Xanthophyll Cycle and Its Molecular Mechanism in Photoprotection

When plants absorb more light than that can be used for photosynthesis, the excessive energy can cause photoinhibition and even photooxidation of photosynthetic apparatus. Xanthophyll cycle-dependent photo-protection is believed to be the main mechanism for plants to deal with excessive light energy. This review focuses on molecular biological aspects and regulations of violaxanthin de-epoxidase and zeaxanthin epoxidase involved in xanthophyll cycle. We will summarize the functions of xanthophyll cycle, especially recent advances in its thermal dissipation mechanism of photoprotection. Some interesting issues deserving further study will be discussed.

PSⅡ Photochemistry and Xanthophyll Cycle in Two Superhigh-yield Rice Hybrids, Liangyoupeijiu and Hua-an 3 During Photoinhibition and Subsequent Restoration

PSⅡ photochemistry and xanthophyll cycle during photoinhibition (exposed to strong light of 2 000 μmol photons·m -2 ·s -1 ) and the subsequent restoration were compared between two superhigh_yield rice hybrids (Liangyoupeijiu and Hua_an 3, the newly developed rice hybrids from two parental lines) and the traditional rice hybrid Shanyou 63 developed from three parental lines. The results showed that the maximal efficiency of PSⅡ photochemistry ( Fv/Fm ), the efficiency of excitation energy capture by open PSⅡ centers ( Fv′/Fm′ ), and the yield of PSⅡ electron transport ( Φ PSⅡ ) of the three rice hybrids decreased during photoinhibition. However, a greater decrease in Fv/Fm , Fv′/Fm′ , and Φ PSⅡ was observed in Shanyou 63 than in Liangyoupeijiu and Hua_an 3. At the same time, the components of xanthophyll cycle, antherxanthin (A) and zeathanxin (Z) increased rapidly while violaxanthin (V) decreased considerably. Both the rate of accumulation and the amount of A and Z in the two superhigh_yield rice hybrids were higher than that in Shanyou 63. The de_epoxidation state (DES) of xanthophyll cycle increased rapidly with the fast accumulation of A and Z, and reached the maximal level after first 30 min during photoinhibition. Of the three hybrids, the increasing rate of DES in Liangyoupeijiu and Hua_an 3 was higher than that in Shanyou 63. After photoinhibition treatment, the plant materials were transferred to a dim light (70 μmol photons·m -2 ·s -1 ) for restoration. During restoration, both chlorophyll fluorescence parameters and xanthophyll cycle relaxed gradually, but the rate and level of restoration in the two superhigh_yield rice hybrids were higher than those in Shanyou 63. Our results suggest that Liangyoupeijiu and Hua_an 3 had higher resistance to photoinhibition and higher capacity of non_radiative energy dissipation associated with xanthophyll cycle, as well as higher rate of restoration after photoinhibition, than Shanyou 63 when subjected to strong light.

Xanthophyll Cycle and Its Relative Enzymes

Light is a fundamental source of energy but is also potentially harmful to organisms. Plants have evolved a variety of regulatory mechanisms to respond to the naturally varying light conditions. Xanthophyll cycle is now recognized as a key regulator and photoprotective mechanism found ubiquitously in plants. Xanthophyll cycle has multiple functions, such as thermal dissipation, protection against oxidative stress caused by light, modulation of the structure of thylakoid membrane, involving in blue light signal transduction and regulating the synthesis ofABA （Abscisic acid）. VDE （Violaxanth de-epoxidase） and ZE （zeaxanth epoxidase）, are involved in xanthophyll cycle. This paper outlined the functions of xanthophylls cycle and its relative enzymes.

Xanthophyll Cycle and Inactivation of Photosystem Ⅱ Reaction Centers Alleviating Reducing Pressure to Photosystem Ⅰ in Morning Glory Leaves under Short-term High Irradiance

