Huihui Zhang?Peng Feng?Wei Yang?Xin Sui?Xin Li?Wei Li?Rongtao Zhang?Siyu Gu?Nan Xu

Introduction

Due to change in the global climate and deterioration of the environment,extreme weather events have occurred with greater frequently in recent years.One of the major consequences of climate change is the increase in floods resulting from above average seasonal precipitation and poor drainage-irrigation systems in cities(Fan et al.2014).Due to inadequate drainage of urban green areas,most garden plants are susceptible to flood-induced damage.We studied the physiological responses of garden plants under flood stress.Flooding can directly change the physical,chemical and biological characteristics of soil,leading to inadequate supply of soil oxygen(Parolin et al.2006).In addition, flooding can directly or indirectly affect the physiological metabolism of plants,including hormone metabolic disturbance(Geigenberger 2003;Kaelke and Dawson 2003),outbreak of reactive oxygen(Islam and Macdonald 2004),aggravation of the degree of membrane lipid peroxidation(Arbona et al.2008),increased content of substances for osmotic adjustment(Olgun et al.2008),and suppression of mineral absorption by roots(Hank et al.2006).All of these suppress plant growth,and potentially lead to plant death(Jackson and Armstrong 1999).The low oxygen environment of root systems caused by flooding stress also results in a reduction of root activity and respiratory rate,even leading to anaerobic respiration and thus the generation of toxic substances including ethyl alcohol and lactic acid.Photosynthesis is the basis for plants’energy generation,and about 95%of the dry matter of plants is directly derived from photosynthesis.Flooding stress significantly decreases the carbon assimilation capacity of photosynthesis in plants(Ahmed et al.2002).First,chlorophyll synthesis is suppressed,limiting the absorption of light energy in plants.The stomata close in response to the decreased CO2supply(Ashraf and Arfan 2005),reducing the transport of photosynthetic products(Ushimaru et al.1995),which in turn leads to excessive energy consumption(Huang et al.2008).Flooding damages the ultrastructure of leaf cells and decreases photosynthetic enzyme and PS II reaction center activity levels(Du et al.2010).Some studies suggest that the damage to the plant’s photosynthetic apparatus mainly manifests in PSII positions(Mathur et al.2014).Recently,more studies have investigated the effects of flooding stress on the physiological function of agricultural crops including Oryza sativa L.(Nishiuchi et al.2012),Triticum aestivum Linn(Yavas et al.2012)and Gossypium spp.(Cao et al.2012).

Physocarpus species are deciduous shrubs of the Rosaceae and Spiraeoideae with abundant varieties,including P.amurensis Maxim and P.opulifolius ‘Diabolo’.Physocarpus is of high value as flower,leaf and fruit ornaments,and is extremely drought and cold tolerant,making it suitable for cold,arid and semi-arid areas(Zhang et al.2016).In addition,the triterpenoid extracted from the bark of Physocarpus has anti-tumor effects.P.amurensis is endemic to a narrow region of the Mao’er Mountain in Heilongjiang Province,Wuling Mountain and Chengde in Hebei Province in China.Due to human activity in the area and poor pollination of Physocarpus plants,the natural populations of Physocarpus in China are in decline(Li et al.2000a,b).P.amurensis is ranked as an endangered and protected plant species in China(Li et al.2009a,b).Various Physocarpus species,including P.opulifolius were introduced from North America to China for ornamental purposes.P.opulifolius can overwinter in the open field in north and northeast China,is suitable for inlay materials for landscaping,and has significant ornamental value,which has had to a series of studies on its cultivation and crossbreeding with the endemic P.amurensis(Yu et al.2010;Zhou 2000).

Genetic differences among germplasm resources in plants lead to significant variation in physiological functions that impact growth and natural distribution of plants.Levels of stress resistances of plant species and variants also differ.For example,the stress resistance of different varieties of Zea mays L.(Prasad et al.1994),Capsicum annuum L.(Chartzoulakis and Klapaki 2000)and woody plants of Vitis vinifera L.(Li et al.2013)and Ficus carica Linn.(Qi et al.2015)are significantly different.The high level of stress tolerance shown by P.opulifolius ‘Diabolo’’compared to P.amurensis Maxim suggests that genetic engineering or generation of hybrids could aid in preserving the declining P.amurensis Maxim population.Little is known about the photosynthetic response to flooding and adaptive mechanisms.Therefore we conducted a pot-culture study under complete flooding conditions to compare the characteristics of morphological features and the eff iciency of the photosynthetic mechanism in P.amurensis Maxim and P.opulifolius ‘Diabolo’.We also compared the differences of flooding tolerance of photosynthetic characteristics between the two Physocarpus species.We aimed to lay a theoretical foundation for establishing evaluation systems of flooding tolerance in Physocarpus plant varieties,and provide a scientific basis for the selection of Physocarpus varieties in the garden greening of rainy regions.

