Introduction

Water availability and temperature are considered to be the main variables limiting photosynthesis,affecting growth and survival of plants(Niu et al.2008;Ghannoum and Way 2011;Gago et al.2013).Photosynthesis is sensitive to environmental variables which will be profoundly affected by future climate change,including elevated air temperatures and decreased water availability(Luo 2007;Gunderson et al.2010;Lin et al.2012;Ashraf and Harris 2013).Meanwhile,the ability of plants to modify photosynthesis in response to high temperatures and/or drought stress has been shown to be species-speci fi c resulting from different photosynthetic acclimation potentials(Way and Oren 2010;Ashraf and Harris 2013;Way and Yamori 2014).Therefore,the potential for photosynthetic acclimation to new growing conditions plays a central role in the effects of climate change on plant growth and survival(Smith and Dukes 2013;Sendall et al.2015).

Photosynthetic acclimation can alter the short-term abiotic factor-response functions of photosynthesis associated with maintaining leaf gas exchanges under different growing conditions(Smith and Dukes 2013;Zhang et al.2015;Aspinwall et al.2016).For example,thermal acclimation of photosynthesis hasresulted in a shift in optimum temperature(Topt)and/or a change in sensitivity,revealed,for example,by a change in the shape of the temperature response curves(see Berry and Bjo rkman 1980;Hikosaka et al.2006;Way and Yamori 2014;Sendall et al.2015).Furthermore,the potential for plants to maintain photosynthetic capacity when wateravailability decreases depends on the sensitivity of leaf gas exchange to drought(Limousin et al.2013).However,plants have at least three different strategies for maintaining photosynthesis in response to changing temperatures or water availability.First,photosynthetic capacity is closely linked to stomatal conductance(Cond)(Flexas and Medrano 2002;Hikosaka et al.2006;German and Roberto 2013).Long-term elevated air temperatures or drought may alter the sensitivity of stomatal apertures,thus limiting photosynthesis(Zhang et al.2001;Reddy et al.2004;Gao et al.2009;Greer and Weedon 2012).Second,elevated temperatures or drought may lead to changes in leaf anatomy and density,for example,changing the leaf mass per area(LMA)to affect the mesophyll conductance of CO2(Poorter et al.2009;Yamori et al.2009;Vasseur et al.2012;Heroult et al.2013;Drake et al.2015).These two strategies could in fl uence water loss and constrain CO2exchange from leaves.The third,photosynthesis also be limited through biochemical processes which was temperature and water availability dependent(Way and Sage 2008;Lin et al.2012;Limousin et al.2013;Aspinwall et al.2016).In fact,the biochemical processes may be related to photosynthetic attributes such as the amount and/or ef fi ciency of enzymes and photosystem II(PSII)activity.Nitrogen content per mass(Nmass)is an important feature of photosynthetic apparatus,indicative of the relative proportion of enzymes in photosynthetic processes(Yamori et al.2009;Aspinwall et al.2016).In addition,the maximum quantum yield of PSII(Fv/Fm)is a reliable diagnostic indicator of photosynthetic activity(Reddy et al.2004;Gao et al.2009;Ma et al.2010;Wang et al.2014).

Picea crassifolia Kom.and P.wilsonii Mast.are two endemic species in Western China which often form a dominant component in coniferous forests(Farjón 2001;Fu et al.1999;Zhao et al.2008).Due to their signi fi cant commercial and ecological value,P.crassifolia and P.wilsonii are widely used for afforestation in Northern China.For example,in Qinghai Province alone,there are 14,000 ha of planted spruce forest,accounting for～5.4‰of the total spruce forest areas(Han et al.2015).However,current climate models suggest increasing temperatures(raising about 10°C by 2100)and droughts in these regions(Zou et al.2005;IPCC 2013).Coniferous forests may be particularly sensitive to climate change,which may result in changes in carbon exchange and a serious threat to survival(Zhang et al.2015;Aspinwall et al.2016;Kroner and Way 2016).Therefore,the objectives of our study were to examine the effects of long-term high temperatures and drought on photosynthesis of these two important conifers and to determine which was likely to be of more bene fi t to future forest stability in Northern China.