Under 30-min high irradiance （1500μmol m^-2 s^-1）, the roles of the xanthophyll cycle and D1 protein turnover were investigated through chlorophyll fluorescence parameters in morning glory （Ipomoea setosa） leaves, which were dipped into water, dithiothreitol （DTT） and lincomycin （LM）, respectively. During the stress, both the xanthophyll cycle and D1 protein turnover could protect PSI from photoinhibition. In DTT leaves, non-photochemical quenching （NPQ） was inhibited greatly and the oxidation level of P700 （P700^＋） was the lowest one. However, the maximal photochemical efficiency of PSII （Fv/Fm） in DTT leaves was higher than that of LM leaves and was lower than that of control leaves. These results suggested that PSI was more sensitive to the loss of the xanthophyll cycle than PSII under high irradiance. In LM leaves, NPQ was partly inhibited, Fv/Fm was the lowest one among three treatments under high irradiance and P700^＋ was at a similar level as that of control leaves. These results implied that inactivation of PSII reaction centers could protect PSI from further photoinhibition. Additionally, the lowest of the number of active reaction centers to one inactive reaction center for a PSII cross-section （RC/CSo）, maximal trapping rate in a PSll cross-section （TRo/CSo）, electron transport in a PSll cross-section （ETo/CSo） and the highest of 1-qP in LM leaves further indicated that severe photoinhibition of PSII in LM leaves was mainly induced by inactivation of PSII reaction centers, which limited electrons transporting to PSh However, relative to the LM leaves the higher level of RC/CSo, TRo/CSo, Fv/Fm and the lower level of 1-qP in DTT leaves indicated that PSI photoinhibition was mainly induced by the electron accumulation at the PSI acceptor side, which induced the decrease of P700^＋ under high irradiance.

PROTECTIVE ROLES OF D1 PROTEIN TURNOVER AND THE XANTHOPHYLL CYCLE IN TOMATO(SOLANUM LYCOPERSICUM)UNDER SUB-HIGH TEMPERATURE AND HIGH LIGHT STRESS

D1 protein turnover and the xanthophyll cycle(XC)are important photo-protective mechanisms in plants that operate under adverse conditions.Here,streptomycin sulfate(SM)and dithiothreitol(DTT)were used in tomato plants as inhibitors of D1 protein turnover and XC to elucidate their photoprotec-tive impacts under sub-high temperature and high light conditions(HH,35°C,1000 limol m 2.S1).SM and DTT treatments significantly reduced the net photosynthetic rate,apparent quantum efficiency,maximum photochemical efficiency,and potential activity of photosystem II,leading to photoinhibition and a decline in plant biomass under HH.The increase in reactive oxygen species levels resulted in thylakoid membrane lipid peroxidation.In addition,there were increased non-photochemical quenching and decreased chlorophyll pigments in SM and DTT application,causing an inhibition of D1 protein production at both transcriptional and translational levels.Overall,inhibition of D1 turnover caused greater photoinhibition than XC inhibition.Additionally,the recovery levels of most photosynthesis indicators in DTT-treated plants were higher than in SM-treated plants.These findings support the view that D1 turnover has a more important role than XC in photoprotection in tomato under HH conditions.

Disruption of LEAF LESION MIMIC 4 affects ABA synthesis and ROS accumulation in rice

Lesion mimic mutants(LMMs) are advantageous materials for studying programmed cell death(PCD).Although some rice LMM genes have been cloned, the diversity of functions of these genes indicates that the mechanism of cell death regulation in LMMs needs further study. In this study, we identified a rice light-dependent leaf lesion mimic mutant 4(llm4) that showed abnormal chloroplast structure, photoinhibition, reduced photosynthetic protein levels, massive accumulation of reactive oxygen species(ROS), and PCD. Map-based cloning and complementation testing revealed that LLM4 encodes zeaxanthin epoxidase(ZEP), an enzyme involved in the xanthophyll cycle, which functions in plant photoprotection,ROS scavenging, and carotenoid and abscisic acid(ABA) biosynthesis. The ABA content was decreased,and the contents of 24 carotenoids differed between the llm4 mutant and the wild type(WT). The llm4mutant showed reduced dormancy and greater sensitive to ABA than the WT. We concluded that the mutation of LLM4 resulted in the failure of xanthophyll cycle, in turn causing ROS accumulation. The excessive ROS accumulation damaged chloroplast structure and induced PCD, leading eventually to the formation of lesion mimics.

Photoinhibition in Shaded Cotton Leaves After Exposing to High Light and the Time Course of Its Restoration

