Determination of the optimum transplanting date based on GDD and the best plant density in rice (Oryza sativa cv. Tisa)

In future climatic conditions, rice yields depend on environmental conditions of the growing season, such as temperature and available radiation rate, which in unfavorable conditions can lead to yield loss in some rice producing countries (Welch et al., 2010). Achievement to the appropriate plant density per unit area is one of the methods in increasing crop yields through crop management that as a result, all environmental factors fully utilized, while intra- and inter-plant competitions be minimized and on the other hand, suitable plant density can provide sufficient space for agronomical operations and high quality access. In general, suitable rice density is a density that is not detrimental for proper crop development and high grain yield. As the density increased, the number of primary and secondary panicles and spikelet per panicle decreased and the number of panicles per m2 increased linearly and with decreasing density, stem thickness increased (Ebrahimi Rad et al, 2018). Knowing the heat indices, such as the accumulated heat unit, which in most sources are referred to as growing degree days (GDD), can provide basic principles for determining the appropriate phenological stages and planting dates. Evaluation of cumulative GDD for different development stages will allow adjustment of planting date for rice genotype with the aim of adapting the environmental conditions in the growth period. Choosing the appropriate planting date is one of the important factors in effective crop management which has a great role in production control by adapting physiological, morphological and phenological processes of the plant such as germination and emergence, vegetative growth, flowering and maturity with favorable climatic conditions. Management of planting time for optimal production of each cultivar is one of the main factors affecting production that can be easily managed by the farmer based on environmental conditions. Proper planting date will optimize the use of yield factors, while inappropriate planting date will disrupt the balance of yield components in the plant (Esmaeilzadeh et al., 2017). In order to determine proper transplanting date based on GDD and the best plant density in rice (Tisa cultivar), an experiment was conducted at Rice Research Institute of Iran, Tonekabon station during 2015 and 2016. This experiment was carried out as split plot in a randomized complete block design with three replications. The main plot was three transplanting dates with 10 days intervals (May 4, May 14 and May 24) and the sub plot was allocated to plant density at three levels (12×30, 15×30 and 18×30 cm). The results indicated that there was a significant difference between the years of experiment only for the panicle length. The effect of transplanting date was significant in the majority of the measured parameters. The highest grain yield (6573.6 kg/ha) was obtained on May 24 planting date, which did not show any significant difference with the transplanting date of May 14. Although plant density treatments had no significant effect on grain yield, 28 plants per m2 (30×12 cm) had the highest grain yield (6445.2 kg/ha). With delay in transplanting, GDD and number of days from transplanting to tillering stage as well as the time required for panicle emergence and complete maturity increased. The highest and the lowest growth degree-day (GDD) from transplanting to tillering, panicle emergence and full ripening stage were obtained on May 24 and May 4, respectively. According to the results of this research, it can be stated that the farmers of the region can transplant this cultivar between May 14 and May 24 with a plant distance of 12×30 cm (density of 28 plants per square meter).