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Effect of Growth Temperature on the Activity of Cyclic Electron Flow in Tobacco Leaves

生长温度对烟草叶片环式电子传递活性的影响



全 文 :生长温度对烟草叶片环式电子传递活性的影响∗
黄  伟ꎬ 胡  虹
(中国科学院昆明植物研究所资源植物与生物技术重点实验室ꎬ 昆明  650201)
摘要: 高等植物的光合机构在环境胁迫条件下非常容易产生光抑制ꎬ 环式电子传递在光合机构的光保护中
发挥着重要的作用ꎮ 但是ꎬ 生长温度对环式电子传递的影响并不清楚ꎮ 本研究测定了在 24 / 18 ℃和 32 / 26 ℃
条件下生长 40天的烟草 (K326) 叶片的气体交换、 叶绿素荧光和 P700氧化还原态的光响应曲线ꎮ 结果表
明ꎬ 烟草叶片在两种生长温度下的的光合能力、 光化学淬灭、 非光化学淬灭和通过光系统 II 的电子传递速
率 (ETR II) 均没有差异ꎮ 但是ꎬ 在强光条件下ꎬ 生长在 24 / 18 ℃的叶片比生长在 32 / 26 ℃的具有更高的通
过光系统 I的电子传递速率 (ETR I) 和 ETR I / ETR II比值ꎮ 短时间的强光处理后ꎬ 生长在 24 / 18 ℃的叶片具
有较高的光系统 II最大量子产额 (Fv / Fm)ꎬ 表明环式电子传递活性的上调有助于缓解生长在 24 / 18 ℃的叶
片光系统 II受到的光损伤ꎮ 综上所述ꎬ 环式电子传递活性的增强是植物适应较低生长温度的重要策略ꎮ
关键词: 环式电子传递ꎻ 生长温度ꎻ 光抑制ꎻ 光保护ꎻ 光系统 II
中图分类号: Q 945              文献标志码: A              文章编号: 2095-0845(2015)03-283-10
Effect of Growth Temperature on the Activity of Cyclic
Electron Flow in Tobacco Leaves
HUANG Weiꎬ HU Hong
( Key Laboratory of Economic Plants and Biotechnologyꎬ Kunming Institute of Botanyꎬ
Chinese Academy of Sciencesꎬ Kunming 650201ꎬ China)
Abstract: Cyclic electron flow (CEF) around photosystem I (PSI) is an important mechanism for photoprotection in
higher plants under environmental stresses. Howeverꎬ the response of CEF activity to growth temperature has not
been clarified. We here monitored gas exchangeꎬ chlorophyll fluorescenceꎬ and the P700 redox state over a range of
light intensities in leaves of tobacco cultivar ‘k326’ grown at 24 / 18 ℃ and 32 / 26 ℃ (day / night) . No significant
difference was found in the capacity of photosynthetic CO2 assimilation between the plants grown at 24 ℃ and 32 ℃ .
In additionꎬ the light response changes in the photochemical quenching of photosystem II (Y(II)) and non ̄photo ̄
chemical quenching (NPQ) did not differ significantly between those plants. Light response curves indicated that the
plants grown at 24 ℃ and 32 ℃ displayed the same level of electron flow through PSII (ETR II) irrespective of light
intensity. Howeverꎬ under intense lightꎬ plants grown at 24 ℃ showed significantly higher electron flow through PSI
(ETR I). The ETR I / ETR II ratio was significantly higher in plants grown at 24 ℃ when exposed to intense light.
Furthermoreꎬ after short ̄term treatment with strong light at 24 ℃ꎬ the maximum quantum yield of photosystem II
(Fv / Fm) was significantly higher in plants grown at 24 ℃ than that grown at 32 ℃ . Taken togetherꎬ our results sug ̄
gest that enhancement of CEF activity in plants grown at 24 ℃ alleviates PSII photoinhibitionꎬ which is an important
strategy in tobacco for acclimating to a relatively low growth temperature.
