Effects Of Different Light Intensities On The Constitutive Resistance Of Larix Gmelinii

Larix gmelinii is among the most important timber species in northeastern China due to its cold hardiness, drought resistance and rapid growth. It is ecologically and economically very important, but is seriously affected by Lymantria dispar L., a defoliating insect pest. Plants, including trees, employ an array of physical and chemical constitutive defense mechanisms which play an important role in protection from insect herbivory. Constitutive defenses are always present, and because they are maintained even in undamaged plants. Trees constitutive resistance with many factors of instability is easy to change with the maturity of environmental factors, plant nutrition and even the plants ripening degree which can affect the behavior, growth and reproduction of insect. In many organisms light is a crucial environmental signal influencing natural physiological and developmental processes. There are different effects on the phenotypic and physiological characteristics even if the same plant species with the long-term growth in different light condition. Different light intensities have a effect on the defense protein activity, biosynthesis and accumulation of phenolic acids and condensed tannin which have a certain impact on the constitutive resistance. However, the pests can detoxicate by its protection and detoxification system to adapt the plant defense system.In order to understand the impact of light on the constitutive resistance of Larix gmelinii, sunlight control treatments with three intensities:100%of natural sunlight(NS), about50%and25%of natural sunlight(NS1, NS2) were conducted to simulate the light conditions of forest edge, forest gap and understory where the Larix gmelinii grows. In order to demonstrate the effects of different light intensities on the constitutive resistances of the dahurian larch, we determined the defense protein activity, contents of phenolic acids and condensed tannin. We test the effect of the Larix spp. that under different sunlight intensities on the physiological indicators, development of Lymantria dispar. The results showed as follows,1.Effects of different light intensities on the activity of primary defense protein in larch needlesThe primary defense proteins consist of protective enzymes, e.g. peroxidase (POD), superoxide dismutase (SOD) and catalase (CAT); defense enzymes, e.g. polyphenol oxidase (PPO) and phenylalanine ammonia-lyase (PAL); protease inhibitors, e.g. trypsin inhibitor (TI) and chymotrypsin inhibitor (CI). The activities of the primary defense proteins in the needles of Larix gmelinii seedlings grown under different light intensities were tested. The results showed that the activities of POD, SOD, PAL, PPO and CI under shade conditions were significantly (P<0.05) higher than control both in June, July and August. Low light intensity can significantly improve the defense protein activity, thereby effectively to enhance the constitutive resistance. Shading has a significant effect on TI activity (P<0.05) with volatile changes which indicate there is a significant response to the light environmental changes. The activities of SOD, PAL, PPO and CI were highest in June, indicating a stronger resistance of the larch in the early stage of growth.2.Effects of different light intensities on the accumulation of defensive secondary metabolites in the larch needlesSecondary metabolites of plants play an very important role to improve the protection, survival, competitiveness and coordinate the connection with the environment. Phenolic acids and condensed tannin are the mainly defensive secondary metabolites. Shade could ifluence the biosynthesis and accumulation of the phenolic acids and condensed tannin significantly(P<0.05). The results showed that the constituents and contents of the phenolic compounds were both increased compared with control. Twelve phenolic acids were identified (Table1); and eight were found in the larch seedling needles under the natural light (NS) condition; while ten compounds were detected from the seedling needles under the NS1and NS2conditions. Nine phenolic acids were identified from samples taken in June while ten and eight compounds were found from samples taken in July and August, respectively. In addition, phenylacetic acid was the most most dominant component among all the phenolic compounds detected.The significant positive relation between condensed tannin content and light intensity was also observed. The condensed tannin concentrations in Larix gmelinii seedling needles significantly decreased as light intensity decreased (from100%to25%:control->NS1->NS2). The condensed tannin concentrations of each light treatment remained unchanged during the three sampling months. The decrease of the condensed tannin concentrations with the changes of the compositions and the contents of the phenolic acids suggested that there is a dynamic balance between the concentrations of the condensed tannins and other chemical defense substances.3.Effects of the Larix gmelinii under different light intensities on the development and metabolizing enzymes of the Lymantria disparPests can detoxicate the toxic substances ingested by its protection and detoxification system. In order to explore the respond of the Lymantria dispar L. to the changes of constitutive resistance of Larix gmelinii caused by light environmental. We test the effect of Larix gmelinii which under different light intensities on the physiological indicators, growth and development of Lymantria dispar. The results showed that the activities of detoxification enzymes both in fourth and fifth instar larve of Lymantria dispar were significantly affected by the Larix gmelinii which under different light intensities. Compared with control, the activities of protective enzymes SOD, POD, CAT and detoxification enzymes ACP, AKP, CarE, GSTs, MFO were significantly inhibited after fed on the needls of Larix gmelinii grown under shade conditions(P<0.05) as well as the growth and development. In addition, the SOD, POD, CarE, GSTs and MFO activities in the fourth instar larve were significantly higher than fifth instar larve (P<0.05) while the ACP and AKP activities in the fifth instar larve were significantly higher than fourth instar larve (P<0.05). Our research indicate that insects activate different detoxification enzyme in different instar and the phosphatase activity was significantly stimulate in the fifth instar.Overall, the defense protein activity and phenolic acids content were significantly increased under shade conditions while the development and detoxification enzyme activityof the larve were significantly inhibited. This research indicate that low light environments is helpful to improve the constitutive resistance of the larch. Furthermore, Larix gmelinii could likely be used in forestry systems designed to limit light with increasing forest canopy density or in the schattenseite. Our findings suggest to provide theoretic reference for the pest resistance management, forest treatment, andother routine management methods by the natural control ability during the burliness stage of Larix spp.