Principles of Plant Genetics and Breeding

Nature, types, and economic importance
Photoperiod and temperature are two major environmental
factors that influence crop adaptation through
their effect on days to flowering. Photoperiod is also
known to affect photosynthate partitioning in some
species such as peanut, in which researchers found a
reduction in the partitioning of dry matter to pods, in
certain genotypes, under long photoperiods. Decreasing
partitioning to grain favors partitioning to vegetative
parts of the plant, resulting in increased leaf area and dry
matter production. Crop cultivars that are developed for
high altitudes should mature before the arrival of winter
as well as be less sensitive to long photoperiods so that
seed yield is high.
Genetics and germplasm resources
In soybean, a number of maturity genes that influence
flowering under long day length have been reported. Of
these, the E3 locus has the most significant effect on
flowering under long day length. Plants with genotypes
of e3e3 have insensitivity to fluorescent long day length.
The genetic control of photoperiod insensitivity in
wheat is variable among cultivars, including two genes
with major effects, one dominant gene, or one recessive
gene controlling the condition. In sorghum, four
genes (Ma1 , Ma2 , Ma3 , Ma4 ) affect plant maturity and
response to long day length. As J. R. Quimby observed,
the recessive alleles of these genes condition a degree
of sensitivity to longer day length. The genotype did
not affect the time of initiation of flower primordial
under short days (10 hours). However, under long days
(14 hours), the genotype Ma1Ma2ma3Ma4 in sorghum
induced flowering in 35 days, while Ma1ma2Ma3Ma4 flowered in 44 days. The genotype Ma1Ma2Ma3Ma4 flowered in 70 days.
Early maturity
Crop maturity in general is affected by a variety of
factors in the production environment, including photoperiod,
temperature, altitude, moisture, soil fertility,
and plant genotype. Early maturity could be used to
address some environmental stresses in crop production
such as drought and temperature. Maturity impacts
both crop yield and product quality. Sometimes, the
producer desires the crop to attain its maximum dry
matter under the production conditions, before harvesting
is done. In some crops, premature harvesting produces
the product quality for premium prices.
BREEDING FOR PHYSIOLOGICAL AND MORPHOLOGICAL TRAITS 365
Nature, types, and economic importance
There are two basic types of maturity – physiological
and harvest (market) maturity. Physiological maturity
is that stage at which the plant cannot benefit
from additional production inputs (i.e., inputs such
as fertilizer and irrigation will not translate into
additional dry matter or gain in economic product)
because it has attained its maximum dry matter. In
certain crops, the product is harvested before physiological
maturity to meet market demands. This
stage of maturity is called harvest maturity. For example,
green beans are harvested before physiological
maturity to avoid the product becoming “stringy” or
fibrous.
It is desirable for a producer to grow a cultivar that
fully exploits the growing season for optimal productivity.
However, under certain production conditions, it is
advantageous for the cultivar to mature early (i.e.,
exploit only part of the growing season). Early maturity
may allow a cultivar to escape environmental stresses
(e.g., disease, insects, early frost, early fall rain storms,
drought) that may occur later in the season. Also,
early maturing cultivars are suitable for use in multiple
cropping systems, allowing more than one crop to be
grown in a production season. Early maturity has made
it possible to extend the production of some crops to
regions in higher altitudes and with shorter summers, as
well as with low rainfall.
Early maturity has its disadvantages. Because the
plant only partially exploits the growing season, economic
yield may be significantly reduced in species
including corn, soybean, wheat, and rice. In cotton,
earliness is negatively correlated with traits such as fiber
length.
Genetics and germplasm resources
Research reports indicate earliness to be controlled by
either dominant or recessive genes. Modifier genes of
major genes have also been reported. The inheritance
appears to differ among species. Researchers have conducted
QTL analysis for improvement of earliness.
Furthermore, a flowering promoting factor (fpf1 gene)
has been cloned in Arabidopsis. This gene is being used
to transform other species to create early maturity in
those species. In sorghum, four gene loci, Ma1 , Ma2 ,
Ma3 , and Ma4 , influence time to maturity. These are the
same genes that are implicated in photoperiod sensitivity.
Tropical sorghum species are dominant at the Ma1 locus, and do not flower during the long summer photoperiods
in the USA.