The productivity of field crops, especially cereal crops, has increased significantly through the improvement of the plant type (semidwarf and upright leaf) by breeding. Nevertheless, according to the above discussion, it is difficult to explain the high productivity of new varieties based only on the carbon (photosynthesis-respiration) balance theory.
Using high-yielding varieties and slow-release fertilizer, a very high yield was attained for various field crops in Sapporo (43°03'N), Japan (Osaki et al 1991c. 1992c, 1995b; Table 1). These high-yielding varieties (HYVs) were characterized by the fact that the total amount of dry matter (DM,) and the amount of nitrogen absorbed (N, ) in a whole plant increased linearly with growth until harvest (Osaki et al 1991b.c). indicating that photosynthetic rate and root activity remained high during maturation. This type is designated as nitrogen absorption type. In contrast, in standard-yielding varieties (SYVs), nitrogen absorption ceased at the maximum shoot growth stage, and at the same time N, in the nonreproductive organs started to decrease, followed by cessation of the increase in dry matter. This type is designated as nitrogen efficiency type.
For HYVs, the amount of nitrogen absorbed in plants reached a value of 30 to 40 g N itt2, half of which was absorbed within only 30 d during maturation. According to these results, it is impossible to obtain more than 10 t ha-' yield under the current methods of cultivation when nitrogen absorption by SYVs is assumed to amount to only 10 to 20 g N nr:. Consequently, since in the SYVs harvested parts grow by using nitrogen stored or incorporated in leaves and stems before maturation, root activity during maturation does not play a major role. Thus, the basic strategy to achieve high yield should be to transfer the nitrogen efficiency type to the nitrogen absorption type in breeding programs. The issue to address is why the root activity of HYVs remained high during maturation. In this regard, the carbohydrate distribution system is analyzed in the next section.
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