Materials and methods

Experimental materials and treatments

We conducted experiments in the Laboratory of Plant Physiology at Northeast Forestry University in July,2014.Our experimental materials were seedling cuttings of 3-year-old P.amurensis and P.opulifolius provided by Heilongjiang Forest Botanical Garden.We used a local P.amurensis a wild P.amurensis transplanted from the Mao’er Mountain,Shangzhi City,Heilongjiang Province in 2004,and a seed tree of P.opulifolius introduced from Canada in 2006 as seedlings.We cut all material in 2012 and transplanted the surviving seedlings into plastic flowerpots with an opening diameter of 28 cm,bottom diameter of 15 cm,and height of 20 cm.We planted one seedling per pot with culture substrate of soddy soil,cultivated under outdoor natural environments,and watered and weeded at regular intervals.Plants were cultivated until July 2014,when the two test materials entered a vigorous growth period(with plant heights of about 0.5 m),when we conducted the flooding treatment.We applied the doublecasing pot method for our flooding treatment,where we put one black plastic bag around the original flowerpot,and then another flowerpot with the same specification underneath the top one.After watering,we maintained the water level to submerge the surface soil layer to a depth of 2–3 cm,and replenished water regularly during the treatment period.We selected three pots each of Physocarpus for flooding treatment,and determined the photosynthetic gas exchange parameters as well as the chlorophyll fluorescence parameters on days 1,4,7,10,13,and 16 of flooding stress.

Measurement of photosynthetic gas exchange parameters

We measured the net photosynthetic rate(Pn),stomatal conductance(Gs),transpiration rate(Tr),intercellular CO2concentration(Ci),the stomatal limitation value(Ls)and Water use efficiency(WUE)using Ciras-2 Portable Photosynthesis System(Hansatch,UK)according to Zhang et al.(2012a,b).

Measurement of chlorophyll fluorescence parameters

We measured the maximal photochemical efficiencies of PSII(Fv/Fm),actual photochemical efficiency of PSII(ΦPSII),Photochemical quenching coefficient(qP),nonphotochemical quenching (NPQ) using a FMS-2 portable pulse modulated fluorometer(Hansatch,UK)according to Hu et al.(2007).

Measurement of chlorophyll a fluorescence transient

We measured the chlorophyll a fluorescence transient(OJIP curve)with a Handy-PEA fluorometer(Hansatech,UK)(Fan et al.2014),O,J,I and P point relative fluorescence intensity expressed in Fo,FJ,FIand Fmrespectively.We calculated the relative variable fluorescence intensity at 2 ms J-step(VJ)and 30 ms I-step(VI)as:Vt=(Ft-Fo)/(Fm-Fo).In accordance with the JIP-test(Liu et al.2014.),we calculated the Potential photochemical activity of PSII(Fv/Fo),performance index on absorption basis(PIABS),probability that a trapped exciton moves an electron into the electron transport chain beyond(Ψo),quantum yield of absorption flux to dissipated energy(φDo),quantum yield for electron transport(φEo),trapped energy flux per RC(TRo/RC),absorption flux per RC(ABS/RC),dissipated energy flux per RC(DIo/RC)and electron transport flux per RC(ETo/RC).

Data analysis

We repeated each experiment three times.Summary statistics are presented as mean±SE.We used one-way ANOVA and least significant difference to analyze all data,using Excel and SPSS statistical software.We considered differences as significant at p≤0.05.

Results

Morphological characteristics of two cultivars of Physocarpus

Subjecting P.opulifolius and P.amurensis to flooding stress did not result in obvious stress phenotypes on the first day(Fig.1a,b).By day 16 of flooding stress exposure we still observed no significant stress symptoms in P.opulifolius,which had more extended leaves.However,at this point,the flooding stress symptoms of P.amurensis were more apparent with leaf yellowing and blackening,especially obvious on the leaf margin.In addition,at the 16th day of flooding stress,we found necrotic tissue in the growing points of P.amurensis leaves,and lower old leaves began to drop.The morphological characteristics of plants under flooding stress indicated a more apparent flooding stress resistance of P.opulifolius compared to P.amurensis.

Fig.1 Morphological characteristics of 2 cultivars of Physocarpus under flooding stress.1st day of flooding stress exposure(a)and 16th day of flooding stress exposure(b)

Fig.2 Net photosynthetic rate(a),stomatal conductance(b),transpiration rate(c)and intercellular CO2concentration(d)in leaves of two Physocarpus cultivars exposed to flooding stress.Data in the figure are mean±SE,values followed by different small letters denote significant difference(p＜0.05)

Photosynthetic gas exchange parameters

Pn,Gsand Trall significantly decreased in the leaves of both cultivars of Physocarpus with increasing duration of flooding(Fig.2a–d).Pn,Gsand Trin the leaves of P.amurensis were significantly lower than in P.opulifolius at different days of flooding.Pnin the leaves of P.amurensis approached zero at the 13th day under flooding stress,and declined to a negative value on day 16.Both Gsand Trin the leaves of P.amurensis approachd zero.Parameters of P.opulifolius were all significantly higher than those of P.amurensis.During the flooding process,Ciin the leaves of both cultivars of Physocarpus first increased,then decreased.The leaves of P.opulifolius had smaller variation of Ci,however,Ciin P.amurensis rapidly increased from day 10 of flooding stress exposure,and Ciin the leaves of P.amurensis at day 16 was 1.98 times that measured at day 1.