Chlorophyll fluorescence emission, pigment composition and photosynthetic rate of shade-grown cotton ( Gossypium hirsutum L.) plants were measured immediately after suddenly exposing to full sunlight and at regular intervals there after within 15 d. Photoinhibition occurred in shade-grown cotton leaves immediately after exposed to full sunlight. The chlorophyll fluorescence parameter F-v/F-m and PhiPS II, which reflect the efficiency of PS II,obviously decreased in shade-grown leaves, much lower than that of the full sunlight-grown leaves. On the contrary, F-o value was sharply increased. Neither of these parameters could completely recover till next morning. The photoinhibition was chronic and continued for about 4 d, while the F-v/F-m and the net photosynthetic rate ( P-n) continued to decline, then began to increase gradually 6 d later and turned stable after 10 - 12 d, appearing as an acclimation phenomenon. However, the final value of F-v/F-m and P-n did not reach the level as in those leaves grown in the full sunlight ever before. The final P-n was higher by 60% than that before exposure, but lower for more than 40% than that of the full sunlight-grown leaves. The most notable response of chloroplast pigment composition was a pronounced increase in the pool size of carotenoids in xanthophyll cycle over a period of 3 d. The results indicated that when shade-grown cotton seedlings were suddenly transferred to the full sunlight, the decline of F-v/F-m and P-n might associate with the damage of the PS II reaction center. During the light acclimation, photoprotective mechanisms such as the xanthophyll cycle-dependent energy dissipation were increased, so that photodamage in leaves transferred from low to high light might be reduced.

Photoinhibition and Photoprotection in Ginkgo biloba leaves: Influence of Temperature, CO 2 and O 2

In midday ginkgo ( Ginkgo biloba L.) leaves have to bear photon flux density over 1400 μmol·m -2 ·s -1 in combination with high temperatures around 35 ℃ at natural habitat. They show typical midday depression of stomatal conductance and of CO 2 assimilation rate. The zeaxanthin changes with light intensity during the day. The influence of the combination of strong light and temperature on photoinhibition was also examined in the laboratory. A low CO 2 internal conductance (31 mmol·m -2 ·s -1 ) was found in ginkgo leaves, which had been exposed to excessive light at temperature between 15 ℃ and 35 ℃ with reduced CO 2 (80 μL·L -1 ) or oxygen (2%) for 2 h, causing a low CO 2 concentration at the carboxylation site and a high proportion of photorespiration. The ratio of electron transport to CO 2 fixation was rather high in ginkgo (16 e -/CO 2 at 25 ℃) as compared with other plants. It increased with temperature also in 2% O 2 which could not be explained solely as due to change of photorespiration. The reduction of oxygen in 340 or 80 μL·L -1 CO 2 had no effect on the extent of photoinhibition at all temperatures, which indicated that electron flow caused by photorespiration in excess light was negligible in protective effect in ginkgo leaves. However, a decreased CO 2 concentration increased photoinhibition, especially at high temperature. It is concluded that the dissipation of excessive excitation energy in the PSⅡ antennae through the xanthophyll cycle may be the major protective mechanism to preventing from the deteriorated effects of strong light in ginkgo leaves.

Photochemical Efficiency of PSⅡ and Membrane Lipid Peroxidation in Leaves of indica and japonica Rice (Oryza sativa) Under Chilling Temperature and Strong Light Stress Conditions

Relationships between fluorescence parameters and membrane lipid peroxidation in leaves of indica and japonica rice (Oryza sativa L.) during later growth stage were studied under chilling temperature and strong light stress conditions. Results showed that D1 protein contents of PSⅡ in photosynthetic apparatus dropped, the generation of antheraxanthin (A) and zeaxanthin (Z) of xanthophyll cycle were inhibited partly, PSⅡ photochemical efficiency (F v/F m)and non-photochemical quenching (q N) were also decreased obviously. In addition, endogenous active oxygen scavenger—superoxide dismutase (SOD) reduced, superoxide anion radical (O -· 2) and malondialdehyde (MDA) accumulated, as a result, photooxidation of leaves occurred under chilling temperature and strong light stress conditions. Obvious differences in the changes of the above mentioned physiological parameters between indica and japonica rice were observed. Experiments in leaves treated with inhibitors under chilling temperature and strong light conditions showed that indica rice was more sensitive to chilling temperature with strong light and subjected to photooxidation more than japonica rice. Notable positive correlation between D1 protein contents and F v/F m or (A+Z)/(A+Z+V), and a marked negative correlation between F v/F m and MDA contents were obtained by regression analysis in indica and japonica rice during chilling temperature and strong light conditions. According to the facts mentioned above, it was inferred that PSⅡ photochemical efficiency(F v/F m) was the key index to forecast for the prediction of photooxidation under stress circumstances and the physiological basis were the synthetic capacity of D1 protein and the protection of xanthophyll cycle.

