Physiological Mechanisms Of Regulation On Growth And Nitrogen Absorption In Citrus Seedlings By Nitrogen Forms And Levels

Nitrogen?N?is one of the most essential mineral nutrients for the normal operation of plant organism,the ability of plant to absorb,assimilate and utilize nitrogen directly affects its growth and development.Citrus mainly grows in the acidic red soil area of southern China,accompanying with high concentration of ammonium nitrogen environment.Available research showed that excessive ammonium nitrogen fertilizer application can accelerate soil acidification and in turn affect soil nitrogen form conversion and plant nitrogen uptake capacity,and further lead to the decline of citrus yield and quality.This study assumes that citrus is an ammonium insensitive crop based on the high ammonium nitrogen acidic environment of citrus growth.The citrus variety of‘Lugan'?Citrus reticulata Blanco cv.Ponkan?seedlings were fostered with two N forms including ammonium and nitrate?AN and NN?and three N levels including 1,4 and 8 mmol L^-1?AN1,AN4,AN8 and NN1,NN4,NN8?in sand culture,which were used to study the effects of different nitrogen forms and levels on citrus growth?biomass,growth rate and root and leaf morphology?and physiological characteristics?nitrogen metabolism,photosynthetic fluorescence characteristics and protective enzyme changes?.The main results are as follows:?1?Ammonium inhibited the growth and development of the citrus seedlings.AN significantly reduced its biomass,growth rate,and morphological composition of roots and leaves compared with NN,and increased those inhibition with increasing N supply.Inside,the dry weight of root,stem and leaf declined by AN8 than NN8 was 26.49%,47.03%and41.49%,respectively,and AN8 was higher 31.23%than NN8 on root:shoot.?2?Ammonium resulted in the disorder of nitrogen metabolism in citrus seedlings.AN increased N concentration and total free amino acid?FAA?accumulation in roots and leaves and decreased N accumulation and soluble protein content.Meanwhile,in addition to glutamate synthase?GOGAT?activity in roots,nitrate reductase?NR?,glutaminase?GS?in roots and leaves,and GOGAT activity in leaves were decreased,and these changes all increased with increasing N supply.?3?Ammonium impaired the gas exchange performance and photosynthetic system function in citrus seedlings.AN promoted chlorophyll synthesis and accumulation of soluble sugars?glucose,fructose and sucrose?and starch in roots and leaves,impaired supply-side oxygenation complex?OEC?and reduced activity of reaction sites?RC?,reduction of receptor-side Q_A and Q_B,maximum photochemical efficiency?F_v/F_m?,overall electron transfer performance(PI_ABS and PI_CS),net photosynthetic efficiency?Pn?,stomatal conductance?Gs?,and transpiration rate?Tr?,and strengthened with increasing N supply.?4?Ammonium nitrogen leads to the imbalance of antioxidant metabolism and membrane lipid peroxidation in citrus seedlings.AN promoted the synthesis of malondialdehyde?MDA?in roots and leaves,and AN4 increased the activity of superoxide dismutase?SOD?,catalase?CAT?and peroxidase?POD?in roots,whereas opposite trend was observed in AN8,and AN consistently decreased the activity of the three enzymes in leaves.Taken together,citrus is an ammonium sensitive crop.And increasing the supply level can aggravate the disorder of citrus nitrogen metabolism,photosynthesis and antioxidant metabolism process,and then inhibit its growth and development.This result provides a theoretical reference for the rational nitrogen form and dosage selection in the application of nitrogen fertilizer in citrus science and high yield and quality production.