This finding implies that increasing N level under water deficit will be relatively ineffective, perhaps because of an adverse effect of excessive N on yields. Therefore, application of N to levels required to maximize yield under full irrigation is likely to return poor yield when water deficit is high enough to depress yield by 35% or more under optimum N level. These results are in agreement with the findings of Darwish et al. (2006), who stated that productivity of applied nitrogen under the conditions of severe water stress decreased with applied excess nitrogen. Ferreira and Goncalves (2007) also found that potato tuber yield, regardless of irrigation treatment, tended to decline with increased N supply up to a certain limit then remained relatively constant or oscillated as nitrogen levels increased further. However, under the conditions of water deficit, N application levels would not be the limiting factor for increasing potato tuber yields whenever the amount of water applied would probably be the crucial factor. This may indicate that under deficient water conditions the efficiency of N fertilizer is reduced due to inhibition of growth rate as well as impaired N transport to the plant roots (Schjoerring, 1995).