EFFECT OF EXTERNAL N SUPPLY ON THE RECYCLING OF N WITHIN PLANTS

In order to develop more sustainable crop production systems, it is important to ensure that N taken up by the roots and any N already stored in the plant 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 using N inputs ranging from sufficiency to deficiency. The experiments were carried out with young lettuce (Lactuca sativa L.) plants grown hydroponically in the glasshouse in order to maintain strict control over the N supply during growth. Prior to the start of the experiment, the plants were raised in perlite and irrigated using a complete nutrient solution with ¹⁵N Ca(NO₃)₂ at a 10 atom% loading as the N source. At the 6 leaf stage, the plants were transferred to a flowing culture system with 4 levels of ¹⁴N as Ca(NO₃)₂ (8.00, 0.50, 0.05, and 0.00 mM NO₃) and grown for a further 28 days. Plant growth reduced progressively as the level of N supply decreased, the rate of shoot growth being more affected than root growth at the lower N treatments. There was little net movement of ¹⁵N into or out of the roots in the two highest N treatments, but ¹⁵N 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 maintained at consistently high levels for all treatments (including the deficient ones) over the experimental period, with N concentrations approaching 9% of the tissue dry weight when N supply was adequate. Despite this, remobilization of internal N reserves was never efficient enough to maintain the supply of N to the tissues exerting the greatest sink strength at concentrations 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 imply that newly acquired N taken up by the roots tends to be used more effectively than existing N reserves as it can be more easily directed to the sites 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 growth rather than in a single application at sowing or transplanting. There also appeared to be some discrimination in the accumulation of the two N isotopes by the roots and shoots, with a greater ratio of ¹⁵N to ¹⁴N present in the shoots at the beginning of the experiment.