Photosynthesis of Resurrection Angiosperms

Resurrection plants which are able to quickly reactivate after falling into a period of anabiosis caused by dehydration have been very rare among angiosperms, especially among dicotyledons whose chlorophyll content and chloroplast structure little changed in the course of desiccation, therefore has been called homoiochlorophyllous desiccation-tolerant plants (HDTs). Another type of resurrection angiosperms that lost its chlorophyll dining desiccation is called poikilochlorophyllous desiccation-tolerant plants (PDTs). HDTs have been received more attention because of simplicity of protection mechanism which is much easy to the study and utilization of the desiccation tolerance of resurrection angiosperms. Recent advances in studies of photosynthesis of resurrection angiosperms indicate that photochemical activities are sensitive indicators for the study of physiological state of resurrection angiosperms during desiccation and rehydration. Photochemical activities of resurrection angiosperms are inhibited with loss of water similar to those of general plants, however, the magic thing is that they could reactivate rapidly during rehydration even losing more than 95% water. Up-regulations in xanthophyll cycle and antioxidative systems as well as preservation in integrity and stability of photosynthetic membranes during desiccation may be very important to desiccation tolerance of resurrection angiosperms. The fact that phosphate treatment in rehydration stage also strongly influences resurrection indicated importance of studies on rehydration stages of resurrection angiosperms.

Photoinhibition and Photooxidation in Leaves of indica and japonica Rice Under Different Temperatures and Light Intensities

Physiological indices related to the efficiency (F-v/F-m) of light energy conversion in PS II and the peroxidation of membrane lipid were measured in leaves of Oryza sativa L. sp. indica rice cv. 'Shanyou 63' and sp. japonica rice cv. '9516'' under different temperatures and fight intensities for 4 days. No changes in F-v/F-m and membrane lipid peroxidation product (MDA) were observed, so neither photoinhibition nor photooxidation happened in both rice cultivars under moderate temperature and medium light intensity. However, F-v/F-m dropped obviously with no change in MDA contents, and photoinhibition appeared in indica rice cv. 'Shanyou 63' under medium temperature and strong light intensity. Furthermore, both photoinhibition and photooxidation were observed in two rice cultivars under chilling temperature and strong light intensity. Experiments with inhibitors under chilling temperature and strong light intensity showed that indica rice had a decrease in DI protein content and SOD activity, and the extent of inhibition of xanthophyll. cycle and nonphotochemical quenching (qN) was larger, and a higher level of MDA was observed. The photoinhibition and photooxidation in indica rice were more distinct as compared with japonica rice. The authors suggested that PS II light energy conversion efficiency (F-v/F-m) and membrane lipid peroxidation were the key indices for the detection of photooxidation.

Changes in Violaxanthin Deepoxidase Activity and Unsaturation of Thylakoid Membrane Lipids in Indica and Japonica Rice Under Chilling Condition and Strong Light

To explore the differences of sensitivities to chilling and strong light in indica and japonica rice (Oryza sativa L), the changes in unsaturation of thylakoid membrane lipids and xanthophyll cycle were studied under chilling condition and strong light. The contents of unsaturated fatty acids of thylakoid membrane lipids decreased and that of the saturated ones increased with the time of chilling and strong light treatment, resulting in the reduction of the index of unsaturation of fatty acids (IUFA). The activity of violaxanthin deepoxidase (VDE), a key enzyme of xanthophyll cycle, also reduced. The content of violaxanthin (V) increased, and the contents of antheraxanthin (A) and zeaxanthin M decreased, the ratio of (A+Z)/ (A+Z+V) decreased correspondingly. Arrhenius analysis showed that VDE was sensitive to both chilling and unsaturation level of thylakoid membrane lipids. Correlation analysis showed that there was distinctly positive relationships between IUFA of thylakoid membrane lipids and the activity of VDE, Fv/Fm, and D, protein content. Lower IUFA values, less fluidity and stability of thylakoid membrane lipids, lower VDE activity and (A+Z)/(A+Z+V) ratio were found in indica rice cv. Shanyou 63 than in japonica rice cv. 9516 under chilling and strong light.

Comparative Study on Photoinhibition Between Two Wheat Genotypes

Changes in the efficiency of the primary light energy conversion, fluorescence quenching parameters and contents of photosynthetic pigments were compared between two wheat ( Triticum aestivum L.) genotypes in response to high light stress. The contents of chlorophyll and carotenoid in “Jing_411' were slightly higher than those in “Xiaoyan_54'. Under high light stress, photoinhibition as indicated by a sustained decrease in PSⅡ photochemical efficiency was more pronounced in “Jing_411' than in “Xiaoyan_54'. The content of ascorbate and the activity of the deepoxidase were higher in “Xiaoyan_54' than in “Jing_411'. The genotypic difference in resistance to photoinhibition is related to the capacity to dissipate the excess energy nonradiatively.

