Effects of oxygen stress on ¹H-NMR relaxation time (Τ₁) of Vigna radiata seedling

Effects of the oxygen exposure on plants were investigated in intact hypocotyl tissues of etiolated seedlings from mung bean (Vigna radiata L.). We report here that spin-lattice NMR relaxation time (Τ₁) of water proton of intact tissues, which can be measured with a 180° - τ -90° pulse sequence at 20 MHz, is responsible to the oxygen exposure. When the tissues placed in the NMR tube were exposed to 95% oxygen for 5 s, a rapid decrease in the Τ₁ values and a subsequent relaxation process were observed. The value was fully recovered to the initial one during a 40 min incubation. Heat-denatured tissues did not show such recovery response, suggesting that the Τ₁ recovery requires heat-sensitive components contained in the tissues. To examine a possibility that scavenging activities of oxygen radicals is involved in the mechanism for the Τ₁ response, effects of supplemental scavengers were tested. We have found that the shortening of Τ₁ induced by oxygen exposure to membrane fractions was strongly suppressed by the radical scavenger superoxide dismutase (SOD), Tiron and ascorbate. These results suggest that activities of O₂ production in the tissues are involved in the mechanism for abrupt shortening of Τ₁.