Drought-Related Characteristics of Important Cereal Crops - Monitoring and Predicting Agricultural Drought: A Global Study - page 15

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Perhaps the most devastating substrate-mediated response to water def-
icit is lack of seed set. Near the time of pollination, depending on the avail-
ability of assimilates from photosynthesis or stored carbohydrates, plants
establish the number of seeds that will develop further. If drought restricts
carbon dioxide assimilation or the rate or transfer of stored assimilates,
then fewer seeds will develop. Once a plant aborts a seed, the seed can
grow no further and potential yield is irrevocably lost.

Desiccation Effects Even at relatively moderate degrees of drought sever-
ity, plant extremities (such as leaf tips beginning to fire, turning brown, and
desiccate) take place. As drought intensifies, leaves die and abscise. Eventu-
ally, shoots, and then roots, die. Only viable seeds can survive desiccation.
The following sections explain the distribution of the selected major crops
and their response to water deficits.

[15],

Maize

Line
——
-0.0
——
Long
PgEn

D istribution and Use

The center of origin for maize lies in Central America. It is grown widely
throughout the world including both tropical and temperate countries,
with the largest total production estimated at more than 604 million tons
(table 2.1). Industrialized countries generally grow maize for livestock feed.
Among the top five maize producers, significant amounts of maize are
consumed directly by humans only in Mexico. In the developing countries
of Africa and Asia, maize is mostly consumed directly by humans.

[15],

D rought and Water Use Efficiency

Maize has a C 4 photosynthetic mechanism, which gives it high potential
productivity with record yields near 20 Mg/ha (Rhoads and Bennett, 1990).
Estimates of seasonal WUE for maize range from 0.012 Mg ha -1 mm -1 in
a relatively arid environment (Musick and Dusek, 1980) to 0.030 Mg ha -1
mm -1 in a humid environment (Hook, 1985).
With an intermediate level of drought tolerance as compared with other
cereal crops, the majority of global maize production is under rain-fed
conditions. In contrast, because maize has a high water requirement during
tasseling and silking phases, drought during these phases may significantly
reduce productivity. Thus, maize crops are often irrigated in areas that have
frequent rainfall deficit during the early reproductive phase (Rhoads and
Bennett, 1990).
Rather than applying water to maintain soil moisture at a high level,
some farmers allow soils to dry appreciably before irrigation and then do
not replenish the full soil reservoir, a practice called deficit irrigation. If
properly maintained, a deficit irrigation schedule may improve irrigation
WUE of maize. For example, Steele (1994) found that irrigation after soil