对于能源企业来说，在环保工程完工以后，其验收评审标准具有一定的特殊性，在具备应有的工程质量标准的基础上，还要增设环保指数标准。在审核标准构建过程中，要多参考不同环保工程中的环境指标，集中整合多方面标准和数据，根据环保工程实际情况，要制定出合理的审核标准，并不断对审核标准进行细化，还要严格检测和控制环保工程的后续效果。

Materials and methods

Plant materials and growing conditions

农业机械化技术培训工作离不开创新。培训内容乏味、枯燥，在很大程度上增加了培训难度。相关部门应当高度重视农业机械技术的培训工作，创新培训方法、培训内容和培训技术，切实提高农业机械技术培训工作的实效性。

2018年以来，中国家电市场整体面临的压力较大，从下图中的数据我们可以看出，2017年全年，中国家电市场零售额增速为11.8%，而2018年截至11月份，增速只有1.6%，大约只有去年全年增速的七分之一，今年的中国家电零售业可能将迎来整体“失速”的危机。

Experiment 1:Drought experiment

Each fi ve pots of each species were randomly divided into low[80% of maximum fi eld capacity(FC)],mild(60%FC),moderate(40%FC)and severe(20%FC)water stress treatments.Water stress levels continued until October 16,2009,and maintained at these levels by weighing the pots every two days.Pots were assigned a random position in an arti fi cial intelligent greenhouse with growth temperatures controlled at 25/15°C day/night[moderate temperature(MT)]by a temperature control system.All seedlings were grown under 12 h photoperiods with light levels of 300–400 μmol photon m-2s-1at seedling height by artifi cial light sources for automatic control.Unfortunately,all P.wilsonii seedlings grown at 20%FC died during the experimental period.Seedlings of both species in the remaining 35 pots were alive at the end of this experiment(October 15,2009;day t2).

Experiment 2:High temperature experiment

The remaining pots( fi ve pots)of each species were placed in another greenhouse with growth temperatures controlled at 35/25°C[high temperature(HT)].All seedlings were raised under 12 h photoperiods with light levels of 300–400 μmol photon m-2s-1at seedling height.In both greenhouses,the CO2concentration was maintained at～380 μmol mol-1and relative humidity at 50 ± 5%.Seedlings in the high temperature treatment were watered suf fi ciently to avoid any effects caused by extreme water de fi cit.The experimental period continued until October 15,2009(day t2).

我国市场经济的不断发展，使高校的管理体制也从之前的以管为主逐渐向服务型管理模式转变，但是，高校的很多管理体制仍旧存在较多的漏洞和弊端，这些问题的出现很难满足新时代学生的管理需求。

Gas exchange and chlorophyll fl uorescence measurements

Leaf-level gas exchange(Pn,Cond and Tr)measurements were made with a portable open-path gas exchange system and a conifer chamber(Li-6400 and 6400-07,LI-COR Biosciences,Inc.)on fully expanded current year-old twigs for each experiment.Three to fi ve seedlings per treatment and species were randomly selected between 10:00–13:30 h during sunny weather,when the temperatures were 25 or 35°C,depending on each growth conditions. During photosynthetic measurements, CO2 concentration was maintained at 380 μmol mol-1using portable CO2/air mixture tanks with output controlled by a LI-6400-01 CO2injector(LI-COR Biosciences,Inc.).Light levels were maintained at approximately 800 μmol m-2-s-1(saturated light level)at measurement height provided by an external light source.In addition,photosynthetic temperature curves of seedlings in each temperature treatment at 5 °C intervals from 40 to 15 °C were recorded.To ensure that the whole plant was exposed to the desired temperature settings,temperatures were controlled by changing air temperatures of the growth chamber,and micro-changing with the Li-6400 temperature control model.When measurements at one temperature were complete,thechambertemperatureconditionswere adjusted.Seedlings were allowed to equilibrate to chamber conditions for a minimum of 30 min before measuring the same twigs again.All measurements were completed in one day.After measurements had been taken,needles were cut and leaf areas were determined with a LI-3000A portable area meter(LI-COR Biosciences,Inc.)to calculate gas exchange parameters on an area basis.The measured needles were dried at 65°C for 72 h and dry mass determined(LM;Precisa XT120A,Precisa Instruments.Ltd.,Switzerland).These LM and LA values were used to calculate leaf mass per area(LMA).

