Effects of short-term heat stress on PSll and subsequent recovery for senescent leaves of Vitis vinifera L. cv. Red Globe

ZHANG Kun , CHEN Bai-hong HAO Yan, YANG Rui, WANG Yu-an

1 Horticulture College, Gansu Agricultural University, Lanzhou 730070, P.R.China

2 Institute of Fruit and Floriculture Research, Gansu Academy of Agricultural Sciences, Lanzhou 730070, P.R.China

Abstract Heat stress occurs frequently in energy-saving sunlight greenhouses (ESSG) at the late growth stage. Three-year delayed cultivation (DC) of the Red Globe cultivar of Vitis vinifera L. was used to clarify the physiological mechanisms of short-term heat stress on PSII and subsequent recovery from heat stress. By November, the photosynthetic function had declined and the fall in transpiration rate (E) with heating time increased the possibility of heat damage. In July, the most obvious increase was in the relative variable fluorescence at J point at 40°C, and in November it changed to K point. The 5 min of heat treatment resulted in a signiflcant increase of the relative variable fluorescence at 0.3 ms (Wk), and after 10 min of heat treatment, the number of reactive centres per excited cross section (RC/CSo), probability that a trapped exciton moves an electron into the electron transport chain beyond QA– (at t=0) (Ψo) and quantum yield of electron transport at t=0 (φEo)decreased signiflcantly (P＜0.05), suggesting that the reaction centre, donor and acceptor side of photosystem II (PSII)were all signiflcantly inhibited (P＜0.05) and that the thermal stability of the photosynthetic mechanism was reduced. The inhibition of energy fluxes for senescent leaves in November was earlier and more pronounced than that for healthy leaves,which did not recover from heat stress of more than 15 min after 2 h recovery at room temperature.

Keywords: short-term heat stress, leaf senescence, chlorophyll a fluorescence, arid desert region, delayed cultivation

1. lntroduction

The arid desert region of the Hexi Corridor, China has a typical temperate, continental dry climate with abundant light resources. Since the altitude and latitude are high,the ≥10°C active accumulated temperature is not enough forVitis viniferaL.cv. Red Globe to mature in the fleld.Delayed cultivation (DC) for Red Globe grapes in energysaving sunlight greenhouses (ESSG) has been developed to mitigate this problem. In arid desert regions on a sunny day, the max temperature could reach 21°C at noon even in November, and the max temperature in ESSG could rise to nearly 40°C. A previous study found that the optimum photosynthetic temperature is from 25 to 35°C (Mullinset al. 1992; Battagliaet al. 1996), if the temperature is higher than 35°C, photosynthesis will be inhibited (Jianget al. 2017), and temperatures of 38°C for 4 d could injure the photosynthetic system (Xiaoet al. 2017). However,after short-term heat stress, which may occur every day in the summer, plants may recover from injury. But the heat tolerance of different species is very different:V.viniferacultivar can endure 40°C for a short time (Luoet al. 2011);for barley, fully reversible response was observed after temperature stress at 42–43°C (Frolecet al. 2008); and the photosystem II (PSII) function of sweet sorghum could be recovered to the control level 1 d after heat stress at 43°C for 2 h (Yanet al. 2013). However, if the time of heat stress is sufflciently long, temperatures at or above 40°C could cause the plant to die (Chenet al. 2016). The extent of damage to grape leaves by heat stress depends on strength, duration(Koutalianouet al. 2016), leaf age (?esták 1999; Xuet al.2008) and genotype (Kadiret al. 2007) among other factors.Some researchers selected 47°C as the critical temperature(Tc) to evaluate injury in grapes at high temperature (Xuet al. 2014). During the late growing period, the Tcmay decrease mainly because the photosynthetic apparatus has begun to degrade. It was also found that after long-term heat acclimation in the summer, leaf heat acclimatization was improved (Hanet al. 2010). During production, for senescent leaves, the high temperature not only caused leaf yellowing earlier, but also caused serious dehydration of fruit (Hanet al. 2010).

