After 7 days under 3.5% PEG with moderate photosynthesis inhibition,NR activity was located mainly in the leaves. This could be interpreted as that the rate of carbon fixation was not a limiting factor for NO 3−reduction (Larsson et al., 1989). When the water stress was severe (7% PEG), or when the time exposure with PEG was longer(14 days), photosynthetic activity was compromised, and under this extreme situation the behavior between root stocks differed.