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When gas exchange measurements were taken,the maximum quantum yield ofphotosystem II(PSII)was measured for leaves adapted to dark conditions during an acclimatization period of 30 min.Chlorophyll fl uorescencemeasurementswere taken with a portable pulse amplitude modulated fl uorometer FMS-2(Hansatech,King’s Lynn,Norfolk,UK).At least fi ve replicates from each treatment were taken.

Leaf nitrogen measurements

We measured the concentration of nitrogen(N)using samples used for gas exchange measurements.Dried samples were fi nely ground with a mortar and pestle,and sent to the Analytical Testing Center,Lanzhou University for analysis using a CHN analyzer(Vario EL,Elementar,Germany).

Modeling of photosynthetic temperature curves

Photosynthesis data from temperature response curves were used to determine temperature-dependence and fi tted to the following quadratic equation(Gunderson et al.2010;Niu et al.2008;Sendall et al.2015):

The maximum quantum yield of photosystem II(Fv/Fm)was not signi fi cantly different between P.crassifolia and P.wilsonii under the low water treatment.However,as drought stress increased,Fv/Fmfor P.crassifolia only decreased signi fi cantly in the moderate treatment,while Fv/Fmfor P.wilsonii decreased signi fi cantly in both mild and medium treatments.Meanwhile,the value of Fv/Fmin P.wilsonii was lower than in P.crassifolia for mild and moderate treatments(Fig.1;Table 2).Hence,P.wilsonii also appeared more sensitive to drought stress for this character.

Long-term sensitivity of photosynthesis to temperature

We also calculated an index to quantify the degree of thermal acclimation of Pnin response to HT(Way and Oren 2010)based on the following:

Effects of long-term drought stress on photosynthesis

Statistical analysis

Quadratic fi tting was used to estimate temperature response functions for photosynthetic rates(15–40 °C).The experiment was arranged in a completely randomized design with 3–5 replicates.Differences in all traits were determined by analysis of variance(one-way and General Linear Model,Proc GLM)and Tukey’s test for multiple comparisons.All data are presented as mean±SE.Differences were considered signi fi cant at p＜0.05.Statistical analysis was performed using SPSS 16.0(SPSS Inc.,Chicago,IL,USA).

We designed the second experiment to test the effects of temperature as a major limiting factor.As in the low water treatment above,Pn,Cond and Trin P.wilsonii measured at 25°C(MT)were signi fi cantly higher than in P.crassifolia.After long-term 35°C(HT)treatment,Pnand Cond for P.crassifolia were signi fi cantly higher than for P.wilsonii,whereas Trmeasured for P.crassifolia was signi fi cantly lowerwithconsequencesforarelativelygreaterreductionin Pnfor P.wilsonii(Fig.3).Hence,the value of Acclimpnfor P.crassifolia was about 0.75,higher than for P.wilsonii(Fig.4).Meanwhile,there were signi fi cant differences in temperature,species and their interaction(Table 4).

Results

The index of photosynthesis(Acclimpn)for each species was equal to the ratio of Pnin HT leaves(35°C)to MT leaves(25°C).As an index for the degree of acclimation,if Acclimpnis close to 1.0,this indicates that temperature acclimation exhibited is high(Way and Oren 2010;Yamori et al.2009).

As available soil water decreased,Pn,Cond,and Tr decreased signi fi cantly in both species(Table 1).In the low water treatment,Pn,Cond,and Trin P.wilsonii were higher than in P.crassifolia,particularly in the case of the latter two variables(p＜0.05,Table 1).In contrast,P.crassifolia had signi fi cantly higher values of Pn,Cond and Tr under mild and moderate water conditions,leading to a relatively greater decrease of Pn,Cond and Trin P.wilsonii with increasing water stress(Table 1).Furthermore,there were signi fi cant interactions between species and water treatments for Pn,Cond and Tr,suggesting that photosynthetic response to water stress was different in the two species(Table 2).

where PTrepresents the mean net photosynthetic rate at temperature T in°C;and Poptis the photosynthetic rate at the optimum temperature(Topt).Parameter b describes the spread of the parabola(Battaglia et al.1996).For a given Aoptand Topt,parameter b is smaller and the photosynthetic temperature parabola ‘broader’if photosynthesis is less sensitive to short-term temperature changes.