When leaves experience high temperature, the typical symptom is decreased of photosynthetic system function(Thakuret al. 2016). PSII is considered to be the most heatsensitive physiological component (Luoet al. 2011; Rustioniet al. 2015), it is usually inhibited or damaged before other cellular functions (Mathuret al. 2014). The oxygen-evolving complex (OEC) is the most heat-sensitive component of the photosynthetic apparatus, and it is involved in water splitting and oxygen release (Mamedovet al. 1993). The OEC on the donor side is more heat-sensitive than on the acceptor side (Chen and Cheng 2010), and is probably the flrst suppressed target (Huanget al. 2016). The K point of the fast chlorophyll fluorescence induction curve (OJIP) is widely accepted as an index of OEC damage. Chlorophyllafluorescence is a rapid and non-invasive tool that can be used to monitor early temperature stress (Rolfe and Scholes 2010), and the OJIP curve contains a large amount of useful information about the structure and function of the photosynthetic apparatus, especially the PSII (Valcke 2014). The change in light intensity of the OJIP curve mainly depends on the influence on the primary quinone electron acceptors of PSII (QA), and changes in the OJIP curve reflect the transmission of the photosynthetic electron transport chain (Nagyet al. 2012). The OJIP test thus reflects the behaviour of PSII function, including energy absorption, trapping, and electron transport (Ivanovet al.2008) which can reveal the decline of photosynthetic function for senescent leaves during heat stress.

Short-term high temperature may cause severe damage to leaves during aging in ESSG. Monitoring and preventing high-temperature stress could delay the decline of leaf photosynthetic function, and is crucial to maintain the quality of grape fruit. In this study, we evaluated the effects of short-term high temperature on PSII during the late growth period using a series of exposure times at 40°C. This study aims to: (i) evaluate PSII behaviour under short-term high temperature for leaves during aging, and (ii) predict the tolerance of senescent leaves to high temperature and a short recovery time. This study should be useful for ESSG temperature management and will enhance our understanding of the effects of high temperature on plant photosynthetic systems.

2. Materials and methods

2.1. Study site

The experiments were performed in 2015 in Minqin County(38°46′8.23′′N, 103°14′23.78′′E, 1 500 m above sea level),on the edge of the Tenggeli Desert, China. The region has an extremely arid, continental climate, with a mean annual temperature of 7.2°C, 160 frost-free days, and average annual precipitation of 127.7 mm. The average annual evaporation is 2 623 mm, and the ≥10°C effective accumulated temperature is approximately 3 000°C d. The study site has grey-brown desert soil, organic matter is 7.9 g kg–1, total nitrogen is 0.47 g kg–1, effective phosphorus is 6 mg kg–1, and available potassium average is 200 mg kg–1, with a pH value of 8.6.

2.2. Delayed cultivation in ESSG

The goal of ESSG cultivation is to make sure that grapes can mature with enough calories and then delay grape’s phenological phase by artiflcially regulating the temperature in ESSG. For the ESSG in this study, the wall structure was made of rammed earth, and the structure supporting the cover fllm was steel (Fig. 1-A). Some of the main parameters are listed in Table 1.

2.3. Experimental design

Three-year-old cultivated grapevine variety Red Globe was grown in pots in ESSG. Leaves located in the middle of the healthy grown shoots were selected in November when plants were 162 days old. The heat treatments were performed in a light incubator, with photon flux density of 400 μmol m–2s–1, relative humidity of 60%, and CO2concentration dependent on the environment. There were six heat-exposure treatments at 40°C, including 0 min(control), 5 min (M1), 10 min (M2), 15 min (M3), 20 min (M4),

Fig. 1 Basic structure of energy-saving sunlight greenhouses(ESSG) in Northwest China and the temperature during later growth period. A, structure of ESSG. B and C, months and diurnal variation of temperature on sunny days, respectively.L, h and T, width, height and thickness of ESSG, respectively.t, the corridor width on the top of the wall.

25 min (M5), and 30 min (M6).

