Effects of long-term and short-term supply of NO3− or NH4+ on Cytokinin levels and leaf expansion rate in tomato (Lycopersicon esculentum L. cv. Moneymaker)

Abstract

Hormonal effects have been implicated in rapid responses of plant growth, induced by application of different nitrogen (N) forms or different levels of N fertiliser supply. In this study, long-and short-term effects of different N forms on growth responses and cytokinin (CK) levels in root and leaf tissues and in xylem sap of tomato were investigated. Plants were cultivated in a pH-buffered nutrient solution (pH~7), containing either or [2 mM] as sole N source. Long-term treatment (7 d) with decreased plant growth and particularly leaf expansion. This was associated with decreased zeatin+zeatin riboside (Z+ZR) concentrations in roots, xylem sap and leaves. In short-term experiments with preculture, increased leaf expansion was detectable already 12 h after re-application of and coincided with enhanced Z+ZR levels in the xylem sap, in the leaf tissue but not in roots. The results suggest that application rapidly enhances root to shoot translocation of Z+ZR, thereby increasing Z+ZR concentrations in the leaf tissue, which finally stimulate leaf expansion.