The nitrogen economy and primary production of a Helianthus annuus "Man-churian" population were studied with special reference to the pattern of seasonal changes of vertical distributions of dry matter and nitrogen quantities, and its quantitative significance was discussed in relation to the pattern of the plant poulation growth, distribution ratios among organs, and turnover rates of dry matter and nitrogen.
The population was established in plant density of 11.1 plant/㎡ at the experimdntal field of Kyungpook National University, Daegu. During the period of population development(April-September, 1973), the annual inflow rates and outflow rates of dry matter and nitrogen were 5560 gDM/㎡ year and 89 gN/㎡/year, respectively. The distribution ratios of dry matter and nitrogen to leaves were 28% and 45%, to stems 48% and 18%, to roots 13% and 5%, and to flowers and seeds 11% and 32%, respectively. The maximum turnover rates of inflow of dry matter and nitrogen were attained in May-June, and were 216%/month and 210%/month, respectively. The amount of nitrogen demand was 52gN/㎡/year(58%) for the foliage growth, 13 gN/㎡/year(15%)for the stem growth, 20 gN/㎡/year(23%) for the reproductive organs, and 4 gN/㎡/year(4%) for the growth of the underground parts. The amount of nitrogen supply by the nitrogen with-drawn from senescing leaves and stems was 25 gN/㎡/year(28%) and the amount of nitrogen absorption by the root from the environmental soil was 64 gN/㎡/year(72%). The ratio of the a mount of produced dry matter to that of assimilated nitrogen during a year was calculated for this annual plant population as 60, which can be used as the nitrogen utility index.
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