Shahwani, Muhammad Naeem
Studies on abiotic stress tolerance in Hordeum vulgare L. genotypes from arid and temperate regions.
PhD thesis, University of Glasgow.
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Plants growing in arid regions often suffer from high shoot temperatures, low shoot water concentrations, low turgor pressures, and high salinity in the rhizosphere. To investigate which traits confer tolerance on plants in these areas a range of genotypes was studied. These included Local (an uncharacterized landrace grown in south western Pakistan), Soorab-96 and Awaran-2002 (both elite cultivars developed by ICARDA and commercially grown in Pakistan), and Optic (an elite European cultivar). Measurements on germination, growth, and yield suggested that landrace Local is significantly least affected by high salinity (p<0.05) compared with lines Soorab-96, Awaran-2002 and Optic. Further investigations on ion profiling established that landrace Local could maintain low Na+/K+ ratios. This appeared to arise from Local’s ability to prevent Na+ accumulation in the roots and shoots by enhanced exclusion or efflux or both. Probably this unique characteristic of landrace Local helped in maintaining its photosynthetic efficiency, plant water status, and stomatal conductance, which resulted in its better performance and survival in high salinity. There was no evidence that high tissue solute concentrations, high proline levels or life cycle strategies played a role in salt stress tolerance. In addition, there was no evidence that osmotic stress was responsible for the observed suppression of growth in any of the genotypes. The main conclusion from this study is that for glycophytes (which do not complete a full life cycle above 100 mM NaCl; this includes all of the world’s major crops), it is the ionic component of salinity stress that impairs growth processes and yield, not the osmotic component. Further research on salinity stress in crops should focus on understanding the processes that control ionic balance rather than osmoregulation. There is some evidence that long term exposure of plants in the preceding generations to moderately high salt concentrations (e.g. 100 mM NaCl) improves barley halotolerance in succeeding generations, i.e. halotolerance has a transgenerational, epigenetic basis, but there was also evidence that the improved halotolerance in the Local genotype was partly genetic.
In another series of experiments the importance of short periods of high leaf temperatures (Tleaf) on photosynthetic efficiency of barley genotypes Local, Optic, and Soorab-96 was investigated. In all three genotypes light saturated carbon dioxide assimilation rates (Asat) and the carboxylation coefficients (CO2, a measure of the efficiency of CO2 fixation) in the fourth fully expanded leaves were equally suppressed to approximately 20 % of their pre-treatment levels immediately after a short period of heat stress (Tleaf > 40.0 ± 0.5 °C for 20 minutes). Parallel measurements using a range of techniques confirmed that the suppression of Asat and CO2 was not attributable to changes in the light harvesting capacity (leaf absorptance and chla excitation spectra), maximum quantum efficiency of PSII (PSII, Fv/Fm), and to stomatal conductance (gs). It is unlikely that the suppression arose from damage to the electron transport chain¸ or to the capacity to develop or maintain non-photochemical quenching (NPQ, which is dependent on the transthylakoid pH), but these possibilities cannot be dismissed. LC-MS and enzymic analysis of leaf metabolite levels showed that the pools of metabolites feeding into RuBisCO are not affected by heat stress whilst those of the metabolites flowing away from RuBisCO were significantly depleted. The implication is that short periods of heat stress severely impairs RuBisCO, RuBisCO Activase, or processes close to the carboxylation step. Five days after heat stress Asat and CO2 had significantly recovered to approximately 40 % (p<0.05) of their pre-stress levels in landrace Local, but no significant recovery was observed in any of the elite lines including those distributed by ICARDA for arid land production. These findings provide evidence that thermal damage may play a significant role in yield suppressions in arid regions and that there is a genetic basis for thermotolerance in barley .
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