Seeds of each species were collected within their natural range(Picea crassifolia Kom:30.3–37.8°N,126.5–130.5°E,Alt.:2400–3600 m above sea level(a.s.l.);P.wilsonii Mast:33.7–40.8°N,101.6–116.8°E,Alt.:1400–2800 m a.s.l.).In 2005,seeds were germinated and grown indoors at Yuzhong campus,Lanzhou University(35°56′37′N,104°09′05′E,Alt.:1750 m a.s.l.Temperatures ranged during the growing season from 7.7 to 25.8°C)for one year;the seedlings received ample water and light.Seedlings were then transplanted into 24 cm(upper diameter)×16 cm(basal diameter)× 17 cm(depth)pots fi lled with a homogeneous mixture consisting of equal volumes of peat and perlite.One pot was designed three seedlings.All pots were periodically watered to fi eld capacity(FC)according to Ma et al.(2010).On June 15,2009(day t1),25 pots of each species of uniform growth(c.20 cm tall P.crassifolia and c.25 cm tall P.wilsonii)were selected and divided into two groups:one for the water stress experiment,the other for the high temperature experiment.Control pots( fi ve of each species)were used for both experimentsandgrownat25/15°C and80% FC conditions.

Compared with the low water treatment,increasing soil water stress generated a signi fi cant reduction in the value of Nmassfor both Picea species,with the exception of P.wilsonii in the mild treatment.Meanwhile,the variations in Nmassbetween species were only apparent in the low water treatment(Fig.2).Therefore,Nmassdiffered,depending on both the watering treatments and species(Table 2).However,LMA for both species was not signi fi cantly changed in either treatment(data not shown),while the values of LMA for P.crassifolia were obviously larger than those for P.wilsonii(see Table 3).

Effects of long-term high temperatures on photosynthesis

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Table 1 Comparison of gas exchange parameters between Picea crassifolia and P.wilsonii,across different soil water treatments(80% of maximal fi eld capacity(FC),60 and 40%FC).Each value represents a mean and SE.Letters after SE values distinguish between statistically different(p＜0.05)values for different water treatments(A,B,C)and between different species(X,Y)

Treatment Pn(μmol m-2s-1) Cond(mol m-2s-1) Tr(mmol m-2s-1)P.crassifolia P.wilsonii P.crassifolia P.wilsonii P.crassifolia P.wilsonii Low stress(80%FC)3.71±0.15 AX 4.34±0.19 AX 0.025±0.001 AX 0.038±0.0001 AY 0.84±0.05 AX 1.23±0.003 AY Mild stress(60%FC)3.06±0.06 BX 1.49±0.16 BY 0.019±0.0020 BX 0.008±0.0004 BY 0.79±0.07 AX 0.25±0.04 BY Moderate stress(40%FC)1.46±0.02 CX 0.78±0.03 CY 0.009±0.0002 CX 0.003±0.0004 CY 0.49±0.10 BX 0.11±0.01 CY

Table 2 Effects of watering treatment,species and their interaction on fi ve indicators measured during drought treatments

**Difference is signi fi cant at the 0.01 level;***Difference is signi fi cant at the 0.001 level

Parameters Watering treatment Species Watering×species interaction Pn 227.73*** 22.97*** 36.11***Cond 168.63*** 1.42 40.91***Tr 90.30*** 14.36** 41.47***Fv/Fm 12.63** 2.92 1.93 Nmass 19.67*** 6.03* 6.54**

Fig.1 Maximum quantum yield of PSII(Fv/Fm)in seedlings of Picea crassifolia and P.wilsonii under different soil water conditions(80% of maximal fi eld capacity(FC),60 and 40%FC).Each value represents a mean and SE.Letters after SE values distinguish between statistically different(p＜0.05)values for different water treatments(A,B)and between different species(X,Y)

Fig.2 Effects of watering treatments on the nitrogen content per dry mass(Nmass)in seedlings of Picea crassifolia and P.wilsonii under different soil water conditions(80% of maximal fi eld capacity(FC),60 and 40%FC).Each value represents a mean and SE.Letters after SE values distinguish between statistically different(p＜0.05)values for different water treatments(A,B)and between different species(X,Y)

