This two-year experiment with different planting seasons (April 2018 and August 2021) aimed to determine the effect of deficit irrigation (DI) levels (DI0: 100%, DI30: 70%, and DI60: 40% field capacity) on light interception, light extinction coefficient (k), dry biomass and radiation use efficiency (RUE) of two basil cultivars (Green and Purple). The greenhouse experiment was arranged in a split-plot design with three replications. The results showed that the k values ranged from 0.57 to 0.68 and 0.31 to 0.43 in April and August planting, respectively. The cumulative solar radiation interception, dry biomass, and RUE depended on DI levels, harvest numbers, and seasons. In two study years, dry biomass decreased and RUE increased with the reduction in irrigation water at all harvests and their total. Compared to DI0, basil dry biomass did not show a considerable decrease under DI30 in three harvests in April planting and in the first and second harvests in August planting. April planting had the higher cumulative solar radiation interception in the first and third harvest than the August planting. The basil dry biomass and RUE in the April planting were higher than those in the August planting at all three harvests and their total.

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