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Ear-to-row recurrent selection for plant
height is being applied to several composite
populations, as well as to those subjected to
mass selection for yield. Thus far, effects of
plant height reduction on yield have not been
adequately assessed. It seems probable that
modifications of cultural practices must accompany
any major change in plant height.
Sunlight as a Limiting Factor in Dwarf Corns
To investigate the importance of light as a
limiting factor when moisture and soil fertility
were optimum, a variety homocygous for brachytic-2
was grown at EI Roque, Guanajuato
during during the summer of 1969, under conditions
intended to provide four levels of light
interception. (The brachytic 2 gene tends to
telescope the plants with little space in between
the leaves.)
Fou r treatments of canopy organization
were used: (1) leaves above the ear positioned
upright with transparent plastic bands (positioned
at about 10° from the vertical) from the
beginning of the flowering period. (2) plants
oriented so that all leaves emerged and grew
in an East-West direction with seedlings oriented
by hand ten days after emergence. (3)
leaves oriented east-to-west and positioned
upright; and (4) control. (See Table 12.)
These four treatments of canopy organization
were tested in a randomized complete
block experiment with two replicates, and with
a rate of planting of approximately 61,000 plants
per hectare. Resulting yields greatly exceeded
yields normally obtained with locally adapted
hybrids and varietes of normal plant size.
TABLE 12. Mean grain yields at 15.5% moisture
for the 4 systems of canopy organization.
Treatment
Meen Yield
Ton/he
Yield In %
of Control
Leaves positioned upright
Leaves oriented East to West
10.71
11.10
108
112
Leaves positioned upright and
oriented East to West 12.15 122
CONTROL 9.93 100
These yields exceed by far the normal yield obtained with locally
adapted hybrids and varieties of normel plent size.
Treatment 1 showed an increase in productivity
of 8 percent over the control. Treatment
2 showed a 12 percent increase; while
treatment 3 showed a statistically significant
22 percent increase.
The yield increment obtained in treatment
3 was a little more than the sum of the increments
obtained from treatments 1 and 2. This
seems to indicate that light continued to be a
limiting factor for maximum yield per unit area,
even for the treatment "upright leaves and
oriented east to west."
In view of these results, and other recent
findings on the importance of sun light as a
limiting factor for maximum productivity, it
appears that the brachytic-2 plant type is very
inefficient in its use of incoming light.
For corn breeders working with brachytic-2,
it is fortunate that several other genes considerably
modify its expression in several ways.
allowing selection of desirable dwarf types.
For example, two inbred lines were found in
Puebla Group 1 brachytic, which consistently
showed interesting plant types.
One of these lines produces short internodes
and horizontal leaves below the ear,
with normal internodes and erect leaves above
the ear. This growth habit may be of great
utility if easily transmittible to other materials.
The other inbred line produced extremely
dwarfed plants with erect stiff leaves, and two
relatively good sized ears. Thus, the modifications
of the maize plant to obtain more efficient
types appears to be a distinct possibility
through proper breeding methods.
Introduction of the Brachytic 2
Gene into Tuxpefio Types
In 1966, the brachytic-2 gene was crossed
into 10 varieties of the Tuxpeno race and seed
of all 10 crosses was bulked and planted at
San Rafael, Veracruz, where it was sibbed and
crossed to other materials. The original and
other crosses with Tuxpeno types were planted
in an isolated block until 1968. In 1968 one
cycle of mass selection was conducted and
400 plants were selected to begin a program
of modified ear-to-row selection.
The yield performance of these brachytic-2
Tuxpenos appears quite promising, even
though no superiority can be claimed for them
when compared to the normal height corn.
They are however, much less susceptible to
lodging and stalk. breakage. To provide the
next step, the brachytic-2 Tuxpeno population
has been crossed to a wide range of tropical
corns. These were selected to for other attributes
including earliness, narrow and fewer
leaves, resistance to disease, and broader
adaptation to the humid tropics.
Selection for Grain·to-Stover Ratio
The efficiency of a corn plant can be
measured on the basis of the relationship
between production of stover and grain. This
ratio, in tropical com, falls between 0.3 and
0.7, which compares to more than 1.0 in improved
wheat, rice and sorghum varieties.
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