Collective Calculation of Actual Values of Non-Photochemical Quenching from Their Apparent Values after Chloroplast Movement and Photoinhibition

Chlorophyll fluorescence parameters such as Fv/Fm, NPQ and ΦII (YII) are widely used to estimate the fitness and photosynthetic activity of plant leaves and non-photochemical dissipation of excessive excitation energy in photosystem II. The effect of chloroplast movement on these fluorescence parameters reduces the accuracy of estimations of the size of de-excitation processes, but there is no method to calculate correct parameters from altered (fluctuated) parameters. Chloroplast movement was recently identified as the “middle” kinetic component of NPQ. In this paper, we devised a complex but reasonable mathematical method to remove the effect of chloroplast movement on fluorescence parameters, based on our previously reported fluorescence theory. The fraction of “S fluctuation” (designated as σ) was estimated from fluorescence observations and used to calculate the non-fluctuated Fs and F′m. fluorescence yields. From the σ values, the fractional change of light absorbance by a leaf caused by chloroplast movement was estimated at 70% - 100%, which varied according to the experimental conditions and plant species. The effect of photoinhibition on fluorescence parameters was also examined in this paper. The photochemical and non-photochemical de-excitation sizes during photoinhibition (measured by the parameters qPI and qSlow) changed on a single regression line. Using this correlation, qPI and qSlow can be predicted from Fv/Fm, and the non-fluctuated Fm and Fo values can be estimated from the fluctuated F″m and F″o values.

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.

Seasonality of PSII thermostability and water use e ffi ciency of in situ mountainous Norway spruce(Picea abies)

The stability of monocultural,even-aged spruce forests at lower altitudes in Central Europe is seriously threatened by the prospects of global climate change.The thermostability and water use efficiency of their photo synthetic apparatus might play a vital role in their successful acclimation.In this study,photo systemⅡ(PSⅡ)performance(OJIP transient,rapid light curves)and thermostability were analyzed in Norway spruce(Picea abies(L.)Karst.)throughout the growing season of the exceptionally warm year 2018(May-September)in the Western Carpathians,Slovakia.These measurements were accompanied by analysis of pigment concentrations in the needles.In addition,gas-exchange temperature curves were produced weekly from June until September to obtain intrinsic water use efficiencies.At the beginning of the growing season,needles exposed to heat stress showed significantly higher basal fluorescence and lower quantum yield,performance index,critical temperature thresholds of PSII inactivation and nonphotochemical yield in comparison to other months.The overall thermostability(heat-resistance)of PSII peaked in July and August,reflected in the lowest basal fluorescence and the highest quantum yield of PSII,critical temperature thresholds and yield of non-photochemical quenching under heat stress.Additionally,the ratio between chlorophyll and carotenoids was the highest in August and had a positive impact on PSII thermostability.Moreover,the high-temperature intrinsic water use efficiency was significantly higher during July and August than in June.Results show that15-year-old trees of Picea abies at 840 m a.s.l.exhibited acclimative seasonal responses of PSII thermostability and intrinsic water use efficiency during an exceptionally warm year.Our results suggest that mountainous P.abies at lower altitudes can acclimate their photosynthetic apparatus to higher temperatures during summer.

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.

Effects of parasitic plant Cistanche deserticola on chlorophyll a fluorescence and nutrient accumulation of host plant Haloxylon ammodendron in the Taklimakan Desert

The parasitic plant Cistanche deserticola attaches to Haloxylon ammodendron, a perennial shrub with high tolerance to salinity and drought. However, little was known about the parasite-host relation between the two species. Effects of the parasite on chlorophyll a fluorescence and nutrient accumulation in the host plant （H. am- modendron） were investigated in the Taklimakan Desert. Some photosynthetic parameters of both host and non-host H. ammodendron plants were measured by in vivo chlorophyll a fluorescence technology in the field. The assimilating branches of host and non-host plants were collected and nutrient and inorganic ion contents were analyzed. The results from field experiments showed that the infection of C. deserticola reduced the non-photochemical quenching of the variable chlorophyll fluorescence （NPQ） and the potential maximum quantum yield for primary photochemistry （Fv/Fm） of the host. Compared with non-host plants, the host H. ammodendron had low nutrient, low inorganic ion contents （Na~ and K~） and low K~/Na~ ratios in the assimilating branches. It suggested that C. deserticola infection reduced the nutrient acquisition and caused damage to the photoprotection through thermal dissipation of the energy of the photosystem II in the host, resulting in a decrease in the tolerance to salinity and high radiation. It was concluded that the attachment of the parasite plant （C. deserticola） had negative effects on the growth of its host.

