Eco-physiological Basis For Controlling Maize (Zea Mays L.) Airborne Diseases Via Double-genotypes Intercropping

Maize airborne diseases can be controlled efficiently via double-genotypes intercropping, but the mechanisms are little known. We presuppose that the eco-physiological factors such as compound population quality, disease resistant metabolism and maize leaf volatile organic compounds (LVOCs) may be of importance for controlling maize airborne diseases in this intercropped system. In this study, we conducted a field experiment on the farm of Henan Agricultural University, Zhengzhou, China, using six maize hybrids, Zhengdan 958(ZD958, compact plant type, susceptible to maize leaf south rust), Ludan 981(LD981, semi-compact plant type, highly resistant to maize leaf south rust), Zhoudan 041(ZD041, semi-compact plant type, resistant to maize leaf south rust disease), Yuyu 19(YY19, semi-compact plant type, susceptible to maize leaf south rust), Xundan 20(XD20, compact plant type, medium susceptible to maize leaf spot), Huafeng 9(HF9, semi-compact plant type, medium resistant to maize leaf spot). The experimental plots of 6 m×6 m were separated by 1 m. The experiments were planted by hand with rows oriented north-south and spacing of 0.70 and 0.50 m. The ratio of two maize varieties intercropped was 1:1 per row. The planting density was 67 500 plant·hm-2. There were nine treatments: each of the varieties alone, YY19 intercropped with ZD041 (YY19‖ZD041), ZD958 intercropped with LD981 (ZD958‖LD981), and HF9 intercropped with XD20 (HF9‖XD20). There were four replicate plots for each treatment, arranged in a randomized complete block design. We investigated the population quality, resistant physiology and relationship between airborne diseases resistance and maize leaf volatile organic compounds. Our objective was to reveal the eco-physiological basic for controlling maize airborne diseases in intercropping with double-genotypes. The results are as follows:1) Maize intercropping with double-genotypes is favorable for improving plant population structure, enhancing population quality and reducing disease index. In the intercropping treatments both ZD958 II LD981 and HF9 II XD20, the average leaf life of four varieties increased. The leaf area indexes (LAI) of four varieties were no significant difference between sole-cropping and Intercropping before silking. But, the LAI of HF9 and LD981 increased while that of ZD958 and XD20 decreased in the intercropping treatments after silking. The soil and plant analyzer development readings (SPADRs) of ear-leaves of four varieties increased in the treatments 10 days after silking, and the photosynthesis rate of ear-leaves in LD981, HF9 and XD20 increased or increased significantly. There were no differences in the dry matter accumulation (DMA) of four varieties between the treatments of sole-cropping and intercropping before silking. The DMA of semi-compact plant types both HF9 and LD981 increased or increased markedly, while that of compact plant types both XD20 and ZD958 decreased or decreased significantly after silking. In the intercropping treatments both ZD958‖LD98 and YY19‖ZD041, the activities of SOD and POD of leaves increased in the intercropping treatments 10 days after silking, which resulted in decreased MDA contents of leaves. The activities of CAT of leaves in ZD041 and ZD958 increased or increased markedly, while YY19 and LD981 decreased slightly. The changes of activities of SOD and POD and contents of MDA in leaves remained similar 40 days after silking. Meanwhile, the activities of CAT of leaves increased, among them YY19 increased significantly. Transmittance enhanced in the treatment of compact plant type intercropped with semi-compact plant type, which resulted in reduced pathogen spore concentrations in the intercropped colonies, which led to reduced disease index of the intercropped colonies. For example, the disease index of southern rust in the intercropped colonies of ZD958 and HF9 reduced by 59.75% and 50.38%, respectively. Also, yields, land equivalent ratio (LER) and grain quality were enhanced in the intercropped colonies. Results suggested that suitable maize intercropping with different genotypes would increase the activities of SOD, POD and CAT of leaf, prolong leaf life, enhance photosynthesis rate, increase disease index, increase yield and improve