At 25°C(MT),all estimated parameters(b,Toptand Aopt)of the photosynthetic temperature response curves in P.wilsonii were signi fi cantly higher than those for P.crassifolia(Table 3).Following the 35°C(HT)treatment,the shapes of the curves were obviously different between species (Fig.5).Only Toptfor P.crassifolia was signi fi cantly greater,whilst b and Aoptwere reduced for P.wilsonii.Meanwhile,Aoptat 35°C in P.crassifolia was higher than in P.wilsonii;however,there were no significant differences in b and Toptbetween species(Table 3).Interactions between temperature and species for these variables were also highly signi fi cant,indicating that temperature treatments in b,Toptand Aoptwere different between P.crassifolia and P.wilsonii(Table 4).

For growth at 25°C(MT),Fv/Fmwas not signi fi cantly different between the two species.In contrast,Fv/Fmin P.wilsonii was signi fi cantly lower following the 35°C(HT)treatment for 4 months,and its value was clearly less than that for P.crassifolia(Table 3).Variations in LMA and Nmasswere species-speci fi c following the 25°C(MT)treatment;LMA and Nmasswere higher in P.crassifolia(Table 3).In contrast,there were no signi fi cant differences in LMA and Nmassbetween species grown at 35°C(HT)(Table 3).However,there were only signi fi cant species effects with respect to LMA,while the values of Nmasswere signi fi cantly affected by temperature treatments(Table 4).

Discussion

Precipitation and temperature are the most important factors affecting plant growth and distribution because of their in fl uence on photosynthesis(Zhang et al.2009;Way and Oren 2010).In this study,we examined a combination of photosynthetic parameters and leaf morphological characteristics of P.crassifolia and P.wilsonii grown under two temperature conditions and four water supply regimes.We found different patterns in their long-term response to temperature and drought:P.crassifolia exhibited greater photosynthetic acclimation in both treatments as compared with P.wilsonii.

前测试卷为该师范学院2016级大一新生第一学期英语期末考试听力部分。主测试卷题目选自《新视野大学英语:视听说教程》(第三版)第二册前三个单元中的短对话、长对话和短文。调查问卷的目的是为了获得一些关于受试的基本背景信息，以及调查受试是否熟悉前测和主测中的听力材料。

Photosynthetic acclimation to drought

Drought tolerance is essential for the survival and growth of many plants(Reddy et al.2004;Mao and Wang 2011).Pnfor P.crassifolia and P.wilsonii decreased with increasing drought stress(Table 1),suggesting that drought was inhibiting photosynthetic activity(Ashraf and Harris2013).Similar patterns have been observed in other plants(Ma et al.2010).However,photosynthetic responses to drought were different between the two Picea species in this study:the decrease of Pnfor P.wilsonii was much larger than that for P.crassifolia,in mild and moderate water treatments,particularly in the mild water treatment(c.–65%;Table 1),suggesting that the photosynthesis of P.wilsonii was more prone to drought limitations,whilst the photosynthesis of P.crassifolia showed an acclimatory response to drought.In addition,whilst the seedlings of P.crassifolia survived the high stress treatment with water supplied at 20%FC,P.wilsonii seedlings did not(see Materials and methods).This also suggests that high drought stress limits the growth and survival of P.wilsonii,while P.crassifolia is more resistant to drought,especially under extreme water de fi cit conditions.

Table 3 Effects of growth temperature on several parameters (b,Topt,Aopt,Fv/Fm,LMA and Nmass) for Picea crassifolia and P.wilsonii

Values represent the mean±SE.Letters after SEvalues distinguish between statistically different(p＜0.05)values for different temper ature treatments(A,B)and different species inthe sametemperature treatment(X,Y)

