Drought Mitigation

Growing Drought-Resistant Crops

Timing of rainfall and choice of crop variety are critical to avoiding agricultural drought, which can occur even when the annual or seasonal rainfall is normal. Crops may resist or evade drought in two basic ways (Levitt,

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 developed the Katumani composite B and Makueni composite B maize varieties as drought-escaping genotypes that mature early. In the case of drought resistance, 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 varieties 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 equivalent 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 reduce 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 increasing moisture stress. Drought-tolerant crops have a greater capacity to adjust osmotically than those that are drought susceptible, and they recover quickly and resume growth when moisture conditions become favorable. 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,

Table 18.3 Agroecological conditions and drought-resistant status of grain crops grown in Kenya (derived from Jaetzold and Schmidt, 1982; Ashley, 1993, 1999)

Range of

Range of altitude

rainfall (mm)

a.m.s.l. (m)

per growing

according to the

Drought-resistant

Common name

Scientific name

periods

growing periods

status

Barley

Hordeum vulgae

150-650

>2100

Not drought-

resistant

Kidney beans

Phaseolus vulgaris

230-450

>600

Not drought-

resistant

Tepary beans

Phaseolus

120-320

>600

Drought- and

acutifolius

heat-resistant

Soya beans

Flycine max

350-750

<400

Not drought-

resistant

Bonavist beans

Lablab niger

200-2000

1200-2100

Very drought-

or Lablab

resistant

purpureus

Bulrush millet

Pennisetum (syn.

220-800

<1200

Both drought-

(or pearl

P. typhoides,

tolerant and

spiked or

P. typhoideum,

drought-evading

cat-tail millet)

P. glaucum,

P. spicatum)

Cowpeas

Vigna unguiculata

190-700

<1500

Drought-resistant

Finger millet

Eleusine coracana

230-900

900-2400

Drought-resistant

Green gram

Vigna aureus

190-400

<1500

Drought-resistant

Black gram

Vigna mungo

200-400

<1500

Drought-resistant

Groundnuts

Arachihypogea

280-550

<1500

Mildly drought-

(or peanuts)

resistant

Maize

Zea mays

240-1100

<2400

Not drought-

resistant

Pigeon peas

Cajanus cajan

370-800

<1500

Drought-resistant

Rice

Oryza sativa (or

75-1200

<1200

Not drought-

O. glaberrima)

resistant

Simsim

Sesamum indicum

300-600

<1500

Moderately

drought-resistant

Sorghum

Sorghum bicolour

200-450

900-1500

Very drought-

(or Sorghum

resistant

vulgare)

Sunflower

Helianthus annuus

180-650

<2600

Very drought-

resistant

Wheat

Triticum aestivum

350-750

1200-2900

Not drought-

resistant

indicating their agroecological conditions and drought resistance status. Barley, kidney beans, soya beans, maize, and wheat can succumb to drought conditions and rainfall that is not well distributed (table 18.3). The potential effect of drought on agricultural production in Kenya has been minimized through selection of appropriate and timely agricultural practices such as Katumani composite B, Makueni composite B, and Mwezi Moja beans variety.

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