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Although the beneficial role of Fe, Zn, and Mn on many physiological and biochemical processes is well established, effects of each of these elements on chlorophyll (Chl) a fluorescence and photosynthetic pigment contents is not well studied. The objective of this study was to evaluate effects of Fe, Zn, and Mn deficiency in two lettuce cultivars. The parameters investigated could serve also as physiological and biochemical markers in order to identify stress-tolerant cultivars. Our results indicated that microelement shortage significantly decreased contents of photosynthetic pigments in both lettuce cultivars. Chl a fluorescence parameters including maximal quantum yield of PSII photochemistry and performance index decreased under micronutrient deficiency, while relative variable fluorescence at J-step and minimal fluorescence yield of the dark-adapted state increased under such conditions in both cultivars. Micronutrient deficiency also reduced all parameters of quantum yield and specific energy fluxes excluding quantum yield of energy dissipation, quantum yield of reduction of end electron acceptors at the PSI, and total performance index for the photochemical activity. Osmoregulators, such as proline, soluble sugar, and total phenols were enhanced in plants grown under micronutrient deficiency. Fe, Zn, and Mn deficiency led to a lesser production of dry mass. The Fe deficiency was more destructive than that of Zn and Mn on the efficiency of PSII in both lettuce cultivars. Our results suggest that the leaf lettuce, which showed a higher efficiency of PSII, electron transport, quantum yield, specific energy fluxes, and osmoregulators under micronutrient deficiency, was more tolerant to stress conditions than crisphead lettuce.

The mechanism of osmoprotectant action on photosynthesis process is still not well known, especially under salt stress. The objective of this study was to evaluate and explain the effect of glycine betaine (GB) on photosynthetic efficiency and other physiological parameters of cucumber plants grown under salinity stress. Our results indicated that salinity decreased chlorophyll and carotenoids content, Ca 2+ and K + concentrations, and quantum yield parameters, such as probability that a trapped exciton moves an electron in to the electron transport chain beyond QA, quantum yield of electron transport from QA to QB in PSII, quantum yield of reduction of end electron acceptors in PSI, performance index for the photochemical activity, total performance index for the photochemical activity, trapping per reaction centers, and other parameters related to primary photochemical reactions of PSII. However, the exogenously applied GB increased most of tested parameters including the total soluble carbohydrate, proline and GB content, and Ca 2+ and K + concentrations, under salt stress. We suggest that GB can play an essential role as regulator to improve photosynthetic efficiency and thus yield of cucumber plants under salt stress conditions. At the level of photosynthesis process, the application of exogenous GB indirectly enhanced the performance of the photosynthetic machinery of cucumber plant due to the reduction of the dissipated light energy, as heat, and the increase of primary reactions of photosynthesis efficiency. Abbreviations: Chl -chlorophyll; DM -dry mass; ET0/RC -electron transport flux per RC; F0 -minimal fluorescence yield of the dark-adapted state; Fm -maximal fluorescence of the dark-adapted state FM -fresh mass; Fv -maximal variable fluorescence; Fv/Fm -maximal quantum yield of PSII photochemistry; GB -glycine betaine; PIabs -performance index for the photochemical activity; PItot -total performance index for the photochemical activity; RC -reaction center; SM -normalized area (related to the number of electron carriers per electron transport chain); TChl -total chlorophyll; TR0/RC -trapped energy flux per RC; TSC -total soluble carbohydrate; VI -relative variable fluorescence at I-step (30 ms after the start of actinic light pulse); VJ -relative variable fluorescence at J-step (2 ms after the start of actinic light pulse); ΦDo -quantum yield of energy dissipation; φET20 -quantum yield of electron transport from QA to QB in PSII; ΦPo -maximum quantum yield of primary PSII photochemistry; ΦRE10 -quantum yield of reduction of end electron acceptors in PSI; Ψ0 -probability that a trapped exciton moves an electron in to the electron transport chain beyond QA.

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