Key words: Cyclic electron flowꎻ Growth temperatureꎻ Photoinhibitionꎻ Photoprotectionꎻ Photosystem II
植 物 分 类 与 资 源 学 报  2015ꎬ 37 (3): 283~292
Plant Diversity and Resources                                    DOI: 10.7677 / ynzwyj201514098
∗ Funding: National Natural Science Foundation of China (grant 31300332) and the Following Scientific Foundations (110201101003 (TS ̄
03)ꎬ 2011YN02ꎬ 2011YN03)
Received date: 2014-07-01ꎬ Accepted date: 2014-07-25
作者简介: 黄伟 (1986-) 男ꎬ 博士后ꎬ 主要从事植物光合作用研究ꎮ E ̄mail: huangwei@mail􀆰 kib􀆰 ac􀆰 cn
  Plants capture light energy through light ̄har ̄
vesting systems. The absorbed energy then drives
photosynthetic electron flow through the thylakoid
membranes of the chloroplasts. Electrons pass through
the cytochrome b6 / f complex generate a proton gra ̄
dient ( ΔpH) across the thylakoid membrane. The
formation of ΔpH drives ATP synthesis by the ATP
synthase complex. Products of the light reactionsꎬ
ATP and NADPHꎬ are ultimately used in the Cal ̄
vin ̄Benson and photorespiratory cycles. Environmen ̄
tal stresses can decrease stomatal conductanceꎬ and
then limit the operation of photosynthetic CO2 fixa ̄
tion (Murata et al.ꎬ 2007). A constraint in the ca ̄
pacity of the Calvin ̄Benson cycle can increase the
ratio of NADPH / NADP + and depress the operation
of photosynthetic electron flow from photosystem I
(PSI) to NADP + . That then leads to the production
of reactive oxygen species (ROS) that can accelera ̄
te photoinhibition of photosystem II (PSII) (Murata
et al.ꎬ 2007ꎻ Oguchi et al.ꎬ 2009ꎬ 2011). An in ̄
crease in NADPH / NADP + ratio can activate cyclic
electron flow (CEF) around PSI ( Johnsonꎬ 2005ꎻ
Shikanai et al.ꎬ 2007ꎻ Okegawa et al.ꎬ 2008)ꎬ there ̄
by enhancing the formation of ΔpH across thylakoid.
In higher plantsꎬ CEF is suggested to be essen ̄
tial for balancing the ATP / NADPH production and
protecting PSI and PSII from photodamage under
conditions in which the absorbed light is in excess of
the photon requirements for photosynthesis ( Taka ̄
hashi et al.ꎬ 2009ꎻ Miyakeꎬ 2010ꎻ Takahashi and
Badgerꎬ 2011). The CEF ̄dependent generation of
ΔpH across the thylakoid membrane is necessary for
activating non ̄photochemical quenching (NPQ) and
preventing stromal over ̄reduction (Munekage et al.ꎬ
2002ꎬ 2004ꎻ Nandha et al.ꎬ 2007). Activation of
NPQ can harmlessly dissipate excess light energy as
heat and then diminish the production of ROS
(Niyogi et al.ꎬ 1998ꎬ 2001). The main action of
ROS in accelerating PSII photoinhibition can inhibit
the repair of photodamaged PSII complex at the step
of D1 protein synthesis ( Nishiyama et al.ꎬ 2001ꎬ
2004ꎬ 2011). Arabidopsis mutants lacking PGR5 ̄
dependent CEF activity ( pgr5 mutants) show the
same capacity of NPQ as those in which NPQ activi ̄
ty is absent (npq1 and npq4 mutants) (Takahashi et
al.ꎬ 2009). Howeverꎬ pgr5 mutants incur signifi ̄
cantly more PSII photodamage than do npq1 and
npq4 mutants. This indicates that CEF alleviates
PSII photoinhibition through at least two independent
mechanisms: one linked to NPQ that favors the re ̄
pair of photodamaged PSIIꎻ the other oneꎬ inde ̄
pendent of NPQꎬ suppresses photodamage to PSII
(Takahashi et al.ꎬ 2009).
