| Intercropping can not only improve the utilization efficiency of phosphate fertilizer,but also have the ability to use the store phosphorus in the soil efficiently,which is of great significance for controlling phosphate fertilizer input,reducing agricultural non-point source pollution caused by soil phosphorus and maintaining soil fertility.In this study,the combination of field experiment and pot experiment was used to explore the regulation ways of phosphorus absorption advantage of soybean in the maize-soybean relay strip intercropping system.The field experiment was a long-term positioning experiment,with a single set of two planting patterns and two phosphorus levels.(Plants were grown in intercropping and sole system with two P application rates P0:zero-P supply and P20:20 kg P ha-1).In the pot experiment,there were two phosphorus levels with two light conditions.The Sunshade net（light transmittance:50%）was use to simulation the light environment in the field.Shading from sowing to V5 was used to imitate the light condition of soybean in the relay trip intercropping system,while shading from V5 to harvest was used to imitate the light condition of sole soybean.(Plants were grown in shade-light（S-L）and light-shade（L-S）with two P application rates(P0:zero-P supply;P100:100 mg P kg-1 soil).Through the dynamic determination of the changes of light environment in soybean,photosynthetic capacity of leaves,phosphorus uptake,sucrose content of leaves and roots,root auxin content,root morphological changes and related gene expression during the critical period（V5 and R3）,we have made a preliminary clarification.We have preliminarily clarified the mechanism of the relay strip intercropping light environment to promote soybean root growth and phosphorus uptake.The main results were as follows:1.The seed P content,shoot P accumulation and HI of soybean were significantly enhanced in the relay strip intercropping system.During the V5 and R3 stage,the shoot P accumulation of intercropping soybean was significantly lower than sole soybean.At the stage of R8,the shoot growth P accumulation of intercropping soybean was 11-32%higher than sole soybean,which implied that the P uptake of intercropping soybean during reproductive growth was significantly higher than sole soybean.Meanwhile,the seed P of intercropping soybean was significantly higher than sole soybean,which could also prove that the advantage of P uptake of relay strip intercropping soybean came from the reproductive growth period.Grain yields of soybean with 20 kg ha-1 fertilizer P supply were higher than zero P supply treatment,especially in the intercropping system.The grain yield of sole soybean was significantly higher than intercropping soybean in the zero P supply treatments（P0）,and there was no significant difference between intercropping and sole soybean in the conventional P supply treatments（P20）across all three years.The shade during the vegetative growth period inhibited the biomass of intercropping soybean.The increased light interception during the reproductive growth period significantly increased the photosynthetic capacity of leaves,which promoted the transport of photosynthate to the growth center（pod）,and finally increased its harvest index.However,the self-shade during the reproductive growth period decreased the photosynthetic capacity of the middle and lower leaves,affected the formation of pods,and finally reduced the harvest index of sole soybean.2.The light interception and leaf photosynthetic capacity were significantly enhanced in the in the relay strip intercropping system during the reproductive growth period.The single-leaf light interception,leaf number,and total light interception of soybean in intercropping were lower than sole until the maize harvested.After maize harvest,single-leaf light interception of intercropped soybean was sharply increased.And after about three weeks,the whole plant light interception of intercropping soybean was significantly higher than that of sole soybean,meanwhile,the leaf number of intercropping soybean was higher than sole soybean at the stage of R1.No matter at V5 or R3 stage,the leaf area of sole soybean was significantly higher than that of intercropping.However,53-61%of the leaves of sole soybean were in weak light environment,and 24-36%in the intercropping soybean at the stage of R3.There was no significant difference of the youngest leaves between intercropping and sole soybean at the stage of R3.However,the whole leaf photosynthetic capacity of intercropping soybean was significantly higher than that of sole soybean.3.Light-mediated sucrose and low phosphorus signals promoted root growth of intercropping soybean during reproductive growth.The sucrose content of soybean leaves and roots in sole was significantly higher than that in intercropping at the stage of V5.Phosphorus application had no significant effect on the sucrose content in soybean leaves,but the P0 treatment significantly increased the sucrose content in soybean roots.The results of the pot experiment were the same as that of field experiment.And the root biomass,total root length and surface of L-S were significantly higher than that of S-L.At the stage of R3,the sucrose content in leaves and roots of intercropping soybean was significantly higher than that of sole soybean,but the leaf phosphorus content was significantly lower than that of sole soybean.Meanwhile,the expression of low-P response related gene Gm PHR25 in S-L treatment soybean leaves was significantly higher that of L-S.The expression of Gm EXPB2,a gene regulating root growth downstream of Gm PHR25,was also up-regulated in the S-L treatment soybean.The primary root length and surface of L-S were still higher than that of S-L,however,the lateral and total root length and surface of S-L were significant higher than that of L-S.And P0 treatment significantly increased the total root length and surface.4.Light significantly increased the content of auxin and the expression of related genes in soybean roots.At the stage of V5,the IAA concentration of soybean root in L-S was higher than that of S-L,especially in the P0 treatment.In the contrast,the IAA concentration of soybean root in S-L was higher than that of L-S at the stage of R3.Meanwhile,Late light significantly up-regulated the expression of Gm YUCCA14 and Gm TIR1C,while in the P0treatment,increasing light could up-regulate the expression of Gm ARF05.In conclusion,the improved light environment in the relay strip intercropping promoted the synthesis of photosynthates,increased the sucrose content in roots,which provided the C-substrate for the growth of soybean lateral roots during reproductive growth period.More light capture decreased the leaf P content in the intercropping soybean.And the low-P response related gene Gm PHR25 and Gm EXPB2 were up-regulated,which induced the adaptive changes of root morphology to"low phosphorus".Meanwhile,"growth-induced"P-deficiency of intercropping soybean also significantly increased the auxin content of soybean root tip and the expression of auxin response gene during the reproductive period,which promoted the growth of lateral roots and P uptake.The HI of soybean was significantly increased in the relay strip intercropping.P supply significantly increased the grain yield of soybean,especially in the relay strip intercropping system. |
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