Yoshida - 1981 - Fundamentals of Rice Crop Science

24 FUNDAMENTALS OF RICE CROP SCIENCE
1.4.4. Functions of individual leaves
a. Physiologically active centers. The rice plant at any given growth stage is
composed of leaves of physiologically different ages and this suggests that those
leaves are different in their contribution to the growth of the whole plant.
Accumulation of 32 P in plant tissues is usually considered an indication of
high metabolic activity. When 32 P is absorbed through the roots of solutioncultured
rice plants, an accumulation is found in the topmost leaf and the third and
fourth leaves (Fig. 1.23). In the topmost leaf that is still elongating, a 32 P
accumulation is associated with a high respiratory rate. This leaf, however, is low
in photosynthetic activity and depends on the lower leaves for assimilates. In other
words, the topmost elongating leaf is dependent on the lower leaves. The third or
fourth fully developed leaf from the top has the highest photosynthetic activity
among the leaves and exports assimilates to upper leaves. These photosynthetically
active leaves are considered the most important for growth of the whole plant
and are called physiologically active centers (Tanaka 1961a). Any leaf serves as a
physiologically active center at some point in the plant’s life cycle. Since new
leaves develop upward as lower leaves die, the physiologically active centers
move upward as growth advances. Thus, the physiologically active centers may be
7/0 leaf during the tillering stage, move to 9/0 and 10/0 leaves after panicle
primordia initiation, and to 10/0 and 11/0 leaves during milky stage.
b. Division of work among leaves. Leaves are different not only in age but in
position relative to panicles or roots. Physical distance appears to control the
direction of assimilate movement. At the milky stage, the flag leaf, 12/0, exports
assimilates mainly to panicles, whereas the lowest leaf, 8/0, exports a large
amount of assimilates to the roots (Table 1.7). Thus, grain growth appears to be
largely dependent on the upper leaves, and root activity is maintained by assimi-
1.23. Distribution of 32 P, photosynthetic activity, and respiration of individual leaves of the rice
plant (Tanaka 1960).