The effect of various environmental stresses on
CEF activity has been studied extensively (Horvath
et al.ꎬ 2000ꎻ Golding and Johnsonꎬ 2003ꎻ Li et al.ꎬ
2004ꎻ Wang et al.ꎬ 2006ꎻ Yamori et al.ꎬ 2011ꎻ
Huang et al.ꎬ 2011ꎬ 2012). For exampleꎬ CEF plays
an important role in photoprotection under severe
short ̄term stressesꎬ including those associated with
intense lightꎬ droughtꎬ and extreme temperatures
(Li et al.ꎬ 2004ꎻ Wang et al.ꎬ 2006ꎻ Takahashi et
al.ꎬ 2009ꎻ Huang et al.ꎬ 2011ꎬ 2012ꎬ 2013). Such
conditions can induce inhibition of CO2 assimilation
and then lead to over ̄reduction of the electron trans ̄
port chain. Several studies have reported that CEF
activity could be enhanced by temporary heat stress
(Havauxꎬ 1996ꎻ Bukhov et al.ꎬ 1999ꎻ Kou et al.ꎬ
2013) or be accelerated by brief exposure to chilling
temperaturesꎬ as noted with tobaccoꎬ cucumberꎬ
and tropical tree species (Kim et al.ꎬ 2001ꎻ Barth
and Krauseꎬ 2002ꎻ Huang et al.ꎬ 2011). Howeverꎬ
the response of CEF capacity to long ̄term changes in
temperature is unclear.
Temperature is a major limiting factor affecting
the distribution of plants. Photosynthesis has long
been recognized as one of the most temperature ̄sen ̄
sitive processes in plants. Because low temperature
can induce stomatal closure and lead to PSII pho ̄
toinhibitionꎬ plants grown under such conditions
probably have high CEF capacity to increase the
generation of a proton gradient across the thylakoid
membrane (ΔpH) and alleviate this photoinhibition.
Hereꎬ we examined the effect of growth temperature
482                                  植 物 分 类 与 资 源 学 报                            第 37卷
on CEF capacity. Plants of Nicotiana tabacum ( to ̄
bacco) cultivar ‘ k326’ were cultured at 24 / 18 ℃
and 32 / 26 ℃ (day / night) . We hypothesize that to ̄
bacco plants regulate CEF activity to acclimate to
growth temperature and then protect PSII against
photoinhibition.
1  Materials and methods
1􀆰 1  Plants materials and growth conditions
The seedlings of tobacco cultivar ‘k326’ were
cultivated in plastic potsꎬ then transferred to two
phytotrons set to either 32 / 26 ℃ or 24 / 18 ℃ (day /
night)ꎬ hereafter referred to as 32 ℃ or 24 ℃ꎬ re ̄
spectively. These phytotronsꎬ located at Kunmingꎬ
Yunnanꎬ China (elevation 1 900 mꎻ 102°41′Eꎬ 25°
01′N) relied upon sunshine as the light sourceꎬ with
plants receiving approximately 95% of full sunlight.
During the experimental periodꎬ none of the plants
experienced any water or nutrient stresses. The rela ̄
tive humidity was kept at 60% and the atmospheric
CO2 concentration was maintained at 400 μmol mol
-1 .
During 40 and 60 days after transplantationꎬ mature
leaves were used for photosynthetic measurements.
1􀆰 2  Measurement of chlorophyll content
Contents of chlorophyll a and b were deter ̄
mined according to the method of Inskeep and Bloom
(1985).
1􀆰 3  Gas exchange measurement
Rates of CO2 assimilation (An) were measured
at 24 ℃ with an open gas exchange system that in ̄
corporated infrared CO2 and water vapor analyzers
(Li ̄6400ꎻ Li ̄Cor Incꎬ Lincolnꎬ NEꎬ USA). During
the measurementsꎬ the relative air humidity was 60%
and atmospheric CO2 concentration was 400 μmol
mol-1 . The leaf samples were illuminated by either
a quartz halogen light source or red light ̄emitting
diodes ( 656 - 680 nmꎻ Li ̄6400 ̄02ꎬ Li ̄Cor Inc).
Measurements of An in response to incident photosyn ̄
thetic photon flux density ( PPFD) were made be ̄
tween 2 000 and 0 μmol photons m-2 s-1 with a rela ̄
tive air humidity of 60%. Curves for the rate of CO2
assimilation to intercellular concentration of CO2 (A /
Ci) were measured between 2 000 and 0 mmol mol
-1
atmospheric CO2 concentration while PPFD was
maintained at 1 000 μmol photons m-2 s-1 (von Cae ̄
mmerer and Farquharꎬ 1981). Based on those A / C i
curvesꎬ we calculated the maximum rates of RuBP
regeneration (Jmax) and RuBP carboxylation (Vcmax)
according to the method of Long and Bernacchi
(2003).
