ABSTRACT
The natural abundance of carbon and nitrogen isotopes in higher plants integrate both physiological and environmental effects. Higher plants are composed of various tissues of different ages, and carbon and nitrogen are cycled amongst these tissues according to source-sink relationships. The green leaves are the major site of CO2 fixation and the fixed carbon is converted to sugars for export. These sugars are transferred via the phloem to sink organs such as the fruits and roots. This chapter introduces new data on changes in carbon and nitrogen values in castor bean plants grown in the field, based on analyses of xylem and phloem constituents, as compared to whole plant organic matter and soil nitrate. Changes to the nitrogen signals of distinct soil and atmospheric nitrogen sources are brought about through isotopic fractionation which can occur at several steps in the sequence of assimilatory reactions responsible for converting nitrate and ammonia into amino acids and high-molecular nitrogen compounds in higher plants.
