Influence of Nitrogen Nutrition and Season on Photosynthetic Incorporation of C14O2into Sucrose and Other Soluble Compounds of the Sugarbeet

Abstract

Plants of a clone of sugarbeet were grown outdoors in large tiles on a complete nutrient solution. After August 2, half the plants received minus-nitrogen nutrient until harvest. Each plant was exposed to approximately 1 mc of C14O2 in sunlight, and 1 hr later one leaf was removed. The remainder of the plant was harvested 24 hr after exposure and divided into blades, petioles, crown, and root. The distribution of total soluble C14 among the plant parts was ascertained by paper chromatography and radioautography. After 1 hr of photosynthesis, the blades of the -N plants had a significantly greater percentage of C14 in sucrose and a smaller percentage in citric, malic, and amino acids than the blades of the +N plants. The petioles of the -N plants contained a greater percentage of C14 and a greater portion in sucrose than the +N plants. After 24 hr, the -N plants retained approximately 40% less C14 in the blades and proportionately more in the roots than the +N plants. The roots of the -N plants contained 39% of the C14, while those of the +N plants contained only 19%. Of the C14 retained in the blades of -N plants, 36% resided in sucrose and 21% in citric plus malic plus amino acids, whereas the blades of the +N plants contained 16% of the C14 in sucrose and 44% in citric, malic, and the amino acids. Date of harvest had less effect on the distribution of C14 than did nitrogen nutrition. Delaying harvest from September to October increased the percentage of newly fixed C14O2 incorporated into sucrose. At the last harvest (November 1), more C14 was retained in the leaves. Sucrose translocation also was impaired as compared with the earlier harvest dates.