Effect Of Shading On Photosynthetic Physiology And Agronomic Characters Of Soybean

Shade avoidance was one of strategies that plants have evolved in response to shading.For shade avoidance,plants often increased stem and hypocotyl elongation rates,decreased stem diameter and stem breaking strength,which were easier to cause lodging,and even more affected the yield and quality of soybean.Different genotype soybean cultivars had different response to shade,and the shade-tolerant cultivars were more resistant to lodging and could gain higher yields.In order to clarify the response mechanism of soybean to shading,this paper started with the effect of shading on soybean morphogenesis,photosynthetic characteristics,physiological and biochemical reactions,accumulation and distribution of dry matter,and nitrogen metabolism,and studied（1）Physiological adaptability of soybean to light environment of intercropping,（2）Causes of soybean lodging in intercropping,（3）Causes of decreased photosynthetic capacity of shading soybean leaves,（4）Effects of shading on nitrogen metabolism and dry matter accumulation of soybeans.And it was expected to provide a theoretical basis for exploring the response mechanisms of soybeans to shading stress and the breeding of shade-tolerant soybean cultivars.1.The light saturation point of the soybean leaves in intercropping was reduced.In the face of strong light stress at noon,the leaf excitation energy was excess.The soybean leaves reduced the proportion of light energy absorbed by the photosystem II for photochemical processes,limiting the electron transfer of the electron transfer chain,increasing non-photochemical quenching,in time to dissipate excess excitation energy.At the same time,increas the antioxidant enzyme activity of leaves in time to remove reactive oxygen,to prevent the occurrence of light damage.2.The lodging-related indexes of soybean with different lodging resistance were analyzed to discuss the lodging resistance of shade-tolerant soybeans in intercropping.The intercropping pattern significantly changed the mechanical strength and morphology of soybean stem in intercropping,reduced the lodging resistance of soybean and increased the risk of lodging.The lodging-tolerant cultivars had lower plant height,the height of center of gravity and average internode length,which were advantageous for reducing the lodging caused by the elongation of stem.Meanwhile,lodging-resistant cultivars also had higher vascular bundles,xylem ratios,cellulose and lignin content,which could increase stem mechanical strength and reduce the risk of stem lodging.3.The decrease of LCP（light compensation point）,LSP（light saturation point）and Amax（ligh-saturated net photosynthetic rate）in intercropping indicated that the photosynthetic capacity was limited.The increase of AQE（apparent quantum efficiency）,chl b content,PI_ABS andφ_Po indicated that the ability of light-intercepting and the activity of PSII get promoted in light-limited environment conditions,and this was beneficial for higher light utilization efficiency in intercropping.However,The leaf and palisade tissue thickness of soybean leaf in intercropping became thinner,which resulted in the reduction of chloroplast,while the electron transport in the electron transport chain was limited,resulting in a decrease in photosynthetic capacity of soybean leaves.Pot and shade treatment experiments were conducted in order to further clarify the difference in photosynthetic ability of different shade-tolerant soybean cultivars.The results showed that the chlorophyll content,PI_ABS,A_max,R_d（dark respiration rate）and AQE of shade-tolerant cultivar Liaodou 32 were higher than that of Liaodou 29,indicating that the light energy interception capability,PSII activity and photosynthetic utilization efficiency of Liaodou 32 were higher than Liaodou29.4.Under shade condition,soybean showed shade avoidance,with a higher plant height,thinner stem stems,lower dry matter accumulation,reduced seeds and pods number of per plant,which led to a significant decrease in yield.Under 50%shade conditions,there was significant difference in tolerance among different genotypes,and shade-tolerant cultivars could limit stem elongation and maintain lower plant height,which was beneficial to reduce the risk of lodging;meanwhile,higher leaf area helped to intercept more light energy and obtained higher yield.However,in the 75%shade condition the shade stress was severe and the light energy obtained from the leaves could no longer meet the growth requirement of soybean.The growth and development of the two cultivars were severely restricted,and lodging was serious.5.Shade could significantly reduce nitrogen accumulation in soybean plants,but it could significantly increase leaf nitrogen content.Shade would reduce the NR and GS activities of soybean leaves,and the decrease of nitrogen reduction and ammonia assimilation would lead to the accumulation of NO₃^-content and the decrease of soluble protein content,which was not conducive to the accumulation of nitrogen,resulting in the inhibition of carbon and nitrogen metabolism and the decrease of photosynthetic activity.Under 50%shade conditions,the key enzymes for nitrogen metabolism of Liaodou 32 were affected less,and could maintain high levels of NO₃^-and soluble protein,and ultimately could obtain more nitrogen accumulation.