Agriculture Reference
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did not, but results differed greatly among cultivars. As for other crops,
there is a strong relationship between evapotranspiration reduction and
yield reduction by drought, but these drought stress indices and produc-
tion functions must be calibrated for each variety and location rather than
having a general applicable relationship.
Wheat
D istribution
At more than 575 million tons, wheat has the third largest global produc-
tion of any food crop. The center of origin for wheat is in the Middle East
between the Tigris and Euphrates rivers. As a cereal with C 3 photosynthesis
metabolism, wheat productivity generally declines as temperatures increase
above 30°C (Abrol and Ingram, 1996). Thus, wheat is predominantly pro-
duced in the temperate regions of the world (table 2.3). Thirty countries
produce 90% of all wheat produced globally. A small portion of the wheat
production lies in subtropical zones during winter.
Wheat has a dual role in the food system. It provides human nutrition
both by direct consumption and through feeding to livestock. Although
high temperatures restrict the productivity of wheat in the tropics, the
importance of wheat for direct human consumption increases with national
development as people develop a taste for breads and pastas. About half
of the total world wheat production is traded internationally, with a large
portion of that trade occurring from wheat produced in the industrialized
nations and exported to developing countries.
[19],
Line
——
-0.0
——
Norm
PgEn
[19],
D rought and WUE
Although the evapotranspiration WUE of wheat is only 0.010-0.017 Mg
ha -1 mm -1 (Zhang et al., 1999), the wheat root system is generally very deep
and the crop has a relatively high level of drought tolerance (Musick and
Porter, 1990). As is the case for most cereals, wheat is most susceptible to
yield reduction when stress occurs during the heading phase. In the North
China Plain, wheat was most sensitive to water deficit from stem elongation
to heading and from heading to milk dough phases (Zhang et al., 1999).
Irrigation two to four times in North China increased evapotranspiration
WUE from 0.010 to 0.012 Mg ha -1 mm -1 .
There is a wide range of drought tolerance among wheat varieties.
Moustafa et al. (1996) found that a 10-day water deficit during head-
ing reduced yield from 0 to 44% among four wheat cultivars. Although
the physiological drought adaptations of wheat include osmotic adjust-
ment, desiccation tolerance, and cellular elasticity, the principal contribu-
tor to drought resistance appears to be its deep root system (Musick and
Porter, 1990). In Mexico, Calhoun et al. (1994) found that selecting wheat
 
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