Responses of Chlorophyll Fluorescence and Carotenoids Biosynthesis to High Light Stress in Rice Seedling Leaves at Different Leaf Position

In the present study, we investigated the changes of photosynthesis, chlorophyll fluorescence and the content of carotenoid pigments in rice (Oryza sativa L.) seedling leaves and their responses to high light. The results showed that the rate of photosynthesis, the contents of individual and total carotenoids and the pool size of xanthophyll cycle decreased with age increasing of the leaf. When the leaves were exposed to high light for 2 h, the qN of mature leaf (5th leaf) increased more significantly than that of younger (6th leaf) and older leaves (3rd and 4th leaf). Comparing with the leaves before exposure to high light, the excitation pressure on PSⅡ (1- qP ) increased by 44%, 57%, 19% and 45% in the 3rd, 4th, 5th and 6th leaf under high light, respectively. The highest content of carotenoids and the greatest conversion of violaxanthin to zeaxanthin were found in the 5th leaf, and it was consistent with the 5th leaf exhibiting the strongest resistance to high light. Our results suggested that the ability of rice leaf to resist photoinhibition is related to the level of carotenoids and the ability of carotenoids biosynthesis.

Cloning and Expression Analysis of Violaxanthin De-Epoxidase (VDE) cDNA in Wheat

Violaxanthin de-epoxidase (VDE) is the key enzyme in the xanthophyll cycle and protects plant photosynthetic apparatus from the damage of excessive light. A wheat (Triticum aestivum L cv. Xiaoyan 54) VDE cDNA was obtained using RT-PCR method. Its deduced protein sequence shares high identity with that of Arabidopsis and rice. Southern blot revealed that there are three copies of VDE gene per haploid genome of wheat. VDE transcript levels were higher in green leaf than in root, seed and etiolated leaf. Northern blotting analysis indicated that VDE mRNA level is induced during greening process of etiolated wheat seedling and increased by intense light illumination.

Photosynthetic Characteristics of a Super High Yield Cultivar of Winter Wheat During Late Growth Period

The mechanism of high yield of winter wheat in the field at late growth period was investigated by measuring the photosynthetic characteristics of photosystem Ⅱ （PSⅡ） and xanthophylls cycle, which could provide physiological reference for breeding. Weimai 8 （W8）, a super high yield cultivar, and Lumai 14 （L14）, a control cultivar were object. The photosynthetic rate （Pn）, parameters of chlorophyll fluorescence and chlorophyll content were measured. The Pn, maximum photochemical efficiency of PSII （Fv/Fm）, quantum yield of PSII electron transport （ΦPSⅡ）, efficiency of excitation energy capture by open PSII reaction centers （Fv＇/Fm＇）, and photochemical quenching coefficient （qP） were higher in Weimai 8 compared to that in Lumai 14, a commercial high yield cultivar. Furthermore, Weirnai 8 showed a lower non- photochemical quenching coefficient and a lower de-epoxidized ratio of the xanthophyll cycle pigments than of Lumai 14 at late growth period. At mature stage, chlorophyll content of different leaves decreased both in Weimai 8 and Lumai 14. Chlorophyll content in flag, second and third leaf from the top of plant decreased more in Lumai 14 than in Weimai 8. These results suggested that Weimai 8 had more antenna pigments to absorb light energy, and had higher photosynthetic capability and photochemical efficiency of PSⅡ. The yield of Weimai 8 was also higher than that of Lumai 14.

Effects of Cyclic Electron Flow Inhibitor(Antimycin A)on Photosystem Photoinhibition of Sweet Pepper Leaves upon Exposure to Chilling Stress Under Low Irradiance

In chloroplast, there were two pathways involved in the cyclic electron flow around photosystem 1 （PS 1）. One was the NADH dehydrogenase （NDH）-dependent flow and the other was the ferredoxin quinone reductase-dependent flow. It was proposed that the NDH-dependent cyclic electron flow around PSI was related to the xanthophyll cycle-dependent non-photochemical quenching （NPQ） at chilling temperature under low irradiance （CL）. The function of the chloroplastic cyclic electron flow around PS 1 was examined by comparing sweet pepper （Capsicum annuum L.） control with its antimycin A （AA）-fed leaves upon exposure to CL stress. During CL stress, the maximum photochemical efficiency of PS2 （Fv/Fm） decreased markedly in both controls and AA-fed leaves, and P700＋ was also lower in AA-fed leaves than in controls. These results implied that cyclic electron flow around PS 1 functioned to protect the photosynthetic apparatus from CL stress. Under such stress, NPQ and PS2-driven electron transport rate were different between AA-fed leaves and controls. The lower NPQ in AA-fed leaves might be related to an inefficient proton gradient across thylakoid membranes （ApH） because of inhibiting cyclic electron flow around PS 1 under CL stress.

