Interaction Effects Of Soil Moisture And Nitrogen Rates On Growth And Photosynthesis In Seedling Stage Of Alfalfa

The effects of soil moisture and nitrogen levels on growth of alfalfa(Medicago sativa L.)were codetermined by the single factor effect and interaction of two.Numerous studies have reported the single factor effect,whereas little information is available on interaction effects of soil moisture and nitrogen.A semi-controlled pot experiment was carried out to investigate the effects of soil moisture and nitrogen levels on growth and photosynthesis of alfalfa and evaluate the interaction effects.The experiment used two cultivars(Sadie 7 and Icon)and consisted of three different soil moisture levels(50%,70%and 90%of maximum water holding capacity)and four different nitrogen rates(0,90 kg/ha,180 kg/ha,270 kg/ha).The major results showed as follows.1.The dry matter accumulation and nitrogen use efficiency were increased with the increase of soil moisture contents.Under the same soil moisture level,the dry matter accumulation and nitrogen use efficiency increased with the improvement of N rate,and decreased by more nitrogen application.There were significant interaction effects on soil moisture and nitrogen levels.Results of regression analysis showed that the nitrogen fertilizer effects were significant increased with the increase of soil moisture contents.With the increase of soil moisture contents,a moderate amount of nitrogen application was also increased.Therefore,when the soil moisture contents were lower,irrigation and nitrogen application should be carried out simultaneously.In the area with the higher soil moisture contents,more nitrogen application could increase the yield and nitrogen use efficiency.2.Sufficient soil moisture and nitrogen application improved Pn.The effects of nitrogen fertilizer on Pn varied depending on soil moisture contents.Under the lower soil moisture level,appropriate nitrogen application could increase leaf nitrogen contents and photosynthesis pigments,and the highest values of net photosynthetic rate(Pn),transpiration rate(Tr),stomatal conductance(Gs),intercellular concentration of CO2(Ci),apparent quantum yield(AQY),carboxylation efficiency(CE),light saturation point(LSP)and photosynthetic efficiency of photosystem Ⅱ(ΦPS Ⅱ)occurred in N1 treatment.While with the higher soil moisture contents,more nitrogen application significantly improved leaf nitrogen contents and Pn.The highest values of Pn,Tr,Gs,Ci,AQY,CE,LSP andΦPS Ⅱ occurred in N2 treatment.Photo respiratory rate(Rp)and dark respiration rate(Rd)were improved with the increase of soil moisture and nitrogen application rate.It concluded that reasonable nitrogen application under the higher soil moisture content could increase the opening degree of stoma and accelerate CO2 into the leaf tissues on the one hand,on the other hand,it could improve light use efficiency,promote photosynthetic electron transport and photosynthetic phosphorylation,as well as improve CO2 use efficiency,which could help increase photosynthesis and dry matter accumulation of the seedlings.In summary,when the soil moisture contents were higher,more nitrogen application could improve photosynthetic intensity.Then it promoted dry matter accumulation and growth of alfalfa.