Water use efficiency and stomatal limitation value

The trends of WUE and LSwere similar in the leaves of both cultivars of Physocarpus under flooding stress(Fig.3a,b).With increasing days of exposure to flooding,both WUE and LSin the leaves of the two cultivars first increased,then decreased.The levels of fluctuation of WUE and LSin P.opulifolius were significantly less than that in P.amurensis during the flooding process.WUE and LSin P.amurensis were slightly higher than in P.opulifolius between day 1 and day 10 of flooding stress.However,at days 13 and 16 of treatment,WUE and LSin the leaves of P.amurensis were significantly lower,especially by day 16,where both values were negative,while those in P.opulifolius remained at higher levels.

F v/F m and ΦPSII

All Fv/Fmin P.opulifolius were higher compared to those of P.amurensis throughout all days of measurement,and Fv/Fmin the leaves of P.opulifolius did not vary with duration of flooding exposure(Fig.4a,b).Fv/Fmin the leaves of P.amurensis followed a significantly downward trend,declining by days 13 and 16 by 9.5%(p＜0.05)and 14%(p＜0.05),respectively,to levels 21%(p＜0.05)and 25%(p＜0.05)less,respectively,than those recorded for P.opulifolius ‘Diabolo’.Similar to Fv/Fm,ΦPSIIlevels for P.opulifolius were significantly higher than those for P.amurensis at different days of flooding exposure.From the 10th day of flooding stress,ΦPSIIin both cultivars began to decrease,but the extent of decrease of ΦPSIIfor P.amurensis was significantly greater for than for P.opulifolius.

The photochemical quenching and non-chemical quenching

NPQ(non-chemical quenching)and qPin the leaves of both cultivars varied slightly in response to flooding stress(Fig.5a,b).There were no changes in qPin the leaves of both cultivars from day 1 to 7 of flooding stress exposure,however,after day 7,qPin both Physocarpus significantly decreased with increasing number of days under flood treatment.NPQ in leaves of P.amurensis did not vary with flood stress duration.NPQ did not vary in P.opulifolius between days 1 and 7 of flood treatment but increased significantly from day 10 onward.NPQ in P.opulifolius at days 13 and 16 was 2.29 and 2.95 times day 1 values,respectively.

Fast chlorophyll fluorescence kinetic curve

Fig.3 Stomatal limitation value(a)and water use efficiency(b)in leaves of two Physocarpus cultivars exposed to flooding stress.Data in the figure are mean±SE,values followed by different small letters denote significant difference(p＜0.05)

Fig.4 Maximal PSII photochemical efficiency(a)and actual photochemical efficiency of PSII(b)in leaves of two Physocarpus cultivars exposed to flooding stress.Data in the figure are mean±SE,values followed by different small letters denote significant difference(p＜0.05)

Fig.5 Values for photochemical quenching(a)and non-photochemical quenching(b)in light-adapted leaves of two Physocarpus cultivars exposed to flooding stress.Data are mean±SE,values followed by different small letters denoting significant difference(p＜0.05)

The relative intensity of fluorescence Ftin each point of the OJIP curve for P.opulifolius was significantly lowerthan for P.amurensis at day 1 under flooding stress,and the differences increased during the process from O point to P point(Fig.6a,b).Ftin leaves of both cultivars after 16 days of flooding stress were significantly lower compared to valuesrecorded on day 1.In particular,the extentofdecrease recorded for P.amurensis was more severe compared to that for P.opulifolius.Similarly,the extentofdecrease at point P for both cultivars was larger than that at point O,and the OJIP curves of both Physocarpus became smoother after being subjected to flooding stress for 16 days.However,the relative intensity offluorescenceatpointOin theleavesof P.amurensis was higher than recorded for P.opulifolius at day 16,and we observed little differences at point J between the two cultivars.The relative intensities of fluorescence at points I and P for P.amurensis were significantly lower than those for P.opulifolius.After the respective standardization of OJIP curves in both Physocarpus under different treatments,we found little difference on the standardized OJIP curves between the two cultivars at day 1 under flooding stress.The leavesof P.opulifolius were similaron days1 and 16.The relative variable fluorescence at the J point of the standardized OJIP curve in the leaves of P.amurensis at day 16 was higher compared to day 1,while the extent of the increaseatIpointwassignificantly smallercompared to the J point.

Fig.6 Relative intensity of fluorescence Ft(a)and rise kinetics of relative variable fluorescence Vt(b)in light-adapted leaves of two Physocarpus cultivars exposed to flooding stress.The O,J,I and P represent the corresponding time points of the abscissa 0,2,30 and 1000 ms respectively

The radar chart of JIP-test data

Fig.7 Radar plot of fluorescence data in leaves of two Physocarpus cultivars exposed to a 16 day flooding stress