Topt(°C)Aopt(μmol m-2s-1)Fv/FmLMA(g m-2)Nmass(mgg-1)folia P.wilsonii Picea crassifolia P.wilsonii Picea crassifolia P.wilsonii Picea crassifolia P.wilsonii Picea crassifolia P.wilsonii Picea crassifolia P.wilsonii MT(25°C)0.009±0.001AX 0.015±0.001AY 23.40±0.69AX 27.40±0.75AY 3.74±0.15AX 4.46±0.25AY 0.83±0.007AX 0.84±0.008AX 244.24±2.89AX 189.44±2.84AY 17.77±1.07AX 14.06±0.65AY HT(35°C)0.008±0.001AX 0.007±0.001BX 28.96±0.97BX 28.10±0.41AX 3.10±0.09BX 1.70±0.04BY 0.84±0.007AX 0.67±0.039BY 226.94±X 22.74A 190.12±1.63AX 19.74±0.52AX 20.56±0.66BX

Fig.3 Comparison of leaf gas exchange parameters at growth temperature[the net photosynthetic rate(Pn,a),the stomatal conductance(Cond,b)and the transpiration rate(Tr,c)]between Picea crassifolia(P.c)and P.wilsonii(P.w)in the MT and HT treatments.Data are presented as mean±SE.Letters distinguish between statistically different(p＜0.05)values for two temperature treatments(A,B)and between different species(X,Y)

Fig.4 Comparison of the temperature acclimation of photosynthesis(Pn)between Picea crassifolia(P.c)and P.wilsonii(P.w).Letters distinguish between statistically different(p＜0.05)values for two temperature treatments(A,B)

The higher Pnunder drought observed in P.crassifolia could be explained by the conditions affecting two processes.First,water supply was reduced with increasing drought stress,progressively inducing stomatal closure(Flexas and Medrano 2002;Reddy et al.2004;Gao et al.2009;Matteo et al.2014).Therefore,photosynthetic reduction in both Picea species may be partly explained by stomatal limitation.However,P.crassifolia had higher Cond and Trthan P.wilsonii in the mild and moderate treatments.Moreover,larger decreases in Cond of P.wilsonii(decreasing c.80%)were observed in the mild water treatment relative to non-stressed seedlings.These results suggest that the leaves of each Picea species showed different sensitivities to water de fi cit(Ashraf and Harris 2013),and that leaves of P.wilsonii were more sensitive to drought.Second,with increasing water stress,drought can affect biochemical processes(Way and Sage 2008;Lin et al.2012;Limousin et al.2013).Our results reveal that decreases in Fv/Fmfor P.crassifolia were smaller in each treatment.There was only a slight reduction under the mild water treatment(Fig.1),suggesting more stable light capture and utilization in P.crassifolia leaves(Evans 1989;Reddy et al.2004).However,there was a signi fi cant decrease in Nmassin the mild water treatment.In contrast,larger deceases in Fv/Fmwere observed in P.wilsonii,suggesting that increasing water stress may inhibit or damage photosynthetic process in P.wilsonii(Ma et al.2010;Ashraf and Harris 2013).At the same time,steady values of Nmassin P.wilsonii in the mild water treatment did not compensate for a larger reduction in Fv/Fm.Thus,variations in Fv/Fmalso partly explain different photosynthetic acclimations to drought in both Picea species.These results suggest that the photosynthetic process of P.crassifolia has a higher drought tolerance than that of P.wilsonii.

Table 4 Effects of growth temperature,species and their interaction on 10 measured indicators under the two temperature treatments

*Difference is signi fi cant at the 0.05 level;**Difference is signi fi cant at the 0.01 level;***Difference is signi fi cant at the 0.001 level

Parameters Temperature treatment Species Temperature×species interaction Pgrowth 230.42*** 8.79* 62.36***Cond 78.10*** 2.68 51.61***Tr 15.01** 2.23 47.14***Fv/Fm 8.18* 8.53* 11.00**b 18.93** 4.99 11.63**Topt 20.01** 5.05* 12.11**Aopt 140.58*** 5.54* 54.35***LMA 0.72 21.93** 0.85 Nmass 9.03* 10.68* 5.06

Fig.5 Temperature(Tleaf)responses of leaf photosynthetic rates(Pn)in Picea crassifolia(a)and P.wilsonii(b)under the MT and HT treatments.Data are presented as mean±SE

