The Role Of Light In Efficient Phosphorus Use Efficiency Of Maize/Soybean Relay Strip Intercropping System

Belowground interspecific and light interception facilitation in leaves contribute to the resources（light,water,nutrition,et al.）use advantages in intercropping,especially in the cereal-legume intercropping system.However,how the ameliorated light condition aboveground in intercropping mediates the growth,development and function of leaves and roots,and affects the yield and nutrient use efficiency is not clear.Cereal/legume intercrops are prevalent in previous studies,such as the maize/soybean relay strip intercropping system is popular in China,especially in Southwest of China.Southwest of China is a typical area with few solar radiation(from 3350 to 4190 MJ m^-2 one year),in there the light interception advantage of intercrops is particularly prominent,which provides an ideal light environment for studying the mechanisms of shoot-root interactions improving resources use efficiency in intercropping,like phosphorus（P）.In this thesis,maize/soybean relay strip intercropping with two P levels(zero and 35 kg P ha^-1)was studied in pot and field for 4 consecutive years,and focused on how does light environment affect leaf development and longevity,and then regulate root development,growth,morphology,architecture,distribution in soil profile and P uptake,which not only provides a novel clue for studying the mechanisms of efficient P use efficiency in intercropping,but also provides a new strategy for efficient P fertilizer management in the area with low solar radiation.The main results and conclusions were as follows:Compared to sole,maize and soybean in relay strip intercropping intercepted more light.The light interception of maize leaves in intercropping was higher than sole except the uppermost 2-3leaves,and the leaves in the border-row were the major contributors.However,the total light interception in intercropped maize higher than sole occurred post-silking only.During the maize soybean common growth period,the light interception of intercropped soybean was lower than sole;after maize harvested,single-leaf light interception of intercropped soybean was increased sharply,but the total light interception in intercropping over sole observed after the V8 stage.Regrettably,the light interception in maize and soybean was not affected by P application.During the post-silking,ameliorated light condition increased maize yield and P fertilizer use efficiency by extending the middle leaves and roots functional period.During the first 20 days after silking（but not pre-silking）,both 35 kg P ha^-1 supply and increased light interception decreased the export of P and increased the duration of photosynthesis and the expression of gene Zm See2β（that influences leaf longevity by regulating chlorophyll catabolism）in middle leaves in intercropping compared to the sole system,likely providing more assimilates for grain formation and maintaining longer functional period in roots because roots and leaves had higher concentration of sucrose.The ideotype root system architecture of maize,including border-row extension and higher proportion of root distributed in the upper 15-cm soil layer,combined with longer root function period,conferring 14.3%higher fertilizer P use efficiency in intercropping（23.7%）than the sole system（15.8%）.The ameliorated light condition in intercopping altered maize rhizosphere bacterial community and feedback on soil P availability.10 days after silking,the carbon exudation of intercropped maize with 35 kg P ha^-1 supply was 0.49μg C cm^-124h and was higher than sole 29.8%;on the contrary,the carbon exudation was no difference between intercropping and sole with none P apply.Compared to sole system,the OTU numbers,as well as Chao,Ace,and Shannon index advantages in intercropping occurred 35 kg P ha^-1 supply only.Additionally,Proteobacteria and Acidobacteria phyla involved in metabolism of root exudates in the rhizosphere,and Proteobacteria,Acidobacteria,Firmicutes phyla and Massilia,norank＿c＿Acidobacteria genus involved in siddolving insoluble P in rhizosphere soil in intercropping was higher than sole.Interestingly,the microbial biomass P and rhizosphere soil availability in intercropping was higher than sole existed with 35 kg P ha^-1 supply only（no different when none P supply）.The variable light condition in intercropping mediated soybean leaves development to change the root architecture and improve P fertilizer use efficiency.During the maize/soybean intercropping period,the shoot-,root-biomass,as well as root length especially main root length in intercropping was lower than sole.After maize harvested,the ratio of leaf to stem biomass in intercropping was higher than sole.At R3 stage,the number of leaves,total photosynthesis and sucrose concentration in leaves and roots in intercropping was higher sole,but the leaves P concentration in intercropping was lower than sole,especially with zero P supply.More light capture and low P concentration in leaves increased the sucrose transport to root and the genes Gm PHR25、Gm EXPB2、Gm ACP1 expression,which increased the lateral root length,density,and acid phosphatase activity.Furthermore,more than 90%root in intercropping distributed in 0-15cm soil layer and with border-row extension.The grain filling requirements combined with P efficient root architecture increased grain P concentration,P harvest index,and P use efficiency（intercropping,14.3%,sole,9.0%）.However,compared with sole,yield of intercropping did not increased,even decreased slightly.The ameliorated light condition in intercropping mediates Auxin to shape P efficient soybean root system.Intercropped soybean sustained 35-40 days shading condition by common growth maize,which decreased Auxin concentration in root,and inhibited root growth,especially main root growth,but up-regulated the expression of genes Gm YUCCA14 and Gm TIR1C especially under zero P supply.At R3 stage,the genes Gm YUCCA14 and Gm TIR1C were highly expressed in intercropping soybean with low leaves P concentration and high sucrose concentration in roots,which increased the Auxin concentration and sensitivity of Auxin receptor in root improving lateral root initiation,formation,density and length.Simultaneously,shading at the early stage inhibited the main root growth,altered the center of auxin synthesis from one（main root cap）to more（lateral root caps）,helping increased the lateral formation site and lateral root density of intercropping soybean.Within this Ph D thesis,we found intercropped maize and soybean shaped a shallow and inner row inhibition border-row extension root distribution.The yield and P use efficiency advantages of maize in intercropping over sole were mainly attributed to the longer leaves and roots function period during post-silking conferring increased carbon capture,rhizosphere P availability,and P uptake.In intercropping soybean,the variable（shading-lighting）light environment shaped ideotype plant morphology causing inhibition of main root growth and facilitation of lateral root density and length.The ideotype root system architecture combined with continual grain filling demand conferring efficient P（P in fertilizer or store in soil pools）use efficiency in intercropping soybean.A rational light management strategy is a new path for improving nutrient use efficiency in intercropping.