Effect of Planting Date on Agro-Meteorological Indices at Different Phenological Stages of Potato Cultivars

Growing degree days (GDD) and helio thermal units (HTU) are the temperature based agro-meteorological indices that play important role in predicting crop growth and yield. Growing degree days is based on the concept that the actual time to achieve a phenological stage is linearly related to base temperature (Tb) and optimum temperature. The efficiency of conversion of heat and radiation energy to dry matter depends on genetics factors and planting date. Heat summation units (HSU), that mainly called growing degree days, and its derivations such as helio thermal units are the most necessary to determine phenological stages and the suitable planting dates for cultivars of different crop. Growth of potato can be divided into five stages: sprout development, vegetative growth, tuber initiation, tuber bulking and maturation. There is little information concerning of growing degree days and helio thermal units for potato in the sub-tropics. This experiment was undertaken to define growing degree days and helio thermal units at phenological stages and effect on yield for potato cultivars established at different planting dates.

The research was conducted in the research farm of Behbahan Agriculture Research Station for two years (2015-2017). The experiment was arranged as a split-plot in a randomized complete block design with three replicates. Planting dates from 21 December to 20 January at a 10-day interval were the main plot and the Cozima, Savalan, and Elmera cultivars were considered as sub-plot. When the diameter of the swollen tip of stolon was twice as long as stolon diameter, considered as tuber initiation. Tubers were harvested in mid May. Growing degree days and helio thermal untis were calculated at all phenological stages and after harvest thermal use efficiency and helio thermal unit use efficiency. Results from experiments of 2 years were combined for analysis. Data were statistically analyzed by MSTAT-C. Significant differences among treatments means were determined at p ≤ 0.05 by Duncan’s multiple test range.

The longest of the sprout development stage was recorded for 21 Dec. planting date. With the postponement of planting date, due to increase temperature, duration of sprout development significantly shortened, and growing degree days and helio thermal unit decreased. The duration of vegetative stage was 15 days. Early exposure of plants to favorable climatic conditions for tuber initiation, short day and cool night temperature, caused the duration of vegetative growth, growing degree days and helio thermal unit were low. Tuber initiation is thought complete within 2-6 weeks of that event, but since the majority of the tubers are formed over a period of 15 days, duration of this stage is usually considered about 15 days. Although the duration of tuber initiation was equal in all planting dates, however there was significant difference between the amount of growing degree days and helio thermal unit in some treatment dates due to unsimultaneous timing of tuber initiation. The duration of tuber bulking, depend on planting date and cultivar, ranged from 38 to 44 percent days. Growing degree days and helio thermal units at tuber bulking stage were higher than previous stages, such as depend on planting date 61.52 to 66.27 percent of total growing degree days belong to tuber bulking stage. Maturation stage was not observed. There were no significant differences for yield and helio thermal unit use efficiency among all planting dates, however the highest thermal use efficiency was recorded in 20 Jan. planting date. The highest yield, thermal use efficiency and helio thermal units use efficiency belong to Savalan cultivar. Climatic conditions at sprout development, vegetative growth and tuber initiation stages were relatively more favorable for Savalan cultivar in 20 Jan. than other planting dates. Moreover, plants of Savalan cultivar in 20 Jan. planting date, during incidence of late blight in effect of less age were more tolerant as compares with other planting dates. Therefore, the yield of Savalan cultivar in 20 Jan. was significantly higher than other planting dates.

The duration of sprout development and vegetative stages are long and short respectively. In this experiment low temperature during sprout development and high temperature from mid tuber bulking stages caused the less tuber yield than temperate regions of the country. According to results, planting of Savalan cultivar in mid January is recommended for winter potato production in Khuzestan province. Planting from mid December to mid January is recommended for Cozima and Elmera cultivars.