Photosynthetic acclimation to high temperatures

Grown at elevated temperatures,Pnmeasured at 35°C decreased in both Picea species compared to seedlings grown at 25°C,indicating that photosynthesis of these two Picea species did not completely acclimatize to elevated temperatures(Berry and Bjo rkman 1980).However,the AcclimPnfor P.crassifolia was higher than that for P.wilsonii(Fig.4),suggesting that thermal acclimation of photosynthesis in P.crassifolia is more effective than in P.wilsonii(Yamori et al.2009;Way and Oren 2010).P.crassifolia has an inherently lower photosynthetic sensitivity to short-term temperature fl uctuations(e.g.parameter b in Table 3 and Fig.5;see Battaglia et al.1996;Niu et al.2008;Gunderson et al.2010;Sendall et al.2015).Photosynthesis of P.wilsonii was more sensitive to short-term temperature fl uctuations(Table 3;Fig.5)and therefore the thermal sensitivity of photosynthesis in P.wilsonii was limited.Meanwhile,there was an upward shift in the Toptof P.crassifolia that reduced high temperature stress;this was not observed in P.wilsonii(Gunderson et al.2010;Way and Yamori 2014;Sendall et al.2015).Therefore,the photosynthetic process of P.crassifolia has a higher thermal tolerance than that of P.wilsonii.

The regulation of Pnis related to changing stomatal conductance,leaf development,and biochemical processes during prolonged exposure to high temperatures(Way and Sage 2008;Lin et al.2012;Heroult et al.2013).Changes in stomatal conductance(Cond)could reduce Pnirrespective of biochemical effects(Hamerlynck and Knapp 1996;Zhang et al.2001;Hikosaka et al.2006;Greer and Weedon 2012).High temperatures are associated with increasing leaf-to-air vapor pressures leading to increasing drought stress.Potentially plants could limit Cond to reduce Tr(Day 2000).Our results show that decreased Cond for P.wilsonii limited Trat elevated temperatures(Fig.3),indicating increased heat-induced physiological drought stress for P.wilsonii even though water de fi cits were avoided by providing abundant water during the experimental period(German and Roberto 2013).In contrast,increased Trin P.crassifolia meant that P.crassifolia plants were exposed to thermal stress as a result of increasing water loss from leaves even when there was abundant soil moisture(Fig.3c).Hence,variations in Cond between temperature treatments were important in explaining treatment differences in Pn(German and Roberto 2013).On the other hand,LMA was identical between temperature treatments for the two species(Tables 3,4),suggesting that high temperatures did not affect leaf structure and density.Leaf developmental processes are therefore unlikely to explain temperature treatment differences in Pn.In addition,biochemical processes may partly explain the smaller proportional decreases in Pnat high temperatures.Our results showed that Fv/Fmwas only signi fi cantly reduced by high temperatures in P.wilsonii,indicating that high temperature inhibits or damages the photosynthetic apparatus in P.wilsonii leaves.Hence,increasing Nmassin P.wilsonii may be assumed to repair chlorophyll and thylakoids(Evans 1989;Hikosaka et al.2006;Hikosaka and Shigeno 2009;Wang et al.2014).This was not compensated for by greater photosynthetic range.These results suggest that thermal acclimation of photosynthesis in P.crassifolia is more effective than in P.wilsonii(Fig.4).

Conclusions

Overall,our results suggest that stress caused by drought and high temperatures reduce the Pnof the seedlings of both Picea species.However,P.crassifolia exhibited higherphotosyntheticacclimation to both increasing drought and temperature than P.wilsonii.In addition,we recorded higher Cond and Fv/Fmfor P.crassifolia with increasing drought and temperature,indicating that these were responsible for the improved acclimation compared to P.wilsonii.Moreover,the photosynthetic apparatus in P.crassifolia leaves exhibited an inherently lower temperature-sensitivity and higher thermostability(see parameter b).Further,severe drought stress(20%FC)killed P.wilsonii.In conclusion,our results indicate that P.wilsonii is more susceptible to drought and high temperatures;P.crassifolia is more appropriate to plant to survive future climate increases and to sequester carbon.

Acknowledgements We thank Dr.David Blackwell for correcting the English in the fi nal manuscript.

现实总是困难重重：页岩气井一旦进入“老龄化”，产量递减极为严重。为延缓气井产量递减，涪陵页岩气公司决定实行增压开采，并选定焦页49号平台开展试验，由米瑛负责开采试验的动态分析。

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