Effect Of Nitrogen Fertilization On The Growth And Physiological And Biochemical Characteristics Of Hemp （Cannabis Sativa L.） Under Salt Stress

To alleviate the land use competition between grain and economic crops,it is proposed by China Agricultural Research System for Bast and Leaf Fiber Crops to grow fiber crops using marginal land（saline-alkali land,hillside land and winter fallow land）.Hemp（Cannabis sativa L.）is a bast fiber with great potential value.Using salt-affected land to grow hemp could avoid land use competition with grain crops,thereby promoting a sustainable development of hemp industry.On salt-affected soil the negative impacts of salts as well as improper management of nitrogen fertilization could limit plant growth.It could support the nitrogen fertilization of hemp on salt-affected soil by understanding the effect of nitrogen application on the growth and physiological and biochemical characteristics of hemp under salt stress.Therefore,in the present thesis the widely cultivated hemp cultivar Yunma 7 was studied in the experimental greenhouse of Yunnan University in two trials from 2019 to 2020.The growth and physiological and biochemical characteristics of hemp plant were studied in response to different nitrogen levels（0～3 g（urea）/kg（peat））under 5 levels of salt stress（0,6,12,18,24 g（Na Cl）/kg（peat））.The main results presented as follows:（1）When the applied salt exceeds 18 g/kg,the emergence of hemp seedlings was postponed,the seedling emergence rate,plant height,stem thickness and biomass were decreased significantly.Under this condition,urea application more than 1 g/kg before sowing could reduce the rate of seedling emergence and cause seedling damage.（2）With a low urea application（0～1 g/kg）,the relative electrical conductivity,soluble protein content,and malondialdehyde（MDA）content of leaves showed an upward trend with an increase in salt level,while the chlorophyll content decreased.With a high urea application（2～3 g/kg）,the relative electrical conductivity of leaves showed an upward trend with an increase in salt level,while the soluble protein content and MDA content did not change significantly.Compared with low urea application,high urea application resulted in a significant increase in soluble protein content and chlorophyll content under all the tested salt levels.（3）With an increase in the salt level,the Na⁺content in roots and stems increased continuously,whereas the change of K⁺content was not significant;the K⁺content in leaves showed an increase trend,but the Na⁺content did not change significantly.The content of Na⁺in roots,stems and leaves were negatively correlated with the dry weight of roots,stems,and leaves,respectively;the content of K⁺in roots was positively correlated with the dry weight of roots,and the content of K⁺in stems and leaves were negatively correlated with the dry weight of stems and leaves,respectively.There was a negative correlation between Na⁺content and K⁺content in roots,and a positive correlation between Na⁺content and K⁺content in stems and leaves.Nitrogen application had a significant effect on the Na⁺content and K⁺content in the roots,stems and leaves of the plant under salt stress,but it had no significant effect on the relationship between the Na⁺content and K⁺content.Salinity affected the uptake of nitrogen nutrition of hemp plant,but the effect varied among different nitrogen levels.Compared with low nitrogen treatment（0～0.5 g/kg urea application）,high nitrogen treatment（2～3 g/kg urea application）resulted in an increase of nitrogen uptake by 7%to 460%.（4）Under low nitrogen treatment（0 g/kg urea application）,the leaf net photosynthetic rate（Pn）decreased with an increase in the salt level,while the intercellular CO₂ concentration（Ci）,stomatal conductance（Cond）and transpiration rate（Tr）increased at a low salt level and decreased at a high salt level;under high nitrogen treatment（3 g/kg urea application）,the Pn,Ci,Cond and Tr did not change significantly with an increase in the salt level.Nitrogen application resulted in a significant increase in Pn,Ci,Cond and Tr under all the tested salt levels.In conclusion,a high salt content（the applied salt exceeds 18 g/kg）in the soil could significantly inhibit the growth of hemp plant,and change its physiological and biochemical characteristics.Appropriate nitrogen application（2～3 g/kg urea application）after seedling emergence could increase the soluble protein content and chlorophyll content in the leaves,increase the uptake of nitrogen,enhance the photosynthetic capacity,and thereby alleviate the adverse effects caused by excessive salinity.