Physiological indices of leaves of jujube ( Zizyphus jujuba ) damaged by Apolygus lucorum

Apolygus lucorum Meyer-Duer (Hemiptera: Miridae) is an important type of pest, damaging various agricultural and forest crops such as cotton, fruits, and weeds. A. lucorum feeds on young buds, leaves, and various other tissues of host plants with its piercing and sucking mouthparts, and it is an important secondary insect pest in agricultural production in China. With the extensive cultivation of Bt cotton since 1997 in China, the wide-spread use of this insect-resistant cultivar has drastically reduced the need for insecticides to control many key lepidopteran pests. However, this reduction in insecticide use has led to increased population densities of various mirids, among which A. lucorum is considered as the most important pest in cotton production in northern China. The large-scale cultivation of fruits in recent years has provided abundant food and appropriate hibernacle for A. lucorum. Consequently, it has gradually become a significant pest for fruit trees, especially jujube (Zizyphus jujuba), causing serious damage in orchards in northern China. After being damaged by A. lucorum, the young leaves of jujube (Zizyphus jujuba) showed many needle-like reddish-brown dots, which formed into irregular holes as the leaves grew. Growth of the floral buds ceased, and black spots appeared on the young fruit. Many studies have shown that pest damage induces plants to produce stress responses. Therefore, certain physiological indices in plants will change during the response to pest damage. To examine the responses of jujube leaves to insect damage, we conducted biochemical analyses to study changes in various physiological indices of jujube leaves damaged to different degrees by A. lucorum. The soluble sugar content first increased and then decreased, while the protein content decreased and free amino acids content increased with increasing severity of leaf damage. The activities of protective enzymes also changed with increasing severity of leaf damage. Superoxide dismutase (SOD) activity first increased and then decreased, while peroxidase (POD) activity increased. The catalase (CAT) activity was not significantly different between damaged and undamaged jujube leaves. In summary, jujube leaves damaged by A. lucorum showed changes in soluble sugar, protein, and free amino acids contents, and changes in the activities of defense enzymes. Except for CAT, the changes in soluble sugar and protein contents and SOD and POD activities were closely related to the s