Response of Cowpea Genotypes to Drought Stress in Uganda

Moisture stress is a challenge to cowpea production in the drought prone areas of eastern and north eastern Uganda, with yield losses of up to 50% reported. Genotypes grown by farmers are not drought tolerant. This study was therefore, undertaken at Makerere University Agricultural Research Institute Kabanyolo to identify cowpea genotypes tolerant to drought. Thirty cowpea accessions comprising of Ugandan landraces and released varieties, Brazilian lines, Makerere University breeding lines, elite IITA germplasm and seven IITA?drought tolerant lines as checks were screened for drought tolerance at vegetative and reproductive stages. The experiment was designed as a 2 × 37 factorial and laid out in a split-plot arrangement, 37 genotypes of cowpea at two soil moisture stress levels (T1, no stress and T2, severe stress) with all factorial combinations replicated two times in a screen house. The genotypes showed considerable variability in tolerance to drought. Genotypes were significantly different for chlorophyll content (P ≤ 0.01), efficiency of photosystem II (P ≤ 0.05), non-photochemical quenching (P ≤ 0.05), recovery (P ≤ 0.01), delayed leaf senescence (P ≤ 0.01), grain yield (P ≤ 0.01), 100 seed weight (P ≤ 0.05), number of pods per plant and number of seeds per pod (P ≤ 0.001). There was a highly significant positive correlation between chlorophyll content and efficiency of photosystem II (r = 0.75, P ≤ 0.001) implying that chlorophyll content and efficiency of photosystem II could be used as efficient reference indicators in the selection of drought tolerant genotypes. Genotypes SECOW 5T, SECOW 3B, SECOW 4W, WC 30 and MU 24 C gave relatively high yields under stress and no stress conditions, maintained above mean chlorophyll content, efficiency of photosystem II and had good recovery scores from stress and thus were tolerant to drought stress induced at both vegetative and reproductive stages.

Relationships between D1 protein, xanthophyll cycle and photodamage-resistant capacity in rice (Orysa sativa L.)

Relationships between D1 protein, xanthophyll cycle and subspecific difference of photodamage-resistant capacity have been studied in O. japonica rice varieties 02428 and 029 (photoinhibition-tolerance) and O. indica rice varieties 3037 and Palghar (photoinhibition-sensitivity) and their reciprocal cross F, hybrids after photoinhibitory treatment. It was shown that PS II photochemical efficiency (Fv /Fm) decreased, and xanthophyll cycle from violaxanthin (V), via anaxanthin (A), to zeaxanthin (Z) was enhanced and non-photochemical quenching (g/v) increased accordingly in SM-pretreated leaves of rice when the synthesis of D1 protein was inhibited, and that there was a decrease in qN and, as a result, more loss of D1 protein and a big decrease in Fv/Fm in DTT-pretreated leaves when xanthophyll cycle was inhibited. O. japonica subspecies had a higher maintaining capacity of D1 protein and a decrease of Fv /Fm in a more narrow range, and exhibited more resistance against photodamage, as compared with O.

SUPPRESSOR OF VARIEGATION4, a New var2 Suppressor Locus, Encodes a Pioneer Protein that Is Required for Chloroplast Biogenesis

VAR2 is an integral thylakoid membrane protein and a member of the versatile FtsH class of metalloproteases in prokaryotes and eukaryotes. Recessive mutations in the VAR2 locus give rise to variegated plants （var2） that contain white sectors with abnormal plastids and green sectors with normal-appearing chloroplasts. In a continuing effort to isolate second-site suppressors of var2 variegation, we characterize in this report ems2505, a suppressor strain that has a virescent phenotype due to a missense mutation in At4g28590, the gene for a pioneer protein. We designated this gene SVR4 （for SUPPRESSOR OF VARIEGATION4） and the mutant allele in ems2505 as svr4-1. We demonstrate that SVR4 is located in chloroplasts and that svr4-1 single mutants are normal with respect to chloroplast anatomy and thylakoid membrane protein accumulation. However, they are modestly impaired in several aspects of photochemistry and have enhanced non-photochemical quenching （NPQ） capacity. A T-DNA insertion allele of SVR4, svr4-2, is seedling-lethal due to an early blockage of chloroplast development. We conclude that SVR4 is essential for chloroplast biogenesis, and hypothesize that SVR4 mediates some aspect of thylakoid structure or function that controls NPQ. We propose that in the suppressor strain, photoinhibitory pressure caused by a lack of VAR2 is ameliorated early in chloroplast development by enhanced NPQ capacity caused by reduced SVR4 activity. This would result in an increase in the number of chloroplasts that are able to surmount a threshold necessary to avoid photo-damage and thereby develop into functional chloroplasts.