Abstract
An experiment with lucerne plants (Medicago sativa L.) previously labelled with 15N and grown in hydroponic culture, was undertaken to define sink and source behaviour of different organs in defoliated and intact plants. Changes in 15N contents of plants during 24 days of regrowth on an unlabelled medium after defoliation were used to estimate flows of exogenous and endogenous nitrogen. The 15N content of regrowing stems and leaves increased as a result of remobilisation mainly from lateral and tap roots which, therefore, acted as source organs. Nitrogen remobilisation reached a plateau after 10 days of regrowth and, until this time, nearly all N for shoot regrowth came from endogenous N in roots and crown. Between 25 and 35% of N reserves were translocated to regrowing stems, the remainder to regrowing leaves. Amino acid-N was the most readily available form of N while protein-N was the largest storage pool. Nitrogen uptake from the medium and accumulation in source organs (roots and crown) was significant only between days 6 and 14, and almost all was subsequently translocated to regrowing tissues. Defoliation induced changes in source-sink relationships for N. Whereas stems and tap roots were the main sink organs in intact plants, regrowing shoots exerted a stronger sink behaviour in defoliated plants.
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