Performance of Sunflower cv. Sakha-53 as Influenced by Accumulated Heat Units under Different Sowing Dates and Plant Spacing at Aswan Conditions

The present study was carried out during the two seasons of 2019 and 2020 at the Experimental Farm of Faculty of Agriculture and Natural Resources , Aswan University, to estimate the crop phenology and vegetative growth, yield and thermal indices of sunflower cv . Sakha-53 under four sowing dates i.e. (SD 1 : 1 st July, SD 2 : 15 th July, SD 3 : 1 st August and SD 4 : 15 th August) and four plant spacing (P 1 : 15 cm, P 2 : 20 cm, P 3 : 25 cm and P 4 : 30 cm with constant width, 60 cm). The results refer to significant influences of sowing date, plant spacing and their interaction for all studied traits across both seasons. Early sowing date on 1 st July significantly increased thermal indices and the seed yield associated with as increase of other studied traits. With regard to plant spacing, increasing plant spacing from 15, 20, 25, and up to 30 cm recorded increase gradually of crop phenology, number of leaves plant -1 , leaf area plant -1 , stem diameter, head diameter, 1000-seed weight, seed weight plant -1 , daily seed weight plant -1 , growing degree days (GDD) and heliothermal units (HTU) while narrow spacing at 15 cm produced the tallest plants , heaviest seed, oil yield, highest heliothermal use efficiency (HTUE), heat use efficiency (HUE) and photothermal index(PTI). The interaction among sowing dates and plant spacing had a significant effect on all studied traits, revealing that the heaviest seed yield (1300 and 1327 kg fed -1 ) was produced from the interaction of SD 1 × P 1 in both seasons.