1􀆰 4  Determinations of chlorophyll fluorescence
and the P700 redox state
Using a Dual ̄PAM ̄100 Measuring System (Heinz
Walzꎬ Effeltrichꎬ Germany) connected to a comput ̄
er with control softwareꎬ we conducted synchronous
measurements for the light responses of chlorophyll
fluorescence and the P700 redox state in the 24 ℃
phytotron. The relative air humidity was 60% and at ̄
mospheric CO2 concentration was 400 μmol mol
-1 .
Five mature leaves were light ̄adapted (1 000 μmol
photons m-2 s-1) for at least 20 min to induce stoma ̄
ta opening prior to determining the light response
curves. Values for light ̄adapted photosynthetic pa ̄
rameters were recorded after 3 min of exposure to
1 976ꎬ 1 618ꎬ 1 311ꎬ 1 052ꎬ 849ꎬ 555ꎬ 363ꎬ 240ꎬ
and 119 μmol photons m-2 s-1 . The fluorescence pa ̄
rameters were calculated as follows:
Fv / Fm =(Fm-Fo) / Fm
Fo′=Fo / (Fv / Fm+Fo / F m′)
    (Oxborough and Bakerꎬ 1997)
qL=(Fm′-Fs) / (Fm′-Fo′)×Fo′ / Fs
    (Bakerꎬ 2008)
Y(II)= (Fm′-Fs) / Fm′ (Gentyꎬ 1989)
Y(NPQ)= Fs / Fm′-Fs / Fm(Kramer et al.ꎬ 2004)
Y(NO)= Fs / Fm(Hendrickson et al.ꎬ 2004ꎻ
    Kramer et al.ꎬ 2004)
where Fv / Fm represents the maximum quantum yield
of PSII after dark adaptationꎬ making it a useful in ̄
dicator for estimating PSII activityꎻ qL represents
the proportion of PSII centers in the open state (with
oxidized primary quinone acceptor QA)ꎻ Y(II) is the
effective quantum yield of PSIIꎻ Y(NPQ) is the frac ̄
tion of energy dissipated as heat via the regulated
non ̄photochemical quenching mechanismꎻ and Y(NO)
5823期      HUANG and HU: Effect of Growth Temperature on the Activity of Cyclic Electron Flow in Tobacco Leaves     
is the fraction of energy that is passively dissipated
in the forms of heat and fluorescence.
The P700 redox state was measured with a dual
wavelength unit ( 830 / 875 nm) according to the
method of Klüghammer and Schreiber (2008). Satu ̄
ration pulses (10 000 μmol photons m-2 s-1) were
also applied to assess the P700 parameters (Klügha ̄
mmer and Schreiberꎬ 1994ꎬ 2008). The P700+ sig ̄
nals (P) can vary between a minimal (P700 fully
reduced) and a maximal level ( P700 fully oxi ̄
dized). The maximum levelꎬ Pmꎬ was determined
with application of a saturation pulse after pre ̄illu ̄
mination with far ̄red light. Pm′ was determined simi ̄
larly to Pmꎬ but with background actinic light in ̄
stead of far ̄red illumination. The photochemical
quantum yield of PSIꎬ i􀆰 e.ꎬ Y( I)ꎬ was calculated
as Y(I)= (Pm′-P) / Pm . Because ETRII is responsi ̄
ble for linear electron flow (LEF) and ETRI involves
LEF and CEFꎬ if CEF were activatedꎬ the value of
ETRI would be higher than the value of ETRII. As a
resultꎬ the higher value of ETRI than that of ETRII
is regarded as an indicator of CEF activation (Huang
et al.ꎬ 2011ꎬ 2012ꎬ 2013ꎻ Yamori et al.ꎬ 2011).
Hereꎬ the light response change in ETR I / ETR II
ratio was used to compare the CEF activity in plants
grown at 24 ℃ versus 32 ℃ .
