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Radiation use efficiency (RUE) measures the efficiency of a crop to use radiant energy. It may be strongly affected by drought in arid and semi-arid areas. Deficit irrigation (DI) methods may improve crop water use efficiency (WUE) while maintaining high yields under soil water restriction. Research was carried out on processing tomato 'Brigade' in a typically Mediterranean environment, where the influence of reduced irrigation volumes at two plant densities (P1 and P2, 2.5 and 5 plants/m(2), respectively) on RUE and Irrigation Water Use Efficiency (IWUE) was assessed. A severe stress (no irrigation) affected canopy development at both plant densities. Under DI (50% evapotranspiration. ETc. restoration) the crop developed a leaf area index (LAI) similar to that under full irrigation (F, 100% ETc), at 5 plants/m(2). The extinction coefficient (k value of the regression line of PAR intercepted vs. LAI) at 2.5 plants/m(2) (P1) was 35% lower (0.703) than that at 5 plants/m(2) (1.087). At the low plant density, RUE was negatively affected by the increasing soil water deficit, whilst at the high plant population (P2) RUE did not differ between D and F regimes. DI caused a minor fruit yield decrease (11 and 13%, in P1 and P2, respectively) but a great water saving (47%), as compared to conventional irrigation (F), confirming the validity of using this saving water strategy in semi-arid environments. Unlike RUE, WUE was greatly improved with DI, mostly at 5 plants/m(2) density. In conclusion, DI at a 5 plants/m(2) density may be beneficial for processing tomato in semi-arid environments, since the crop exhibits the same RUE as under well irrigated conditions, IWUE was improved, yield decrease was negligible and a water saving >40% was achieved.