2.4. Measurement of gas exchange

Gas exchange was measured with a portable photosynthesis system (LI-6400, Li-Cor, Inc., Lincoln, NE, USA) in July and November when plants were not experiencing heat stress.Before the formal experiment, each leaf was acclimated in the chamber for 30 min when net photosynthetic rate(Pn) achieved a stable value. Photosynthetic light response curves (Pn/PAR) were made by measuring the value ofPnto varying photosynthetically active radiation (PAR).ThePARvalue was reduced from 1 800 to 0 μmol m–2s–1, in steps of 200 μmol m–2s–1, CO2concentration was 450 μmol mol–1and the temperature was 26°C. Each stablePnvalue was saved automatically. Parameters including light compensation point (LCP), light saturation point (LSP),and apparent quantum yield (AQY) were generated by evaluating the equations of thePnresponse toPAR. TheAQYwas deflned as the slope of the regression curve atPAR≤200 μmol m–2s–1.

Gas exchange in high-temperature treatments was measured in a light incubator immediately after hightemperature treatment,PARwas set at 400 μmol m–2s–1,CO2concentration was 450 μmol mol–1, and relative humidity was 60%.Pn, stomatal conductance (Gs), intercellular CO2concentration (Ci), and transpiration rate (E) were recorded simultaneously.

2.5. Measurement of chlorophyll a fluorescence transient

Chlorophyllafluorescence transients of normally grown leaves in July and November were measured directly with a Handy-PEA fluorometer (Hansatech Instruments Ltd.,Norfolk, UK) in the ESSG. After acclimation to total darkness for 30 min, the leaves were exposed to a saturating light pulse (3 000 mmol m–2s–1photon flux density (PFD)) for 2 s and parameters were automatically recorded.

The chlorophyllafluorescence transients of leaves in two periods were measured after heat treatments in an incubator in the dark. Plant recovery occurred at room temperature for 2 h with the PFD of 300 μmol m–2s–1.

2.6. Statistical analysis

Data were analysed and charts were generated using Excel 2003 and the Microsoft Draw tool. One-way ANOVA was carried out using SPSS 19.0 (SPSS Inc., Chicago,IL, USA). Signiflcant differences between means were determined through a Student’st-test, and differences were considered statistically signiflcant whenP＜0.05. The results are presented as the mean±SE.

3. Results

3.1. Effects of leaf senescence on Pn/PAR and OJlP curve

The photosynthetic light response curve (Pn/PAR) and chlorophyllafluorescence transients changed signiflcantly from July to November, the O and J points increased, and the I and P points decreased (Fig. 2-B). ThePnmax, LSP,AQY and dark respiration rate (Rday) were signiflcantly decreased by 53.71, 29.03, 34.55 and 16.88%, respectively(P＜0.05), and theLCPwas signiflcantly increased by 26.99% (P＜0.05) in November compared to in July(Fig. 2-A and Table 2).

Table 1 Basic parameters of energy-saving sunlight greenhouses (ESSG) in arid desert area of Hexi Corridor, China1)

Fig. 2 Performance comparison of photosystems in leaves under normal environment. A, the response curves of net photosynthetic rate (Pn) to irradiance measured in 450 μmol CO2 mol–1 at room temperature using the same leaf in July (26°C) and November(18°C). PAR, photosynthetically active radiation. B, fast chlorophyll a fluorescence induction curves (OJIP) measured directly in the fleld at the same time. The OJIP parameters O, J and I are the fluorescence intensity at 50 μs, 2 ms, and 30 ms, respectively,and P is the maximum fluorescence intensity.

Table 2 The photosynthetic parameters in July and November of the same leaf 1)

3.2. Effects of heat stress on gas exchange

At 40°C, as treatment time increased,Pndecreased signiflcantly from M5 to M6 (P＜0.05) in July, and in NovemberPndecreased signiflcantly after 15 min of heat treatment (from M3 to M6) (Fig. 3-A). After treatment for 30 min,PnandGsdecreased by 17.68 and 19.94%,respectively, in July, and by 70.97 and 73.65%, respectively,in November (Fig. 3-B). In July,Ciincreased from M1 to M6, and 30 min of heat treatment had no signiflcant effect onCi(P＜0.05); in November,Cifrom M4 to M6 signiflcantly decreased compared with control (P＜0.05) (Fig. 3-C). In July, the changes inEincreased, but in November their values decreased gradually as high-temperature treatment time increased (Fig. 3-D).

