Physiological Responses Of Soybean Seedlings To The Interactive Effect Of Shade And Drought Stresses

Intercropping results in suppression of weeds,control of diseases and pests,improve soil fertility and nitrogen acquisition in legume or non-legume intercropping systems that are being used globally especially in Southwest of China.However in this system maize plants change the microclimate of soybean and expose them to numerous abiotic stresses simultaneously,including limited light energy（shade）and water availability（drought）that affect the relay strip intercropping systems.Shade and drought are considered the most drastic abiotic stress factors which restrict normal crop growth especially for those high-density and intercropped crops in arid and semi-arid areas.However,the relationship and mechanisms of the interactive effects of these two stress factors on crops are elusive.Henceforth,here we report our identification of the relationship between shade and drought and their interactive effects on soybean plants.In order to test this relationship,we performed two experiments.In the first experiment,12^th days old healthy soybean seedlings were selected that were uniform in growth and were treated with six treatments of shade（0,2 h,4h,6h,8 h and 12 h）.Our quantitative-real time PCR（q-PCR）results described that shade significantly up-regulated abscisic acid（ABA）biosynthesis（Gm NCED3 and Gm AAO3）and signaling（Gm ABI4 and Gm ABI5）genes.Moreover,shade massively enhanced the ABA contents.Based on these results,we subjected soybean seedlings to the pretreatment of shade（Pre-SH）and normal light（Pre-NL）followed by polyethylene glycol 6000 induced drought stress.The drought resulted in the reduction of relative water content,morphophysiological and biochemical parameters under both light regimes.However,plants grown under Pre-SH treatment protected their cells from damaging effects of drought by scavenging more reactive oxygen species（ROS）through the enhancement of its defense system as compared to Pre-NL plants.Conclusively,our findings demonstrated that plants grown under Pre-NL conditions were more affected with drought stress than those of Pre-SH ones which suggested that pretreatment of shade can enhance the drought resistance of soybean seedlings.In the second experiment,the interactive effect between shade and drought stresses on soybean seedlings was analyzed.The 12^th days old soybean seedlings were subjected to drought stress induced by polyethylene glycol 6000（PEG-6000）under shading and non-shading conditions.Drought significantly reduces the relative water contents,carbohydrates and chlorophyll contents under both light variables.A significant increase was observed in osmoprotectants,reactive oxygen species and antioxidant enzymes in soybean seedlings.Moreover,the shaded plants accumulated more sugars and proline than non-shaded ones under drought stress.Henceforth,our results have revealed that non-shaded grown plants were more sensitive to PEG-induced drought stress than those exposed to shading conditions,which proposed that shade can boost up the drought confrontation in soybean seedlings.