干旱胁迫对银杏叶片光合系统Ⅱ荧光特性的影响

Abstract

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(PSⅡ) 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 PSⅡ structure,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 PSⅡ 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 trees were grown in pots in a greenhouse and subjected to one of four drought treatments(20,30,40,or 50 days without watering) and compared with control trees(0 days without watering).We determined 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 PSⅡ destruction and instability.The damage to PSⅡ structure was accompanied by changes in the fluorescence characteristics.The minimal fluorescence(F0) increased and maximal fluorescence(Fm) decreased gradually with increasing levels of drought stress.The absorption flux per reaction center(RC)-ABS/RC,electron transport flux per RC(ET0/RC),trapped energy flux per RC(TR0/RC),and trapped energy flux per cross section(CS)-TR0/CS0 increased significantly,while the electron transport flux per CS(ET0/CS0) 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 PSⅡ acceptor side.During drought stress,there were gradual increases in the approximated initial slope of the fluorescence transient(M0) and the relative variable fluorescence intensity at the J-step and I-step(VJ and VI).The dissipated energy flux per RC(DI0/RC) and dissipated energy flux per CS(DI0/CS0),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/CS0),Fv/Fm and performance index on an absorption basis(PIabs),which reflect the photochemical efficiency of PSⅡ,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 PSⅡ function in ginkgo leaves was due to an inbalance in energy flux allocation,instability of PSⅡ units,inactivation of reaction centers,disturbance of electron transport,and damage to the oxygen-evolving complex under drought stress.QA accumulation on the PSⅡ acceptor side may have played a major role in the decrease in PSⅡ electron transport activity that accompanied reaction center inactivation.PIabs was more sensitive than Fv/Fm to drought stress,and may be used as a biomarker to determine the extent of drought stress in ginkgo leaves.