Photosynthesis produces the sources of assimilates which plants use for growth and development. Temperature and radiation influence the rate of photosynthesis. However, plants also have an obligatory development in time, which must be met if the photosynthetic assimilates are to be converted into economically useful yields of satisfactory quantity and quality. Temperature (and day-length in case of photosensitive crops) influences the developmental sequence of crop growth in relation to crop phenology.
Evolutionary changes that have occurred in the biochemical and physical characteristics of photosynthesis have resulted in a large variation between crops in both their optimum temperature requirements and the responses of photosynthesis to changes in temperature, radiation, and composition of the atmosphere. These responses depend on the nature of the photosynthetic pathway. In general, the C3 pathway of assimilation is adapted to operate at optimum rates under lower temperature conditions than the C4 assimilation pathway. However, breeding and selection (both natural and under human influence) have changed temperature responses of photosynthesis in some C3 and C4 species. It is therefore necessary to make a division of the major food crops according to their assimilation pathway and corresponding temperature requirements. Four groups have been recognized in global AEZ:
Group I
| C3 species adapted to lower temperatures (e.g., wheat, potatoes); |
|---|---|
Group II
| C3 species adapted to higher temperatures (e.g., soybean, rice, cassava); |
Group III
| C4 species adapted to high temperatures (e.g., millet, sorghum, maize, sugarcane); |
Group IV
| C4 species adapted to lower temperatures (e.g., sorghum, maize). |
To cater for differences in thermal requirements of crops, an adequate characterization of the temperature regimes is required, applicable for a wide range of locations. The characterization of the temperature regimes in the present approach comprises four parts, namely:
The thermal climates were obtained through classifying monthly temperatures corrected to sea level (with an assumed lapse rate: 0.55°C/100m). The latitudinal thermal climates distinguished in global AEZ are the following: tropics, subtropics with summer rainfall, subtropics with winter rainfall, temperate, boreal and polar/arctic. The temperate and boreal belts have been further subdivided according to continentality into three classes, namely: oceanic, sub-continental and continental.
| Thermal climate classification |
| Thermal climates |
Temperature seasonality is expressed in terms of the number of days falling into periods with respectively increasing and decreasing temperatures. A complete account of time periods of individual temperature intervals provides a year-round temperature profile. These profiles have been calculated for each grid-cell.
 | Examples of average temperature profiles for Bangkok, Harbin, Manaus, Marseilles, Nairobi and Vienna |
Temperature growing periods and accumulated temperatures
In addition to thermal climates and temperature profiles, temperature growing periods (LGP t ) have been inventoried. For instance, LGP t=5 of 5°C, i.e., the number of days when mean daily temperature exceeds 5°C, represents the period with temperatures supporting crop growth. Similarly LGP t=10 of 10°C approximates the frost-free period.
| Temperature growing periods (LGPt=5) |
| Frost-free periods (LGPt=10) |
For various base temperatures, accumulated temperatures have been calculated for each grid-cell. These indicators are widely used in the agronomic literature and have been applied in matching crop requirements to grid-cell characteristics.
| accumulated temperatures (Tmean> 0 Deg. C) |
| accumulated temperatures (Tmean> 10 Deg. C) |
