Principles of Plant Genetics and Breeding

emoved with forceps, the cut may be made across the
anthers. The anthers can be extracted by using a vacuum
pump or by forceps.
Pollination
The stigma remains receptive for about 4–5 days, so
pollination should be made sooner than later. Pollen
remains viable for a shorter period (a few minutes to
about half a day). Pollination should be conducted during
the period of peak anther dehiscence, with pollen
gathered just before anther dehiscence. The male panicles
are cut and the flag leaf removed. These panicles are
watched closely for anther extrusion, and used thereafter
for pollination. The bag is taken from the female
and the pollen shaken over it. The bag is replaced and
clipped securely against the stem. Other techniques are
also used for pollination.
Natural pollination
Rice is highly self-pollinated. Where commercial F 1
hybrid seed production is undertaken, natural crosspollination
is often inadequate. Consequently, natural
pollination is supplemented with hand pollination. The
flag leaf may be cut and the leaf sheaths that enclose the
panicle torn, to help the release of pollen. Also, a rope or
pole may be dragged across the field at the level of the
panicles each day during the flowering period to aid
pollen dehiscence.
Seed development and harvesting
The success rate in artificial pollination of rice is about
50% or higher depending on the technique used. Ovule
swelling starts 3–4 days after pollination. The developing
F 1 seed lacks a complete covering because the
glumes were cut during the preparation of the female for
pollination.
Rice does not mature uniformly in the head. Grain
harvest moisture is critical to yield and produce quality.
Chang, T. 1997. Rice. In: Genetics, cytogenetics and breeding
of crop plants, Vol. 2 (Bahl, P.N., P.M. Salimath, and
A.K. Mandel, eds). Science Publishers, Enfield, NH.
BREEDING RICE 507
References and suggested further reading
The recommended grain moisture content is between
23 and 28%. At this stage the grains in the top portion
of the head are ripe but those in the lower portion are
in the hard-dough stage. Harvesting at this stage will
include some immature grain but delaying harvesting
increases the chances of shattering and checking of
grains in susceptible varieties.
Common breeding objectives
1 Grain yield. Grain yield in rice depends on yield
potential under the most favorable conditions, yield
stability (across seasons), and crop productivity. The
components of yield are panicle number per unit area,
the number of filled grains per panicle, and grain
weight. Some breeders use spikelet number per unit
area and grain weight. The dramatic yield increases
observed in the tropics in the 1960s were as a result
of the development and use of semidwarf (sd1 gene)
cultivars that were environmentally responsive.
2 Grain quality. Grain quality traits of interest vary
from one region to another. They include grain size
and shape, color of kernel, aroma, stickiness, and
protein content.
3 Disease resistance. Host resistances to many major
diseases and insect pests have been identified in rice.
Many of these are under oligogenic control and
hence are susceptible to changes in the pest resulting
in the pest becoming resistant to a resistant cultivar.
The strategy of durable resistance is favored by many
breeders. Resistance to insect-transmitted viral infections
is complex to breed because the insect-resistance
aspect (e.g., non-preference of the vector) can mask
the disease-resistance component.
4 Resistance to environmental stresses. Breeding
resistance and tolerance to various environmental
stresses is important in rice breeding. Drought and
flooding frequently alternate with each other. Research
indicates that regarding moisture stress, escape and
avoidance mechanisms are important in dryland culture,
while tolerance and recovery mechanisms are
important under rainfed wetland culture. It should
be pointed out that plant reactions to environmental
stresses are site- and growth stage-specific.
Coffman, W.R., and R.M. Herrera. 1980. Rice. In: Hybridization
of crop plants (Fehr, W.R., and H.H. Hadley, eds),
pp. 511–522. American Society of Agronomy, Madison, WI.