COMPARISON OF PRODUCTIVITY BETWEEN TROPICAL AND TEMPERATE MAIZE .1. LEAF SENESCENCE AND PRODUCTIVITY IN RELATION TO NITROGEN NUTRITION

To identify the difference in maize productivity between tropical and temperate regions, maize (Zea mays L.) plants were grown in two regions: in the fields of two CIMMYT Experimental Stations in Mexico and in a field of Hokkaido University in northern Japan. Results obtained were as follows. 1) The grain yield of tropical maize was lower than that of temperate maize. As the harvest index of dry matter of tropical maize was not appreciably low, the biological yield determined the grain yield of tropical maize. 2) Amount of dry matter and amount of nitrogen absorbed in tropical maize did not increase during maturation in spite of adequate amount of nitrogen applied, while in temperate maize the values continued to increase until late maturation. 3) Dry weight and amount of nitrogen in leaves and stems of tropical maize decreased steeply after flowering regardless of the growth conditions and in spite of the adequate amount of nitrogen applied. Especially, leaf senescence of tropical maize became rapid just after flowering. It was impossible to prevent the leaf senescence from proceeding by additional nitrogen application at flowering. Therefore, leaf senescence in tropical maize is considered to be regulated by an autonomous process. 4) When the lower leaves received a sufficient amount of light, tropical maize could absorb nitrogen after flowering, and the decrease in the rate of chlorophyll in leaves became less pronounced, indicating that the higher the activity of lower leaves the higher the root activity. In conclusion, low productivity of tropical maize was mostly caused by rapid leaf senescence after flowering, then partially by the plant architecture. Since, after the rapid decrease of the nitrogen content from leaves, which is another aspect of leaf senescence, nitrogen absorption in plant decreased in spite of the adequate supply of nitrogen nutrient in soils, leaf senescence and root activity of tropical maize were regulated by leaf autonomy.