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 metho d for determination of NR activity (NRA) (Robin et al., 1983) The exog enous 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 o n 50 mM exogenous NO3-. Under these conditions, leaf NO3- accumulation reached 30 to 50 mu moles g(-1) FW. In leaves harvested from the adul t 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- w as supplied through the xylem to the leaves of the adult trees, where it induced a noticeable level of NRA. This is interpreted as an eviden ce for the occurrence of leaf NO3- reduction in peach-trees grown unde r normal orchard conditions. The actual rate of this reduction cannot be estimated using in situ NRA only. However, in agreement with the in ducibility of NR, both availability of NO3- in the soil solution and l eaf (+NO3-) NRA followed parallel time courses during the growing seas on. This suggests a possible use of in situ NRA as an instantaneous qu alitative indicator of the exogenous NO3- availability for the peach t ree.