Maize - Bioenergy Crop Plants - page 407

900

Grain
Stover

800

700

600

500

400

300

200

100

0

Corn Wheat Rice Barley Oats Rye Sorghum

Crop

FIGure 16.2 Global annual production of grain and crop residue from cereal crops. (From U.S. Department
of Agriculture/National Agricultural Statistics Service, Agricultural Statistics Database , 2009. Available at
http://www.fas.usda.gov/wap/current/toc.asp)

This not only contributes to environmental pollution but also adversely affects soil microflora. The
availability of a market for the crop residue would help to mitigate these problems. As agriculture
becomes more modernized along with industrialization of developing nations and crop productivity
increases, correspondingly larger amounts of crop residue will become available.
The advantages of maize as a candidate for a bioenergy crop are numerous, the most important
being the depth of genetic knowledge and commercial success of its transgenic products. Another
major advantage corn stover holds over alternative crops in that it is already produced with grain as
the target and does not require dedicated land (Dhugga 2007).

16.2.2 o rigin and E volution
The center of origin of maize, a member the genus Zea in the grass family Poaceae, previously
referred to as Gramineae, is agreed upon to be southern and southwestern Mexico. The genus Zea
has five species: diploperennis, perennis, luxurians, nicaraguensis, and mays L.; the first two are
perennial and the last three annual. All the species are diploid with the exception of Zea perennis ,
which is tetraploid. The species Zea mays L. is highly polymorphic and consists of four subspe-
cies: huehuetenangensis, mexicana, parviglumis , and mays . All Zea species except cultivated
maize, Zea mays L. ssp. mays , are collectively referred to as teosinte. Based on multiple pieces
of evidence, which include molecular tools and fossil records, maize domestication occurred
approximately 10,000 years ago, which was preceded by human migration to the Americas
around 15,000 years ago.
Teosinte remarkably differs from maize in morphological features, which led some scientists to
propose that the progenitor of maize was extinct. However, the geneticists led by Beadle, arrived
at the conclusion that teosinte was the immediate progenitor of maize, mainly based on the ability
to obtain fertile crosses of maize with teosinte, but also several other observations, such as chro-
mosome number and positioning of knobs in the chromosomes. The latter view is now generally
accepted as additional evidence from evolving molecular technology has provided abundant sup-
porting evidence in its favor.
From screening a large F 2 population derived from a cross between teosinte and maize, Beadle
arrived at a conclusion that as few as five loci could explain the major differences between these
two subspecies. In keeping with his foresight, single genes, for example, teosinte branched-1 ( tb1 )