Mechanisms And Effects Of NH₄～+-N/NO₃(?) Ratio In Nutrient Solution On Growth And Quality Of Spinach (Spinacia Oleracea L.)

Spinach (Spinacia oleracea L.) is an important cool-season leaf vegetables, which iswidely cultivated from south to north in our country. More and more attention is being paidto spinach since it is high in vitamin C, vitamin E and micronutrients, such as iron (Fe) andzinc (Zn). However, it is prone to accumulate large amounts of nitrate and total oxalatecontent in the shoot, and both of which areharmful to human being.Previous reports showed that not only yield of vegetables can be significantlyincreased, but also nitrate and oxalate content of vegetables can be markedly decreased byproperly increasing ammonium concentration in the nutrient solution. Therefore, the qualityof vegetables can be improved. However, at present, the mechanisms of the effect ofdifferent nitrogen forms and NH₄⁺-N/NO₃ -N ratios on leaf vegetable growth and foodquality are little known.The dissertation is aimed to the related mechanisms from the view point of plantnutrition and plant physiology and biochemistry. Hydroponic experiments were conductedin the greenhouse, Anhui Science and Technology University, P. R. China. Measurementsincluded nitrate and ammonium uptake; Vc, soluble sugar and crude protein contents;nitrate and soluble oxalate contents; phosphorus and potassium uptake and accumulation;chlorophyll contents, stomatal conductance, Rubisco activity and net photosynthetic rate;organic acids and amino acids content; NR, GS and GDH activity, et al Moreover, anothersolution culture experiment was conducted to study effects of NH₄⁺-N/NO₃ -N ratio andphosphorus level on growth, nitrogen uptake nitrate reductase activity (NRA), andglutamine synthetase activity (GSA) of spinach therein. Results obtained were as follows.1 Effects of the ratios of NH₄⁺-N/NO₃ -N on growth and biomass of spinach werenotable. Fresh weight of spinach shoots was gradually increased with the percentage ofNO₃ -N in the total nitrogen which concentration was 12 mmol L^-1 in nutrient solution, andspinach supplied with NO₃ -N as its sole nitrogen source was the highest in biomass, butthere was no significant difference (P=0.05) in dry matter weight between treatments, 25:75 and 0:100 in NH₄⁺-N/NO₃ -N ratio.2 Increasing ammonium in the nutrient solution could improve the quality of spinach.Content of nitrate and nitrite in spinach shoots was increased with the decrease in NH₄⁺-N/NO₃ -Nratio. Obviously, proper partial replacement of NO₃ -N by NH₄⁺-N was beneficialto decreasing content of nitrate and nitrite in spinach. Oxalate content was the highest atNH₄⁺-N as the sole nitrogen resource, while lowest at 25ï¼…replacement of NO₃ -N byNH₄⁺-N. When all NH₄⁺-N in solution was replaced by NO₃ -N, oxalate content increasedsignificantly than that in the 25ï¼…replacement. Nutritional quality of spinach alsoresponsed to NH₄⁺-N/NO₃ -N ratio. Supplied with nitrate as the sole nitrogen resource, Vccontent of spinach shoot was significantly lower than that of supplied with ammonium, orother treatments combining nitrate and ammonium, and the highest content was observed atthe NH₄⁺-N/NO₃ -N ratio of 50:50. Soluble sugar content of spinach shoots were graduallydecreased with the NH₄⁺-N/NO₃ -N ratio, and crude protein content was the highest intreatment 25:75 in NH₄⁺-N/NO₃ -N ratio.3 Effects of different NH₄⁺-N/NO₃ -N ratio on nitrogen metabolism, phosphorus,and potassium uptake of spinach were remarkable. Nitrate contents of spinach shoot androot was increased with the the decrease in NH₄⁺-N/NO₃ -N ratio, while ammoniumcontent of root decreased sharply, and there was no significant effect existed in ammoniumcontents of shoot when NH₄⁺-N/NO₃ -N ratio was changed. Effects of NH₄⁺-N/NO₃ -Nratio on nitrogen contents of spinach organs were slight. Nitrogen accumulation in spinachplant increased with the decrease of NH₄⁺-N/NO₃ -N ratio. Proper partial replacement ofNO₃ -N by NH₄⁺-N was beneficial to both of phosphorus and potassium uptake andaccumulation, which were the highest in treatment 25:75 in NH₄⁺-N/NO₃-N ratio.4 Chlorophyll a, chlorophyll b and the total chlorophyll content in spinach leafincreased linearly with the decrease of NH₄⁺-N/NO₃ -N ratios; while net photosyntheticrate of spinach leaf was gradually increased. Furthermore, net photosynthetic rate ofspinach supplied with nitrate was 6.18 time-fold that of supplied with ammonium. With thedecline of NH₄⁺-N/NO₃ -N ratios, stomatal conductance and initial Rubisco activity ofspinach leaf increased gradually, while there was no significant change in total Rubiscoactivity. The result with grey correlative analysis showed that effect degrees ofNH₄⁺-N/NO₃ -N ratios on net photosynthetic rate in spinach were in the following order:Rubisco activation state > the initial Rubisco activity > chlorophyll b content > totalchlorophyll content> chlorophyll a content > stomatal conductance > total Rubisco activity. 