We normalized the measurements of chlorophyll fluorescence parameters on day 1 of flooding stress,and found that Fo,FJ,FIand Fmin the leaves of both Physocarpus were significantly lower after 16 days of flood treatment(Fig.7).All parameters showed a declining trend.FJ,FIand Fmof P.opulifolius were all higher than in P.amurensis at day 16,except for Fo,which was higher in P.amurensis.VJ,VIand Fv/Foi of P.opulifolius were also normalized to 1 at day 16,and there was no significant difference compared to day 1 under flooding stress.VJand VIin the leaves of P.amurensis were 1.25 and 1.07,respectively at day 16,with Fv/Foratio of 0.71.At day 16,PIABSin the leaves of P.opulifolius decreased by 21%compared to day 1 of the treatment,while PIABSdecreased by 79%in P.amurensis,where the extent of decrease was greater than in P.opulifolius.There were non-significant differences in Ψo, φEoand φDoof P.opulifolius after 16 days of flooding stress.Ψoand φEoin the leaves of P.amurensis decreased by 21 and 43%,respectively,while φDoincreased by 100%.At the 16th day of exposure to flooding stress,ABS/RC,TRo/RC,ETo/RC and DIo/RC in P.opulifolius were all significantly higher than at day 1.Except for ABS/RC and DIo/RC showing an increasing trend compared to day 1,TRo/RC and ETo/RC in P.amurensis decreased by 20 and 36%,respectively.

Discussion

Plant leaves have stomatal(Li et al.2006)and non-stomatal factors(Seemann and Sharkey 1986;Meyer and Kouchkovsky 1993)that restrict photosynthesis.The factors limiting photosynthesis under short-term stresses are based on stomatal limitation,while non-stomatal limitation prolongs stress time or increases the degree of stress(Satoh et al.1983).We found that Pnin the leaves of both Physocarpus cultivars declined significantly with prolonged flooding stress and was accompanied by declines in Gsand Tr.Our results suggest that the decrease in photosynthetic carbon assimilation capacity of both Physocarpus cultivars were directly related to a decrease in stomatal conductance,which was consistent with previously published studies(Havaux and Davaud 1994;Fukao and Bailey-Serres 2004,2008).In stress conditions,reduced stomatal conductance in plant leaves is considered an adaptive behavior.The reduced stomatal conductance effectively reduces water loss caused by transpiration,and increases water use efficiency.In addition,it also increases the diffusion resistance of CO2into leaves,which is counterproductive to the photosynthesis of plants(Abiko et al.2012;Chaves et al.2009;Yan et al.2012).

Prolonged flood stress exposure leads to a significantly larger reduction in Pncompared to Gsand Tr,especially at later periods of flooding stress(from day 10 to day 16).Pnin the leaves of both Physocarpus cultivars declined linearly with elongated exposure to flooding,while the changes of Gsand Trwere slower with more consistent variation trends.This indicated that Pnin the leaves of both cultivars was mainly restricted by stomatal factors during the first 10 days of flooding stress.Ciin the leaves of both cultivars increased with prolonged flood treatment from day 10 onwards and Lsalso decreased significantly.Based on the viewpoint of Farquhar and Sharkey(1982),the observed decrease of Pnwas mainly restricted by nonstomatal factors.Comparing our two cultivars,Pn,Gsand Trof both decreased with increasing duration of flooding stress during the first 10 days.The extent of the decrease for each photosynthetic gas exchange parameter in P.opulifolius was significantly lower than for P.amurensis,indicating that the photosynthetic gas exchange capacity of P.amurensis was more sensitive to flooding stress than was that of P.opulifolius.The increase in Ciand decrease in Lsin the leaves of P.amurensis were significantly greater than for P.opulifolius after day 10 of flooding stress,indicating that the limitations on photosynthesis caused by nonstomatal factors,such as the decrease of photosynthetic carbon assimilation enzymes or damage of mesophyll cells,occurred at a higher rate in P.amurensis exposed to longterm flooding stress.

Under flooding stress,stomatal and non-stomatal factors lead to an adaptive response through changes in how the plant responds to light and CO2,decreasing carbon assimilation capacity.In particular,non-stomatal factors play an important role during later periods of flooding stress(Zhang et al.2013).In non-stomatal factors,the decrease in activity of PSII photochemistry,and the resulting disturbance of reactive oxygen,is considered as one of the main limitation factors(Fernandez 2006;Yuan et al.2012).The chlorophyll fluorescence technique is widely used to study PSII function under stresses to understand plant stress physiology.Especially the fast chlorophyll fluorescence curve(OJIP)contains abundant information about the PSII primary photochemical reaction,and the injured sites of photosynthesis under stresses can directly be analyzed(Li et al.2009a,b).In our study,Fv/Fmand ΦPSIIi of both Physocarpus cultivars began to significantly decrease after 7 days of exposure to flooding stress.The timing of the decline lagged behind the photosynthetic gas exchange parameters Pn,Gsand Tr,indicating that the limitation on the photosynthetic carbon assimilation capacity both of Physocarpus cultivars at the initial stage of flooding stress mainly resulted from a decrease of stomatal conductance.The gradual decrease of PSII photochemistry activity of both Physocarpus cultivars at the later period of flooding stress suggests that the limitation of non-stomatal factors in photosynthesis played a major role.

