Photosynthetic Biochemistry and Kaempferol Profiles of Soybean Under Leaf Wounding and Abscisic Acid Treatments

Soy flavonoids such as kaempferols are well-known anticancer phytochemicals biosynthesized especially under environmental stress.  Leaf wounding results in both biotic and abiotic stresses, and abscisic acid (ABA) is the most common plant stress hormone. Thus, we conducted greenhouse experiments to evaluate association of biochemical parameters of photosynthesis and the yield of anticancer kaempferols in soybean under two types of leaf wounding—hole punching and piercing, and two levels of ABA foliar spray—100 and 200 µM.  After three treatment cycles between V6 and R1 stages, leaves were measured and sampled at R2 stage.  Net photosynthesis was reduced up to 37% by ABA spray and 16% by hole-punching but not by leaf piercing.  Although reduced by ABA treatment, stomatal conductance (g_s) and intercellular CO₂ concentration (C_i) were not affected by mechanical wounding.  Photosystem II efficiency was 22% less under 200 µM ABA treatment compared with leaf piercing.  Electron transport between photosystem II and I was decreased by ABA treatment and hole-punching, but not by leaf piercing.  Transpiration rate (E) was reduced by ABA treatments, but not by mechanical wounding.  Plants treated with 200 µM ABA were >1 °C warmer than control or wounded plants.  Triose Phosphate Utilization and CO₂ Compensation Point were not different among treatments. Although unaffected by wounding, Rubisco Carboxylation and RuBP Regeneration rates were greatest in ABA 200 µM treated plants, 43% and 48% more than the controls. However, concentrations of major leaf kaempferols increased under both wounding and ABA treatments.  Association of kaempferol accumulation to biochemical variables of photosynthesis in soy leaves was found to be specific variable- and stressor-dependent.  Both leaf wounding and ABA treatments are promising cultural practices for producing soy kaempferols.