tuberization in potato cultivars that are unable to develop effectively under continuous radiation (Tibbitts, Bennet, and Cao, 1990).

Two important aspects of the environment influencing induction of flowering in mango are photoperiodism and thermoperiodism. Studies in the Maharashtra region of India (Lad, Pujari, and Magdum, 1999) indicated that minimum temperature below 10°C and above the freezing point stimulated heavy flowering in mango. Furthermore, flowering occurred only in a single flush, compared to two to three flushes under normal environmental conditions.

Many crop seedlings will grow perfectly well at a constant temperature, but others, such as celery, germinate best at fluctuating temperatures. The emergence of carrot seedlings from soil is faster in fluctuating temperatures than at constant temperature. Solanum elaeagnifolium is a weed that produces solasodine (a steroidal alkaloid used for the production of cortico-steroids). It has a strict requirement for alternating temperature to germinate. A constant day/night temperature prevents the germination process. Seeds become sensitive to alternating temperature five days after the start of imbibition. After that, three daily cycles of alternating temperature are required for 50 percent germination (Trione and Cony, 1990).

The ecological significance of this response to a diurnal alteration of temperature may be that it promotes germination of those seeds close to the soil surface. When such fluctuations do not occur, germination, especially of more deeply seated seeds, may remain suppressed.

Apart from daily fluctuations in temperature, seasonal fluctuations are important in the development of many plants. Annual plants do not need a cold period during their development, except for plants that germinate in autumn and flower in the spring or summer after a cold winter. An example is winter wheat. Peaches cannot flower at high temperatures, but the vegetative growth phase continues. They need a period of cold weather before flower buds can open. No flower primordia are laid down under conditions of continued high temperature.

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