Vergara - 1976 - Physiological and morphological adaptability of ri

RICE GROWTH IN DIFFERENT ENVIRONMENTS 441
extension of the growth period by using protected seedbeds was very effective in
improving grain yield. Because of the 'I'_vpe B pattern, top-dressing of nitrogen
was not recommended because it might accelerate the tillers which come out
after the panicle-ptimordia initiation stage. On the other hand, in southern
Japan, such as in Kyushu, there were l6 leaves on the main stem of mediumduration
varieties, and growth pattern tvas Type C. About four leaves developed
during the vegetative-lag phase. and the grain yield was not high due to a small
grain-straw ratio, although the total drir-matter production was high. Splitapplication
of nitrogen during growth was an indispensable technique in avoiding
excessively vigorous vegetative growth. In central Japan, such as in Tohoku.
there were I4 leaves on the main stem of medium-duration varieties, the growth
pallem W38 Type A, and very high yields were frequently reported because of
high values of total dry-matter production as well as the grain-straw ratio.
In recent years in Japan this tendency disappeared due to improvements in
varieties and cultural practices. In northern Japan the growth duration has been
extended by using improved seedbeds. new varieties suitable to such a condition
have been bred, response to top-dressing with nitrogen is favorable, and the
grain yield has become higher by an increase of total dry-matter production. In
southem Japan varietal improvements have been made. especially in plant type.
Moreover. as the rice season has been shifted to earlier in the spring by using
HOHSCHSOHHl varieties, transplanting is done when the temperature is lower.
These improvements have resulted in a larger grain-straw ratio, and very high
grain yields are often reported.
The tendency observed in Japan some years ago can be extended to the situation
existing in the tropical rice-growing countries in Asia (Tanaka et al.. I964).
In these countries the varieties popular among farmers are vigorous in vegetative
growth because this characteristic, combined with a primitive cultural technology.
is effective in competing with weeds or overcoming other unfavorable
growing conditions (Jennings and Jesus. I968). The vegetative growth is
excessively vigorous. especially with nitrogen application. and the number of
leaves on the main stem is frequently more than 20. These conditions cause
serious mutual shading which promotes the elongation of excessive internodes
and results in a decrease of the NAR during later growth stages.
An additional condition which aggravates excessive vegetative growth is that
phottiperiod-scnsitive, long-duration varieties are used preferentially in the
tropics because of various agroecologieal conditions. In a given environmental
condition. there is a tendency for shtirt-. medium-. and late-duration varieties to
assume Type B. Type A. and "Type C patterns. respectively (Tanaka et al., I959).
There is. therefore. a tendency‘ in the tropics toward the Type C grovv-‘th pattern.
the total dry-matter production is high. but the grain yield is low due to a low‘
grain-straw ratio.
However, it has been demonstrated that such a situation is not inevitable in
the tropics because of the introduction of new‘. photoperiod nonsensitiire varieties
with a good plant type and with a short grovcth duration. New varieties