The effect of Piriformospora indica seed bio-priming and paclobutrazol foliar spraying on tolerance to chilling stress in green beans (Phaseolus vulgaris L.)

Introduction Green bean (Phaseolus vulgaris L.) is one of the four species of the genus Phaseolus with the highest protein among a variety of commercial plants (Nasri and Khalatbari, 2011(. This plant can be grown in different times in Mazandaran weather conditions (Motaghian et al., 2010). Green beans as well as other crops during their growth stages faced with environmental stresses such as chilling. Chilling stress referred to conditions as temperature between 0 and 10 ᵒC which resulted in damages to the plant (Baninasab, 2009). Chemical and biological inducers can be used to increase chilling stress tolerance of plants. Paclobutrazol (PBZ) is a member of the Tryazol families with growth regulating effect (inhibitors) and increased stress tolerance through changes in plant growth and morphological, anatomical and biochemical changes (Berova et al., 2002; Chaturvedi et al., 2009). Meanwhile, Piriformospora indica fungus is one of the soil microorganisms which form a symbiotic relationship with plants and could increase plant growth and resistance to environmental stresses throughout systemic changes or defense induction (Deshmukh et al., 2006). Hence, the present research was aimed to determine PBZ and P. indica efficiency in green bean tolerance against chilling stress.  Materials and Methods The experiment was conducted under controlled conditions at Genetics and Agricultural Biotechnology Research Institute, Sari Agricultural Sciences and Natural Resources University in factorial arrangement based on completely randomized design with three replications in 2013. Treatments were three levels of chilling stress (control, three and six days exposure to five centigrade degree temperature), two levels of fungi inoculations (control and inoculation with P. indica) and three levels of PBZ (0, 40 and 80 mg/L). To inoculate the plants with P. indica, fungal spore suspension was prepared (5×105 spore/ml). The sprouted seed were soaked for four hours in fungal suspension and were sown in pots containing two kg of sterilized soil. PBZ was sprayed twice at two stages of five and six week after sowing. Then pots were transferred to growth chamber after eight weeks from sowing and then chilling stress was imposed. After 12 hours of stress, aerial dry weight, SPAD value, chlorophyll a, b and a+b, carotenoid content and electrolyte leakage were measured. The data were analyzed using SAS statistical software and significant differences between means were estimated using least significant difference (LSD) test at P