Ecological Effects Of Rice-Water Spinach Intercropping System And Its Mechanism Of Promoting Silicon Absorption Of Rice

Intercropping is a sound agricultural practice for crop production,pest control and nutrient utilization,but aquatic crops(e.g.rice)intercropping is relatively rare.In many areas of China,there is a shortage of silicon in paddy fields.Silicon is an essential element for rice and silicon deficienty will have a serious impact on rice production.So far there is limited study about silicon nutrition of rice under intercropping conditions.In the present study,rice and water spinach were chosen to explore the ecological effects of intercropping system and its intrinsic mechanism of promoting silicon absorption of rice.First of all,two traditional rice varieties(Huang-hua-zhan and Gui-nong-zhan)were respectively intercropped with water spinach(line ratio 4:2)in a field trial for consecutive two years(2012,2013)/four seasons,to study the effects of intercropping on system productivity,nutrient absorption,and pest and disease control.Based on the trials of 2012,2013,only one rice varieties(Huang-hua-zhan)was intercropped with water spinach at three intercropping ratios(2:2;3:2;4:2)in a field trial for consecutive two years(2014,2015)/four seasons,focusing on the interspecies relationships in rice and water spinach intercropping system and its effects on rice silicon and nitrogen nutrition and rice pests and diseases control.Finally,the intrinsic mechanism about intercropping to promote silicon absorption of rice was in-depth studied by setting up a series of control experiments.The main results are as follows:(1)Through four years/ eight seasons of field trials,it was found that rice and water spinach intercropping had no significant effect on plant height of rice,but significantly increased tiller number of rice plant,and the number of tiller was gradually increased as the proportion of rice rows decreased.Rice and water spinach intercropping significantly increased rice yield per unit area,but reduced water spinach yield.The land equivalent ratio(LER)of rice and water spinach intercropping in all seasons were greater than 1,indicating that rice and water spinach intercropping can increase comprehensive productivity of system.Based on LER,3:2 intercropping(LER is up to 1.06~1.20)was the best intercropping pattern among three intercropping ratios(2:2;3:2;4:2).By testing rice yield composition,we found rice yield was increased under intercropping conditions mainly by increasing effective spike number,grain number per spike and seed setting rate.In addition,in rice and water spinach intercropping system,rice had stronger competitiveness than water spinach,especially in early seasons.(2)Field investigation showed that,rice and water spinach intercropping could significantly reduce the incidence rate of rice sheath blight and leaf folders,with a reduction of rice sheath blight by 33.2~50.6% in early seasons and 17.3~34.9% in late seasons,while rice leaf borer by 33.6~43.0% in early seasons and 52.8~58.2% in late seasons.These results show that this intercropping system has practical guidance significance for chemical reduction and environmentally friendly agriculture.(3)In field trial,rice and water spinach intercropping significantly increased the absorption of silicon and nitrogen of rice plant;meanwhile,intercropping significantly increased the silicon content of rice leaves at ripening stage,but didn’t increase nitrogen content and even slightly reduced it at ripening stage,resulting in an improvement of nutritional composition of rice especially in late seasons.The effects of rice and water spinach intercropping on soil available nutrients were mainly related to available silicon,ammonium nitrogen,and available potassium,but the difference was not significant,and the intercropping system didn’t affect total nutrient content in soil.(4)Rice and water spinach intercropping can significantly increase the silicon content of rice plant,due to the root interaction under intercropping conditions,which can effectively promote rice absorption of silicon,and this effect will be decreased with the increae of distance from water spinach.More interesting,our results indicated that rice-water spinach intercropping induces the up-regulated expression of silicon transporter gene in rice roots(OsLsi1,OsLsi2)and stems(OsLsi6),and intercropping increases silicon availability in soil.Compared with rice monocropping,rice and water spinach intercropping greatly increased the soil water-soluble silicon(up to 168.1%),which can be directly absorbed by rice.(5)Regarding the effectiveness of soil silicon,our studies found that rice and water spinach intercropping reduced soil pH(by 0.17 units in pot experiment),which sitimilate the transformation of soil active silicon into water-soluble silicon.Compared with rice,the root exudate of water spinach is beneficial to increase soil water-soluble silicon,and pH environment is one of the reasons.However,the reasons doesn’t include organic acids,because organic acids content in rice roots exudate is much higher than that in water spinach roots exudate.(6)Organic acids have a significant effect on the forms change of silicon in soil.First,organic acids can extract water-soluble silicon from soil solid phase to aqueous solution,which is more convenient for crop absorption.In the soil with high available silicon(calcareous soil),except oxalic acid,other organic acids can dissolve soil amorphous silicon and soil active silicon in the system so that increaseing water-soluble silicon content.In the soil with low available silicon(acidic paddy soil),most organic acids can dissolve soil amorphous silicon in the system to increase soil active silicon and water-soluble silicon content.In addition,regardless of calcareous soil or acidic paddy soil,citric acid has important effect on increasing water-soluble silicon content.In summary,rice and water spinach intercropping is a feasible ecological cultivation mode for paddy fields,which can reduce application of pesticides and chemical fertilizers while increasing crop yields.More importantly,rice and water spinach intercropping can promote silicon absorption of rice,which will play an important role for rice production.