3.3. Effects of heat stress on chlorophyll a fluorescence transients

The relative variable fluorescence ΔVtin July and November were compared after high-temperature treatments. In July, as treatment time increased, the increase in J was more obvious than in I or K (Fig. 4-A). In November, the change in the K point was the most signiflcant (Fig. 4-B).As heat treatment time increased, the J and K points increased.

3.4. Effects of heat stress on Fo, Fm, Fv/Fm and PIABS

Fig. 3 The gas exchange parameters of the same leaf in July and November after heat treatments at 40°C. A, net photosynthetic rate (Pn). B, stomatal conductance (Gs). C, intercellular CO2 concentration (Ci). D, transpiration rate (E). Control and M1–M6,high temperature treatments at 40°C for 0, 5, 10, 15, 20, 25, and 30 min, respectively. The values are mean±SE (n=5). Different lowercase letters indicate signiflcant differences at P＜0.05.

Fig. 4 Fast chlorophyll a fluorescence induction curves of the same leaf in July (A) and November (B), respectively. The relative variable fluorescence ΔVt=Vt treatments–Vt control, Vt=(Ft–Fo)/(Fm–Fo), in which, ΔK, ΔJ and ΔI are the ΔVt at 300 μs, 2 ms and 30 ms,respectively. M1–M6, high temperature treatments at 40°C for 5, 10, 15, 20, 25, and 30 min, respectively. The values are mean±SE(n=8). Different lowercase letters indicate signiflcant differences at P＜0.05.

Before high-temperature treatment,Foincreased by 39.47%andFmdecreased by 7.53% in November compared to July due to leaf senescence. Heat treatment of 40°C for 10 min led to a signiflcant increase inFoin November(P＜0.05), and the time of signiflcant increase was 10 min earlier than in July. After 30 min,Foincreased by 26.28 and 51.57% in July and November, respectively (Fig. 5-A). A signiflcant decrease inFmoccurred after 20 min of heating in July (P＜0.05), and occurred only 5 min later in November(Fig. 5-B). TheFv/Fmdecreased signiflcantly after 30 min of heat treatment in July (P＜0.05), but in November,10 min of heat treatment caused it to decrease signiflcantly(P＜0.05) (Fig. 5-C). In November, before high-temperature treatment,PIABSdecreased by 81.01% compared with that in July, and as treatment time increased, thePIABSvalue decreased rapidly. In July, high-temperature treatment time over 15 min causedPIABSvalues to decrease signiflcantly(P＜0.05) (Fig. 5-D).

Fig. 5 Comparison of chlorophyll a fluorescence parameters. A, the minimal fluorescence (Fo). B, the maximal fluorescence(Fm). C, the maximum quantum yield of photosystem II (PSII) (Fv/Fm). D, performance index (PIABS). Control and M1–M6, high temperature treatments at 40°C for 0, 5, 10, 15, 20, 25, and 30 min, respectively. The room temperatures in July and November were 26 and 18°C, respectively. The values are mean±SE (n=8). Different lowercase letters indicate signiflcant differences at P＜0.05.