Fv/Fmand ΦPSIIin the leaves of P.opulifolius were all significantly higher than in P.amurensis at all durations of flood exposure.The extent of decrease at the later period of flooding stress was also lower than in P.amurensis,indicating that PSII in the leaves of P.opulifolius was significantly less sensitive to flooding stress than it was in P.amurensis.This was one of the reasons that P.opulifolius had higher photosynthetic capacity under flooding stress.To some extent,qPre flected the opening degree of the PSII reaction center,and had a positive correlation with the electron activity of PSII(Lu et al.2003),while NPQ was positively correlated with heat dissipation depending on the xanthophyll cycle(Li et al.2000a,b).qPin the leaves of both Physocarpus cultivars showed significant decreasing trends with increasing duration of flood stress.Variation in NPQ of P.amurensis was less during the flooding process.However,NPQ of P.opulifolius significantly increased from day 10 of flooding stress,indicating that loss of excess excitation energy to the reaction center in both Physocarpus cultivars could be triggered by a closure of the reaction center under flooding stress.The normal physiological function of PSII could be protected in P.opulifolius through starting heat dissipation depending on the xanthophyll cycle.This protective mechanism was less active in P.amurensis under flooding stress,potentially leading to a significant suppression of the function of the PSII reaction center in the leaves of P.amurensis at the later period of flooding stress,as well as resulting in a significant decrease of the capacity for photosynthetic gas exchange.

PIABSmeasures the capture of luminous energy by the PSII reaction center,but can also re flect the state of electron transport in the downstream of the PSII reaction center.Therefore,the PIABSserves as a representation of plant photochemistry activity in comparison to Fv/Fm(Havaux and Davaud.1994;Strauss et al.2006;Salmela et al.2011).In this study,the variation of PIABSin the leaves of both Physocarpus cultivars was higher after 16 days of flooding stress.The change in Fv/Fmin P.opulifolius during flooding stress was not significant,but PIABSof P.opulifolius decreased by 21%compared to the 1st day of treatment.Similarly,the extent of variation in PIABSin P.amurensis was larger than in Fv/Fmunder flooding stress.Therefore,the application of PIABSseemed to more accurately re flect the function of PSII reaction center of Physocarpus experiencing flooding stress.

The relative intensity of fluorescence on each point of the OJIP curve of both cultivars,after 16 days of flooding stress were significantly lower than on the 1st day of treatment.There is typically a larger effect of external factors on the variability of the original OJIP curve(Qu et al.2012),therefore,we applied mathematical methods to standardize the OJIP curves:The relative intensity of fluorescence at the O point in all OJIP curves was defined as 0,while that at point P was defined as 1.The standardized OJIP curve of P.opulifolius after 16 days of flooding stress showed less variation than it did on day 1.VJand VIin the leaves of P.amurensis increased,and the extent of increase in VJwas significantly larger than that of VI.A blockage of the electron transport at the receptor site of PSII is the major source of excess electron accumulation in the photosynthetic electron transport chain under stressful conditions.The time needed to transfer an electron to QAthrough pheophytin(pheo)to generateis very short(250–300 ps,while the time needed for an electron to transfer fromto QBis 100–200 μs.Therefore,the electron transport fromto QBcan easily be blocked under stress conditions.easily accumulates,while a variation of relative fluorescence intensity at point J(2 ms)on the OJIP curve exactly reflects the accumulation of QA-as it could indirectly reflect the transport capacity of electrons at the receptor side of the PSII reaction center from QAto QB.The variation of relative fluorescence intensity at point I(30 ms)reflects the heterogeneity of the PQ pool(Strasser et al.1997;Li et al.2009a,b).

After 16 days of flooding stress,VJin the leaves of P.amurensis increased significantly.Along with this,the degree of Ψodecreased,indicating that electrons at the receptor site of PSII in the leaves of P.amurensis were transferred from QAto QBat the 16th day of treatment.This would lead to over-reduced QAand heavily accumulated,thus reducing the receptivity of QBin the leaves of P.amurensis under flooding stress.However,the extent of variation VJand Ψoin P.opulifolius was lower at day 16,suggesting that this cultivar has a higher electron transport rate under flooding stress,and that this is caused by a suppression of electron transport at the electron acceptors of PSII during the transport process from QAto QB.P.opulifolius showed higher receptivity ofelectrons in QB,which was not determined by the character of the PQ pool.

Previous studies suggested that the electron transport mediator QBfunctions along D1 proteins in the chloroplast.Stress conditions would lead to suppressing D1 protein synthesis or increasing D1 degradation,leading to the separation of QBand D1 proteins,leading to a loss of the capacity of QBto receive electrons(Cheng et al.2016a;Nadia et al.2006).Therefore,the decreased electron transport rate from QAto QBin the leaves of P.amurensis under flood stress may be related to the suppression of the D1 protein in the chloroplast or to the acceleration of its degradation caused by flooding.Flooding did not influence D1 protein levels in the leaves of P.opulifolius,and the underlying causes for this will need further investigation.The photosynthetic electron transport chain is the main source of reactive oxygen(ROS)in the photosynthetic apparatus under stress(Cheng et al.2016b).PSII excitation energy increases under stress,and this energy exists in the form of electrons.When electron transport in the photosynthetic electron transport chain is blocked,an outbreak of ROS results in the oxidative damage of proteins,lipid nucleic acids,and the photosynthetic system(Kreslavski et al.2007).In this study,we found that the leaves of P.opulifolius had higher electron transport rates when subjected to flooding stress,which could reduce reactive oxygen levels in the photosynthetic apparatus,thereby protecting the physiological function.

