Photosynthetic, Antioxidant and Anticancer Properties of Soybean Under Drought and Mechanical Wounding

Drought and tissue wounding are common environmental stresses that limit crop productivity.  We hypothesized that drought and leaf wounding would change photosynthetic variables, and such changes would be correlated to concentrations of phytoalexin flavonoids of importance to human health, in soybean.  Thus, a greenhouse study was undertaken to determine the effects of drought and mechanical wounding on leaf gas exchange, photosystem II efficiency, total antioxidant capacity and the yields of anticancer kaempferols and glyceollins in soybean cultivar, IA 2032.  Drought and wounding were imposed twice, each time by withholding water for three days, and making 10 holes on the left leaflet of all leaves of the fourth branch from bottom with a hole puncher, respectively, during pod filling.  Droughted plants were allowed to recover by watering between treatments for four days.  By the end of second treatment cycles, photosynthesis dropped by 98% accompanied by 6% and 90% decreases in photosystem II efficiency and stomatal conductance, respectively, in droughted plants compared with control.  Drought decreased chlorophyll content by 28% with no effect on carotenoid levels but increased total antioxidant capacity by 88%.  In wounded plants, photosynthesis increased by 25% with 8% and 79% increases in photosystem II efficiency and stomatal conductance, respectively, compared with control.  Wounding had no effect on chlorophylls, carotenoids and antioxidant capacity.  During recovery, four days after the second drought and seven days after second wounding, photosynthesis was still 44% less in droughted plants acomponied by 17% and 53% decreases in photosystem II efficiency and stomatal conductance.  Chlorophyll and carotenoid contents were 45% and 44% less, but total antioxidant capacity was still 41% greater in droughted plants.  Wounded plants had the same photosynthesis, photosystem II efficiency and antioxidant capacity, but 31% greater stomatal conductance with 24% and 29% less chlorophylls and carotenoids, respectively, compared with control.  Drought stress but not wounding enhanced total antioxidant capacity in soybean leaves.  Preliminary results suggest that concentrations of four leaf kaempferols were reduced by stress, especially wounding.  Drought-elicited antioxidant capacity was not due to chlorophylls, carotenoids or kaempferols in soy leaves.  Changes in photosynthetic variables under stress did not appear to be correlated with leaf kaempferol levels.  Leaves had no detectable levels of glyceollins while stems, seeds and roots are currently analyzed.