Introduction
The common bean is an important legume cultivated by farmers in various regions using low-input practices. It originated in South and Central America and is now cultivated in all tropical and temperate regions worldwide. Asia accounts for 43% of global bean production, while the Americas (North, Central, and South America) account for 29%, and Africa for 26%. Bean seeds, containing approximately 22% protein, are a valuable alternative to animal protein sources. Iran is an arid and semi-arid country that faces the problem of resource shortages, especially water. Undoubtedly, modifying consumption patterns as well as optimizing the use of agricultural inputs will lead to increased food security.
 
Materials and Methods
The experiments were conducted at the research farm of the Campus of Agricultural and Natural Resources (34°, 19´ N, 47°, 50´ E and altitude 1320 m) of Razi University in the Kermanshah region, located in western Iran, over three years from 2021 to 2023. In terms of climatic divisions, this region is located in the temperate mountainous regions with a Mediterranean climate. The experiments were conducted in a split plot in a randomized complete block design with three replications. The treatments included irrigation water amount as the main factor (providing 100% of the water requirement, equivalent to 7300 m³ ha^-1 (IR100%), providing 80% of the water requirement, equivalent to 5840 m³ ha^-1 (IR80%), and providing 60% of the water requirement, equivalent to 4380 m³ ha^-1 (IR60%) for 2021. Because the amount of water required by the plant is determined according to climatic factors, the amounts of irrigation water in each treatment were determined as 6800, 5440, and 4080 m³ ha^-1 for 2022 and 7200, 5760, and 4320 m³ ha^-1 for 2023, respectively. Three common red bean cultivars, including Ofogh, Yaghoot, and Derakhshan, were also considered as secondary factors. The soil water status, leaf area index (LAI), light absorption, total dry weight (TDW), light use efficiency (LUE), and grain yield (GY) were measured.
 
Results and Discussion
The soil volumetric water content fluctuated as a function of the amount of water entering the soil (irrigation) and the amount of water leaving the soil (evaporation from the soil surface and transpiration by the plant). Soil volumetric water content varied in different irrigation treatments. Regardless of the red bean cultivars, the highest and lowest soil water content were observed in the IR100% and IR60% treatments during 2021-2023, respectively. The volumetric soil water content also varied for the studied red bean cultivars during the experimental period. Regardless of irrigation treatments, the highest and lowest soil water content were observed for the Ofogh and Derakhshan cultivars, respectively. The Ofogh cultivar had a shorter growth period than the other cultivars, so it required less irrigation water. It seems that the Ofogh cultivar absorbed water from the soil more quickly in the early stages of growth, which led to a sharp decrease in soil moisture content. But at the end of the growing season, as the air gradually cooled, soil moisture content increased again due to a decrease in water absorption by the Ofogh cultivar. The Yaghoot cultivar required more irrigation water than the Ofogh cultivar due to its unlimited growth, and in the middle of its growth period, due to the greater development of leaves in the canopy, it absorbed more water from the soil, which led to a more severe decrease in soil moisture. The changes in soil water content for the Derakhshan cultivar were almost similar to the Yaghoot cultivar. Due to the longer growth period of Derakhshan cultivar compared to other cultivars, more irrigation water was absorbed by this cultivar, which led to a sharp decrease in soil volumetric water content at the end of the growth period. The results showed that soil moisture content varied significantly among irrigation treatments and cultivars. Severe water deficit stress reduced LAI, TDW, and GY. The highest LUE in the vegetative and reproductive stages for the Yaghoot cultivar compared to other cultivars were 1.36 and 0.54 g MJ^-1, respectively. In 2022, the evaluated traits were higher than in 2021 and 2023, due to the lower average temperature (27°C) during the growing season. Regardless of year, the highest grain yield (192.5 g m^-2) belonged to Ofogh cultivar under optimum irrigation conditions, and the lowest grain yield (80.9 g m^-2) belonged to Derakhshan cultivar in severe water stress conditions.
 
Conclusion
The results of this study showed a positive effect of irrigation on the light absorption and LUE of common bean cultivars. Severe water stress reduced the ability of the crop to maintain leaf area during the growth period. Finally, the reduction in light absorption, LUE, and the length of the bean growth period due to drought stress led to lower TDW and GY.