Study The Effect of Planting Date on Agro-meteorological Indices at Phenological Stages and Yield of Behbahan bred Onion

Temperature based agro-meteorological indices such as growing degree days and helio thermal unit have been reported quite useful to predict the growth and yield of crops. The efficiency of converting heat and radiation energy to dry matter depends on the genetic factors, sowing date and crop type. Hence, the knowledge of calculation heat thermal summation unit, mostly called the growing degree days and its further mathematical derivations like helio thermal unit and heat and radiation use efficiency will be the basic prerequisite of phenology periods and optimum planting date for different crop varieties. In spite of much researches have been done about onion in country, no paper have already published related to study the agro-meteorological indices at phonological stages of this crop. This experiment was conducted to study the effect of planting date on agro-meteorological indices at phenological stages and yield of Behbahan bred onion.

This research was conducted in randomized complete block deign including four planting dates from 6 September to 21 Octobers with 15 days interval contain four replications at Behbahan Agriculture Research Station for two years (2014-16). In this experiment studied genotype was Behbahan bred onion. Seedlings were transplanted (at two or three leaf stage). Fertilizers were applied based on soil analysis results according the recommendation of Soil and Water Research Institute.Time of bulbing was estimated based on bulbing ratio and cumulative sums. Agro-meteorological indices including growth degree days, helio thermal unit and photo thermal index were calculated at seed germinations, vegetative growth and bulbing and bulb development stages. Bulbs were harvested when 50-80% of foliage top had fallen and collapse. Statistical analysis was performed using MSTATC software and mean comparisons were also performed using Duncan’s multiple range test at P≤0.05.

The lowest duration between plant to seedling emergence and minimum growth degree days at seed germination stage were appointed to planting date at 6th September. The maximum photo thermal index during seed germination stage belonged to 21st September and 6th Octeber planting dates in the first and second year of experiment, respectively. The maximum growth degree days and helio thermal unit during transplant production belonged to 21st September planting date as well as with delaying planting date these indices decreased. The differences of obtained growth degree day in all studied planting dates were not considerable in plant growth period (Except of sowing date at 6th October in the first year) during two years. The highest yield, thermal use efficiency and helio thermal ones were recorded for 6th and 21st September planting dates in the first and second year, respectively. Reduction of growth and development of bulb in sowing date at 21st October and being exposed to high temperature at the late of this stage led to produce the lowest yield at this planting date.

According to the results, the best planting date is 21st September for Behbahan bred onion and delaying sowing date caused in yield reduction.