quality.2) Maize intercropping with double-genotypes is favorable for enhancing resistant metabolism. The artificial inoculation test showed that, in the intercropping treatment of ZD958‖LD98, the activities LOX, PAL and PPO of leaves increased rapidly and maintaining at the high activity level for a long period after inoculation. There was a significant difference for resistance action against southern rust between susceptible variety (ZD958) and resistant variety (LD981). LOX, PAL and PPO would play a key role in ZD958 against southern rust, while PPO would be very important in the same action for LD981. Results suggested that maize intercropping with different resistance genotypes would be favorable for improving defense ability of the intercropped colonies. 3) Maize intercropping with double-genotypes increases the genetic diversity of colony, which results in enriching leaf volatile organic compounds (VOCs) and the ability to resistant maize leaf south rust and maize leaf spot maize leaf spot. Maize leaf VOCs were extracted by steam distillation and analyzed by GC-MS. Results showed that there were great differences of the leaf VOCs among varieties, and the VOCs were changed in the intercropping treatments. The results of exposure to VOCs test indicate that the urediospore germinations of Puccinia polysora Underw were inhibited by leaf VOCs of LD981, HF9 andXD20, while conidia germinations of Helminthos porium maydis Nisik & Miy were marked inhibited by leaf VOCs of HF9 andXD20. In order to prove antimicrobial activity of maize leaf VOCs, a further test was conducted by five maize leaf volatile organic components such as (Z)-3-hexenol, (E)-2-hexenal, benzyl alcohol, vanillin and a-pinene. Results revealed that the urediospore germinations of Puccinia polysora Underw were inhibited significantly by the five components, the inhibitory rate from high to low were (Z)-3-hexenol, (E)-2-hexenal, benzyl alcohol, vanillin and a-pinene. Also, the conidia germinations of Helminthos porium maydis Nisik & Miy were inhibited markedly by the five components, and the inhibitory rates of the conidia germination were benzyl alcohol>vanillin>(Z)-3-hexenol> (E)-2-hexenal>α-pinene. Meanwhile, the mycelium growth of Helminthos porium maydis Nisik & Miy were significant inhibited markedly by the five components. Among them, the inhibitory rates of mycelium growth of vanillin, (E)-2-hexenal and (Z)-3-hexenol were more significant. The sporulation of the mycelium treated with high and low concentration of (Z)-3-hexenol was promoted, and the germination of spores generated from the mycelium was also promoted. In the treatments both (E)-2-hexenal and vanillin, the sporulations of mycelium were inhibited significantly, while the germinations of spores inhibited slightly. In the treatment of benzyl alcohol, the growth and sporulation of mycelium were mightily inhibited, and the mycelium was significant variation, the color of aerial mycelium was white with hardly sporulated. The treatment of a-pinene had a slight inhibition effect on the mycelium growth, while the sporulation quantity and the spore germination rate were lower. Results also indicated that the inhibitory effects of the treatments of different combination with the five volatile components on Puccinia polysora Underw and Helminthos porium maydis Nisik & Miy were more significant than the treatment of a single volatile organic component, which accord with natural condition of plant resistance.In conclusion, the higher resistance against Puccinia polysora Underw and Helminthos porium maydis Nisik & Miy in maize intercropped system is based on the intercropped colonies ecology (including chemistry ecology) and physiological resistance. Suitable maize intercropping with different genotypes not only optimizes the population structure, improves the conditions of ventilation and transmission in the intercropped colonies, creates a micro-environment which is benefit for plant growth and makes against the pathogen growth. It also increases the genetic diversity of disease-resistant gene in the compound population, which results in increasing secondary metabolisms and leaf VOCs, and the chemical communication net of the compound population becomes more sophisticated and consequently improves the population resistance. Thus, leaf life would be prolonged, which be favor of improving grain yield and quality.