干旱胁迫对银杏叶片光合系统Ⅱ荧光特性的影响

Effects of drought on fluorescence characteristics of photosystem Ⅱ in leaves of Ginkgo biloba

为了探讨银杏叶片在长期干旱胁迫下光合性能的变化,采用盆栽控水法对银杏幼苗进行干旱处理,并分别在处理后第0、20、30、40、50天,对荧光动力学曲线和荧光参数进行了测定和JIP-test分析。结果表明:随着干旱的加剧,叶片叶绿素含量逐渐递减。由于干旱导致银杏叶片的光合系统Ⅱ(PSⅡ)结构破坏和稳固性减弱,其荧光动力学曲线出现了K相和L相的升高,在干旱处理40 d后升高加剧。PSⅡ结构的变化导致了一系列荧光参数的变化,初始荧光F0逐渐升高,最大荧光Fm逐渐降低;代表单位反应中心活性的参数ABS/RC、ET0/RC、TR0/RC逐渐升高;单位面积捕获的光能TR0/CS0高于对照,单位面积用于电子传递的光能ET0/CS0后期下降;表示受体侧电子传递活性的参数Sm、ψ0、φE0逐渐下降,M0、VJ、VI逐渐升高;表示热耗散的参数DI0/RC、DI0/CS0随干旱时间延长而急剧升高;活性反应中心数目RC/CS0减少;代表PSⅡ光合效率的参数Fv/Fm、PIabs逐渐下降;代表PSⅡ供体侧活性的参数WK随干旱时间延长而逐渐升高,OEC逐渐减少。干旱胁迫导致银杏叶片PSⅡ反应中心失活,能流分配失衡,电子传递受阻,PSⅡ放氧复合体失活和PSⅡ稳固性减弱,从而破坏PSⅡ光合功能。而反应中心失活和受体侧电子Q-A的累积是造成PSⅡ电子传递活性减弱的主要原因。干旱条件下,银杏PSⅡ的PIabs比Fv/Fm对干旱的反应更灵敏,可作为银杏叶片受到伤害的指标。

In plants, photosynthesis is one of the physiological processes that are sensitive to drought stress. Leaves of drought-stressed plants show decreases in biomass and chlorophyll content and changes in chlorophyll fluorescence parameters. Photosystem Ⅱ （PS Ⅱ ） is the most sensitive component of the photosynthetic apparatus to environmental stresses. Analyses of chlorophyll fluorescence dynamics are useful to determine the effects of environmental stresses on PSⅡstructure, and to study the response mechanisms of the photosynthetic machinery. Ginkgo （Ginkgo biloba ） is often subjected to drought during its growth season. However, little is known about the physiological mechanisms underlying changes in the photochemical activities of PS Ⅱ in Gingko. In this study, therefore, we analyzed changes in the fluorescence characteristics of PSⅡ in chloroplasts of mesophyll cells in drought-stressed leaves of the G. biloba cultivar ＇ Taixingdafuzhi＇. Five-year-old ginkgo treatments （20, 30, 40, or 50 days trees were grown in pots in a greenhouse without watering） and compared with control and subjected to one of four drought trees （0 days without watering）. Wedetermined chlorophyll fluorescence dynamic curves and parameters and performed a JIP-test. The chlorophyll content in ginkgo leaves decreased gradually with increasing levels of drought stress. The fluorescence dynamics curves showed increased values at K and L phases. These increases in the values of fluorescence dynamics curves were particularly significant at 40 days of drought treatment, and were attributed to PS Ⅱ destruction and instability. The damage to PS Ⅱ structure was accompanied by changes in the fluorescence characteristics. The minimal fluorescence （F0 ） increased and maximal fluorescence （F） decreased gradually with increasing levels of drought stress. The absorption flux per reaction center （RC）-ABS/RC, electron transport flux per RC （ETo/RC）, trapped energy flux per RC （TRo/RC）, and trapped energy flux per cross section （CS）-TRo/CSo increased significantly, while the electron transport flux per CS （ETo/CSo ） decreased as the period of drought lengthened. During drought stress, there were gradual decreases in the normalized total complementary area above the O-J-I-P transient （ Sm） , the probability that a trapped exciton will move an electron into the electron transport chain beyond QA（ψ0 ）, and quantum yield for electron transport （φE0 ）, which reflect electron transport activities of the PS Ⅱ acceptor side. During drought stress, there were gradual increases in the approximated initial slope of the fluorescence transient （ Mo ） and the relative variable fluorescence intensity at the J-step and I-step （ Vj and V1）. The dissipated energy flux per RC （DIo/RC） and dissipated energy flux per CS （DIo/CSo ）, which reflect heat dissipation, significantly increased in response to drought stress. As the period of drought stress lengthened, there were decreases in the density of RCs （RC/CSo ）, Fv/Fm and performance index on an absorption basis （Plabs ）, which reflect the photochemical efficiency of PS Ⅱ , while there were increases in relative variable fluorescence at 300 μs of the chlorophyll fluorescence transient （ WK） , reflecting electron transport activities of the donor side. Moreover, we observed degradation of the oxygen- evolving complex （OEC） as the period of drought stress lengthened. Taken together, these results indicated that the decline of PS Ⅱ function in ginkgo leaves was due to an inbalance in energy flux allocation, instability of PS Ⅱ units, inactivation of reaction centers, disturbance of electron transport, and damage to the oxygen-evolving complex under drought stress. QA accumulation on the PS Ⅱ acceptor side may have played a major role in the decrease in PS Ⅱ electron transport activity that accompanied reaction center inactivation. Plabs was more sensitive than FJFm to drought stress, and may be used as a biomarker to determine the extent of drought stress in ginkgo leaves.