1996; Kornyeyev et al. 2010); however,

the level of photoinhibition is inversely proportional to the level of photoprotection and to the ability to repair photodamaged PSII elements. Many studies show that both the photoprotection and the repair ability increase with longtime exposure to high excitation pressure, mostly at HL intensities (Tyystjärvi et al. 1992; Niinemets and Kull 2001). Together with a very low ETR and non-photochemical quenching (of Chl fluorescence), similar to that in sun plants, selleck kinase inhibitor we could expect severe photoinhibitory damage in shade plants exposed to HL treatment. However, low differences in photoinhibitory effects (q I) between sun and shade leaves did not correspond with high differences in excitation pressure. One possible explanation is that the values of the excitation find more pressure may have been estimated inaccurately and 1-qP values are really not the true estimates of the PSII redox poise. Rosenqvist (2001) has discussed the possible “inaccuracy” of the calculated values of photochemical quenching, qP, as it probably inaccurately estimates the fraction of oxidized QA due to “connectivity among PSII units” (Joliot and Joliot 1964; Paillotin 1976; Joliot and Joliot 2003). The concept of connectivity among PSII units

is included in many models; however, there is still a lack of reliable data for the correct values of probability parameter p in different plant species. Kramer et al. (2004), based on the data published by Lazar (1999), have reported that the p value in higher Thymidine kinase plants is usually higher than 0.6 (supported by Joliot and Joliot 2003, who obtained p = 0.7); in such a case, the qL would

reflect fully the redox state of QA. On the other hand, the data published by Kroon (1994) show p values between 0.25 and 0.45. Further, GSK458 molecular weight Strasser and Stirbet (2001), using direct measurements of fast ChlF kinetics, found a value of p 2G around 0.25, using both ChlF curves in the presence and the absence of DCMU; it represents a p value of ~0.5 (Stirbet 2013). Although the connectivity is estimated from the initial part of chlorophyll fluorescence curve, it does not mean that it is valid only for the initial phase. According to the theory of PSII connectivity, the migration possibilities for excitons that are inferred from the sigmoidal shape of fluorescence induction also influence the efficiency of utilization of absorbed light for trapping electrons in the RC and hence, it has an effect on the entire fluorescence kinetics (Lavergne and Trissl 1995). Recently, Tsimilli-Michael and Strasser (2013) documented that the p 2G can be correctly calculated even if only some of the RCs are inactive as well as in the case when the true F m (all RCs closed) is not reached experimentally.