Agriculture Reference
In-Depth Information
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1972): drought tolerance and drought escape. Drought escape is the ability
to complete the plant's life cycle before serious soil and plant moisture
deficits develop; plants mature early. Plant breeders in Kenya have devel-
oped the Katumani composite B and Makueni composite B maize varieties
as drought-escaping genotypes that mature early. In the case of drought re-
sistance, a plant develops the ability to advance into the next phenophase.
For instance, a mild water deficit for wheat between floral initiation and
anthesis phases may hasten the plants to maturity. Table 18.3 gives some
of the main drought-resistant crops in the region. In modern cereal vari-
eties and genotypes, drought resistance and yield stability in drought areas
with less reliable rainfall have been achieved mainly by reducing time to
maturity (Fischer and Maurer, 1978).
According to research on drought-resistance in crops (Sanchez-Diaz and
Kramer, 1971; Turner, 1974; Turner and Jones, 1980; ICRISAT, 1987),
the factors that confer drought resistance can generally be classified into
enhancement of water acquisition from the soil by plants and restriction of
transpiratory losses. At the seedling stage the aerial portions of drought-
resistant crops, such as sorghum, tepary beans, bonavist beans, and bulrush
millet, grow slowly until the root systems are well established. For more
drought tolerance, the plants develop more lateral and adventitious roots,
which are better suited to extract water from the soil (Ashley, 1993, 1999).
In the semiarid areas of Laikipia in Kenya, for instance, Liniger (1991)
found that maize, a less drought-resistant crop than sorghum, grew vertical
roots to beyond a depth of 1.5 m and more lateral roots to adapt to the lack
of water in this semiarid area. Sanchez-Diaz and Kramer (1971) established
that the sorghum leaves and stems are covered with a white, waxy bloom
to reduce net radiation and cuticular transpiration.
Drought-tolerant plants have high water-use efficiency. For example,
sorghum requires about 20% less water than maize to produce an equiva-
lent amount of dry matter. In drought-resistant plants, photosynthetic and
growth rates under mild water stress are equal to or more than those of
the nonstressed plants. Moderate stress enables continued root growth,
even when aerial growth has stopped. In some grass crops (e.g., sorghum),
the leaves become more erect and roll inward along their lengths to re-
duce energy load, which would increase respiratory losses. The stoma of
the drought-tolerant crops close at relatively low water potential. The leaf
stomata retain viability during periods of wilting that last about two weeks
or more. Functional recovery follows the restoration of leaf turgidity. This
shows a lower rate of decline in relative turgidity when subjected to in-
creasing moisture stress. Drought-tolerant crops have a greater capacity
to adjust osmotically than those that are drought susceptible, and they re-
cover quickly and resume growth when moisture conditions become favor-
able. Drought-tolerant crops induce premature leaf senescence to reduce
transpiratory water loss. These crops are adaptable to high temperatures.
Table 18.3 shows grain crops grown in eastern Africa, including Kenya,
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