In situ nitrate reductase activity in leaves of adult peach trees

The inducibility of nitrate reductase (NR) in the leaves of five year old orchard-grown peach trees was investigated using the in situ method for determination of NR activity (NRA) (Robin et al., 1983) The exogenous supply of NO3- to cut young shoots resulted in the induction of leaf in situ NRA. Full induction of the enzyme activity (i.e. increase to values of 2 mu moles NO2- h(-1) g(-1) FW) was obtained after 6 h on 50 mM exogenous NO3-. Under these conditions, leaf NO3- accumulation reached 30 to 50 mu moles g(-1) FW. In leaves harvested from the adult trees in the orchard, and not subjected to the induction treatment, leaf NO3- accumulation was no longer detected, but the addition of 100 mM NO3- in the assay medium for in situ NRA (+NO(3)(-)NRA) led to an activity far from negligible (up to 1 mu mol NO(2)(-)h(-1) g(-1) FW). These data reveal that although not accumulated in the tissues. NO3- was supplied through the xylem to the leaves of the adult trees, where it induced a noticeable level of NRA. This is interpreted as an evidence for the occurrence of leaf NO3- reduction in peach-trees grown under normal orchard conditions. The actual rate of this reduction cannot be estimated using in situ NRA only. However, in agreement with the inducibility of NR, both availability of NO3- in the soil solution and leaf (+NO3-) NRA followed parallel time courses during the growing season. This suggests a possible use of in situ NRA as an instantaneous qualitative indicator of the exogenous NO3- availability for the peach tree.