Decline of tolerance in leaf photooxidative-stress with age in sunflower

Sunflower seeds were planted under field conditions in order to investigate the behavioral pattern of photooxidation tolerance in plants in response to leaf aging. During pollination stage, the leaves were theoretically divided into three sections including young, middle aged and senescent leaves, so that the cell defense potential would change with leaf aging, and then modeling of different intensities of light and its effects on light oxidation of biological materials in the leaves of different ages was investigated. The results from the first series of experiments showed that despite of no change of chlorophyll content in leaves due to leaf aging, total protein content and membrane stability index were declined. Furthermore, lipid peroxidation and hydrogen peroxide were increased in respond to leaf aging. Meanwhile, the activity of hydrogen peroxide scavenging enzymes and glutathione S-transferase in senescent leaves were higher than that of young leaves. Overall, the results showed that with any increase in leaf age, defense potential of cells slowed down and the production of oxidants exceeded the scavenging potential of the cells. The results of modeling of different intensities of light revealed that in senescent leaves under high light intensities, hydrogen peroxide reached to its highest levels. However, under those light condition, hydrogen peroxide scavenging enzymes and glutathione S-transferse activities were recorded at amounts lower than that of low light intensities. The outcome of the above mentioned factors lead to oxidation of chlorophyll and subsequently its content reduction under highlight intensities in aged leaves. In total, under high light intensities, the damage of oxidation stress on aged leaves was more intensified compared to young leaves mainly due to diminished defense potential of stress faced cells.