The Effect of Lenient Defoliation on the Nitrogen Economy of White Clover: The Contribution of Mineral Nitrogen to Plant Nitrogen Accumulation During Regrowth

Single plants of white clover (Trifolium repens L.) were grownfrom stolon cuttings rooted in sand. All plants were inoculatedwith Rhizobium trifolii, and for 14 weeks received nutrientsolution containing 0.5 mg N each week, as either ammonium ornitrate. Plants were then leniently defoliated or were leftintact and a ¹⁵N-labelled N source was applied at intervalsof 4 d to replace the unlabelled N. Lement defoliation removedfully expanded leaves only; the remaining immature leaves accountedfor 39–44% of the total. At harvests over the following21 d, leaf numbers were counted and dry matter (DM), N contentsand ¹⁵N enrichments of individual plant organs were determined. Rates of leaf emergence and expansion were accelerated in defoliatedplants; numbers of young leaves were similar in defoliated andintact plants. Total DM and N content were less in defoliatedthan intact plants and were not affected by form of N supplied.DM of young leaves, growing points and stolons and N contentof young leaves were, however, greater when ammonium ratherthan nitrate N was supplied. Rates of increase in the contentof plant total N were 8.2 ± 1.36 mg N d^-1 and 10.2±1.82 mg N d^-1 in defoliated and intact plants respectively.The increases were predominantly due to N₂ fixation, since recoveryof ¹⁵N showed that less than 1% of the increment in plant totalN was assimilated mineral N. Nevertheless, the contributionof mineral N to plant total N was 50% more in defoliated thanin intact plants; higher amounts of mineral N were found particularlyin young leaves and growing points. Partitioning of mineralN to nodulated roots increased over time and was greater whenammonium rather than nitrate N was present. White clover, Trifolium repens L. cv. S184, lenient defoliation, N accumulation, N₂ fixation