Simulating the risk of heat stress on grain maize production under arid and semi-arid conditions

 Heat stress is one of the most important threats and concerns for maize production, which mostly occurs in hot and dry areas. Heat stress reduces grain yield and the plant's photosynthesis rate and increases transpiration. Maize is very sensitive to heat stress and extreme temperatures at the flowering stage because extreme temperatures decrease pollen germination ability, and thus, decrease grain yield. However, there are some strategies to prevent the maize flowering stage from being exposed to heat stress. Careful management practices including Adjusting the sowing time and cultivar can be considered as useful strategies to deal with heat stress. Crop simulation models can be used to investigate these practices. Therefore, the present study was carried out to evaluate the risk of heat stress (frequency and intensity of heat) on grain maize of Iran and evaluate the risk window for grain maize using the modeling approach.

 In order to evaluate the risk of heat stress in maize agroecosystems of Iran, a simulation experiment was designed in five regions (Iranshahr, Dezful, Parsabad, Kermanshah, and Kerman), three sowing times (common: farmers sowing time in each region; late: 20 days after common sowing time; early: 20 days before common sowing time), and two cultivars (SC704 and SC260 as late- and early-maturity cultivars, respectively). To do this, the long-term climatic data of each region including minimum and maximum temperatures, rainfall, and radiation were collected from Iran Meteorological Organization. These data were applied as inputs of the crop simulation model. In this study, the APSIM model was employed to simulate the growth and development of the maize plant. In order to assess the risk of heat stress on grain maize, three dimensions including the critical stage of grain maize to extreme temperatures (flowering), frequency of extreme temperatures at the critical stage, and intensity of extreme temperatures at the critical stage were evaluated. Furthermore, the risk window for maize flowering in each region was equal to the first day of the year with a temperature of over 36 °C until the last day of the year with a temperature above 36 °C.

 The highest risk window of extreme temperatures was recorded in Iranshahr County (183 days) as a hot and dry region and the lowest risk window was simulated in Parsabad (14 days) as a semi-arid and temperate region. Moreover, the percentage of the number of maize flowering days with temperatures above 36 °C and the mean maximum temperature during the flowering period were 63.5% and 37.09 °C, respectively. This issue reduced the grain yield of maize in Iran so that the grain yield was simulated 6196.5 kg ha-1. However, in the spring season, the early sowing time and the early-maturity cultivar decreased the percentage of the number of maize flowering days with temperatures above 36 °C (37.2%) and mean maximum temperature during the flowering period (35.1 °C) and increased grain yield (7486.9 kg ha-1). Overall, in the summer, the percentage of the number of maize flowering days with temperatures above 36 °C and mean maximum temperature during the flowering period were decreased 38.9% and 35.3 °C, respectively, and grain yield was boosted to 7743.6 kg ha-1 under the combination of late sowing time and late-maturity cultivar.

 The results showed that grain maize is currently cultivated by farmers under high-risk conditions of heat stress. In order to reduce the risk and increase grain yield, farmers in each region should apply the optimal sowing times and cultivars according to the growing season.