Exposure to 16 days of flooding stress led to relatively small changes in φEoand φDoof P.opulifolius ‘Diabolo’’compared to levels recorded on day 1.φEoof P.amurensis increased by 43%,while φDoincreased by 100%,indicating that the light use capacity of the PSII reaction center of P.amurensis was significantly lower than that of P.opulifolius under flooding stress.The specific activity parameters per unit of reaction centers not only directly reflect the absorption and use of luminous energy in plants,but can also indirectly reflect the activity and quantity of plant reaction centers.We found that the luminous energy ABS/RC absorbed per unit of reaction center in both Physocarpus cultivars increased with prolonged flood treatment.Flooding stress might have led to an increased proportion of inactivated PSII reaction centers in the leaves of Syringa oblate Lindl.seedlings and a decrease in the quantity of activated reaction centers.However,as a stress adaptation,the functions of the rest of the activated reaction centers in plants increase,in order to retain normal energy generation under stress(Strasser et al.1997).While ABS/RC in the leaves of both Physocarpus cultivars increased,TRo/RC,ETo/RC and DIo/RC increased to varying extents in P.opulifolius.Both TRo/RC and ETo/RC in P.amurensis declined through the 16 days of flood treatment.In contrast,DIo/RC increased dramatically.These findings suggest a decrease in the proportion of luminous energy absorbed per unit of reaction center in P.amurensis under flooding stress,and an increase in the dissipation of thermal energy.In addition,the extent of the o bserved TRo/RC decrease of P.amurensis was signi ficantly smaller than that for ETo/RC,indicating that flooding had no significant effect on the transport of electrons from Pheo to QA(as QAwas reduced toThis measures the energy absorbed from luminous energy per unit of reaction center at the PSII receptor site in the leaves of P.amurensis under flooding stress for the reduction of QA.The electron blockage in the electron transport chain mainly occurred during the transport from QAto QBwhich was consistent with the changing characteristics of relative fluorescence intensity at point J.

Conclusions

P.opulifolius had significantly higher photosynthetic capacity under flood stress than did P.amurensis.At the initial stage of flood stress,photosynthetic gas exchange was limited through stomatal factors in both Physocarpus cultivars.Non-stomatal factors began to play a role from day 10 onward,especially in P.amurensis.Non-stomatal factors included changes in photochemistry activity and light use capacity of PSII reaction centers.Maximal PSII photochemical efficiencies and actual photochemical eff iciency in P.opulifolius under flooding stress were signif icantly higher than those of P.amurensis.Excess luminous energy in P.opulifolius might have been dissipated through NPQ to protect the PSII reaction center from injury caused by excess excitation energy.The flooding stress of 16 days had little effect on the transport of electrons at the receptor site of PSII in the leaves of P.opulifolius.Transport of electrons at the receptor site was suppressed in P.amurensis,which presented a block in the transport process of electrons from QAto QB.There was a decrease in the proportion of luminous energy absorbed per unit of reaction center and PSII reaction center in the leaves of P.amurensis,which might have accounted for the lower flooding tolerance of P.amurensis compared to P.opulifolius.

Abiko T,Kotuia L,Shiono K,Malik A,Colmer TD,Nakazono M(2012)Enhanced formation of aerenchyma and induction of a barrier to radial oxygen loss in adventitious roots of Zea nicaraguensis contribute to its waterlogging tolerance as compared with maize.Plant,Cell Environ 35(9):1618–1630

Ahmed S,Nawata E,Hosokawa M,Domae Y,Sakuratani T(2002)Alterations in photosynthesis and some antioxidant enzymatic activities of mungbean subjected to waterlogging.Plant Sci 163(1):117–123

Arbona V,Hossain Z,Clemente RP,Cadenas AG(2008)Antioxidant enzymatic activity is linked to waterlogging stress tolerance in citrus.Physiol Plant 132(4):452–466

Ashraf M,Arfan M(2005)Gas exchange characteristics and water relations in two cultivars of Hibiscus esculentus under Waterlogging.Biol Plant 49(3):459–462

Cao G,Wang XG,Liu Y,Luo W(2012)Effect of water logging stress on cotton leaf area index and yield.Proc Eng 28(12):202–209

Chartzoulakis K,Klapaki G(2000)Response of two greenhouse pepper hybrids to NaCl salinity during different growth stages.Sci Horti 86(3):247–260

Chaves MM,Flexas J,Pinheiro C(2009)Photosynthesis under drought and salt stress:regulation mechanisms from whole plant to cell.Ann Bot 103(4):551–560

Cheng DD,Liu MJ,Sun XB,Zhao M,Chow WS,Sun GY(2016a)Light suppresses bacterial population through the accumulation of hydrogen peroxide in tobacco leaves infected with pseudomonas syringaepv.tabaci.Front.Plant Sci 7(512):1–11

Cheng DD,Zhang ZS,Sun XB,Zhao M,Sun GY(2016b)Photoinhibition and photoinhibition-like damage to the photosynthetic apparatus in tobacco leaves induced bypseudomonas syringaepv.Tabaciunder light and dark conditions.BMC Plant Biol 16(1):1–11

