Study On Physiological Mechanism Of Soybean Growth Under Shade Stress By Regulating The Carbon To Nitrogen Ratio

Cultivated soybean is a major agricultural crop that provides a crucial source of edible protein and oil.Light is a necessary condition for plant growth.In the maize-soybean relaystrip intercropping system,shade from maize severely affects the soybean photosynthetic carbon assimilation,growth and development.Bragg and two of its induced nodulation mutants(NOD139,non-nodulating and NTSl007,extreme supernodulator)and soybean Nandou12 were compared.In the maize-soybean relay-strip intercropping system,different cultivar’s character of agriculture and photosynthesis,metabolism of carbon and nitrogen,yield were studied,the principal results were shown as follows:(1)Under shade conditions,the shoot and root biomass of four varieties of soybeans were decreased significantly,and the shoot biomass of shade-tolerant cultivar of soybean Nandou-12 was minimum decreased.The volume and number of root tips of soybean Nandou-12 increased significantly under sucrose treatments,whereas the total root length and surface area decreased significantly.The total root length and number of root tips of Bragg increased significantly,but the effect on the root system of mutant NOD139 and mutant NTS1007 was nonsignificant.Furthermore,under sucrose treatments,the stem diameter of four varieties of soybeans were decreased significantly and the height of soybean Nandou-12 increased significantly,whereas Bragg decreased significantly.(2)Under shade conditions,the net photosynthetic rate of four varieties of soybeans was decreased significantly compared to normal light.The net photosynthetic rate of Bragg increased significantly under nitrogen treatments.The stomatal conductance and transpiration rate of soybean Nandou-12 and Bragg increased significantly under sucrose treatments.(3)Under shaded conditions,the C content in shoot and root of Bragg and mutant decreased significantly and the N content increased significantly at the same time,which caused the C/N decreased significantly.However,the C and N content in shoot of soybean Nandou-12 were nonsignificant compared to normal light.The C and N content in root of soybean Nandou-12 followed a similar pattern with mutant NOD139 and Bragg which showed good light contracting ability and growth coordination ability.The C and N content and C/N of mutant NOD139 in root were decreased significantly,and the changes in root followed a similar pattern with mutant NOD139 and Bragg.(4)Under shaded conditions,the carbon assimilation of soybean is adversely affected,But the accumulation of total nitrogen was higher than that under normal light.The nitrogen content of plants increased under nitrogen treatments,which exacerbated the imbalance of the carbon-to-nitrogen ratio of soybeans.Although the response of different varieties of soybeans showed different trends under nitrogen or sucrose treatments,in general there is no significant effect on the C content of soybean’s root under sucrose or nitrogen treatments after two weeks.The C content of Bragg,Nandou 12 and mutant NOD139 in shoot increased significantly under sucrose treatments after four weeks,and the C content of Nandou 12 and Bragg in root decreased significantly.The N content in shoot and root of different varieties of soybeans increased significantly under nitrogen treatments.The N content in root of mutant NOD139,Bragg and NTS1007 increased significantly under sucrose treatments,also the C/N of mutant NOD139,Nandou 12 and Bragg in shoot.The mutant NTS1007 requires a large amount of nitrogen during the vegetative growth period,so the C content and C/N of mutant NTS1007 both increased significantly under nitrogen treatments.(5)Compared with normal light,the yield of four varieties of soybeans in the shade environment is decreased significantly under sucrose treatments.The yield per plant and the theoretical yield of mutant NOD139 increased significantly.However,the yield per plant and the theoretical yield of mutant NOD139 were nonsignificant under nitrogen or both sucrose and nitrogen treatments.The yield per plant and the theoretical yield of Nandou 12 increased significantly,while those were decreased significantly under nitrogen treatments and were nonsignificant under both sucrose and nitrogen treatments.The yield per plant and the theoretical yield of Bragg increased significantly under sucrose or both sucrose and nitrogen treatments while those were nonsignificant under nitrogen treatments.The yield per plant and the theoretical yield of mutant NTS1007 increased significantly under nitrogen or both sucrose and nitrogen treatments while those were nonsignificant under sucrose treatments.Among them,the yield per plant and the theoretical yield of Bragg increased the most,56.5% and 55.5% respectively.Generally,application of sucrose can increase the production of mutant NOD139,Nandou 12 and Bragg,and application of nitrogen can increase the production of mutant NTS1007.