Principles of Plant Genetics and Breeding

Table 18.1 Calculating heat units and degree-days.
that is often used is staggered planting, whereby the
inbred lines are planted at different times to ensure
that pollen will be available for both early and later
flowering females.
4 Field layout. Female and male inbred parents should
be arranged in the field such that pollen distribution
is effective and efficient. Effective layout patterns vary
among species. Male and female lines may be planted
in alternating rows, or a certain number of males can
alternate with a certain number of females according
to the pollen-producing ability of the pollinator.
5 Plant density. Hybrid seed is harvested from only
the female parent (i.e., the space occupied by the
male is not available for seed production). It is important
to maximize plant populations in seed fields. The
female plants may be increased relative to the pollen
source.
BREEDING HYBRID CULTIVARS 347
Scientists use the correlation between temperature and plant growth to predict crop harvesting times and also to determine
adaptability to a given area. To do this, plant growth is measured in heat units (the number of degrees the mean daily
temperature exceeds a base minimum growth temperature):
Daily minimum temperature + daily maximum temperature
Heat units = ———————————————————————— —
2
The base temperature varies with the species (e.g., 4.5°C for small grains, 10°C for corn and soybean, 15.5°C for cotton). A
modification of the heat units called growing degree-days (GDD) is used where, for the particular species, limits are placed on
the daily minimum and maximum temperature (obtained from weather records). For example, if the critical maximum
temperature is 30°C and the minimum is 10°C, the GDD for corn for that particular day is:
10 + 30
GDD = —— — − 10 = 10
2
The GDD are summed over the growing season to give the total needed for commercial production of these crops.
A generalized equation for estimating accumulated temperature above a threshold is:
GDD = T mean − T base
The application of this technology is in determining the staggering of planting dates of male and female parents in a hybrid
production program in the field to ensure high cross-pollination. GDD is also used for predicting vegetative growth and other
plant growth processes. Certain horticultural crops such as temperate fruits need to accumulate a certain critical amount of
chilling days without which fruiting would not occur.
The estimation of chilling dates is the opposite of GDD. Field crops such as spring wheat are sown in the fall so that they receive
cold treatment (vernalization) during the winter. This treatment triggers the reproductive phase of plant development in spring.
Some species are frost tolerant while others are frost sensitive. Frost damage is most critical when flower buds start to open.
The grower should pay attention to and use frost forecast information provided by the US Department of Agriculture.
Temperature is also an important requirement for germination. Some species (e.g., oat) can germinate in cool soils (3°C), while
others (e.g., sorghum) prefer a warm temperature (5°C) for germination. Temperature is a key factor in the Hopkins Bioclimatic
Law (crop production activities (e.g., planting, harvesting) and specific morphological developments are delayed 4 days for each
1° of latitude, 5° of longitude, and 122 m (400 ft), as one moves northward, eastward, and upward, respectively).
Maintenance
Once established, the crop should be properly maintained
for optimal growth and development. Field care
should include weed control, irrigation, proper fertilization,
and disease and insect control. To enhance the
development of the female line, and reduce the chance
of contamination at harvesting, the male line may be
removed after pollination is completed. This “thinning”
of the field provides additional growth resources for the
female line, as a result of reduced competition.
Where pollen control was implemented by mechanical
means (as opposed to the use of the male-sterility
system), workers need to walk through the field several
times during hybrid seed production to remove any pollen
sources that may have survived mechanical emasculation.