1􀆰 5  Photoinhibitory treatment
Detached leaves were placed on moist papers
and treated at 2 000 μmol photons m-2 s-1ꎬ 24 ℃ꎬ
and an atmospheric CO2 concentration of 400 μmol
mol-1 . Before and after exposure to highlightꎬ the
value of Fv / Fm was measured via Dual ̄PAM ̄100
Measuring System following 20 min of dark ̄incuba ̄
tion at 24 ℃ .
1􀆰 6  Statistical analysis
The results were displayed as mean values of at
least five individuals. A one ̄way ANOVA was per ̄
formed (α= 0􀆰 05) to determine any significant diff ̄
erences among treatments.
2  Results
The light response curves indicated thatꎬ under
intense illuminationꎬ stomatal conductance (gs) and
intercellular CO2 concentration (C i) were higher in
plants grown at 24 ℃ compared with those grown at
32 ℃ (Fig􀆰 1Aꎬ B). Photosynthetic CO2 assimila ̄
tion ( An ) did not significantly differ between the
plants grown at 24 ℃ and 32 ℃ (Fig􀆰 1C). Moreo ̄
verꎬ at 1 000 μmol photons m-2 s-1ꎬ the response of
CO2 assimilation to C i did not differ significantly
in leaves grown at 24 ℃ and 32 ℃ (Fig􀆰 2). The
value of Jmax was 84 μmol m
-2 s-1 and 78 μmol m-2
s-1 in leaves grown at 24 ℃ and 32 ℃ꎬ respectively
(Fig􀆰 3A). Values for Jmax were 84 μmol m
-2 s-1 and
78 μmol m-2 s-1 in leaves grown at 24 ℃ and 32 ℃ꎬ
respectivelyꎬ while those for Vcmax were 78 μmol m
-2 s-1
and 76 μmol m-2 s-1ꎬ respectively (Fig􀆰 3A). Their
corresponding Jmax / Vcmax ratios were 1􀆰 08 and 1􀆰 03.
Furthermoreꎬ Jmaxꎬ Vcmaxꎬ and the Jmax / Vcmax ratio
were not significantly changed in plants grown at 24℃
and 32 ℃. Although the content of chlorophyll b in
plants grown at 24 ℃ was slightly higher than that at
32 ℃ꎬ the contents of chlorophyll a and total chloro ̄
phyll were not significantly altered in response to
temperature (Fig􀆰 3B). Thereforeꎬ these results in ̄
dicated that the plants grown at both temperatures
had the similar capacity to capture light energy and
assimilate CO2 .
Light response changes in Y( II)ꎬ Y(NPQ)ꎬ
and Y(NO) did not vary significantly between plants
grown at 24 ℃ and 32 ℃ (Fig􀆰 4A-C)ꎬ indicating
that plants grown at both temperatures were equally
capable of utilizing and dissipating absorbed light
energy in PSII. Furthermoreꎬ plants grown at 24 ℃
and 32 ℃ showed the same value of 1 ̄qL under all
light intensities (Fig􀆰 4D)ꎬ indicating no alterations
occurred in the stromal redox state between them.
Light response curves indicated that ETRII did
not differ between plants grown at 24 ℃ and 32 ℃ꎬ
irrespective of light intensity ( Fig􀆰 5A ). Plants
grown at 24 ℃ and 32 ℃ showed the same value of
ETRI under light intensities below 1 052 μmol pho ̄
tons m-2 s-1 (Fig􀆰 5B). Howeverꎬ when illuminated
at light intensities above 1 311 μmol photons m-2 s-1ꎬ
682                                  植 物 分 类 与 资 源 学 报                            第 37卷
the value of ETRI was significantly higher in plants
grown at 24 ℃ than that grown at 32 ℃ (Fig􀆰 5B).
