The Effects of Piriformospora indica Inoculation on the Seed Yield, Light Absorption and Radiation Use Efficiency of Soybean (Glycine max) Under Water Stress Conditions

Water stress is one of the most important limiting factors in crop production, especially in arid and semi-arid regions. More than 45% of agricultural land on earth is subjected to continuous or frequent water deficiency, and it can cause ~50% loss of grain yield, on average. Soybean growth is affected by drought stress. Drought stress has been estimated to reduce seed yield of soybean by 24 to 50 percent. Improvement the light absorption in the crop plant increase the crop yield. All plants, at least during their vegetative growing period, produce and store dry matters using sunlight. One of the most important strategies to increase tolerance to dehydration and improve the growth performance in crops is to establish associations with the beneficial of fungal symbiosis. Piriformospora indica is one of the cultivable root-colonizing endophytic fungi that has a symbiotic relationship with the roots of most crops and improves the growth and yield of plants by increasing the absorption of nutrients such as phosphorus and some micro- elements and can enhance the resistance to biotic and abiotic stresses (Oelmuller et al., 2009). The aim of the present investigation was to study the impact of P. indica on the light absorption, radiation use efficiency and grain yield of soybean under different levels of irrigation. Two field experiments were carried out at the Agricultural Faculty, University of Bu-Ali Sina, Hamedan, Iran (35º1ʹN, 48 º31ʹE; 1690 m a.m.s.l.) in 2014 and 2015. This region has a cold and semi-dry climate. The experiments were carried out as split-plot based on a randomized complete block design with three replications. The Main factors consisted of three irrigation treatments (irrigation after 60 (well-watered), 90 (mild stress) and 120 (severe stress) mm cumulative evaporation from pan class A) and sub plots included of two levels of fungus P. indica (inoculated and non-inoculated). All main plots were irrigated immediately after sowing. Water-stress treatments as described above were applied after seedling establishment. Irrigation was performed via polyethylene pipes, and a water meter was used to measure the volume of irrigation water in each main plot. In order to maintain the specified soil-moisture regimes, the amount of used water was calculated by using crop water requirement as described by Doorenbos and Pruitt (1992). In both years, drought stress decreased leaf area and dry matter of soybean. Inoculation with fungus, increased leaf area and dry matter of soybean plants in different irrigation levels. Daneshian et al. (2011) studied the effect of drought stress on dry matter and soybean growth indices. Due to the decrease in leaf area, drought stress reduced the amount of dry matter accumulation in the soybean plants. The highest radiation use efficiency (1.75 and 1.85 g MJ-1 in 1st and 2nd year, respectively) was obtained from inoculated soybean plant under well-watered, and the lowest one (1.10 and 1.15 g MJ-1 in 1st and 2nd year, respectively) was observed in control plant (non-inoculated) under severe drought stress. Drought stress reduces the amount of radiation use efficiency by reducing photosynthetic rates and decreasing leaf area index. Severe drought stress significantly decreased grain yield of soybean by about 57.20 percent. Application of P. indica caused an increase in grain yield of soybean by about 13.67, 22.85 and 22.14 percent under well-watered, mild and severe drought stress, respectively, compared to control (non-inoculate). Inoculation with P. indica fungus increases the light absorption and radiation use efficiency by increasing the amount of vegetative growth, leaf area index and photosynthetic material production, which improves the yield of soybean. The results showed P. indica fungus had a positive effect on absorption and radiation use efficiency of soybean in different irrigation levels, so that the application of fungus mitigated the effects of drought stress and improved the yield of soybean under drought stress.