Effects Of Elevated Atmospheric CO₂ On Maize Insect-Resistant Compounds

As one of the main green house gas, impact of elevated CO₂（eCO₂） concentration of atmospheres has scientifically become the main concern worldwide. eCO₂ will generally lead to the increase of plant metabolism and generate plants with higher C/N. On the other hand, eCO₂ may also mediate the changes in plant secondary metabolism because of alterations in the plant allocation to defence chemistry and chemical signalling, which will affect the plant-herbivore interaction. Corn has been ranked the first grain crop for food, feed, and industry. The Asian corn borer（ACB）, Ostrinia furnacalis（Guenée）, is the morst economic insect pest on corn, which is periodically outbreak and results in serious yield losses. Utilization of host plant resistance for control of ACB is a primary component of Integrated Pest Management for this insect because of its economic, easy application, and environmental friendly. Reasearch for impact of eCO₂ on changes of corn plant biochemistry could provide scientific evidence for understanding the evolution rule of corn plant metabolism and secondary metabolism under climate change, as well as clarify the trend and potential use of corn plant resistance to ACB. This research simulates the growth and development of a corn line Zheng 58 and a line expressing Bt Cry1 Ac toxin（GM-line）, ACB and their interactions under ambient and two eCO₂（³90 μL/L,550μL/L,750μL/L） environments with open top chambers（OTCs） and closed dynamic chambers（CDC）. Corn plant chemistry, especially the C and/or N contents as well as 2, 4 dihydroxy-7-methoxy-2H-1, 4 benzoxazin-3（4H）-one（DIMBOA） of corn plants as well as Cry1 Ac toxin were assessed. The fitness of ACB larvae were bioassayed in the field with artificial infestation under OTCs and in vitro bioassays.Corn leaves were sampled in plants grown in OTCs in 45 days after planting. Non-structured carbohydrates（starch, soluble sugar, TNC） and nitrogen content of the leaves were quantified. The results showed that the starch and soluble sugar content, TNC: N were significantly increased in the tissues from plants under eCO₂（750 μL/L） OTC, but the N contents in leaves were not significantly different among CO₂ concentrations. However, the N content in leaves of Zheng 58 was significantly higher than that in GM-line.The contents of DIMBOA in leaves were measured by UPLC and Cry1 Ac toxins in GM-line leaves were assayed using Bt-Cry1Ab/Ac ELISA kits. There was no significant difference in the DIMBOA content among the treatments. Although the amount of Cry1 Ac toxin expressed in GM-line leaves showed decline along with atmospheric CO₂ level increase, there was no significant difference among atmospheric CO₂ treatments.ACB neonates were feed on corn leaves of Zheng 58 and GM-line plants that were grown under ambient and two eCO₂ OTCs. The experiments were conducted in the same CO₂ levels of corn plants grown or ambient condition. eCO₂ did not significant affect the mortality of ACB in 48 h. The survival rate was significantly low in GM-line. The interaction between CO₂ and corn lines was not significantly difference on larval survival. Field infestation under OTC condation indicated that CO₂ did not affect the corn plant resistance levels to ACB. Larval survival and plant damage ratings were significantly low in GM-line comparing to Zheng 58. However, larval survival and plant damage ratings were apparently increased in GM-line grown under eCO₂（750 μL/L）, which was consistent the trend of Cry1 Ac expressing levels. In addition, those parameters were apparently increased eCO₂（550 μL/L） compared to ambient, but decreased on eCO₂（750 μL/L） compared to eCO₂（550 μL/L）. This indicated that the nutrition quality was declined while C/N increased in corn plants grown under eCO₂（550 μL/L）, which resulted in the alternation of food consumption to compensate for these change in ACB. Meanwhile, the larval fitness were also declined along with food quality decline, especially when eCO₂ reached to 750 μL/L, which results in decline in laval survival and plant damage.In conclusion, corn plant had higher TNC:N ratios under eCO₂ than at ambient CO₂, which could resulted in a nutritional deficiency in the plants as the host of ACB and reduced the fitness of ACB. Larvae might alter its food intake to compensate the poor nutrition, which will lead to more serious damage to corn plants. eCO₂ did not influnce the DIMBOA content in corn plant. eCO₂ may influnce the expressing level of Cry1 Ac toxin in transgenic Bt corn.