Agriculture Reference
In-Depth Information
CHAPTER TWO
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[11],
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Norm
PgEn
Humans cultivate more than 200 species of plants, but this chapter re-
views responses of 5 important cereal crops to drought. These crops are
maize (
Zea mays
L.), rice (
Oryza sativa
L.), wheat (
Triticum aestivum
and
Triticum turgidum
L. var. durum), sorghum (
Sorghum bicolor
[L.]
Moench), and pearl millet (
Pennisetum glaucum
[L.] R. Br), which provide
the majority of food in the world. In general, farmers cultivate millet in the
most drought-prone environments and sorghum where a short growing
season is the greatest constraint to production. Some sorghum cultivars set
grain in as short as 50-60 days (Roncoli et al., 2001). Rice is grown un-
der a wide range of environments, from tropical to temperate zones, from
deep water-flooded zones to nonflooded uplands. Rice productivity is lim-
ited mostly by water (IRRI, 2002). Drought limits, to a varying extent, the
productivity of all of these crops. Although water is likely the most impor-
tant manageable limit to food production worldwide, we should recognize
that water management cannot be isolated from nutrient, crop, and pest
management.
[11],
C
rop Water Use
Life on earth depends on green plants, which capture solar energy and store
chemical energy by the process of photosynthesis. Although plants use a
small amount of water in the reactions of photosynthesis and retain small
amounts of water in plant tissues, as much as 99% of the water that plants
take up is lost through transpiration (i.e., gaseous water transport through
the stomata of leaves). Stomata, which are small pores on leaf surfaces,
must open to allow carbon dioxide to enter leaf tissues for photosynthesis
and plant growth, but open stomata also allow water to escape.
In addition to transpiration, there are several other avenues of water loss
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