Abstract
In this study, the effects of exogenously applied salicylic acid (0.5 and 5 mM SA) on the rates of photosystem II (PSII) activity was analysed in 4-week-old barley (Hordeum vulgare‘Bahman’ ) seedlings using chlorophyll (Chl) a fluorescence transient (OJIP) measurements. No evident changes in Chl and carotenoid contents as well as chlorophyll fluorescence transient curves were observed in either of the studied concentrations after 24 h of SA application. After 5 d, low SA concentration (0.5 mM) increased PSII activity, Chl b and carotenoid contents in barley seedlings. In contrary, 5 days after 5 mM SA treatment, the maximal quantum efficiency of PSII (Fv/Fm) and the Performance Index (PIABS), as an indicator of PSII structure and functioning, were significantly decreased. This lower Fv/Fm and PIABS coupled with lower levels of Chl b and carotenoids, and lower values of photosynthetic electron transport chain components including the electron transport flux (φEo) and the inferred oxygen evolving complex activity (Fv/Fo). By monitoring the chlorophyll a fluorescence rise kinetics, from the initial “O” level to the “P” (the peak) level, a dramatic increase in “OJ” phase was detected, which coincides with an increased photo-reduction of QA as a result of blockage of electron flow. This study provided the evidence that the high concentration of SA induced damage to different sites of the PSII.
Highlights
Salicylic acid (SA), as a common plant-produced phenolic compound, plays an important role in plant growth and development as well as in tolerance to biotic and abiotic stresses (Li et al, 2014; Khan et al, 2014; Janda and Ruelland, 2015)
The performance index that is calculated as: (RC/ABS)×(φPo/(1−φPo))×(ψo/(1− ψo)), where, RC is for reaction center; ABS is for absorption flux; φPo is for maximal quantum yield for primary photochemistry; and ψo is for the quantum yield for electron transport
We found that the low concentration of SA (0.5 mM) caused a significant increase in Chl b and carotenoid content compared with the control (Fig. 1)
Summary
Salicylic acid (SA), as a common plant-produced phenolic compound, plays an important role in plant growth and development as well as in tolerance to biotic and abiotic stresses (Li et al, 2014; Khan et al, 2014; Janda and Ruelland, 2015). At low concentrations (0.1–0.5 mM for most plants), it enhances the efficiency of the antioxidant system and the efficiency of PSII photochemistry (Chen et al, 2016), whereas at higher concentrations (1-10 mM for most plants) it increases oxidative damage (Hara et al, 2012; Miura and Tada, 2014). Plants respond to high SA depend on PSII response to this stress (Chen et al 2016). To address this issue, the chlorophyll (Chl) a fluorescence transient (OJIP) measurements were used to study photosynthetic apparatus functioning in response to various SA concentrations and incubation times in this study. In order to improve our knowledge of barley photosynthetic apparatus in response to SA treatment, the OJIP fluorescence transient was measured in barley plants in responses to different concentrations of SA
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