5 Organic acids in spinach shoot, such as pyravate, citrate,α-ketoglutarate,succinate, fumarate and malate increased with the decline of NH₄⁺-N/NO₃ -N ratios innutrient solution, and were positively correlated with nitrate content in nutrient solution.Inspinach roots, pyravate, citrate, succinate and malate content significantly increased withthe decreasing of NH₄⁺-N/NO₃ -N ratios, which didn't significantly affected fumaratecontent. With the decline of NH₄⁺-N/NO₃ -N ratios, ratios of five organic acids, i.e.,pyravate,α-ketoglutarate, succinate, fumarate and malate content, of root to that of shoot inspinach tended to decrease, while the ratio of citrate content gradually increased. Starchcontent in both of shoot and root in spinach tended to decrease with the decrease inNH₄⁺-N,/NO₃ -N ratio, and starch content in both of shoot and root was negativelycorrelated with the NO₃ -N content in nutrient solution.6 The total free amino acids content in spinach shoot decreased with decreasingNH₄⁺-N/NO₃ -N ratio, and the value at 0:100 was 34.4ï¼…of that at 100:0 of NH₄⁺-N/NO₃ -Nratio. Effect of NH₄⁺-N/NO₃ -N ratio on the total free amino acids content inspinach root was insignificant compared with shoot. When ammonium was supplied as solenitrogen source, the main composition of free amino acids in spinach shoot were glutamine(39.8ï¼…), arginine (20.2ï¼…) and glutamate (8.9ï¼…), while nitrate was supplied as solenitrogen source, the main amino acids were glutamate (30.3ï¼…), aspartate (18.6ï¼…) andserine (8.5ï¼…). Percentage of glutamine content to the total content of free amino acids inspinach root significantly decreased with the decrease of NH₄⁺-N/NO₃ -N ratio, whilepercentages of aspartate, glutamate and glycine significantly increased.7 With the increase of the NH₄⁺-N/NO₃ -N ratio, NR activity of both shoot and rootin spinach increased markedly, but there was no significant difference (P=0.05) betweentreatments, 25:75 and 0:100 in NH₄⁺-N/NO₃ -N ratio. GDH activity of both of shoot androot in spinach decreased significantly with the decrease of NH₄⁺-N/NO₃ -N ratio innutrient solution. With ammonium as the sole nitrogen resource, GDH activity of spinachroot was higher than that of shoot, while nitrate was supplied as the sole nitrogen resource,GDH activity of spinach shoot was higher than that of root.8 Phosphorus nutrition affected significantly uptake and assimilation of different nitrogen form in spinach. Biomass of spinach increased with decreasing NH₄⁺-N/NO₃ -Nratio in nutrient solutions, when phosphorus was supplied as the same level, while therewas no significantly difference between the treatments 25:75 and 0:100 in NH₄⁺-N/NO₃ -Nratio. Biomass of spinach increased with rising phosphorus level in the nutrient solutions when NH₄⁺-N/NO₃(?) ratio was supplied as the same. Nitrate content of spinach shootsincreased with decreasing in both of NH₄⁺-N/NO₃(?) ratio and phosphorus level. Nitrogencontent of spinach increased with increasing phosphorus level, but did not show significantdifference between treatments different in NH₄⁺-N/NO₃(?) ratio, while nitrogenaccumulation in spinach significantly increased with decreasing in both of NH₄⁺-N/NO₃(?)ratio and increasing phosphorus level, since biomass of spinach changed. Phosphorusdeficiency significantly inhibited both of NH₄⁺-N and NO₃(?) uptake in spinach, when themixture of NH₄⁺-N and NO₃(?) was supplied as nitrogen source, and the inhibit effect onNO₃(?) uptake was more servious than NH₄⁺-N. With increasing phosphorus level, NRA ofspinach markedly increased, but with higher NH₄⁺-N/NO₃(?) ratios, NRA of spinach wasdepressed, and it increased when the ratio was lower. Moreover, phosphorus deficiencydepressed GSA seriously. At the same phosphorus level, GSA of spinach shoot increasedsignificantly with increasing NH₄⁺-N in nutrient solutions.Summary of this total thesis showed that: Spinach prefers to nitrate nutrition. With theincrease of proportion of nitrate in nutrient solution, chlorophyll content, initial Rubiscoactivity tended to increase, so did net photosynthetic rate. Meanwhile, metabolism oforganic acids enhanced to provide enough energy and carbon skeleton for protein synthesis.Therefore, spinach biomass gradually increased, but no significant difference (P=0.05)was observed between treatments, 25:75 and 0:100 in NH₄⁺-N/NO₃(?) ratio. With nitrateas the main nitrogen resource, optimal enhance ammonium nutrition could reduced both ofnitrate and oxalate contents in spinach significantly, and improved nutritional quality byinceasing Vc, soluble sugar and crude protein content, At optimal NH₄⁺-N/NO₃(?) ratio,metabolism of organic acid and amino acid in spinach were improved to cooperate withcarbon and nitrogen metabolism each other. Considering the total effects of nitrogen formon biomass, food safety and nutritional quality of spinach, the optimal NH₄⁺-N/NO₃(?)ratio in nutrient solution should not exceed 25:75 for cultivating spinach.