The major driving force that pushes wheat through its successive developmental stages from emergence through double ridge, to flowering and maturity is temperature. Development can occur over a range of temperatures; below 0° C is too cold for most varieties and much above 30° C is too hot. As mean temperature rises between these bounds called the base and optimum temperatures respectively, development goes progressively faster. Growth, or increase in size, doesnt necessarily follow these rules because solar radiation (p 89) as well as temperature affects size and number.
Every phase of development requires a minimum accumulation of temperature before that stage can be complete and the plant can move to the next stage. In effect, the plant senses the temperature every day and adds the average for that day to a running total up to the total required for the stage. This running total is called thermal time or the heat sum for the phase and the thermal units are degree days (°Cd). You can calculate it in the same way by totalling the mean temperatures for each day during the phase. Mean temperature is maximum + minimum divided by 2. So if the day had a maximum of 35° C and a minimum of 15° C the mean would be 25° C (35 + 15 divided by 2) and the heat sum for the day would be 25° Cd.
Minimum heat sums for developmental phases
The accompanying diagram shows the minimum number of °Cd needed for each phase. For example, to move from emergence to double ridges there must have been a running total of at least 200° Cd (10 days with a mean of 20° C or 20 days with a mean of 10° C). Similarly, from sowing to heading needs at least 760° Cd.
If you want to calculate for your location approximately how many days it would take for a short-duration wheat to complete the above phases, divide the heat sum given in the diagram by the mean temperature at your site. For example, if your mean temperature is 15°C, the phase from sowing to heading will take at least 50 days (760/15 = 50). You can also work out how a period of lower or higher temperature would affect development.
Conceptually, base temperature is the temperature at which development stops through cold. As temperature increases above the base, the process goes progressively faster until it reaches the optimum temperature. The optimum is the temperature at which development is fastest. Higher temperatures than the optimum can slow development and at temperatures well above the optimum development may stop altogether and the plant may die.
Change in optimum and base temperature with development
For wheat, base and optimum temperatures arent always 0° C and 25° C respectively. They actually start lower and rise with development. The figure shows that although plants can grow at 0° C during the seedling stages, they make slow progress at the heading stage if temperatures are much below 10° C. Fortunately, varieties differ in their base and optimum by as much as 7° C at any stage (see Slafer and Rawson 1995 and associated papers). In general, winter wheat can develop at lower temperatures than spring wheat.
If you need to be accurate in your calculation of heat sums, you should work out the mean temperature for each day as maximum + minimum divided by 2 as above, then subtract the base temperature for the stage.
At particular stages in development, there are other factors that can modify some of the effects of temperature. Minor modifiers include drought, nutrition and solar radiation. The major modifiers however are photoperiod (hours between first and last light each day) and vernalization (temperatures between 0 and 12° C). These modifiers differ in strength with stage and with wheat variety (see Rawson et al 1998). Shorter days slow development as do the absence of temperatures in the vernalization range during seedling growth, though the latter applies mainly in winter wheat. Bear this in mind when working out how long developmental phases might take. For example, sowing to heading could take twice as long with a photoperiod of 10 hours than with one of 15 hours (1500° Cd instead of the 760° Cd shown in the diagram of minimum heat sums).