Influence of salinity stress on PSII in barley (Hordeum vulgare L.) genotypes, probed by chlorophyll-a fluorescence

Chlorophyll-a fluorescence is an efficient tool to determine the photosynthetic capacity of plants and the health status of plants under normal or stress conditions including salinity stress. This study was aimed to elucidate changes in the efficiency of photosystem II (PSII) in barley genotypes differing in degree of salt tolerance, which can be used for phenotyping in the breeding program for developing salt-tolerant cultivars. Twelve barley (Hordeum vulgare L.) genotypes were subjected to salt stress and salt stress reduced the growth of all barley genotypes, which is associated with a decline in chlorophyll and K+ contents (roots and leaves) and increase in Na+. Of the 12 barley genotypes, one salt-tolerant (B-10008) and one salt-sensitive barley genotype (B-14011) was selected to further investigate the structural stability of PSII using fast chlorophyll a kinetic analysis under salinity stress. The shape of OJIP transients changed due to salt stress in both salt-sensitive and salt-tolerant barley genotypes indicating a disturbance in structural stability at various points of PSII. The detailed analysis of JIP-test parameters suggested that salt stress caused photoinhibition of PSII due to enhanced inactive reaction centers, reduced absorption flux (ABS/RC), low transfer of electrons per reaction center (ETO/RC) and enhanced accumulation of QA¯ (VJ) thus reducing primary photochemistry (TRO/RC, ɸPO). These changes in PSII resulted in the reduction of the maximum quantum yield of PSII (Fv/Fm) and performance index (PIABS). Moreover, salinity stress enhanced dissipation energy flux per reaction center (DIO/RC) as a protective measure to save PSII from photooxidative damage in thylakoid membrane. The appearance of positive K and L-bands supported the idea that salt stress dissociated the light-harvesting complex from core proteins of PSII, damaged oxygen-evolving complex and reduced the structural stability of PSII by disturbing the electron transfer between acceptor and donor sides of PSII especially in salt sensitive genotype (B-14011). Moreover, such an adverse effect of salt stress on PSII was less in salt-tolerant barley genotype (B-10008). Thus, some JIP-test parameters can be used as potential phenotype marker for screening salt-tolerant genotypes.