Under 1 618 μmol photons m-2 s-1ꎬ ETR I was 245
and 216 μmol electrons m-2 s-1 in plants grown at
24 ℃ and 32 ℃ꎬ respectively. The value of ETR I
under 1 976 μmol photons m-2 s-1 was 251 and 216
μmol electrons m-2 s-1 in plants grown at 24 ℃ and
32 ℃ꎬ respectively. Light response change in ETR
I / ETR II ratio is regarded as an indicator of activa ̄
tion of CEF. Plants grown at 24 ℃ and 32 ℃ showed
no significant difference of ETR I / ETR II ratio when
Fig􀆰 1  Responses of stomatal conductance ( gs )ꎬ intercellular CO2
concentration (Ci) and CO2 assimilation (An ) to incident photosyn ̄
thetic photon flux density (PPFD) in leaves of tobacco grown at 24 ℃
and 32 ℃ . Values are means ± SE (n= 5)
exposed to light intensities below 1 311 μmol photons
m-2 s-1 (Fig􀆰 5C). When exposed to light intensities
of 1 618 and 1 976 μmol photons m-2 s-1ꎬ the ETR I /
ETR II ratio was significantly higher in plants grown
at 24 ℃ than that grown 32 ℃ (Fig􀆰 5C). These re ̄
sults indicated that plants grown at 24 ℃ had signifi ̄
cantly greater CEF activity than that grown at 32 ℃.
Fig􀆰 2  Response of CO2 assimilation (An) to incident intercellular
CO2 concentration (Ci) in leaves of tobacco grown at 24 ℃
and 32 ℃ . Values are means ± SE (n= 5)
Fig􀆰 3  Maximum rates of RuBP regeneration (Jmax ) and RuBP car ̄
boxylation (Vcmax)ꎬ Jmax / Vcmax ratioꎬ and chlorophyll content in leaves
of tobacco grown at 24 ℃ and 32 ℃. Values are means ± SE (n = 4~
5). Significant differences (shown by asterisks) between plants grown
at 24 ℃ and 32 ℃ were examined via one ̄way ANOVA (P < 0􀆰 05)
7823期      HUANG and HU: Effect of Growth Temperature on the Activity of Cyclic Electron Flow in Tobacco Leaves     
Fig􀆰 4  Responses of Y(II)ꎬ Y(NPQ)ꎬ Y(NO)ꎬ and 1 ̄qL to inci ̄
dent photosynthetic photon flux density ( PPFD) in leaves of tobacco
grown at 24 ℃ and 32 ℃ . Y(II)ꎬ effective quantum yield of PSIIꎻ Y
(NPQ)ꎬ fraction of energy dissipated as heat via regulated non ̄photo ̄
chemical quenching mechanismꎻ Y(NO)ꎬ fraction of energy passively
dissipated in forms of heat and fluorescenceꎻ qLꎬ proportion of PSII
centers in open state. Values are means ± SE (n= 4~5)
Fig􀆰 5  Responses of ETRIIꎬ ETRIꎬ and ETRI / ETRII ratio to inci ̄
dent photosynthetic photon flux density ( PPFD) in leaves of tobacco
grown at 24 ℃ and 32 ℃ . Values are means ± SE (n= 4~5). Signifi ̄
cant differences (shown by asterisks) between plants grown at 24 ℃
and 32 ℃ were examined via one ̄way ANOVA (P < 0􀆰 05)
When plants normally grown at 24 ℃ were trea ̄
ted for 1 h at 24 ℃ and 2 000 μmol photons m-2 s-1ꎬ
their leaf values of Fv / Fm decreased from 0􀆰 78 to
0􀆰 57 versus a decline from 0􀆰 78 to 0􀆰 50 for plants
normally grown at 32 ℃ (Fig􀆰 6). After 2 h of treat ̄
ment with this highlight intensityꎬ those Fv / Fm val ̄
ues decreased to 0􀆰 47 and 0􀆰 39 in plants normally
grown at 24 ℃ and 32 ℃ꎬ respectively (Fig􀆰 6). This
882                                  植 物 分 类 与 资 源 学 报                            第 37卷
demonstrated thatꎬ under strong illumination at 24 ℃ꎬ
plants originally exposed to 32 ℃ had significantly
more PSII photoinhibition than those that had re ̄
mained at 24 ℃ throughout this experimental period.