Du KB,Xu L,Tu BK,Shen BX(2010)Influences of soil flooding on ultrastructure and photosynthetic capacity of leaves of one-year old seedlings of two poplar clones.Sci Silv Sin 46(6):58–64

Fan XL,Zhang ZS,Gao HY,Yang C,Liu MJ,Li PM(2014)Photoinhibition-like damage to the photosynthetic apparatus in plant leaves induced by submergence treatment in the dark.PLoS ONE 9(2):1633–1641

Farquhar GD,Sharkey TD(1982)Stomatal conductance and photosynthesis.Ann Rev Plant Physiol 33:317–345

Fernandez MD(2006)Changes in photosynthesis and fluorescence in response to flooding in emerged and submerged leaves of Pouteria orinocoensis.Photosynthetica 44(1):32–38

Fukao T,Bailey-Serres J(2004)Plant responses to hypoxia:issurvival a balancing act.Trends Plant Sci 9(9):449–456

Fukao T,Bailey-Serres J(2008)Ethylene-A key regulator of submergence responses in rice.Plant Sci 175(S1–2):43–51

Geigenberger P(2003)Response of plant metabolism to too little oxygen.Cur Opin Plant Biol 6(3):247–256

Hank W,William A,Timothyd C(2006)Conditions leading to high CO2(＞5 kPa)in waterlogged flooded soils and possible effects on root growth and metabolism.Ann Bot 98(1):9–32

Havaux M,Davaud A(1994)Photoinhibition of photosynthesis in chilled potato leaves is not correlated with a loss of photosystem-II activity-preferential inactivation of photosystem I.Photosynth Res 40:75–92

Hu YB,Sun GY,Wang XC(2007)Induction characteristics and response of photosynthetic quantum conversion to changes in irradiance in mulberry plants.J Plant Physiol 164(8):959–968

Huang SB,Colmer TD,Millar AH(2008)Does anoxia tolerance involve altering the energy currency towards PPi?Trends Plant Sci 13(5):221–227

Islam MA,Macdonald SE(2004)Ecophysiological adaptations of black spruce(Picea mariana)and tamarack(Larix laricina)seedlings to flooding.Trees 18(18):35–42

Jackson MB,Armstrong W(1999)Formation of aerenchyma and the processes of plant ventilation in relation to soil flooding and submergence.Plant Biol 1(3):274–287

Kaelke CM,Dawson JO(2003)Seasonal flooding regimes influence survival,nitrogen fixation and the partitioning of nitrogen and biomass in Alnus incana ssp. Rugosa. J Plant Soil 254(1):167–177

Kreslavski VD,Carpentier R,Klimov VV,Murata N,Allakhverdiev SI(2007)Molecular mechanisms of stress resistance of the photosynthetic apparatus.Biochem Suppl Ser A Membr Cell Biol 1(3):185–205

Li PM,Cheng LL,Gao HY,Peng T(2000a)Heterogeneous behavior of PSII in soybean(Glycine max)leaves with identical PSII photochemistry efficiency under different high temperature treatments.J Plant Physiol 166(15):1607–1615

Li XP,Bjorkman O,Shi C,Grossman AR,Rosenquist M,Jansson S,Niyogi KK(2000b)A pigment-binding protein essential for regulation of photosynthetic light harvesting. Nature 403(6768):391–395

Li HB,Chen WF,Li QY(2006)Responses of rice leaf photosynthetic parameters to light intensity under NaCl stress.Chin J Appl Ecol 17(9):1588–1592

Li G,Gao HY,Zhao B,Dong ST,Zhang JW,Yang JS,Wang JF,Liu P(2009a)Effects of drought stress on activity of photosystems in leaves of maize at grain filling stage.Acta Agron Sin 35(10):1916–1922

Li PM,Cheng LL,Gao HY,Jiang C,Peng T(2009b)Heterogeneous behavior of PSII in soybean(Glycine max)leaves with identical PSII photochemistry efficiency under different high temperature treatments.J Plant Physiol 166(15):1607–1615

Li Y,Du YP,Fu YD,Heng D(2013)Physiological responses of waterlogging on different rootstock combinations of cabernet sauvignon grape.Acta Hortic Sin 40(11):2105–2114

Lindroth RL,Reich PB,Tjoelker MG,Volin JC,Oleksyn J(1993)Light environment alters response to ozone stress in seedlings of Acer saccharum Marsh,and hybrid Populus L.New Phytol 124(4):637–646

Liu MJ,Zhang ZS,Gao HY,Cheng Y,Fan XL,Cheng DD(2014)Effect of leaf dehydration duration and dehydration degree on psII photochemical activity of papaya leaves.Plant Physiol Biochem 82(3):85–88

Lu CM,Qiu NW,Wang BS,Zhang J(2003)Salinity treatment shows no effects on photosystem II photochemistry,but increases the resistance of photosystem II to heat stress in halophyte Suaeda salsa.J Exp Bot 54(383):851–860

Mathur S,Agrawal D,Jajoo A(2014)Photosynthesis:response to high temperature stress. J Photochem Photobiol Biol 137(8):116–126

Meyer S,Kouchkovsky Y(1993)Electron transport,photosystemreaction centers and chlorophyll protein complexes of thylakoids of drought resistant and sensitive lupin plants.Photosynth Res 37(1):49–60

