ObjectivePopulus euphratica, commonly known as a desert poplar, is a pioneer tree species with stress resistance and heteromorphic leaves in western China. Previous studies have demonstrated that the structural characteristics of the diversiform-leaves of P. euphratica are related to its eco-adaptability. In the present study, to elucidate the regulation mechanisms of stress tolerance, we focused on the antioxidant capacity for three types of heteromorphic leaves of P. euphratica.

MethodIn this paper, the changes of ·O₂⁻ production rate, MDA content, antioxidant enzyme activities (SOD, POD, CAT, APX and GR) and antioxidative substance contents (flavone, carotenoid, AsA and GSH) in lanceolate leaves, ovate leaves and dentate broad-ovate leaves, three typical morphological leaves of adult P. euphratica, were studied in different seasons.

ResultThe results were as follows: (1) the ·O₂⁻ production rate and MDA content: with the change of seasons, the rate of ·O₂⁻ production was progressively increased both in dentate broad-ovate leaves and ovate leaves. While it firstly increased and then decreased in lanceolate leaves. The content of MDA decreased in dentate broad-ovate, firstly decreased and then increased in ovate leaves, while it increased in lanceolate leaves all the time. It is noteworthy that compared with the other types of leaves, ·O₂⁻ production rate was the lowest while MDA content was the highest in lanceolate leaves in September. (2) The activities of antioxidant enzymes: with the change of seasons, both in dentate broad-ovate leaves and ovate leaves, the activity of SOD firstly decreased and then increased, while the activity of CAT firstly increased and then decreased. There was a progressively increase of the activity of POD, and a decrease of the activity of GR. The activity of APX firstly increased and then decreased in dentate broad-ovate, and it progressively increased in ovate leaves. Remarkably, the activities of SOD, POD, CAT, APX and GR in lanceolate leaves all firstly increased and then decreased. Overall, the activities of antioxidant enzymes were the highest in dentate broad-ovate leaves, followed by ovate leaves, and was the lowest in lanceolate leaves. Furthermore, the difference of POD activity was the most obvious among the three types of leaves. (3) The antioxidative substance contents: in the three heteromorphic leaves of P. euphratica, a large amount of flavone and carotenoid accumulated in July, and AsA content was the highest while GSH content was the lowest in September. The contents of carotenoid and flavone firstly increased and then decreased both in dentate broad-ovate leaves and ovate leaves, while they were increased in lanceolate leaves. The content of AsA progressively increased both in dentate broad-ovate leaves and ovate leaves, while it firstly decreased and then increased in lanceolate leaves. In addition, the content of GSH all decreased in three types of morphological leaves. (4) Correlation analysis: the ·O₂⁻ production rate was significantly and positively correlated with POD activity and AsA content in dentate broad-ovate leaves (P < 0.01). In addition to POD and AsA, there was a significant and positive correlation between the ·O₂⁻ production rate and APX activity, carotenoid content in ovate leaves (P < 0.01). While the activities of antioxidant enzymes (POD, CAT and APX) and the contents of flavone and AsA were all highly significantly and positively correlated with the ·O₂⁻ production rate in lanceolate leaves (P < 0.01).

ConclusionThese results showed that there was an obvious difference of the antioxidant ability among the three morphological leaves in P. euphratica. It was the strongest in dentate broad-ovate leaves, followed by ovate leaves, and was the weakest in lanceolate leaves. Moreover, there were differences in the antioxidant response mechanisms of three morphological leaves with seasonal changes. In dentate broad-ovate leaves and oval leaves, POD, CAT and APX were the main antioxidant enzymes for scavenging ROS in summer, while the SOD and POD with high activity were the main antioxidant enzymes in autumn. Meantime, APX activity in the ovate leaves in autumn was also maintained at a high level. In addition, SOD, POD, CAT, APX and GR synergistically scavenged ROS in lanceolate leaves. And in three heteromorphic leaves, flavone and carotenoid were the main antioxidants to eliminate ROS in summer, and more ROS was removed through the AsA-GSH circulation in autumn.