Fig􀆰 6  Effect of high light on PSII photoinhibition in leaves of tobacco
grown at 24 ℃ and 32 ℃ . Detached leaves were exposed to light
(2 000 μmol photons m-2 s-1) at 24 ℃ . Values are means ± SE
(n= 5). Asterisks indicate significant differences between plants
grown at 24 ℃ and 32 ℃ (one ̄way ANOVAꎬ P < 0􀆰 05)
3  Discussion
We hypothesized that the plants of tobacco could
regulate CEF activity for acclimating to growth tem ̄
perature. Light response curves showed thatꎬ under
high lightꎬ the plants grown at 24 ℃ significantly
had higher values of ETRI and ETR I / ETRII ratio
than plants grown at 32 ℃ . These results suggested
that CEF activity was up ̄regulated in plants grown at
24 ℃ compared with that grown at 32 ℃ . Because
24 ℃ is lower than the optimum growth temperature
for tobacco (about 30 ℃ꎬ Yamori et al.ꎬ 2010)ꎬ our
results strongly suggest that the enhancement of CEF
activity is an important strategy for tobacco plants to
acclimate to relatively low growth temperature.
The response of CO2 assimilation to incident
light and C i did not differ significantly between
plants grown at 24 ℃ and 32 ℃ (Fig􀆰 1 and Fig􀆰 2).
Likewiseꎬ levels of chlorophyll and values for Jmaxꎬ
Vcmaxꎬ and the Jmax / Vcmax ratio were not significantly
altered ( Fig􀆰 3). Based on these results we could
conclude that 1) N ̄partitioning between the enzymes
related to RuBP carboxylation and regeneration did
not change in plants grown at either 24 ℃ or 32 ℃ꎬ
and 2) the Rubisco activation state did not differ
significantly between plants grown at these two tem ̄
peratures. Furthermoreꎬ the light response change in
1 ̄qL did not significantly differ for plants at 24 ℃ or
32 ℃ (Fig􀆰 4D). Thusꎬ the higher CEF activity in
plants grown at 24 ℃ did not affect photosynthetic
capacity and the stromal redox state. The enhance ̄
ment of CEF activity in plants grown at 24 ℃ proba ̄
bly had other physiological functionsꎬ such as photo ̄
protection for PSII.
The photoinhibition of PSII is a net result when
the rate of photodamage exceeds that of repair. This
repair of photodamaged PSII is based on the newly
synthesis of D1 proteinꎬ which can be inhibited by
ROS (Nishiyama et al.ꎬ 2001ꎬ 2004ꎬ 2011ꎻ Taka ̄
hashi et al.ꎬ 2007ꎬ 2009). Because we found that
the value of Y(NPQ) under high light did not differ
in plants grown at 24 ℃ and 32 ℃ꎬ we assumed that
ROS production was equal. Consequentlyꎬ the rate of
PSII recovery probably did not differ between the two
temperature scenarios. Because ROS not only inhib ̄
its the repair of the photodamaged PSII complex but
also accelerates the rate at which PSII is damaged
(Oguchi et al.ꎬ 2009ꎬ 2011)ꎬ we would have ex ̄
pected to find no growth ̄temperature ̄related change
in the rate of ROS ̄induced PSII photodamage. After
high ̄light treatment and dark ̄adaptation for 20 min
at 24 ℃ꎬ Fv / Fm values were higher for plants grown
at 24 ℃ than for those grown at 32 ℃ ( Fig􀆰 6).
Thereforeꎬ plants at the lower temperature showed
less rate of PSII photodamage. Photodamage to PSII
occurs primarily at the oxygen ̄evolving complex
(OEC)ꎬ which is on the lumenal side of the thyla ̄
koid membrane (Hakala et al.ꎬ 2005ꎻ Ohnishi et
al.ꎬ 2005). Following photodamage to the OECꎬ the
supply of electrons from water to the primary electron
donor of PSII ( P680+ ) is blockedꎬ such that the
level of P680+ remains high (Takahashi and Mura ̄
taꎬ 2008ꎻ Takahashi and Badgerꎬ 2011). P680+ is
a strong oxidant thatꎬ at high levelsꎬ can damage the
9823期      HUANG and HU: Effect of Growth Temperature on the Activity of Cyclic Electron Flow in Tobacco Leaves     
PSII reaction centers (Anderson and Chowꎬ 2002).
Thereforeꎬ the difference in Fv / Fm after high ̄light
treatment found between plants grown at 24 ℃ and
32 ℃ (Fig􀆰 6) may have been caused by variability
in the sensitivity of OEC to photodamage.
