Effects of drought stress and its sudden relief on free radical processes in barley

The generation of free radicals and other paramagnetic species has been investigated by electron paramagnetic resonance (EPR) spectroscopy in leaves of barley plants during and after release of drought stress. Three cultivars were studied; the mlo-resistant spring barley cultivars, Atem and Krona, along with the cultivar Golden Promise, with the wild-type dominant Mlo. In the cultivar Atem, which exhibits a partial loss of resistance to powdery mildew after relief of drought stress, the free radical levels were elevated in the drought-stressed samples compared with those seen in the controls, but they returned rapidly to the control levels after release of the stress. The other two cultivars showed little or no change in the free radical signal for any of the treatments. A sizeable increase in the level of a mononuclear Fe(III) complex was seen in the droughted samples (compared with the levels in the watered controls), and these elevated levels remained after release of the stress. These observations are discussed in the context of molecular processes in the mesophyll cells, where activation of plasma membrane H+-ATPase by 14-3-3 proteins leads to a decrease in pH. This, in turn, perturbs the Fe(II) speciation in favour of more easily oxidized complexes, and results in the generation of the mononuclear Fe(III) complex. The greater change in free radical activity in Atem as a result of drought stress compared with that seen with the other cultivars indicates that it is affected to a greater extent by the stress. (C) 2004 Society of Chemical Industry.