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Title: Identification of some key physiological and biochemical indicators for salt tolerance in Canola (Brassica napus L.)
Authors: Iqbal, Muhammad
Keywords: Biological & Medical Sciences
Issue Date: 2019
Publisher: Bahauddin Zakariya University Multan
Abstract: The study was conducted to evaluate canola germplasm (local and exotic) for salt tolerance at the germination, seedling growth stages and adult growth stages, and to draw the relationships among various physiological and biochemical processes to identify the traits responsible for salt tolerance in canola. In the present study, selected canola cultivars at the germination and seedling stages were further evaluated for salt tolerance at the adult vegetative growth stage using various biochemical and physiological attributes particularly PS-II photochemistry and protein profiling. For this purpose, a series of four independent experiments were conducted to screen out available canola germplasm at varying salinity levels (0, 75, 100, 150 and 200 mM NaCl) at germination and seedling stages. A considerable variation among the canola cultivars has been observed in seed germination ability under salt stress. Among canola cultivars investigated in the present study, a few cultivars showed a consistent degree of salt tolerance with the change in level of salt stress. From all screening experiments at the seedling stage, it was concluded that cvs. DGL, Dunkeld and Faisal canola categorized as salt tolerant while cvs. Ac-Excel and Oscar moderate salt tolerant and cvs. Legend followed by Cyclon as sensitive to salt stress at the seedling stage. Canola cultivars subjected to screening at germination and seedling stages were also evaluated at the vegetative growth stage by subjecting them varying levels of salt stress (0, 75, 100, 150 and 200 mM NaCl). Results from current study demonstrated that accumulation of Na+ in the leaves of all the canola cultivars increased due to salt stress with consistent decrease in K+ concentration. Cultivars Cyclon, Bulbul-98, Oscar and NURYT-202/19 accumulated higher Na+ concentration as compared to cvs. Dunkeld, DGL, Faisal Canola and Punjab canola. However, cultivar DGL accumulated higher K+ concentration whereas reverse was true for cv. Cyclone. Furthermore, higher K+/Na+ ratio was obtained in cvs. Dunkeld and DGL and lowest in cv. Cyclone and Bulbul-98. On the basis of photosynthetic capacity of the canola cultivars, results showed reduction in CO2 assimilation rate and transpiration rate in all canola cultivars. Lowest reduction in photosynthetic rate (A) was found in cv. DGL while highest reduction was found in cv. KN(s)-751V followed by cv. Ac-Excel. Water use efficiency (WUE) increased in most of the canola cultivars however this increase was more pronounced in cv. Chakwal Sarson and this could be the consequence of reduction in transpiration rate (E). Total chlorophyll 172 contents measured as SPAD values increased in all canola cultivars under saline conditions. Increase in total chlorophyll contents might be the consequence of reduction in leaf area under salt stress. Structural and functional stability of photosystem II was assessed using fast chlorophyll a kinetic analysis and dual pulse amplitude chlorophyll fluorescence analysis in selected 13 canola cultivars. The initial fluorescence denoted by Fo did not change significantly in most of canola cultivars. Moreover, there was observed significant reduction in maximal fluorescence Fm in all cultivars. The appearance of L-band in cv. Rainbow followed by cvs. Oscar and Legend showed loss in connectivity of light harvesting complex from core of PS-II. However, appearance of negative L-band cvs. Ac-Excel, CON-II indicated the persistent energetic connectivity between LHCII to PS-II reaction centers. A positive K-band was visible in cv. Legend followed by rainbow representing salt stress induced imbalance of the transfer of electrons at the acceptor and donor side of PS-II. Accentuated negative K-band appeared in cvs. Faisal canola and Dunkeld followed by cvs. Ac-Excel and CON-II suggesting their potential to resist imbalance of the electron flow from oxygen evolving complex (OEC) to reaction centers of accepter side of PS-II. Variable and maximum fluorescence (Fo, Fv, Fm) and their ratios (Fv/Fo, Fm/Fo) decreased in most of the canola cultivars, but salt sensitive or moderately sensitive canola cultivars such as Cyclone, Legend, Oscar exhibited a greater decrease. Salt stress did not decrease Fv/Fm in all canola cultivars. Performance index (PIABS) reduced due to salt stress in canola cultivars except in salt tolerant cultivar DGL where it remained unchanged. Moreover, maximum reduction was observed in salt sensitive Legend and Oscar, whereas no or minimum reduction performance index found in salt tolerant cultivars DGL and Faisal Canola. From these results, it is suggested that salt tolerant canola cultivars maintained energy conversion efficiency by down-regulating PSII activity with increase in DIo/RC. Results from protein profiling study of 13 canola cultivars indicated 18 proteins differentially expressed under saline conditions. Of 18 differentially expressed proteins, six proteins were related with antioxidant activities such as peroxidase 73, chloroplastic Cu-Zn SOD, thioredoxin H5, Glutathione S-transferase DHAR2, five proteins were associated with photosynthesis (chlorophyll biosynthesis and CO2 fixation in Calvin cycle) and two proteins related with ATP metabolism were identified in salt stressed plants of canola such as mitochondrial ATP synthase. 173 In conclusion, salt induced reduction in canola cultivars was due to changes in photosynthetic activity and oxidative stress in addition to changes in sodium and potassium ion accumulation. Chlorophyll fluorescence measured as fast chlorophyll a kinetic analysis or through dual pulse amplitude fluorescence along with proteome analysis provide deeper insights about contribution of photosynthetic activity and antioxidant potential in salt tolerance of canola cultivars. In future studies this study will provide a new avenue for exploring photosynthesis as selection criteria for salt tolerance.
Gov't Doc #: 24204
Appears in Collections:PhD Thesis of All Public / Private Sector Universities / DAIs.

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