Application of Non-photochemical Quenching on Screening of Osmotic Tolerance in Soybean Plants

Non-photochemical quenching (NPQ) values for utilizing them to detect osmotic tolerance in plants were examined with two different soybean cultivars, an osmotic tolerant soybean (Shinpaldalkong 2) and a control soybean (Taekwangkong). Two different stresses were applied to the cultivars as the restricted irrigations of 200 and 50 ml water pot-¹ d-¹ for 5 days for a control and a drought stress, respectively, and a sodium chloride solution of 200 mmol for 6 days for a salt stress. The intact leaves of the two cultivars after treatment were used to measure chlorophyll fluorescence parameters, maximum efficiencies of photosystem Ⅱ photochemistry (Fv/Fm), efficiencies of photosystem Ⅱ photochemistry (Φ∧PSII), CO₂ assimilation rate (P∧N), and NPQ. Leaf water potentials of the two cultivars decreased from –0.2 to –0.8MPa by a drought treatment and from –0.7 to –1.7MPa by a salt treatment. Leaf water content of Shinpaldalkong 2 after a salt treatment was less decreased than that of Taekwangkong. F∧v/F∧m values of both cultivars were not changed, while Φ∧PSII and P∧N were decreased proportionally to leaf water potential decrease. The response of NPQ was occurred in Shinpaldalkong 2 under the drought and salt stresses. With Taekwangkong cultivar, only drought stress referred NPQ response. The cultivar differences on chlorophyll fluorescence parameters were found in the relationships between Φ∧PSII and P∧N, and between NPQ and Φ∧PSII. Although the positive relationships between Φ∧PSII and P∧N were established on all treatments of both cultivars, the decreasing rate of Φ∧PSII to P∧N was smaller in Shinpaldalkong 2 than Taekwangkong. The NPQ was increased according to the decrease of Φ∧PSII by osmotic treatments in Shinpaldalkong 2. The complementary relationships between NPQ and Φ∧PSII were well maintained at all treatments in Shinpaldalkong 2, while these relationships were lost at a salt treatment in Taekwangkong. Taken together, the results suggest that analysis of complementary relationships between Φ∧PSII and NPQ could be more valuable and applicable for determining osmotic tolerance than single analysis of each parameter such as F∧v/F∧m, Φ∧PSII and NPQ.