| Blueberry is rapidly emerging as a major fruit crop in recent years.Due to its high nutritional value and outstanding health care,blueberry is known as the "Queen of Fruits",which is widely cultured in various regions at home and abroad.Currently,with the development of facilities cultivation model,frequent and excessive fertilization are used in blueberry cultivation for the purpose of higher economic benefits,which often lead to soil nutrients accumulations and salt accumulations.The salt damage of greenhouse blueberries was mainly caused by fertilization,especially nitrogen fertilizers.Some studies have shown that blueberry prefers ammonium-N to nitrate-N.However,many issues,such as the effects of different ammonium-N fertilizer and excessive fertilization on blueberry and soil chemical properties,the physiological response of blueberry to high ammonium stress and how to alleviate the high ammonium stress,were all need to be discussed and solved.The current study aims to explore the effects of high ammonium stress on blueberry growth,soil properties and physiological responses.Blueberry seedlings and in vitro cultured blueberry plantlets were used as materials.Six kinds of ammonium-N commonly used in production were applied in this experiment.On this basis,the exogenous substance can alleviate the damage of high ammonium stress on blueberry was screened.The main results were as followed.1)The effects of six ammonium-N applied on blueberry were compared,and the optimum and the high stress amount of N application were obtained.The results showed that(NH4)2HPO4 and NH4H2PO4 had the best effects on blueberry plants,which could significantly promoted plant height and number of branches under suitable application rate,and had the minimal damage under high ammonium stress.NH4Cl and NH4HCO3 had the worst effects on blueberry plants for serious high dead seedling rate under high ammonium stress.The optimum N application rates of six ammonium-N were 0.50-0.75 g kg-1,and 1.00-1.25 g kg-1 for high ammonium stress in blueberry seedlings cultivation.The results provided a reference standard for the selection of fertilizer varieties and nitrogen application rate of blueberry cultivation.2)The effects of high ammonium stress on the growth of blueberry seedlings were studied.The results showed that high ammonium stress resulted in the significantly decreased plant height and branch number of blueberry plants,the over excessive accumulation of N content in leaves(more than the standard value 2.1%),and the obviously decreased chlorophyll content and the increased ratio of chlorophyll a/b.The results provided a support for the nutrition monitoring of blueberry leaves.When the N content in leaves exceeded 2.1%and the ratio of chlorophyll a/b were increased,it is suggested that the amount of nitrogen application plant exceeded the standard and should be reduced.3)The effects of six ammonium-N on soil chemistry properties were studied.The results showed that all of the six ammonium-N reduced soil pH value,increased soil electrical conductivity and alkali-hydrolyzed nitrogen content in a short period of time,especially when plants were under high ammonium stress.(NH4)2SO4,NH4NO3 and NH4Cl had a stronger tendency to decrease the pH value of soil.Moreover,(NH4)2SO4 and NH4Cl could significantly promoted the electrical conductivity and the content of alkali-hydrolyzed nitrogen in the soil,but(NH4)2HPO4 and NH4H2PO4 increased them relatively smooth.Our finding demonstrated that the threshold of soil electrical conductivity was 2.0 mS cm-1 and that the threshold of alkaline nitrogen content was 250 mg kg-1 under high ammonium stress.Meanwhile,the main time points of soil chemical transition were determined with obvious changes 1,4 and 16 weeks after fertilization.The results provided scientific basis for selecting suitable fertilizer varieties under different soil conditions,and reasonable suggestions for soil monitoring standards.In addition,our results revealed the explanation of the different effects by different anionic ammonium-N on blueberry plants in the view of soil chemistry combining with the growth of blueberry seedlings.4)The physiological responses of blueberry to high ammonium stress were studied using in vitro cultured blueberry plantlets.The results showed that high ammonium stress caused the imbalance of mineral elements such as N,K,Fe,Ca,which may be the main cause of poor growth of in vitro cultured blueberry plantlets under high ammonium stress.The contents of N and Fe3+ were significantly higher,and the contents of K+,Fe2+ and Ca2+ were significantly decreased in in vitro cultured blueberry plantlets than those in normal plants.The contents of Malondialdehyde(MDA),Proline(Pro),Soluble protein(SP),substances related to membrane peroxidation and osmotic regulation,were all decreased under high ammonium stress.The activities of Catalase(CAT)and Peroxidase(POD)were increased,and the activities of Superoxide dismutase(SOD)were decreased,All the above indicated that membrane and osmotic regulators played little role in the late stage of high ammonium stress and that antioxidant enzyme activity played a major role in the late stage of high ammonium stress.High ammonium stress resulted in increased Glutamine synthetase(GS)activity,which was beneficial to alleviation of the high ammonium toxicity by the transformation of excessive free NH4+.The results revealed the physiological reasons of the damage induced by high ammonium stress on blueberry,which laid a foundation for the study of the mechanism of high ammonium stress,and provided a research direction for the damage alleviation of high ammonium stress in blueberry cultivation.5)The Fourier Transform Infrared Spectroscopy(FTIR)method was applied into the study,which revealed some reasons for the physiological effect of high ammonium stress on in vitro cultured blueberry plantlets from the point of metabolites.High ammonium stress could induce different kinds of metabolites,produce different polysaccharides,proteins and lipids in in vitro cultured blueberry plantlets.Under high ammonium stress,the contents of most metabolites in in vitro cultured blueberry plantlets were decreased,except some substances in proteins,polysaccharides and lipids showed an upward trend.The part of substances may be responsible for adaptation or mitigation of stress.High ammonium stress also led to the ratio of polysaccharides to protein increased,with the higher value and the worse growth of plant.The ratio of polysaccharides to protein can be used as an indicator of screening resistant plants.6)The alleviation effect of exogenous substances under high ammonium stress on blueberry were screened and verified.The results showed that most of the substances,which had better effect on the damage alleviation induced by high ammonium stress,were related to ion balance and the restoration of protective enzyme system.On the contrary,exogenous substances related with osmotic regulation system had little effect on the damage alleviation.Finally.CaCl2 was selected,which showed good alleviating effect on the six kinds of ammonium-N in the study.The results showed that CaCl2 could significantly alleviate chlorosis of blueberry plants induced by(NH4)2HPO4,NH4H2PO4 and NH4HCO3.The remission concentrations were 10,20 and 30 mmol L-1,respectively.Our findings laid a foundation and provided the research direction for the study of high ammonium stress in blueberry cultivation. |
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