Effects of drought stress on grain yield, photosynthetic and physiological characteristic of corn (Zea mays L.) and mung bean (Vigna radiata L.) in different planting patterns

Introduction Under water stress conditions, intercropping can be used as a strategy for maximum use of sunlight and limited water resources. Due to the difference in morphological and physiological traits, the two plants corn and mung bean may be suitable for intercropping under drought stress; Therefore, this research was conducted with the aim of investigating the effects of drought stress on the physiological traits and yield of corn and mung beans in monocultures and intercropping in order to study the possibility of reducing water consumption in this production system. Materials and methods An experiment was carried out as a split plot based on randomized complete block design with three replications at Ilam University during 2019-2020 growing season. The main plot were four water regimes including (40, 60, 80, and 100% of the plant's water requirement) and the subplots were include the planting pattern at four levels (additive intercropping series 100% corn + 50% mung bean, replacement intercropping series 50% corn + 50% mung bean and monocultures of mung bean and corn). The application of drought stress started after the establishment stage of the plant and continued until the harvest. At the end of the growth period, some physiological traits of both plants were measured, including photosynthetic pigments, photosynthesis rate, transpiration rate, intercellular CO2 concentration, proline amount, leaf relative water content and grain yield. Finally, analysis of variance (ANOVA) was performed using Minitab 14 and SAS 9.1 softwares and the means compared by HSD test at 5% probability level (p≤0.05). Results and discussion The results showed the highest rate of photosynthesis (25.4 µmol CO2.m-2.s-1) was obtained in the treatment of 100% water requirement and the lowest rate of photosynthesis was obtained in 40% water requirement, which showed a decrease of 68.5% compared to the well-watered conditions. The rate of photosynthesis of corn in the replacement intercropping of 50% corn + 50% mung bean was 18.3% higher compared to monocultures corn. The highest rate of mung bean photosynthesis was observed in replacement intercropping under conditions of 100% water requirement and additive intercropping series under 80% water requirement, and the lowest rate of mung bean photosynthesis was observed in intercropping under conditions of 60 and 40% water requirement. Dehydration stress decreased the amount of total chlorophyll in mung beans and corn. The highest intercellular CO2 concentration (475.5 mmol of CO2.m-2.s-1) and the temperature of the corn leaf (36.7 °C) belonged to the 40% water requirement treatment. The rate of transpiration of corn and mung bean decreased by 55.56 and 61.43% in the condition of 40% water requirement compared to well-watered conditions. Water stress reduced the relative water content of corn and mung bean. The highest proline in corn and mung leaves (46.3 and 45.23 µmol.g-1.FW-1, respectively) was obtained in the treatment of 40% water requirement, which had no significant difference with 60% water requirement. Corn grain yield in the treatment of 40% water requirement showed a decrease of 52.76% compared to 100% water requirement. Monocultures (3945.8 kg.ha-1) and intercropping (3875.1 kg.ha-1) had the highest corn grain yield, and the lowest corn grain yield in replacement intercropping was 2700 kg ha-1. At all irrigation levels, the highest grain yield of mung bean was obtained in monocultures, and the yield decreased in additive and replacement intercropping, and additive intercropping had the lowest grain yield. The values of land equality ratio of all intercropping patterns were greater than one, which indicates the advantage of corn and mung bean intercropping patterns to use land and increase yield. Conclusion Considering the occurrence of recent droughts in arid and semi-arid regions of the country and the need for forage in these regions, the cultivation of forage plants, especially corn, is inevitable. Therefore, one of the appropriate strategies to protection in management and water consumption is the intercropping of this plant with plants of the legume such as mung bean. Intercropping can partially moderate the effects of drought stress on plant water conditions. In general, it seems that the patterns of intercropping and substitution, due to having higher grain yield potential, is a suitable strategy for producing higher yield and stability of this plant compared to monocultural under well-watered and stress water. Acknowledgments This research has been carried out using the research credits of Ilam University, by which we express our thanks and appreciation to the respected Assistance of Research and Technology of Ilam University.