| Salinity is one of the major abiotic stresses that reduce the size of arable land and productivity of crops.Cucumber is an important vegetable crop grown worldwide for its edible fruits.Majority of the popular cucumber cultivars grown are either sensitive or moderately sensitive to salt salinity stress.Consequently,their growth and productivity is severely impaired by salinity stress.Although technical methods such as land reclamation and desalination had produced satisfactory results to ameliorate salinity menace,the high capital and energy requirements makes them unsustainable.Development of salt tolerant cucumber is the most efficient approach of growing cucumber under saline soil or irrigation water.Previous researches report genotypic variation in salinity tolerance of cucumber at morphological land physiological levels.However,no reports of genetically improved salt tolerant cucumber are available in literature.The complexity of the salinity tolerance,lack selection criterion and interaction with other abiotic stress makes breeding for salt tolerance in cucumber difficult.The genetics of salt tolerance in cucumber is poorly understood.The goal of this study was to select and develop a mapping population to study the genetics of salt in cucumber at seedling stage.We investigated the following objectives to achieve above goal:ⅰ.Assess salt tolerance of cucumber genotypes to salinity under greenhouse conditions.ⅱ.Study gene action involved in controlling different salt traits under salinity using generation mean analysis.ⅲ.Identify quantitative trait locus associated with salt tolerance in cucumber.We assessed salinity tolerance of four cucumber genotypes(11439S and HH1-8-57 as salt sensitive,11432S and 11411S)according to previous in vitro.We employed pot based screening system consisting of vermiculite and peat in the ratio 2:1(v/v)in a greenhouse to confirm their tolerance.The seedlings were irrigated with full strength Hoagland solution without NaCl or with 80 mM applied gradually to avoid shock.The experiment was terminated after 21 days when most plants seedlings had started showing chlorosis and wilting.At the end of the treatment,plant growth,survival rate,gas exchange characteristics and cell membrane stability as indicated by electrolyte leakage,chlorophyll content and Na+/K+ were determined.We also compared germination responses of the 11439S and 11411S at 0,40,60,80,100 mM NaCl.For genetic studies,we crossed the salt tolerant(female parent,11411S and)and salt sensitive(11439S).We used generation mean analysis to determine the gene effect and mode of inheritance of salinity tolerance in cucumber.The experiment involved six basic generations consisting of salt tolerant parent(P1),salt sensitive parent(P2),F1,B1(P1×F1),B2(P2×F1)and F2.The six basic generations planted in pot mix were subjected to 80 mM salinity stress for 21 days.We measured the following traits salinity tolerance(TOL),relative leaf number at 14 days post-salt treatment(RLN14),area of the second largest fully expanded leaf(LA)and vine length(VL).A,B,C test was applied on the data to test simple additive model while joint scaling test was used to determine non-allelic interactions.To identify quantitative trait loci(QTL)associated with salt tolerance,we surveyed the polymorphism between the salt tolerant,11411S and salt sensitive,11439S using 432 simple sequence repeat(SSR)markers.The polymorphic SSR marker segregation was surveyed on 89 F2 individuals.For phenotypic evaluation,F2:3 families derived from a cross 11411S×11439S by single seed descent were subjected to 80 mM NaCl for 21 days in a pot mix in a greenhouse at Pai Lou Research Base,Nanjing Agricultural University.Data on percentage survival(SU),salt tolerance(TOL),percent green leaves(%GL)and relative leaf number(RLN14)were recorded.The key results of the study are summarized below:1.Physiological and growth responses of cucumber genotypes to NaCl salinity stress Salinity reduced all the parameters except internal carbon dioxide concentration and Na+/K+and electrolyte leakage that increased at high salinity levels.The reduction in fresh and dry weights,survival rate were higher in salt sensitive genotypes.Net photosynthetic rate(Pn),transpiration rate(E),water use efficiency(WUE),carboxylation efficiency(CE)were higher in salt tolerant genotypes.Salt tolerant genotypes had higher survival rate by slightly maintaining higher photosynthesis efficiency,reduced membrane damage and higher Na+/K+.Since Na+ content in the shoot tissue increased in all genotypes at high salinity,we reject the Na+ exclusion theory of salinity tolerance in cucumber.Thus,salinity tolerance in cucumber in cucumber may be due to reduced K+ leakage under salinity.The final germination percentage was not affected by salinity level.However,salinity significantly increased mean germination time of the salt-sensitive 11439S.Under our screening system,80 mM NaCl for three weeks discriminated salt tolerance of cucumber.Comparing our results with the previous in vitro screening,we conclude that the former is suitable for large-scale screening of germplasm.However,it is important to rescreen genotypes showing extreme salinity responses in soil or pot mix growing medium.2.Salt tolerance(TOL)and relative leaf number of cucumber are predominantly under addititive gene effectsWhereas A,B,C scaling test did not detect epistatic gene effects on all the traits,joint scaling test showed the presence of non-allelic interactions for all the traits.Additive gene effect predominantly influenced TOL and RLN14 were predominantly under additive gene effect while dominance and additive gene effects predominantly influenced inheritance LA.Dominant gene effect significantly controlled the inheritance of VL.The narrow sense heritability for TOL,RLN14,VL and LA was 0.57,0.26,0.74 and 0.66,respectively.VL and LA registered significant positive heterobeltiosis.Simultaneous selection for RLN14 and TOL in segregating population at advanced filial generations in this cross may be useful in developing cucumber varieties with increased salinity tolerance.We propose intermating of superior segregants to concentrate the favorable genes for salt tolerance.3.Marker-salt tolerance trait association identified by simple regressionSix SSR markers(SSR20710,SSR13312,SSR1667,SSR13021,SSR23627,SSR23627 and SSR 00398)with unambiguous banding patterns were subjected to simple regression analysis to determine significant marker trait association.SU and RLN14 were higher in salt tolerant parent than sensitive one.The mean scores of F2:3 families exhibited continuous variation but some had values outside the parental means.SSR20710 located on chromosome 3 explained 16.5,7.1,5.6 and 7.8 of variations observed in SU,TOL,RLN14,respectively.Markers SSR00398,SSR23627 and SSR13021 had no significant association with all the traits.Marker SSR13312 and SSR16667 contributed 25 and 59%of RLN14 and TOL,respectively.This study provided valuable information for future genetic studies of salinity tolerance in cucumber.We identified three microsatellite markers with significant association with specific quantitative traits.These markers could be used marker assisted selection for salinity tolerance improvement in cucumber.Overall,our study confirmed that salt tolerance(TOL)and relative leaf number(RLN14)are heritable under salt stress.The study revealed significant variation in both osmotic and ionic salt stress in the genotypes used.However,the low parental polymorphism is probably due to narrow genetic base of the cultivated cucumber.A comprehensive screening of all the cucumber germplasm collection at all developmental stages in controlled conditions and followed by association mapping to maximize the available genotypic variation.Cucumber is a potential model crop for salinity tolerance studies owing to its short life cycle and fully sequence genome. |
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