Alternating Respiratory Pathways Are Involved In The Study Of Leaf Light Adaptation

Light is the energy source of photosynthesis for plants?However, excess energywill be produced when the light energy absorbed by plants beyond their maximalphotonsynthetic capacties. The excess light was deleterious since it can result inaccumulation of reducing equivalents, photosynthetic electron transport chain overreduction and production of active oxygen, so that it can lead to photoinhibition.Under natural environment, especially sunny day, the leaves of plant canopysurface exposed to high light for a long, and the light intensity is strongest at noon.Part of the regional plant leaves are subjected to high irradiance stress due to thecovering of upper branches and leaves, the angle of the sun and transient shade fromeclounds. Moreover, such light can change the photochemical efficiency in tissuesdirectly experiencing high irradiance stress, and in distal tissues indirectlyexperiencing high irradiance stress.Under the natural condition, the occurrence of photoinhibition is inevitable. So inthe long-term evolution, plants have evolved a series of photoprotection mechanismsto ease the photoinhibition?At home and abroad, study of light protection havenumerous reports that focused on changing the blade angle changes, heat dissipation,cyclic electron transport, water-water cycle, chloroplast breathing, light breathing andreactive oxygen enzymatic and non-enzymatic scavenging systems, etc. However, fewstudies have been done about extra-chloroplastic defense mechanisms?With further research?it has been found that excess reducing equivalents wouldbe transferred from the chloroplast to mitochondria etc through multiple pathwaysunder high light intensity to be oxidized?The latest research showed that plant leavesin bright light conditions greatly increases the activity of alternative respiratorypathway?It is speculated that the excess reducing equivalents in mitochondria fromchloroplast activates alternative respiratory pathways?accelerating the dissipation ofexcess reducing equivalents and thus indirectly alleviates the photoinhibition in plantleaves. However, the former researches on the mechanism of the alternativerespiratory pathway are no clear, the study only limited to the use of chemicalinhibitors or single bright light intensity. The report of alternative respiratory pathway in different light intensity and system adaptability is still limited.The first part of this paper is studied that the changes of chlorophyllfluorescence parameters of alternative oxidase gene overexpression lines (XX-2) andwild-type (WT) lines of Arabidopsis thaliana plants under different light intensities.The results showed that the parameter of maximal PS?quantum yield (Fv/Fm) didn'thave significant change in two type leaves. The parameters of effective PSII quantumyield(Y(II)) and coefficient of photochemical quenching (qp) decreased according tothe increase of light intensity; the Y(II) and qp of XX-2leaves increased at the samelight intensity compared with WT. The parameters of coefficient of nonphotochemicalquenching (NPQ) and quantum yield of regulated energy dissipation (Y(NPQ))increased with elevation of light intensity; the NPQ and Y(NPQ) of XX-2leavesdeclined at the same light intensity compared with WT. The quantum yield of nonregulated energy dissipation (Y(NO)) declined with the aggrandizement of lightintensity; the Y(NO) of XX-2leaves decreased at the same light intensity comparedwith WT at the range of165~1855?mol m-2s-1. The parameter of electron transportrate increased at the early stage of light treatment, the highest was found at580?molm-2s-1, then declined according to the increase of light intensity; the ETR of XX-2leaves increased at the same light intensity compared with WT.The second part study that the response of the alternative respiratory pathway onhigh light systemic acclimation of leaves. The results show that the Y(II), qp andefficiency of excitation capture by PS?centers (Fv'/Fm') have declined, NPQ andH2O2content significantly increased in leaves, which were irradiated by high light for40min, it implies that high light can cause photoinhibition to leave. However, oppositeside (indirectly experiencing high irrsdiance stress) leaves also have similar trends, itindicates that high light also cause changes of photochemical efficiency to directlyand indirectly experiencing high irrsdiance stress. At the same time, it shows that highlight stress has systemic effect to light use efficiency of plants.Before one side of leaves were irradiated by LL(low light) or high light(HL),using the alternative respiration pathway specific inhibitor salicylhydroxamic acid(SHAM) treat the other side of the same leaf. The results showed that the chlorophyllfluorescence parameters of Y(II), qp and Fv'/Fm' have declined, NPQ and H2O2 content increased of leaves compared with controls. Moreover, the rangeability in HLwas greater than that in LL. It shows that inhibition of AOX pathway would causeseverer photoinhibition in HL than in LL. The above results demonstrate that AOXpathway played an important role in photoprotection in leaves under HL and LLintensities. In addition, the effect of SAA (systemic acquired acclimation) onacclimation of leaves to oxidative stress was assayed by subjecting leaf discs to HLplus H2O2. The results show that the resistant to oxidative damage of distal leaves, HLleaves and leaves pretreated with SHAM under LL and HL intensity were didn'tpresent. Thus, indicating that systemic acquired acclimation and the influence of AOXon SAA can't reflect the light oxidation resistance of leaves.