changes of radiation interception and red to far red light ratio in some of the sunflower hybrids canopy under drought stress

in order to study of the radiation interception and red to far red light ratio variations in sunflower hybrids under drought stress, nine sunflower hybrids in three separate experiments were compared as randomized complete block designs with three replications at the research station of seed and plant improvement institute in karaj in 2006. first experiment (control) was irrigated based on 60 millimeter evaporation from evaporation pan. the second and third experiments were irrigated at 120 and 180 millimeter evaporation from class a evaporation pan, respectively (medium and sever drought stresses, respectively). average of solar radiation intensity and red to far red light ratio in bottom canopy, solar radiation interception, number of active leaves, total dry weight per plant at seed filling stage and seed yield showed significant difference among irrigation treatments. the intensity of drought stress increased the solar at the bottom of canopy and red to far red ratio by 35 and 45 percentages, whereas caused to decrease the radiation interception of control treatment as 0.95 to 0.88 in sever drought stress. results indicated that allstar hybrid with appropriate leaf area index (lai) and number of active leaves and maximum radiation interception produced the highest dry weight and seed yield, in drought stress conditions.