Yoshida - 1981 - Fundamentals of Rice Crop Science

GROWTH AND DEVELOPMENT OF THE RICE PLANT 29
is affected by mechanical strength, chemical composition, and plant nutritional
status. The mechanical strength is related to culm thickness and tissue strength.
Partly a varietal character, mechanical strength is modified by growing conditions.
Increased nitrogen applications, which are essential for high yields, cause elongation
of the lower internodes, making the crop more susceptible to lodging.
Lodging decreases grain yield drastically, particularly when it occurs right after
heading and when panicles are brought in contact with standing water. The
beneficial and ill effects of heavy applications of nitrogen present a serious
dilemma.
Among organic and inorganic constituents of the culm, holocellulose and
potassium content are highly correlated with breaking strength, but silica content
is not related to culm stiffness (Kono and Takahashi 1961a, c). Before heading,
large amounts of starch accumulate in the culm and sheath. The starch was
believed to contribute to the shoot’s stiffness (Sato 1959). A later examination,
however, showed no correlation between starch content and breaking strength
(Kono and Takahashi 1961a).
The culm’s mechanical strength is increased by potassium applications, which
increase its thickness (Noguchi 1940) and maintain high turgor pressure in the cells
(Kono and Takahashi 1961b). Where lodging is a serious problem, balanced
fertilization is essential for achieving high yields. The presence of the living leaf
sheath accounts for about 30–60% of the shoot’s breaking strength. Because
lodging normally occurs somewhere in the lowest two internodes, the leaf sheaths
enclosing these internodes must be stiff. These are sheaths of the lower leaves
during the ripening period. It is important, therefore that cultural practices be
directed toward preventing the lower leaves from early withering. In spite of work
done on lodging resistance, the only successful approach to increasing lodging
resistance has been to develop short varieties. Short varieties, however, are not
always lodging resistant (Chandler 1969) because other characteristics such as
culm thickness, tissue strength, and rate of senescence of lower leaves modify the
breaking strength of the shoot.
1.6. TILLERING
1.6.1. Tillering pattern
Tillers are branches that develop from the leaf axils at each unelongated node of the
main shoot or from other tillers during vegetative growth. When the fifth leaf on
the main culm emerges, the first leaf of the tiller comes from the axil of the second
leaf on that culm. Similarly, when the sixth leaf on the main culm emerges, the first
leaf of the tiller comes from the axil of the third leaf on that culm. Thus, the n th leaf
on the main culm and the first leaf of the tiller that emerges from the axil of the
( n -3)th leaf grow synchronously. This rule applies not only to the main culm but to
all the tillers.
On the main culm, the coleoptile and the first leaf normally do not produce
tillers; tillering usually starts from the second leaf. On each tiller the prophyll
develops before the first leaf emerges. The prophyll, corresponding to the coleop-