A root-shoot interaction model is proposed to explain the high productivity of high-yielding varieties of several crops, based on high-yielding trials. In the high-yielding varieties, nitrogen is always actively absorbed during the vigorous sink organ-filling stage (maturation or the ripening stage of growth), Thus, photosynthetic rate (dry matter increase) and root activity (nutrient absorption) remained constant during maturation in high-yielding varieties because a high photosynthetic rate maintains a high root activity by supplying a sufficient amount of photosynthate to the roots, a phenomenon referred to as root-shoot interaction for high productivity. On the other hand, in the varieties with standard yield, hereafter referred to as standard or old or low-yielding varieties, the photosynthetic rate decreased, followed by a decrease in root activity because of the reduced carbohydrate supply; nitrogen incorporated into leaves and stems before maturation was retranslocated during maturation. Two carbon-nitrogen (C-N) interaction models are developed. One is DMt = DM0 exp (CNI x N-t) for cereals and the other is DMt = DM0 + CNI ' x N-t for legumes, where DMt is the dry weight of a plant at a given time, N-t is the amount of N accumulated in the plant at a given time, DM, is the initial dry weight, and CNI and CNI ' are the C-N indices. Moreover, the productivity per unit amount of N accumulated in legumes is quite low compared with that in cereals during the vegetative growth stage. This is caused by the low growth efficiencies [accumulated dry matter/(accumulated dry matter + respiration)] of whole plants regardless of nitrogen concentration, indicating that the concept of growth and maintenance respiration is not valid. The fate of photosynthesized (CO2)-C-14 was quite different between rice and soybean. In soybean, a large amount of photosynthesized (CO2)-C-14 is respired in the light compared with that in the dark, but in rice the amount of C-14 retained in the leaves is similar regardless of light conditions. This high respiratory loss of current photosynthate in soybean in the light can be explained partly by the high rate of photorespiration in the leaves. A large portion of photosynthetically fixed (CO2)-C-14 in soybean in the light was distributed into organic acids, amino acids, and protein compared with that in rice, where metabolism of newly fixed carbon is mainly regulated by the activity of sucrose-phosphate synthase (SPS) and especially phosphoenolpyruvate carboxylase (PEPC). Thus, it is assumed that the carbon-nitrogen balance of the whole plant is regulated by (1) whether current photosynthate distributes into the tricarboxylic acid cycle or sucrose metabolism in the light, which is regulated by PEPC or SPS, respectively, and (2) whether photorespiratory activity is high or not. This information will help to improve crop productivity through regulation of carbon-nitrogen metabolism. The distribution of (CO2)-C-14 to chemical compounds was studied in transgenic rice plants that showed high expression of the maize PEPC gene. The C/N ratio decreased in transgenic plants compared with controls because of high C-14 distribution to organic acids. As the transgenic plant could exude much organic acid from the roots, this plant showed aluminum tolerance for high aluminum in solution.
