Just as soil temperature influences crop emergence, the soil and air temperature influence growth and development of the plant. Temperature is just one important factor. Moisture is important; light is important. Many other factors of the environment are important to crop development, but temperature is a very important factor. Thermal time has become a useful factor to evaluate crop development, thermal time may also be defined more commonly as Growing Degree Days (GDD).

If temperatures are cooler than usual, the crop may develop at a slower rate than usual. Using the 50 °F (10 °C) is the base temperature for the growth of corn from Lesson 1a, corn growth is assumed to be 0 at the base temperature. As temperature increases, the growth rate increases somewhat, and then does fairly well. Thereafter, at a certain point, it levels off. The crop grows according to an "S"-shaped curve depending on temperature, with the ideal temperature for crop growth, if everything else is satisfactory such as nutrition and water availability, being somewhere around 93 °F (34 °C) (Fig. 2.11). Common corn varieties will not grow below 49 °F (9.5 °C), will grow fastest at 93 °F (34 °C), will not grow above 115 °F (46 °C), and die at 118 °F (48 °C). The growth rate experienced responds to temperature in an "S-shaped" curve between 49 °F (9.5 °C) and 93 °F (34 °C) (Bonner and Galston, 1952).

Above 93 °F (34 °C), the growth rate begins to drop off again. If the temperature rises above 110 °F (43.3 °C), it drops off very rapidly. True heat stress occurs at a temperature of 110-115 °F (43-46 °C). Between 50 °F and 93 °F, the crop can grow very well.

Fig. 2.12 Relative growth rate of corn as affected by air temperature.

The S-shaped line represents the way a crop develops according to temperature. A straight line might represent mean or average response, a way to evaluate it simply using growing degree days (Fig. 2.13).

Fig. 2.13 Relative growth rate as approximated by the growing degree day.

The straight line approximation to crop growth is known as a growing degree day model. The growing degree day model begins at 50 °F (10 °C) and rises linearly to 86 °F (30 °C). The growth rate of the crop goes from 0 to 1, from nothing to maximum, as temperature increases from 50 to 86 °F. After 86 °F, the assumption is that increased temperature does not have any additional positive effect, but not necessarily a negative effect. The growth rate levels off at that point.