Principles of Plant Genetics and Breeding

expense of the rest of the plant. N. W. Simmonds has
identified three outcomes of competition among plant
parts for assimilates and their implications in plant
breeding.
1 Vegetative growth is sacrificed for reproductive
growth. In this outcome, the breeder reduces vegetative
growth by reducing the plant structure (breeding
for dwarf cultivars, or breeding determinate
cultivars). Other strategies include the reduction in
foliage as in the okra leaf cotton phenotype. Dwarf
cultivars have been developed for many major cereal
crops of world importance (e.g., wheat, rice, sorghum,
barley). Dwarf cultivars are environmentally responsive
(i.e., respond to agronomic inputs – fertilizer,
irrigation, etc.). The success of the Green Revolution
was in part due to the use of dwarf cultivars of wheat
and rice.
2 Reproductive growth is sacrificed for vegetative
growth. In crops in which the desired part is vegetative,
flowering and seed set are either reduced, (e.g.,
yam, cassava, potato) in order to channel resources
into the vegetative parts, or suppressed (e.g. sugar
beet, carrot).
3 One vegetative growth is sacrificed for another
desired vegetative growth. The objective is to allocate
dry matter to the harvestable underground vegetative
structures (e.g., potato).
Improving the efficiency of dry
matter partitioning
Proposed by C. M. Donald, harvest index is the proportion
of the plant that is of economic value. It is
calculated as a ratio as follows:
Harvest index = (economic yield/biological yield)
For cereals, for example, the ratio will be grain yield to
total plant weight (grain + straw). The theoretical value
of the harvest index ranges from 0.0 to 1.0 (the value
may be converted to a percentage by multiplying by
100). The higher the value, the more efficient the plant
is in directing assimilates to the plant parts of economic
value. The harvest index is hence also referred to as the
coefficient of effectiveness. The higher the harvest
index, the more economically desirable the genotype,
because it translocates more of the available assimilates
into the economic parts of the plant.
Some researchers indicate that the dramatic increase
in the grain yield of major world cereal crops is due
BREEDING FOR PHYSIOLOGICAL AND MORPHOLOGICAL TRAITS 355
mainly to increases in the harvest index and to a lesser
extent the biological yield. In maize, for example, the
harvest index changed from 24% in 1950 to 43% in
1970, increasing yield from 3 to 8 metric tons/ha.
Generally, tall cultivars have high biological yield and
low harvest index, whereas semidwarf cultivars have
high harvest index and high biological yield. On the
other hand, full dwarf cultivars have low biological yield
and low economic yield. The breeding question is how
effectively can harvest index be selected to make it a
breeding objective for increasing yield? One difficulty
with selecting harvest index is that it is not a phenotypic
trait that can be readily evaluated. It is calculated from
data obtained from two separate weighings. Such data
are problematic to obtain if experimental plants are
harvested mechanically, as is the case in many large
breeding programs.
The developmental pathway followed by the plant
part or chemical component of economic value affects
the harvest index. In cereal crops (e.g., corn, wheat), the
economic part, the grain, fills in a linear fashion up to
a definite point, and then ceases. The harvest index
in these crops depends on the relative duration of the
vegetative and reproductive phases of the plant life
cycle. However, in crops such as sugar beet and Irish
potato, the economic part follows a protracted developmental
pathway. In these crops, harvest index depends
more on genetics than environmental factors.
The harvest index can be decreased or increased by
manipulating the cultural environment. For example,
increasing plant density and drought or soil fertility
(e.g., nitrogen application), are known to lower the
harvest index in corn. However, planting early maturing
cultivars under good management, increased the harvest
index in rice in some studies. This happened because the
plant was able to allocate assimilates to the seed sooner,
thereby leading to reduced accumulation of reserves in
the leaves.
Harvest index has also been increased in small grain
cereals partly through decreasing plant stature (e.g., by
using the Rht dwarfing gene in wheat or by selection
technique).
Harvest index as a selection
criterion for yield
In spite of the role of the harvest index in increasing
crop yield, using this trait as a selection criterion
for grain yield is problematic for the breeder. This is
largely due to the effect of the environment and cultural