The review deals with objective reasons that limit the use of chlorophyll fluorescence induction kinetics (Chl FIK) method in plant ecology. Based on the ontogenetic approach (analysis and comparison of the dynamics of the Chl fluorescence ratio F p /F s and physiological characteristics of plant leaves in ontogeny) possible criteria for the estimation of general plant resistance of photosynthetic apparatus (PSA) to prolonged stress affects are proposed. One of these criteria is the presence or absence of a steady-state phase in the dynamics of Chl fluorescence ratio F p /F s (or R fd ) of plant leaves after they stop growing. We also propose to use the duration of the steady-state phase and variability of Chl fluorescence ratios R fd and F p /F s in this period for quantitative assessment of plant PSA resistance to prolonged stress during plant leaf ontogeny.Additional key words: leaf age; photosystems 1 and 2; radiation colour; specific leaf growth rate.

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The effects of actinic light (AL) intensity on the age dependence of nonphotochemical quenching of fluorescence (q N ) and effective quantum yield in PSII ( PSII ) were studied in continuously illuminated wheat leaves of the upper tier. Regular changes were revealed in both age dependence of q N at elevated AL intensities and light curves of q N . These changes are related to alterations in strategies of redistribution and use of absorbed light energy by the photosynthetic apparatus at different stages of wheat leaf development. Unlike Ф PSII , q N as a parameter was more sensitive to the differences in the leaf age at a certain range of light intensity. At the same time, the stability of q N at moderate light intensities may serve as an indication of leaf maturity.Additional key words: chlorophyll fluorescence induction; leaf age; photosystem II.--Nonphotochemical quenching of fluorescence (q N ) is commonly used to analyze stress tolerance in plants (Baker 2008, Ashraf and Harris 2013). It is well known that nonphotochemical quenching of chlorophyll fluorescence, q N , involves at least three molecular mechanisms: ΔpH-dependent quenching, which determines energy-dependent quenching of fluorescence (q E or the fast component, q Nf ); transition of the pigment system from state 1 to state 2 (q T or the medium component, q Nm ); and photoinactivation of PSII (q l or the slow component, q Ns ) (Baker, 2008). The contribution of each component to the total nonphotochemical quenching, q N , is determined by the actinic light intensity (Bukhov 2004), and their proportions and significance change during the leaf ontogeny (Hong et al. 1999, Müller et al. 2001, which may affect age variations in the q N as a whole. Indeed, under low and medium intensities of actinic light (AL), differences in the q N between mature and aged leaves are insignificant. These differences, however, become greater as AL intensity increases (Hong et al. 1999). Mechanisms determining nonphotochemical quenching of excited states of chlorophyll are mainly studied for PSII, as this photosystem is considered to be more sensitive to photoinhibition than PSI. Although it is well known that q N is age-and light-dependent, the age dependence of q N under medium intensities of AL and the effect of an increase in the AL intensity have not been sufficiently studied. No data have been reported on light curves of q N for leaves of different ages. The actinic light intensities reported in the literature varied between 200 and 1,500 μmol m -2 s -1 PPFD (Hong et al. 1999;Lichtenthaler et al. 2005; Wang and Chen 2011), and the available experimental data are not sufficient to evaluate the effect of the AL on the age dependence of the q N in the plant leaves. Thus, the purpose of this ---Abbreviations: CFI -chlorophyll fluorescence induction; F0 -minimal fluorescence yield of the dark-adapted state; F0' -minimal fluorescence yield of the light-adapted state; Fm -maximal fluorescence yield of the dark-adapted state; Fm' -maximal fluorescence yield of the light-ad...