VARIETAL DIFFERENCES IN PERENNIAL RYEGRASS FOR NITROGEN USE EFFICIENCY IN LEAF GROWTH FOLLOWING DEFOLIATION - PERFORMANCE IN FLOWING SOLUTION CULTURE AND ITS RELATIONSHIP TO YIELD UNDER SIMULATED GRAZING IN THE FIELD

Varietal differences in rates of NO3- uptake and remobilization of nit rogen (N) during a cycle of severe defoliation and regrowth were asses sed in perennial ryegrass (Lolium perenne L.) varieties Ba11778, Abere lan, Talbot and Gator. Plants were grown in flowing solution culture f or 34 days, prior to a 30 d treatment period, including 21 d of regrow th. Net uptake of NO3- was measured continuously from solutions mainta ined automatically at 10 mmol m(-3) NO3-. N-15-labelled NO3- was suppl ied to the plants in situ for either 21 h immediately prior to was sup plied to the plants in situ for either 21 h immediately prior to defol iation, or for 7-11 days afterwards, to measure remobilization of N fr om roots and stubble into the regrowing leaves. The four varieties dif fered only slightly in total dry matter production over 30 d, but vari ed substantially in post-defoliation specific growth rate by the leaf fraction. Gator was the lowest (0.11g g(-1) d(-1)), Ba11778 the highes t (0.15g g(-1) d(-1)) and Aberelan and Talbot intermediate in leaf reg rowth rate. Varieties did not vary significantly in shoot: root dry we ight ratio, but the rate of NO?-3 uptake per unit of leaf fresh weight during regrowth was 27% lower in Ba11778 than in Gator. Leaves of Ba1 1778 and Aberelan had a lower organic N concentration in the dry matte r at the end of the regrowth period than Talbot, which, in turn, had a lower concentration than Gator. Varieties differed in amount of leaf dry matter produced per unit of N absorbed by the plants after defolia tion, the values (g g(-1) N) being Ba11778 (22.6), Aberelan (20.7), Ta lbot (18.3) and Gator (16.7). More N was remobilized from the roots th an from the stubble after defoliation. There were no significant diffe rences among varieties in the amount and rate of N remobilization foll owing defoliation. In earlier field experiments employing frequent cut ting to simulate grazing, Ba11778 and Aberelan yielded at least 10% mo re dry matter annually than Talbot which, in turn, outyielded the amen ity variety Gator by a similar amount. Thus the present results are co nsistent with the hypothesis that the efficiency of N use in leaf grow th is an important factor determining genetic variation in productivit y under frequent defoliation in the field when the herbage produced co nsists primarily of leaf.