Traits Related to Chilling-Induced Photoinhibition in Leaves of indica and japonica Rice (Oryza sativa)

Physiological indices related to PS II photochemical efficiency (Fv/Fm) and membrane lipid peroxidation were measured in leaves of indica rice cv Shanyou 63 and japonica rice 9516 at different temperatures and light intensities for four days. No obvious changes in Fv/Fm and MDA were observed in both indica and japonica rice at moderate temperature and medium PFD, implying neither photoinhibition nor photooxidation happened in these cases. In indica rice either at medium temperature with higher PFD or at lower temperature with medium PFD Fv/Fm dropped obviously with no changes in MDA contents, and photoinhibition appeared while photooxidation did not occur. However, D1 protein, Fv/Fm, (A+Z)/(A+Z+V), and SOD activities dropped, and O2 - production and MDA content increased accordingly, as well as both photoinhibition and photooxidation appeared in two rice varieties at lower temperature and higher PFD. Experiment with inhibitors at lower temperature and higher PFD showed that as compared with japonica rice the decrements appeared in D1 protein contents, SOD activities, and (A+Z)/(A+Z+V) ratios, the xanthophyll cycle and non-photochemical quench (qN) were inhibited in a more degree, as well as increments of MDA content were greater, thus exhibiting more distinct photoinhibition and photooxidation in indica rice. It is suggested that Fv/Fm and membrane lipid peroxidation product-MDA were the key indices to predict and diagnose photooxidation.

Photoinhibitive and Recovery Properties of Hybrid Rice Ⅱ You 129 under Field Conditions

Photoinhibitive properties of super-high-yielding hybrid rice Ⅱ you 129 and its adaptation mechanism to strong light stress were investigated by measuring the light-response curve, diurnal variations of net photosynthetic rate and chlorophyll fluorescence parameters of Ⅱ you 129 leaves and compared with Shanyou 63. Photoinhibition of rice flag leaves under field conditions mainly resulted from the increase of thermal dissipation, especially for thermal dissipation depended on the xanthophyll circle, but no destruction of photosynthetic apparatus occurred. Potentially super-high-yielding hybrid rice Ⅱ you 129 was more tolerant to photoinhibition than Shanyou 63, because it had higher light saturation intensity and maximum net photosynthetic rate; more active xanthophyll cycle, and more rapid recovery ability after photoinhibition.

High non-photochemical quenching of VPZ transgenic potato plants limits CO_(2) assimilation under high light conditions and reduces tuber yield under fluctuating light

Under natural conditions,photosynthesis has to be adjusted to fluctuating light intensities.Leaves exposed to high light dissipate excess light energy in form of heat at photosystem II(PSII)by a process called non-photochemical quenching(NPQ).Upon fast transition from light to shade,plants lose light energy by a relatively slow relaxation from photoprotection.Combined overexpression of violaxanthin de-epoxidase(VDE),PSII subunit S(PsbS)and zeaxanthin epoxidase(ZEP)in tobacco accelerates relaxation from photoprotection,and increases photosynthetic productivity.In Arabidopsis,expression of the same three genes(VPZ)resulted in a more rapid photoprotection but growth of the transgenic plants was impaired.Here we report on VPZ expressing potato plants grown under various light regimes.Similar to tobacco and Arabidopsis,induction and relaxation of NPQ was accelerated under all growth conditions tested,but did not cause an overall increased photosynthetic rate or growth of transgenic plants.Tuber yield of VPZ expressing plants was unaltered as compared to control plants under constant light conditions and even decreased under fluctuating light conditions.Under control conditions,levels of the phytohormone abscisic acid(ABA)were found to be elevated,indicating an increased violaxanthin availability in VPZ plants.However,the increased basal ABA levels did not improve drought tolerance of VPZ transgenic potato plants under greenhouse conditions.The failure to benefit from improved photoprotection is most likely caused by a reduced radiation use efficiency under high light conditions resulting from a too strong NPQ induction.Mitigating this negative effect in the future might help to improve photosynthetic performance in VPZ expressing potato plants.