Utilization of nitrogen from storage and current‐year uptake in walnut spurs during the spring flush of growth

Abstract

[ 15 N]‐depleted (NH 4 ) 2 SO 4 applied to the soil in 1985 resulted in residual labeling of about 16% of the storage nitrogen (N) pool of mature walnut ( Juglans regia L. cv. Serr) trees in 1987. Application of [ 15 N]‐depleted (NH 4 ) 2 SO 4 fertilizer to a different set of mature walnut trees in 1987 allowed monitoring of the kinetics and utilization of N from current year uptake in 1987 and resulted in >20% labeling of fruit N following completion of leaf expansion. Redistribution of storage N to the new growth predominated during the spring flush of growth although N derived from the soil during current‐year uptake contributed increasingly during leaf expansion. Labeled N from current year uptake accumulated preferentially in the leaves as compared with reproductive organs during leaf expansion but subsequent to leaf expansion, fruit were more highly labeled with N derived from current‐year uptake than leaves. Pistillate flower abortion was coincident with an apparent competition for N among developing vegetative and reproductive organs and preceded the period of significant N contribution from current‐year uptake.