Recovery from inactivation of the OEC can be
suppressed by calcium ̄channel blockersꎬ thereby in ̄
dicating that stability of that complex is dependent
upon the Ca2+ in the lumen of the thylakoid mem ̄
brane (Kerieger and Weisꎬ 1993). Acidification of
the lumen could drive a Ca2+ / H+ antiport to sequester
Ca2+ in the lumen. This has been demonstrated by Et ̄
tinger et al. (1999)ꎬ who monitored the movement of
approximately 4 mM Ca2+ into the lumen from an ex ̄
ternal concentration of 15 μM. Thereforeꎬ one might
speculate that the generation of ΔpH is necessary for
the stabilization of OEC. Furthermoreꎬ the formation
of ΔpH through PGR5 ̄dependent CEF activity can
suppress PSII photodamage in Arabidopsis regardless
of NPQ activation (Takahashi et al.ꎬ 2009). Once this
OEC photodamage is alleviated by the CEF ̄depend ̄
ent generation of ΔpHꎬ the PSII reaction centers can
be further protected. Thereforeꎬ the enhancement of
CEF activity in plants grown at 24 promoted the gen ̄
eration of ΔpH under intense illumination and sup ̄
pressed photodamage to PSII activity.
Field ̄grown plants are frequently exposed to low
temperatures and high light intensities. Low tempera ̄
tures decrease the stomatal conductance and activity
in the Calvin ̄Benson cycleꎬ thereby inducing the o ̄
ver ̄accumulation of NADPH and an over ̄reduction
of photosynthetic electron chains under strong light
(Murata et al.ꎬ 2007). Under such conditionsꎬ an
increase in NADPH / NADP + ratio causes activation
of NDH ̄dependent and PGR5 ̄dependent CEF path ̄
ways (Johnsonꎬ 2005ꎻ Shikanaiꎬ 2007). The higher
CEF activity in plants grown at 24 ℃ may be part of
a strategy used by cold ̄sensitive species such as to ̄
bacco to acclimate to relatively low growth tempera ̄
tures. Clear biochemical and spectroscopic evidence
has been found in higher plants for the occurrence of
at least two CEF pathways: PGR5 and NDH (Shika ̄
nai et al.ꎬ 1998ꎻ Munekage et al.ꎬ 2002ꎬ 2004ꎻ
Johnsonꎬ 2011). Although our results indicated that
the plants grown at the lower temperature had greater
CEF activityꎬ it is unclear which pathway was up ̄
regulated. In riceꎬ NDH ̄dependent CEF activity has
an important role in regulating photosynthesis at low
temperatures (Yamori et al.ꎬ 2011). Several cold ̄
tolerant species show substantial up ̄regulation of the
NDH ̄complex in response to low growth temperature
and high light (Teicher et al.ꎬ 2000ꎻ Streb et al.ꎬ
2005ꎻ Laureau et al.ꎬ 2013). Howeverꎬ the rate of
NDH ̄dependent CEF is estimated to be very low and
its contribution to the formation of a proton gradient
is not significant (Munekage et al.ꎬ 2002ꎬ 2004ꎻ
Okegawa et al.ꎬ 2008). The promotion of CEF activ ̄
ity under high light is similar between Arabidopsis
mutants lacking NDH ̄dependent CEF (ccr6 mutants)
and the control type (Yamori et al.ꎬ 2011). The main
role of NDH may be to regulate the rate of PGR5 ̄de ̄
pendent CEF by poising the redox state of intersys ̄
tem electron carriers (Peltier and Cournacꎬ 2002).
Thereforeꎬ we speculate that the tobacco plants
grown at 24 ℃ probably up ̄regulate the components
involved in PGR5 ̄dependent CEF to enhance the
generation of ΔpH under more intense light.
In summaryꎬ our results suggested that PGR5 ̄
dependent CEF activity was up ̄regulated in tobacco
plants grown at 24 ℃ compared with that grown at
32 ℃ꎬ which alleviated PSII photoinhibition under
intense light. In natural environmentsꎬ temperatures
often fluctuate in seasons or days. When strong illu ̄
mination is coupled with low temperatureꎬ PSII ac ̄
tivity usually decreases. Enhancement of CEF activi ̄
ty at a relatively low growth temperature helps in the
generation of ΔpH and protects PSII against photo ̄
damage under high light. This is regarded as an im ̄
portant strategy by chilling ̄sensitive plants to accli ̄
mate to relatively lower growth temperatures.
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