The Responses Of Germination And Growth Of Castor Bean (Ricinus Communis L.) To Saline Stress

Soil salinization widely occurred in many areas of China.The area of saline soil in Jiangsu Province covera an area of more than 687000 hm2,which was the biggest area in China and also the main farming land resource for agricultural development.Saline stress is becoming one of the critical abiotic stresses affecting utilization of saline soil,crop growth and production.Castor bean(Ricinus Communis L.),as one of desirable energy plants with certain salinity resistance,has better saline resistance capacity than conventional crops like rice and wheat,which could be planted in saline soils as a pioneer crop.However,when castor bean was planted in soils with high saline levels,it is sensitive to saline stress at germination and seedling stages,resulting into lower germination percentage and poor seedling establishment.In our study,the responses of germination and seedling growth of castor bean to saline stress were studied through measuring the effects of different saline levels on seed water absorption and germination,seedling growth and physiological properties and root morphological establishment.In the study,we treated two castor bean cultivars,Zibi and Zibi 9 with 5 NaCl levels(0,25,50,100 and 150 mM NaCl).The results were the following.1.The effects of saline stress on seed germination of castor beanWe studied the response of seed water absorption and germination features for two castor bean varieties(Zibi 5 and Zibi 9)treated with different NaCl levels.Saline stress inhibited water absorption percentage and water uptake rate of seed.With increased saline level,water uptake percentage and water uptake rate were significantly decreased.Water absorption mainly happened between 8 to 16 h after the initiation of water absorption,was maintained at a relative low level and was increased again since 48 h after the initiation of water absorption.Therefore,the dynamic pattern of water uptake percentage and water uptake rate can be expressed with a fast-slow-fast model.Saline stress retarded seed germination,decreased germination percentage and inhibited radicle growth.However,the effects of salinity on seed germination and radicle growth were different.At the lower saline level(25 mM NaCl),there was no significant difference in seed germination in comparison with the control,but radicle length was decreased significantly.Germination rate,germination percentage and radicle length were higher for Zibi 5 than for Zibi 9 in saline treatments,but the relative salinity injury rate was lower for Zibi 5.2.The effects of saline stress on seedling growth parameters of castor beanWith Zibi 5 and Zibi 9 as the experimental varieties,a sand-filled pot study was done to investigate the effects of saline levels on plant height,stem diameter,leaf area and aboveground biomass.Our results showed that the effects of saline stress on plant height,stem diameter,leaf area and dry biomass were different at different saline levels.In the lower saline treatment(25 mM NaCl),there was no significant difference in plant height,leaf area,fresh and dry weight in comparison with the control.With increased saline stress,the negative impacts of saline stress on plant growth were gradually increased.At the 100 and 150 mM NaCl levels,plant height,leaf area,fresh and dry matter were decreased significantly as compared with the control or the lower NaCl treatment.At the lower saline levels,both genotypes Zibi 5 and Zibi 9 grew better,while the reductions of Zibi 9 in plant height,leaf area and the weight of above fresh and dry matter were more significant than Zibi 5 at higher saline levels.3.The effects of saline stress on physiological properties of castor beanWith Zibi 5 and Zibi 9 as the experimental varieties,a sand-filled pot study was done to investigate the effects of saline levels and stress duration on chlorophyll content,photosynthetic production and antioxidant system.Our results showed that with the increase of saline level,the contents of chlorophyll a,chlorophyll b and chlorophyll(a+b)as well as photosynthetic performance(photosynthetic rate,stomatal conductance and transpiration rate)were decreased in the two examined cultivars.With increased saline level,the activity of CAT and POD and the content of soluble protein and malondialdehyde(MDA)were increased.With the prolongation of saline treatment,chlorophyll content was increased in control and lower saline treatment,while chlorophyll content was reduced under high saline treatment.As the prolongation of saline treatment,the activity of CAT and POD as well as soluble protein content were decreased.The reduction in chlorophyll and photosynthetic performance were not significant for Zibi 5 and Zibi 9 in the control and the lower saline treatments,but with the prolongation of saline treatment and increased saline level,more negative effects on chlorophyll and photosynthetic production were observed in Zibi 9 than Zibi 5.4.The effects of saline stress on root growth and development of castor beanWith Zibi 5 and Zibi 9 as the experimental varieties,a sand-filled pot study was done to investigate the effects of different saline levels on root morphological development(root volume,length and surface area),root biomass and root activity.Our study showed that saline stress affected plant root development and root biomass accumulation.In the 25 mM NaCl treatment,there was no significant difference in root biomass weight.With increased saline level,root volume,length and surface area and root activity were significantly decreased.In the 50 and 100 mM NaCl treatments,the biomass of root was decreased significantly.Compared Zibi 5 with Zibi 9,there were no significant difference in root morphological development in the control and the lower saline treatment,and the negative effects on root biomass and root morphological establishment for Zibi 9 were significant than those for Zibi 5.In conclusion,the effects of saline stress on growth and physiological properties depended on saline levels and cultivars.At the lower saline level(25 mM NaCl),the reductions in germination percentage and water absorption percentage,plant height,stem diameter and biomass accumulation,the content of chlorophyll and photosynthetic production as well as root development and establishment were not significant.However,the adverse effects of saline stress on germination,plant growth and physiological properties were significant with the increase in saline level.With the increase in saline level and the prolongation of saline treatment,the negative effects of seed germination,plant growth and physiological properties as well as root establishment for Zibi 5 were more significant than those for Zibi 9,indicating that Zibi 5 is more salt tolerant as compared with Zibi 9.