Plants possess an essential ability to rapidly down-regulate light-harvesting in response to high light. This photoprotective process involves the formation of energy-quenching interactions between the chlorophyll and carotenoid pigments within the antenna of Photosystem II (PSII). The nature of these interactions is currently debated, with, among others, ‘incoherent’ or ‘coherent’ quenching models (or a combination of the two) suggested by a range of time-resolved spectroscopic measurements. In ‘incoherent quenching’, energy is transferred from a chlorophyll to a carotenoid and is dissipated due to the intrinsically short excitation lifetime of the latter. ‘Coherent quenching’ would arise from the quantum mechanical mixing of chlorophyll and carotenoid excited state properties, leading to a reduction in chlorophyll excitation lifetime. The key parameters are the energy gap, $$\\Updelta \\varepsilon =\\varepsilon_{\\rm Car}-\\varepsilon_{\\rm Chl},$$\n \n Δ\n ε\n =\n \n ε\n Car\n \n -\n \n ε\n Chl\n \n,\n \n and the resonance coupling, J, between the two excited states. Coherent quenching will be the dominant process when $$-J<\\Updelta \\varepsilon <J,$$\n \n -\n J\n <\n Δ\n ε\n <\n J\n,\n \n i.e., when the two molecules are resonant, while the quenching will be largely incoherent when $$\\varepsilon_{\\rm Chl}>(\\varepsilon_{\\rm Car}+J).$$\n \n \n ε\n Chl\n \n >\n \n (\n \n ε\n Car\n \n +\n J\n )\n \n.\n \n One would expect quenching to be energetically unfavorable for $$\\varepsilon_{\\rm Chl}<(\\varepsilon_{\\rm Car}-J).$$\n \n \n ε\n Chl\n \n <\n \n (\n \n ε\n Car\n \n -\n J\n )\n \n.\n \n The actual dynamics of quenching lie somewhere between these limiting regimes and have non-trivial dependencies of both J and $$\\Updelta \\varepsilon.$$\n \n Δ\n ε\n.\n \n Using the Hierarchical Equation of Motion (HEOM) formalism we present a detailed theoretical examination of these excitation dynamics and their dependence on slow variations in J and $$\\Updelta \\varepsilon.$$\n \n Δ\n ε\n.\n \n We first consider an isolated chlorophyll–carotenoid dimer before embedding it within a PSII antenna sub-unit (LHCII). We show that neither energy transfer, nor the mixing of excited state lifetimes represent unique or necessary pathways for quenching and in fact discussing them as distinct quenching mechanisms is misleading. However, we do show that quenching cannot be switched ‘on’ and ‘off’ by fine tuning of $$\\Updelta \\varepsilon$$\n \n Δ\n ε\n \n around the resonance point, $$\\Updelta \\varepsilon =0.$$\n \n Δ\n ε\n =\n 0\n.\n \n Due to the large reorganization energy of the carotenoid excited state, we find that the presence (or absence) of coherent interactions have almost no impact of the dynamics of quenching. Counter-intuitively significant quenching is present even when the carotenoid excited state lies above that of the chlorophyll. We also show that, above a rather small threshold value of $$J>10\\,\\mathrm{cm}^{-1}$$\n \n J\n >\n 10\n \n \n \n cm\n \n \n -\n 1\n \n \n \n quenching becomes less and less sensitive to J (since in the window $$-J<\\Updelta \\varepsilon <J$$\n \n -\n J\n <\n Δ\n ε\n <\n J\n \n the overall lifetime is independent of it). The requirement for quenching appear to be only that $$J>0.$$\n \n J\n >\n 0\n.\n \n Although the coherent/incoherent character of the quenching can vary, the overall kinetics are likely robust with respect to fluctuations in J and $$\\Updelta \\varepsilon.$$\n \n Δ\n ε\n.\n \n This may be the basis for previous observations of NPQ with both coherent and incoherent features.