3.5. Effect of heat stress on reaction centre, donor and acceptor side of PSll

In November, the density of PSII reaction centres per excited cross-section (RC/CSo) was 29.30% lower than in July before high-temperature treatment. In November, heat treatment at 40°C for 10 min caused a signiflcant decrease inRC/CSo(P＜0.05); in July, the signiflcant reductions occurred 20 min later (P＜0.05) (Fig. 6-A).Wkis widely used to analyse the effects of stress on OEC, and the initialWkvalues of leaves in July and November were 0.461 and 0.554, respectively, without high-temperature stress;5 min of heat treatment resulted in a signiflcant increase ofWk(P＜0.05) and a rapid increase was found as the treatment time increased in November. After 30 min of heat treatment,Wkvalues increased by 17.78 and 38.30% in July and November, respectively (Fig. 6-B).Ψorepresents the efflciency with which a trapped exciton can move an electron into the electron transport chain beyond(att=0), andφEorepresents the quantum yield of electron transport (att=0). After heat treatment for 10 min, signiflcant decreases inΨoandφEo(P＜0.05) were found in November (P＜0.05),and in July their values were signiflcantly decreased after 20 min of heat treatment (Fig. 6-C and D).

3.6. Energy fluxes in reaction centre response to heat stress

Speciflc activity parameters can more accurately reflect the absorption, transformation and dissipation of light energy by photosynthetic organs.ABS/RCis the absorption flux per RC; leaf senescence increasedABS/RC, and hightemperature promoted it. In November,ABS/RCsigniflcantly increased after 10 min of heat treatment (P＜0.05), which was 5 min earlier than in July. After 30 min of heat treatment,the value increased by 102%, far exceeding that in July(Fig. 7-A).TRo/RCis the trapped energy flux per RC, and in November, 10 min of high-temperature treatment allowed it to increase signiflcantly (Fig. 7-B); in July, this change took place 25 min later.ETo/RCis the electron transport flux per RC, and before high-temperature treatment, the value in November was 0.75, and 30 min later it decreased to 0.13 (Fig. 7-C), 78.33% lower than July. Under the same conditions the increase ofDIo/RCin November was higher than in July, and a signiflcant increase occurred after only 5 min of heat treatment (P＜0.05) (Fig. 7-D).

3.7. Recovery after heat stress

After 2 h of recovery at room temperature, in July theVtchanged slightly even after 30 min of high-temperature stress (Fig. 8-A). In November, the J and K points from M4 to M6 were still signiflcantly increased compared with control (Fig. 8-B). TheFv/Fmof treatments from M1 to M6 were not signiflcantly reduced compared with control in July(P＜0.05), but in November from M4 to M6 theFv/Fmcould not be restored to the control level (Fig. 8-C). PIABSof treatments from M4 to M6 and M3 to M6 in July and November,respectively, were signiflcantly reduced compared with the control (P＜0.05) (Fig. 8-D).Foand RC/CSowere difflcult to recover within 2 h, if the preheat treatment time exceeded 15 min in November (Fig. 8-E and F).

Fig. 6 Changes of the reaction centre, donor and acceptor side of PSII. A, the number of reactive centres per excited cross section,RC/CSo. B, ratio of variable fluorescence at 300 μs to the amplitude F j–F o,=(F300 μs–Fo)/(F j–F o). C, probability (at t=0) that a trapped exciton moves an electron into the electron transport chain beyond (ψo(hù)). D, quantum yield of electron transport at t=0 (φEo). Control and M1–M6, high temperature treatments at 40°C for 0, 5, 10, 15, 20, 25, and 30 min, respectively. The values are mean±SE (n=8). Different lowercase letters indicate signiflcant differences at P＜0.05.

Fig. 7 Energy fluxes in reaction centre response to heat stress. A, absorbed energy flux per reactive centre (ABS/RC). B, trapped energy flux per reactive centre (TRo/RC). C, electron transport flux per reactive centre (ETo/RC). D, dissipated energy flux per reactive centre (DIo/RC). Control and M1–M6, high temperature treatments at 40°C for 0, 5, 10, 15, 20, 25, and 30 min, respectively.The values are mean±SE (n=8). Different lowercase letters indicate signiflcant differences at P＜0.05.

Fig. 8 Photosystem II (PSII) recovery for 2 h at room temperature (26°C in July and 18°C in November, respectively) after heat stress. A and B, the relative variable fluorescence (Vt, Vt=(F t-Fo)/(Fm-Fo)) in July and November, respectively. C, the maximum quantum yield of PSII (Fv/Fm). D, performance index (PIABS). E, the minimal fluorescence (Fo). F, the number of reactive centres per excited cross section (RC/CSo). Control and M1–M6, high temperature treatments at 40°C for 0, 5, 10, 15, 20, 25, and 30 min,respectively. The values are mean±SE (n=8). Different lowercase letters indicate signiflcant differences at P＜0.05.