Nadia AA,Dewez D,Didur O,Popovic R(2006)Inhibition of photosystem II photochemistry by Cr is caused by the alteration of both D1 protein and oxygen evolving complex.Photosynth Res 89(89):81–87

Nishiuchi S,Yamauchi T,Takahashi H,Kotual L,Nakazono M(2012)Mechanisms for coping with submergence and waterlogging in rice.Rice 5(2):1–14

Olgun M,Kumlay AM,Adiguzel MC,Abdullah C(2008)The effect of waterlogging in wheat(Triticum aestivum L.).Acta Agric Scand Sect B Soil Plant Sci 58(3):193–198

Parolin P,Armbruster N,Junk WJ(2006)Two Amazonian floodplain trees react differently to periodical flooding.Trop Ecol 47(2):243–250

Prasad TK,Anderson MD,Martin BA,Stewart CR(1994)Evidence for chilling-induced oxidative stress in maize seedlings and a regulatory role for hydrogen peroxide.Plant Cell 6(1):65–74

Qi L,Ma N,Wu WW,An YY,Xu JC,Qin XH,Wang LJ(2015)Physiological responses and tolerance evaluation of fig cultivars to waterlogging.Acta Hortic Sin 42(7):1273–1284

Qu NW,Zhou F,Gu ZJ,Jia SQ,Wang XA(2012)Photosynthetic functions and chlorophyll fast fluorescence characteristics of five Pinus species.Chin J Appl Ecol 23(5):1181–1187

Salmela MJ,Cavers S,Cottrell JE,Lason GR,Ennos RA(2011)Seasonal patterns of photochemical capacity and spring phenology reveal genetic differentiation among native Scots pine(Pinus sylvestris L.)populations in Scotland.For Ecol Manag 262(6):1020–1029

Satoh K,Smith CM,Fork DC(1983)Effect of salinity on primary processes of photosynthesis in the red alga porphyraperforata.Plant Physiol 73(3):643–647

Seemann JR,Sharkey TD(1986)Salinity and nitrogen effects on photosynthesis, ribulose-1,5-bisphosphate carboxylase and metabolite pool sizes in Phaseolus vulgaris L.Plant Physiol 82(2):555–560

Strasser RJ,Srivastava A,Govindjee(1997)Polyphasic chlorophyll a fluorescence transients.Photosynth Res 52(2):147–155

Strauss AJ,Kruger GHJ,Strasser RJ,Heerden PDRV(2006)Ranking of dark chilling tolerance in soybean genotypes probed by the chlorophyll a fluorescence transient O–J–I–P.Environ Exp Bot 56(2):147–157

Ushimaru T,Ogawa K,Ishida N,Tsuji H(1995)Changes in organelle superoxide dismutase isoenzymes during air adaptation of submerged rice seedlings:Differential behavior of isoenzymes in plastids and mitochondoria.Planta 196(3):606–613

Yan K,Chen P,Shao H,Zhao S,Zhang L,Zhang L,Xu G,Sun J(2012)Responses of photosynthesis and photosystem IIto higher temperature and salt stress in sorghum.J Agron Crop Sci 198(3):218–226

Yavas I,Unaya A,Aydin M(2012)The waterlogging tolerance of wheat varieties in western of Turkey.Sci World J 4:1–7

Yu YY,Zhang HY,Pan J,Tan Z,Ma L(2010)Cross-breedingof Physocarpus plant.J Northeast For Uni 38(7):16–18

Yuan L,Zhang LQ,Gu ZQ(2012)Responses of chlorophyll fluoresescence of aninsive plant Spartina alterni florato continuous waterlogging.Acta Sci Circ 30(4):882–889-

Zhang XK,Zhou QH,Cao JH,Yu BJ(2011)Differential Cl/Salt tolerance and NaCl-induced alternations of tissue and cellular ion fluxes in Glycine max,Glycine soja and their Hybrid Seedlings.J Agron Crop Sci 197(5):1–11

Zhang HH,Zhang XL,Li X,Ding JN,Zhu WX,Qi F,Zhang T,Tian Y,Sun GY(2012a)Effects of NaCl and Na2CO3stresses on the growth and photosynthesis characteristics of Morus alba seedlings.Chin J Appl Ecol 23(3):625–631

Zhang ZS,Li G,Gao HY,Zhang LT,Yang C,Liu P,Meng QW(2012b)Characterization of photosynthetic performance during senescence in stay-green and quick-leaf-senescence Zea mays L.Inbred Lines.PLoS ONE 7(8):e42936

Zhang BB,Xu JL,Cai ZX,Ma RJ(2013)Relationship between photosynthetic characteristics and environmental factors in leaves of two plum rootstock varieties under waterlogging stress.J Nanjing Agric Univ 36(5):39–44

Zhang HH,Zhong HX,Sui X,Xu N(2016)Adaptive changes in chlorophyll content and photosynthetic features to low light in physocarpus amurensis maxim and physocarpus opulifolius‘diabolo’.Peer J 4(3):e2125

Zhou YZ(2000)Studies on the propagation in vitro of Physocorpus opulifolius ‘Lutein’.Acta Hortic Sin 27(2):148–150