| Cucumber(Cucumis sativus L.)is one of the most important economic vegetables in the world and is classified as salt sensitive.However,its production is considerably altered due to biotic and abiotic stresses,among which salinity is major constrains against its production.Salinity is a major abiotic stress which acutely affects crop productivity through,osmotic stress,alter metabolism and disrupts plant physiology by accumulation of osmolytes,ionic toxicity,protein dysfunction,causes electrolyte leakage and eventually death of a tissue.Salts stress is devastating stress and affecting approximately 7%of the world land.In order to increase crop productivity,various physiological and agronomic practices are applied to minimize the damage due to salt stress.however,significant progress has been made in understanding the role of plant growth regulators and microbes in mediating a wide range of physiological and metabolic responses which affect plant growth and development.Brassinosteriods(BRs)are recognized as a new class of phytohormones which are ubiquitously distributed in the plant kingdom and have a high potential for promoting growth and development and have a critical role in root/stem growth,cell differentiation,cell elongation,gene modulation,development of flowers and fruits etc.Plants are colonized by various microbes and mycorrhizal fungi which coordinate to involve in arbitrating various plant physiological functions especially nutrient uptake and tolerance to various stresses.AM fungi(AMF)are symbionts and form a beneficial symbiotic relationship in the rhizosphere and play an important role in improving plant growth.Considering the potential roles of HBL(28-Homobrassinolide)and AMF(Glomus versiforme)in ameliorating the stress conditions upon the receiver plants,and more importantly due to the less known activity of these entities as a combined treatment,it seems a dire need to investigate the strategic interaction of these two factors.The current study is therefore among the first to investigate the combined role of HBL and AMF in the context of plant antioxidant enzymes,photosynthesis and root activity of cucumber plants under saline stress and at various stages of the plant growth.Our thesis comprises of three major studies as following.(1)HBLat 1?mol/L level enhances growth of cucumber under mild and high(60and 120 mmol/L)NaCl stress.BRs are plant steroidal hormones which are ubiquitous in plant kingdom and regulate a wide range of physiological and stress responses.In a plastic tunnel,pot bioassay experiment,three levels of HBL at 1,3 and 5?mol/L were tested on growth and physiological attributes of cucumber cultivars Jinyou 1~#(in short,JY1,salt sensitive)and Changchun Mici(in short,CCMC,salt tolerant),grown at various salt stress conditions(60 and 120 mmol/L).Cucumber plants of both cultivars subjected to salt stress exhibited reduced growth attributes and altered antioxidant enzymes at 60 and 120 mmol/L.However,the deleterious effects of salt stress were partially improved by HBL.The foliar application of HBL enhanced shoot and root fresh and dry weight,chlorophyll content at 60 mmol/L and 120 mmol/L NaCl stress in cv JY1 and cv CCMC,respectively.HBL application also altered the antioxidant enzyme levels by increasing super oxide dismutase(SOD,EC 1.15.1.1),peroxidase(POD,EC 1.11.1.7)levels and decreased the malondialdehyde content(MDA)under 60 and 120 mmol/L stressed plants in both cultivars.Overall results showed that HBL could enhance growth attributes,chlorophyll content and antioxidant enzymes for both of the cultivars under mild and high NaCl stress.(2)HBL in combination with AMF shows ameliorative role in growth,physiology and yield of cucumber under NaCl stress.Salinity is among the major hurdles of food security as it causes severe damage to the physiology and growth of the plants.BRs are a new class of plant phytohormones ubiquitously distributed in the plant kingdom and have a high potential for promoting growth.AMF are biotrophs,which inhibits plant rhizosphere and form symbiotic interactions with the host plant.Plants grown in saline soil conditions lead to specific ion toxicity which disrupts cell structures affects physiology and causes oxidative stress,which further alters plant antioxidant enzyme system.Studies show that HBL and AMFcan enhance tolerance to salt stress.However,the mechanism of the combined effect of HBL and AMF in cucumber under salt stress is still unknown.In the present study,the protective role of HBL,AMF and their combined effect on growth,physiology,antioxidants enzymes and mineral uptake under NaCl(100mmol/L)on cucumber cultivars(salt sensitive and tolerant)was investigated.Our result shows that the combination of HBL and AMF enhance growth,yield,chlorophyll content,photosynthetic rate,leaf relative water content and decrease electrolyte leakage as compared to plants under saline conditions.The antioxidant enzyme activity,SOD and CAT(catalase;EC 1.11.1.6)showed a gradual increase after every 10 days in both cultivars,while,the POD activity declined after 30 days of treatment in both cultivars.Moreover,the sodium ion concentration in shoots and roots was significantly decreased as compared to their respective controls.This research suggests that the enhanced tolerance to salinity was mainly related to elevated levels of antioxidant enzymes and lower uptake of sodium ion which lowers the risk of ion toxicity and decreases cell membrane damage.(3)HBL spraying after15daysinterval in combination with AMF improves growth and physiology of cucumber under NaCl stress.Salinity is one of the major obstacles in the agriculture industry and causing huge losses in productivity.Several strategies like plant growth regulators with AMF has been used to decrease the negative effects of salt stress.In our experiment,HBL with spraying intervals was combined with AMFin cucumber cultivars JY1(salt sensitive)and CCMC(salt tolerant)under NaCl(100 mmol/L).Studies have documented that the foliar application of HBL and AMF colonization can enhance tolerance to plants under stress conditions.However,the mechanism of the HBL spraying intervals after 15 and 30 days in combination with AMF in cucumber under salt stress is still unknown.Our results revealed that the HBL spraying interval after 15 days in combination with AMF resulted in improved growth,photosynthesis and decreased sodium toxicity under NaCl.Moreover,the antioxidant enzymes SOD and POD activity showed a gradual increase after every 10 days,while,the CAT increased after30 days of salt treatments in both cultivars.This research suggests that the enhanced tolerance to salinity was mainly related to elevated levels of antioxidant enzymes and lower uptake of Na~+which lowers the risk of ion toxicity and decreases cell membrane damage. |
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