为了探讨喀斯特植被恢复树种青冈栎对干旱环境的适应机制,以当年生青冈栎实生幼苗为材料进行盆栽控水试验,设置正常浇水(-0.1 MPa,对照)、轻度干旱(-0.5 MPa)、中度干旱(-0.9 MPa)和重度干旱(-1.5 MPa)胁迫处理,研究持续干旱处理(15、30、45、60和90天)对其幼苗叶片生长及叶绿素荧光参数的影响.结果表明: 随着干旱胁迫强度的加剧, 叶片的单叶面积、健康叶片数量、叶片含水率、总叶绿素、类胡萝卜素、最大荧光、最大光化学量子产量和潜在光化学效率均显著下降,而枯叶数量和初始荧光显著增加.这些参数在轻度干旱胁迫处理和对照之间均无显著差异.在轻度干旱胁迫处理下,青冈栎幼苗叶片PSⅡ单位反应中心吸收的光能(ABS/RC)、捕获用于还原q_A的能量(TR_o/RC)、单位面积内有活性的反应中心数目(RC/CS)、单位面积捕获的光能(TR_o/CS)和单位面积内用于电子传递的光能(ET_o/CS)均与对照无显著差异,其中RC/CS总是略高于对照,TR_o/CS和ET_o/CS均在第45天达到峰值,分别为606.12和440.78;而中度和重度干旱胁迫处理叶片的ABS/RC、TR_o/RC、ET_o/RC、DIR_o/RC、RC/CS、TR_o/CS和ET_o/CS均低于对照,且随干旱胁迫时间的延长,重度干旱胁迫处理下降更显著.随干旱胁迫的加剧和时间的延长,叶片最大量子效率、其他电子受体的概率和电子传递的量子比率均下降,而用于热耗散的量子比率增加.轻度干旱胁迫下青冈栎幼苗表现出较强的适应性,中度干旱胁迫引起部分叶绿素荧光参数和光合色素指标下降,导致幼苗生长缓慢,而重度干旱则对幼苗生长的影响较为严重,但幼苗未出现死亡现象.因此,青冈栎幼苗有较强的干旱忍受能力,适合在喀斯特地区植被恢复重建和造林工程中应用.

To uncover adaptation mechanism of <i>Cyclobalanopsis glauca</i> to the arid environment of Karst areas, current-year seedlings of <i>C. glauca </i>were potted and grown under four soil water conditions: Normal water supply (-0.1 MPa), light drought stress (-0.5 MPa), moderate drought stress (-0.9 MPa), and severe drought stress (-1.5 MPa). We measured leaf growth and parameters of fast chlorophyll fluorescence induction dynamics after treated by 15, 30, 45, 60 and 90 days. With the increase of drought stress intensity, leaf area, number of green leaves, leaf water content, the contents of chlorophyll a+b and carotenoids, the maximum fluorescence, maximum photochemical quantum yield and potential photochemical efficiency significantly decreased, while the number of dead leaves and the initial fluorescence significantly increased. There was no significant difference in these parameters between light drought and normal water treatments. There were no significant difference in the absorption flux per reaction center (ABS/RC), captured light energy used to restore <i>q</i>_A (TR_o/RC), number of active reaction centers per unit area (RC/CS), light energy captured per unit area (TR_o/CS) and light energy used for electron transfer per unit area (ET_o/CS) between light drought and normal water treatments. Among these parameters, RC/CS was slightly higher under light drought treatment than that of normal water treatment. TR_o/CS and ET_o/CS reached peaks at the 45th day, being 606.12 and 440.78, respectively. Leaf ABS/RC, TR_o/RC, ET_o/RC, DIR_o/RC, RC/CS, TR_o/CS and ET_o/CS of <i>C. glauca </i>seedlings under mode-rate drought and severe drought treatment were lower than those of normal water treatment, and the parameters under severe drought stress decreased more significantly with the extension of drought stress time. With increasing intensity and duration of drought stress, the maximum quantum efficiency, probability of other electron acceptors, and quantum ratio of electron transfer decreased, but quantum yield for energy dissipation increased. These results demonstrated that <i>C. glauca </i>seedlings under light drought condition showed some degree of adaptability and resistance to drought. Mode-rate drought treatment caused a decline in chlorophyll fluorescence and photosynthetic pigments, leading to slow growth of seedlings. Severe drought had a serious impact on growth of <i>C. glauca </i>seedlings, but did not lead to seedling death. Therefore, <i>C. glauca</i> seedlings showed strong drought tolerance, which were suitable for the application of vegetation restoration and reforestation projects in Karst areas.