CHANGES IN SOURCE SINK RELATIONSHIP FOR NITROGEN DURING REGROWTH OF LUCERNE (MEDICAGO-SATIVA L) FOLLOWING REMOVAL OF SHOOTS

An experiment with lucerne plants (Medicago sativa L.) previously labelled with N-15 and grown in hydroponic culture, was undertaken to define sink and source behaviour of different organs in defoliated and intact plants. Changes in N-15 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 N-15 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.