Abstract:During the natural senescence of wheat flag leaves in Triticum boeoticum Boiss and "Yangmai No.5" ( T. aestivum L. ), the decrease of their ability of scavenging active oxygen was an unbalanced process. During photosynthetic active duration (relative steady phase of chlorophyll content), the activity of SOD decreased slightly, but that of catalase (CAT) decreased rapidly, while the activity of ascorbate peroxidase (ASP) increased at first and then decreased. The changing disparity among the SOD, CAT, ASP activities during natural senescence of wheat leaves consequently lead to the accumulation of hydrogen peroxide, which was associated with the rapid senescence of the leaf (sharp fall phase of chlorophyll content). Finally, SOD activity decreased remarkably, That the system of scavenging active oxygen appeared earlier in an unbalanced state could be one of the causes of early senescence in T. boeoticum as compared with "Yangmai No.5". The rapid accumulation of hydrogen peroxide in wheat leaves was closely related to the priming of senescence.