4. Discussion

During production, leaf health is important to maintain the quality of grapes. In open flelds, high-temperature stress does not occur during the fruit ripening period, but in ESSG,it occurs frequently (Fig. 1-B and C). High-temperature produces strong negative effects on photosynthetic function for senescent leaves (Franklin and Agren 2002). Most previous research has focused on PSII regulation during high-temperature stress (Zhaet al. 2015) and recovery following stress using healthy leaf tissue (Yordanovet al. 1997; Guoet al. 2016), but senescent leaves during the late growth period in ESSG have rarely been investigated.

4.1. Gas exchange response to heat stress

The stomata control the supply of CO2and the loss of water in the leaves (Cowan 1977). In this study, theGsin July decreased gradually as treatment time increased, but theCionlychanged slightly (Fig. 3-C), showing that stomatal conductance was sensitive to high temperature stress and decreased flrstly. A slight decrease inGsdid not result in a decrease inE, which was beneflcial for reducing the damage of high-temperature to blade (Zandalinaset al. 2016), but when the time or intensity of heat stress is long enough or high enough,Ewill be decreased with the decrease ofGs. In November, the decrease ofGslimited CO2diffusion,causingCito decrease (Fig. 3-C and D), suggesting that the decrease ofPnwas mainly caused by stomata conductance. The change ofGswas positively correlated with the change ofE, and this change is the best way to achieve optimal water use (Yanget al. 2008). However,in a high-temperature environment, the decrease ofEwill increase the temperature of the blade surface, making it more vulnerable to high-temperature damage, which may be one of the main reasons why old leaves were not resistant to high-temperature stress.

4.2. Response of photosynthetic performance to heat stress

Fv/Fmis usually approximately 0.83 for most healthy plant species (Samsonet al. 1999), but in November,Fv/Fmwas 0.75 without heat stress (Fig. 5-C), which showed that the potential photosynthetic efflciency had been decreased(Tartachnyk and Blanke 2004). The reduction ofFv/Fmwas mainly due to an increase inFoand a decrease inFm(Yamaneet al. 1997). The rise inFoalso means that the inactivation of the reaction centre was serious.PIABScontainsRC/ABS,φPoandψo(hù)of three independent parameters(Huang and Netravali 1994),and is more sensitive thanFv/Fmto heat stress (Weiet al. 2012; Sui 2015). This study showed that after heating for 30 min,Fv/Fmwas decreased by 36.30%(Fig. 5-C) compared with the control, butPIABSdecreased by 37.13% after only 10 min in November (Fig. 5-D).

4.3. The reaction centre, donor and acceptor side of PSll

In November, 10 min of heat treatment resulted in a signiflcant decrease of RC/CSo(P＜0.05), 10 min earlier than in July under the same temperature condition; lower RC/CSosuggested that the load increases per RC. The appearance of the K-point indicated the OEC injury was caused by inhibiting efflcient electron donation to the RC(Mathuret al. 2011b). The ΔK peak is associated with the degree of injury to the OEC (Strasser 1997), and ΔK-band＞0 suggests that the OEC is destroyed to varying degrees,whereas the ΔJ-band ＞0 means the accumulation of QA–increased relative to control. Thus, for senescent leaves the OEC was the main high-temperature injury site because the relative variable fluorescence at 300 μs was increased,the ability to supply electrons downstream was weaker, and the inhibition or injury was aggravated by prolonging heat treatment time. In July, the major high-temperature injury site was the J point, proving that high temperature had some influence on electron transfer and had little effect on the structure of the photosynthetic system.Wkexpresses the K step in the OJIP test, which reflects the degree of restriction on the donor side of the PSII relative to the receptor side,the larger theWkvalue, the greater the injury to the donor side (Jinet al. 2015). Compared with control, heat treatment for 5 min at 40°C signiflcantly increasedWKin November(P＜0.05) (Fig. 6-B). Xuet al. (2014) found that 10 min of heat treatment at 47°C could cause a signiflcant increase inWk, showing that the OEC of the donor side was damaged more seriously than on the receptor side. After 2 h of recovery at 18°C, the relative fluorescence of the K and J points for M4, M5 and M6 were still higher than in the control(P＜0.05) (Fig. 8-B), showing that inhibition or damage of the OEC still occurred, and the electron transport between QAand QBwas still blocked in November. However, in July, this difference between treatments and control was very weak.φEoandψo(hù)reflect the change in the receptor side of PSII, and after heating 5 min,φEoandψo(hù)did not signiflcantly decrease compared with control in November (P＜0.05) (Fig. 6-C and D), this result indicated that the electron transport chain of the receptor side of PSII was not inhibited, but this inhibition occurred 10 min later in November, 5 min later than the time of signiflcant increase inWk, indicating that the donor side was more vulnerable than the receptor side.

