When dry matter is produced through photosynthesis, in fact absorbed energy of solar radiation is stored as chemical compounds. Existence of these compounds indicates radiation energy application and restoration in carbon assimilation function. If sufficient water and nutrients be supplied, the producted dry matter is dependent upon intercepted and absorbed solar radiation by plant as well as radiation use efficiency. This investigation was conducted in year 2000 in Esfahan agricultural research center to study the relationship between planting date as well as density, and radiation transmission and extinction in different safflower genotypes canopies. Five safflower genotypes were compared at two seeding dates (April 4 and July 6) and three planting densities (400, 200 and 133 thousand plants per hectare) in a split split plot design with four replicates. The results indicated that radiation absorption in the whole depth of canopy as well as in all genotypes in spring planting was greater than that in summer planting. In spring planting, intercepted radiation for cultivar Arak- 2811 was highest (829 W/m2) and in summer planting intercepted and extinction radiation in Line 1 was higher than in the others. This difference was low in comparison with cultivar Arak -2811. Solar radiation absorption ability in all genotypes was affected by planting density. Maximum density was accompanied by the most radiation absorption. Line 1 had the least interception radiation at upper half of the canopy. Dry matter and grain yield rates were affected by total intercepted radiation. Upper and lower half of canopy affected these rates differently. Planting density affected extinction coefficient (K) because of leaf area index (LAI) difference. Intercepted radiation in maximum planting density in either spring or summer seeding dates were near %95 of total solar radiation.
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