THE EFFECT OF DEFOLIATION ON THE NITROGEN ECONOMY OF WHITE CLOVER - REGROWTH AND THE REMOBILIZATION OF PLANT ORGANIC NITROGEN

Single plants of white clover (Trifolium repens) were established from stolon cuttings rooted in acid-washed silver sand. All plants were inoculated with Rhizobium trifolii, and received nutrient solution containing 0·5 mg 15N as either ammonium or nitrate weekly for 12 weeks (i.e. 6 mg 15N in total). Plants were then leniently defoliated or left intact, and the labelled N supply was replaced with unlabelled N. Lenient defoliation removed fully expanded leaves only, leaving immature leaves which accounted for 50-55% of the total; growing point numbers were not reduced. Nodules, leaves and growing points were counted over the following 21 d period, and d. wts, N contents, and 15N enrichments of individual plant organs were determined.Defoliated plants had fewer nodules, but numbers of growing points were unaffected by defoliation. The rates of both leaf emergence and expansion were accelerated in defoliated plants; in consequence the number of young leaves remained less than in intact plants until day 21. Total dry matter (DM) and N accumulation were less in defoliated plants, and a greater proportion of total plant DM was invested in roots. About 97 % of plant total N was derived from fixed atmospheric N, but there was incomplete mixing of fixed and mineral N within the plant. Relatively more mineral N was incorporated into roots, whereas there was relatively more fixed N in nodules. There was isotopic evidence that N was remobilized from root and stolon tissue for leaf regrowth after defoliation; approximately 2 % of plant N turned over daily in the 7-d period after defoliation, and this contributed about 50% of the N increment in leaf tissue. © 1990 Annals of Botany Company.