Yoshida - 1981 - Fundamentals of Rice Crop Science

216 FUNDAMENTALS OF RICE CROP SCIENCE
considered the first high yielding indica rice variety. It demonstrated that increasing
the yield potential of indica rices can be achieved by improvements within
indica rices. Thus, it pointed the way for tropical rice breeding.
In 1962, IRRI began research at Los Baños, Philippines. Its plant breeders
crossed Dee-geo-woo-gen and Peta at IRRI. Peta, originating in Indonesia and
popularly grown in the Philippines, is a tall, high-tillering variety. From the cross
IR8 was selected and released in 1966. IR8 is erect leaved, high tillering,
photoperiod insensitive, and about 100 cm tall with stiff culms. It responds well to
nitrogen and produces about 6 t/ha in the wet season and 9 t/ha in the dry.
Occasionally, the yield exceeds 10 t/ha. Figure 6.1 compares the nitrogen
response of IR8 and traditional varieties. IR8 is considered the first high yielding
indica rice variety adapted to tropical climates. With good management dwarf
rices such as IR8 have enormous potential to boost rice yields in Asia. Their impact
was elegantly described as Dwarf rice — a giant in tropical Asia (Chandler 1968).
Dee-geo-woo-gen was used as a genetic source of semidwarf stature in both
TN1 and IR8. Its usefulness is comparable to that of Norin 10 for Mexican wheat.
Both Dee-geo-woo-gen and Norin 10 originated in Asia.
6.3. PLANT-TYPE CONCEPT
6.3.1. Evolution of concept
The term plant type refers to a set of morphological characters associated with the
yielding ability of rice varieties. The plant-type concept in rice, similar to the wheat
ideotype (Donald 1968), emerged gradually from many observations and comparisons
between low and high yielding varieties in Japan (Baba 1954, 1961;
Tsunoda 1964).
Better varieties of crop species must have been selected for higher yields in the
past. In practice, the high yields have been achieved by increased inputs, particularly
nitrogen fertilizers. Hence, varieties must have been selected on the basis of
their response to nitrogen fertilizers. Tsunoda (1964) compared low and high
yielding varieties of rice, sweet potato, and soybean, and summarized their
morphological characteristics:
• Low nitrogen responders have long, broad, thin, drooping, pale-green
leaves, and tall, weak stems.
• High nitrogen responders have erect, short, narrow, thick, dark-green leaves,
and short, sturdy stems.
Tsunoda postulated from physiological knowledge of crop photosynthesis that
the thick, dark-green leaves lose less light through reflection. Reduced leaf size
and erect habit permit uniform light distribution to all leaves and reduce respiration.
Consequently, dry matter and yield increase even under low light conditions.
The uniform light penetrations and the short, sturdy culms minimize lodging; thus
there is little or no yield loss once panicles are well developed (Tsunoda 1964,
Jennings 1964).