Effects of elevated CO_2 concentration on nutrients and secondary metabolites in Medicago sativa leaf under different damage degrees of pea aphid (Acyrthosiphon pisum, Hemiptera: Aphididae)
Due to massive exploitation and use of fossil fuel such as petroleum, coal and natural gas, atmospheric CO_2 concentration has been increasing, which not only accelerated global warming, but also affected the survival and distribution of animals and plants on the earth with far-reaching impacts on the ecosystem. This research was carried out to explore the effects of elevated CO_2 concentration and pest population density of Acyrthosiphon pisum on chemical substances in the leaves of Medicago sativa. The objective was assessed the effects of elevated CO_2 concentration and pea aphid density on the physiology and biochemistry of M. sativa. The nutrients and secondary metabolites in M. sativa leaves were determined by cultivating M. sativa seedlings attacked by 10-day old pea aphids of 10 head·plant~(-1), 20 head·plant~(-1), 30 head·plant~(-1) and 0 head·plant~(-1) (CK) for one week under three CO_2 concentrations [380 muL·L~(-1) (CK), 550 muL·L~(-1) and 750 muL·L~(-1)] in CO_2 gradient chamber. The results indicated that the contents of soluble protein, soluble carbohydrate and starch increased after aphid sucking of 30 head·plant~(-1) with increasing CO_2 concentration. At 750 muL·L~(-1) CO_2 concentration, they were respectively 11.62 times, 0.49 times and 0.24 times higher than those under CK, respectively. Also the contents of flavone, total polyphenols and simple phenols increased significantly. Furthermore, the contents of starch and simple phenols increased and then decreased with increasing degree of damage under the same CO_2 concentration. Comparatively, contents of soluble carbohydrate, total polyphenols and tannin were significant differences between aphid densities of 30 head·plant~(-1) and 0 head·plant~(-1), which increased by 1.66 times, 1.49 mg·g~(-1) and 1.09 mg·g~(-1) (P < 0.05) compared with those of CK under the highest level of CO_2 concentration. The results indicated that nitrogen fixing legume plants were more likely to adapt to increased CO_2 concentration and thereby enhanced self-induced resistance to insect pests.