Effect of external N supply on the recycling of N within plants

In order to develop more sustainable crop production systems, it is importa nt to ensure that N taken up by the roots and any N already stored in the p lant is used to maximum effect. This study was designed to examine the way in which both internal and external sources of N are utilised by plants usi ng N inputs ranging from sufficiency to deficiency. The experiments were ca rried out with young lettuce (Lactuca sativa L.) plants grown hydroponicall y in the glasshouse in order to maintain strict control over the N supply d uring growth. Prior to the start of the experiment, the plants were raised in perlite and irrigated using a complete nutrient solution with N-15 Ca(NO 3)(2) at a 10 atom% loading as the N source. At the 6 leaf stage, the plant s were transferred to a flowing culture system with 4 levels of N-14 as Ca( NO3)(2) (8.00, 0.50, 0.05, and 0.00 mM NO3-) and grown for a further 28 day s. Plant growth reduced progressively as the level of N supply decreased, t he rate of shoot growth being more affected than root growth at the lower N treatments. There was little net movement of N-15 into or out of the roots in the two highest N treatments, but N-15 increasingly accumulated in the roots of the 0.05 and 0.00 mM nitrate treatments under these more deficient conditions. The initial N concentrations of the youngest leaves were maint ained at consistently high levels for all treatments (including the deficie nt ones) over the experimental period, with N concentrations approaching 9% of the tissue dry weight when N supply was adequate. Despite this, remobil ization of internal N reserves was never efficient enough to maintain the s upply of N to the tissues exerting the greatest sink strength at concentrat ions necessary for maximum growth. These results show that N is not used uniformly within and between tissues, even in young plants. Remobilization of N always occurs after a localised deficiency has been created in the more actively expanding tissues and not before a reduction in growth has already taken place. The results also impl y that newly acquired N taken up by the roots tends to be used more effecti vely than existing N reserves as it can be more easily directed to the site s of highest demand. This suggests that crops grown in low input production systems may make better use of N if the supply is maintained throughout gr owth rather than in a single application at sowing or transplanting. There also appeared to be some discrimination in the accumulation of the two N is otopes by the roots and shoots, with a greater ratio of N-15 to N-14 presen t in the shoots at the beginning of the experiment.