Effect of nitrogen nutrition on distribution of photosynthetically incorporated 14CO2 in Lolium perenne

Abstract

Ryegrass plants grown in low NO3-N, high NO3-N, or high NH4-N nutrient culture were labeled by assimilation of 14CO2 by the tops. Plants were harvested 2, 6, and 24 h after labeling and percentage, 14C incorporation was measured in ethanol-soluble, chloroform-soluble, and ethanol-insoluble fractions of leaves, stems, and roots. Low N roots accumulated the greatest and high NH4-N roots accumulated the least 14C at the end of 24 h. Low NO3-N plants had higher levels of ethanol-soluble 14C and lower chloroform-soluble and ethanol-insoluble 14C than high NO3-N plants. Analysis of the ethanol-soluble fraction revealed higher, 14C content in sugars and less in amino acids in low NO3-N plants than in high NO3-N plants. Labeling patterns in high NH4-N plants suggested incipient ammonium toxicity since 14C metabolism was retarded, yet the plants showed no visual toxicity symptoms. When using percentage conversion of label from the ethanol-soluble to ethanol-insoluble fraction as a criterion for plant metabolism, roots appeared to be more sensitive to N deficiency than were tops. This result implies that reduced root growth and (or) function (nutrient uptake) may be one of the first effects of N deficiency.