Effects of excessive nitrogen fertilizer and soil moisture deficiency on antioxidant enzyme system and osmotic adjustment in tomato seedlings

Long-term excessive application of nitrogen fertilizer induces secondary salinization of soil,which results in inhibiting plant growth.In addition,soil moisture deficiency also affects plant growth.To investigate the effects of excessive nitrogen fertilizer and soil moisture deficiency on the antioxidant enzyme system,plant water relations analyzed through pressure-volume(P-V)curve,and photosynthetic light response parameters in tomato(Solanum lycopersicum L.Myoko)seedlings,an indoor experiment of about 50 d was conducted using two irrigation water amounts based on field capacity(soil moisture deficiency:50%-80%;adequate water:70%-80%),two nitrogen fertilizer rates(moderate nitrogen;excessive nitrogen fertilizer:0.585 g/pot)and two types of irrigation water(tap water and microbial diluent).The results showed that excessive nitrogen fertilizer(N)and soil moisture deficiency(W)reduced the biomass of tomato seedlings.In comparison to CK(combination of adequate water and tap water quality),microbial dilution(EM)increased plant biomass by 5.2%.Also,the nitrogen application increased chlorophyll relative contents(SPAD).The maximum net photosynthetic rate(Pc)decreased with nitrogen application and increased with EM application and irrigation amount.Excessive nitrogen application increased the plant nitrate reductase activity(NR).The plant NR in the N treatment showed a 13.0%increase compared to CK,and the plant NR in the treatment of nitrogen application with water deficiency(WN)increased 34.0%compared to water deficiency(W).After applying excessive nitrogen,N,EM-N,WN,EM-WN respectively increased the plant nitrate reductase activity by 13.0%,22.9%,34.0%,and 28.6%,compared with the corresponding treatment with moderate nitrogen(i.e.,CK,EM,W and EM-W).In addition,the activities of antioxidant enzymes[superoxide dismutase(SOD),peroxidase(POD)and catalase(CAT)]in four treatments of nitrogen application(N,EM-N,WN,EM-WN)also increased significantly.Both soil moisture and nitrogen fertilizer significantly affect the parameters of osmotic adjustment,which is manifested in the reduction of osmotic potential(π_(FT)),and the increase in the osmotic concentration(C_(osm))and concentration difference(ΔC_(osm)).But the decrease in the relative water content of apoplast(ζ_(ap))indicated that water deficiency and excessive nitrogen reduced the water absorption and water retention capacity of tomatoes to a certain extent.In conclusion,excessive nitrogen application and soil moisture deficiency inhibit plant growth significantly in this experiment.Meanwhile,microbial dilution can alleviate excessive nitrogen fertilizer and water stress to some extent,but the effect was not significant.

Osmotic adjustment and up-regulation expression of stress-responsive genes in tomato induced by soil salinity resulted from nitrate fertilization

Concerns about the soil salinity caused by excessive fertilization have prompted scientists to clarify the detailed mechanisms and find techniques to alleviate the damage caused by this kind of soil salinity.Aims of this study were to elucidate the effect of soil salinity caused by nitrate fertilization and the differences in salinity effect between nitrate salts and NaCl salt with analyses at various levels of crop physiology and molecular biology.A microbial inoculation was also tried to verify whether it could alleviate the salinity-induced loss and damages.In three experiments(Exp I,II and III),nitrate salts(NS)of Ca(NO_(3))_(2) and KNO_(3) were applied to potted tomato plants to simulate the soil salinity caused by fertilization and a microbial inoculant(MI)was applied.Photosynthesis was measured using Li-6400.Osmotic adjustment was analyzed using the mathematically modeled pressure-volume curve;O_(2)-concentration and activity of SOD and nitrate reductase were measured.Expression of nitrate reductase gene and the stress-responsive gene DREB2 was analyzed using the real-time PCR method.In Exp I and II,where the applied NS amount was moderate,NS application at low concentration induced increases in O_(2)-and MDA concentrations and plants acclimated to the soil salinity as the treatment prolonged for weeks.The acclimation was contributed by osmotic adjustment,activation of SOD and re-compartmentation of cell water between symplasm and apoplasm.These adjustments might be ultimately attributed to up-regulation of stress-responsive genes such as DREB2 as well as the nitrate reductase(NR)gene.However,in Exp III,applications of NaCl and NS at high concentration could not show positive effects as NS did.Application of MI synergistically increased the xerophytophysiological regulation caused by NS and alleviated the salinity damage in addition to its own positive effects on the tomato plants.Different from NaCl,nitrate salts at low application rate increased the total biomass and fruit yield of tomato and induced up-regulation expression of stress-responsive genes and the consequent active osmotic adjustment.However,nitrate application at high level negatively affected tomato plants irrespective of the gene up-regulations.Application of MI alleviated the salinity damage and synergistically increased the xerophytophysiological regulation caused by the soil salinity in addition to its positive effects on the tomato crop but the detailed mechanisms needed to be clarified in future further studies.