4.4. Energy fluxes in reaction centre response to heat stress

When the structure of PSII is changed, energy conversion is affected. After heating for 10 min, signiflcant changes were found in ABS/RC and ETo/RC (P＜0.05). After 15 min of heat treatment the TRo/RC and DIo/RC were signiflcantly changed at 40°C (P＜0.05) in November. ABS/RC, TRo/RC and DIo/RC increased mainly because the RC/CSodecreased and the efflciency was enhanced per RC (Mathuret al. 2011a).This result suggested that RC controlled much more of the energy conversion tasks. The increase of ABS/RC indicates that the degradation rate of pigment in the PSII increased,and in November, this rate of increase was faster than in July.ABS/RC and TRo/RC could measure the size of the antenna pigment, because when both values increase the antenna pigment size will also increase (Bresticet al. 2012). This was caused by the change in the number of light-harvesting complexes per RC, further explaining the partial deactivation of the RC. The absorption and capture of light energy did not increase in unit area energy for electron transport in ETo/RC, but a sharp increase of DIo/RC showed that the RC must increase heat dissipation for self-protection (Mathuret al. 2011a) (Fig. 7-D).

4.5. Recovery of PSll after heat stress

PSII photo-damage can be quickly repaired in an appropriate environment (Huanget al. 2010). In the Hexi Corridor, the temperature can change widely within a day, and excessive inhibition of high temperature would increase the time for recovery or cause the loss of photosynthetic organ function.We set the recovery time at 2 h, mainly based on production needs, and focused concern on the PSII recovery over this short time period. We found that the photosynthetic system of senescent leaves was very sensitive to high temperature during the late growth season. Yordanov and Weis (1984)found thatPhaseolus vulgaristhermal stability declined during leaf aging and was almost absent in senescent leaves. In this study, although only 5 min of heating could signiflcantly decreasePIABS(P＜0.05), the effects were reversible. If the heat treatment time exceeded 15 min then thePIABSwas difflcult to restore to the control level after 2 h of recovery (Fig. 8-C), and the relative fluorescence of the K and J points were still higher than control, meaning that the harm of high temperature to OEC and the accumulation of QA–had not been alleviated for leaves during aging. After a comprehensive analysis of the indicators after recovery, we believe that if the preheat treatment time was more than 15 min at 40°C, the recovery time would be increased, and the possibility of irreversible inactivation would be increased.

5. Conclusion

The temperature tolerance of senescent leaves decreased,but chlorophyll fluorescence parameters were still sensitive to short-term heat stress. At 40°C, stress time for more than 15 min caused serious damage to the PSII, which could not recover to the control level in 2 h, either because it need more time to recover, or because irreversible injury may have occurred. Therefore, we suggest that the temperature management should be considered during the leaf senescence period in the greenhouse.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (31660585), the Experimental Station for Scientiflc Observation of Fruit Trees in the Northwest of China (10218020) and the earmarked fund for China Agriculture Research System (CARS-30-21).