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Keywords:

  • heat stress;
  • isoprene;
  • photosynthesis;
  • reactive oxygen species;
  • rubisco;
  • thylakoid reactions

ABSTRACT

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. HEAT STRESS EFFECTS ON THYLAKOID REACTIONS
  5. HEAT-STRESS EFFECTS ON RUBISCO ACTIVATION
  6. THERMOTOLERANCE INDUCED BY ISOPRENE
  7. CONCLUSION
  8. ACKNOWLEDGMENTS
  9. REFERENCES

Photosynthesis is particularly sensitive to heat stress and recent results provide important new insights into the mechanisms by which moderate heat stress reduces photosynthetic capacity. Perhaps most surprising is that there is little or no damage to photosystem II as a result of moderate heat stress even though moderate heat stress can reduce the photosynthetic rate to near zero. Moderate heat stress can stimulate dark reduction of plastoquinone and cyclic electron flow in the light. In addition, moderate heat stress may increase thylakoid leakiness. At the same time, rubisco deactivates at moderately high temperature. Relationships between effects of moderate heat on rubisco activation and thylakoid reactions are not yet clear. Reactive oxygen species such as H2O2 may also be important during moderate heat stress. Rubisco can make hydrogen peroxide as a result of oxygenase side reactions and H2O2 production by rubisco was recently shown to increase substantially with temperature. The ability to withstand moderately high temperature can be improved by altering thylakoid lipid composition or by supplying isoprene. In my opinion this indicates that thylakoid reactions are important during moderate heat stress. The deactivation of rubisco at moderately high temperature could be a parallel deleterious effect or a regulatory response to limit damage to thylakoid reactions.


INTRODUCTION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. HEAT STRESS EFFECTS ON THYLAKOID REACTIONS
  5. HEAT-STRESS EFFECTS ON RUBISCO ACTIVATION
  6. THERMOTOLERANCE INDUCED BY ISOPRENE
  7. CONCLUSION
  8. ACKNOWLEDGMENTS
  9. REFERENCES

Three papers in this issue of Plant, Cell and Environment describe new understandings of how moderate heat stress affects photosynthesis. Haldimann & Feller (2005) show that pea plants grown in different temperatures exhibit similar deactivation of rubisco at 38 °C but adjust so that thylakoid reactions are more tolerant of high temperature when the plants are grown at high temperature. The other two papers (Peñuelas et al. 2005; Velikova & Loreto 2005) explore thermoprotection of photosynthetic capacity by isoprene. These papers also explore the relationships among heat stress, thermotolerance, and reactive oxygen species. In this opinion I discuss the importance of moderate heat stress effects on photosynthesis and a number of new observations on the mechanisms by which heat may damage photosynthesis.

High leaf temperatures reduce plant growth and limit crop yields. Estimates range up to a 17% decrease in yield for each degree Celsius increase in average growing season temperature (Lobell & Asner 2003). The global mean temperature increased by 0.6 °C from 1990 to 2000 and is projected to increase by another 1.4 to over 5 °C by 2100 (Houghton et al. 2001; IPCC 2001). This is driven in large measure by increasing atmospheric CO2, which is up by 31% since 1750. High leaf temperature is also a consequence of drought because plants lose the ability for transpirational cooling when water availability is limited. It has been specifically shown for cotton that the greater the transpirational cooling, the greater the yield (Radin et al. 1994; Lu et al. 1997), proving that heat-stress-induced reductions of photosynthesis can limit overall yield.

Leaves of many plants are very thin and so have minimal heat capacity and as a result, leaves exposed to full sunlight can warm up substantially above air temperature. Using fine-wire thermocouples it has been shown that leaves with low transpiration rates, such as oak leaves, suffer frequent high-temperature episodes during which leaf temperature can exceed air temperature by as much as 15 °C (Singsaas & Sharkey 1998; Hanson et al. 1999; Singsaas et al. 1999). The high transpirational cooling of cotton can also lead to rapid and large variation in leaf temperature with peak temperatures frequently in the range of 35–40 °C (Wise et al. 2004). Thus, heat stress must be considered in terms of both high air temperature and fluctuations in leaf temperature related to the energy balance of leaves. Leakey, Press, & Scholes (2003) found that high temperature was more deleterious under simulated light-fleck conditions than under steady-state conditions. To understand the effects of moderately high temperature under realistic conditions, the simple ‘cook and look’ methods that have been the most common protocol for exploring effects of moderate heat on photosynthetic capacity up to now, should be supplemented by more realistic temperature regimes.

Even in the absence of heat stress injury, photosynthesis would be expected to decline as temperature increases because photorespiration increases with temperature faster than does photosynthesis (Schuster & Monson 1990). However, it is now well established that moderately high temperature (e.g. 35–40 °C) reduces the rate of photosynthesis more than can be explained by stimulated photorespiration. In the past, photosystem II (PSII) was considered a key weak link (Santarius 1975; Santarius & Müller 1979; Berry & Björkman 1980; Enami et al. 1994) but damage to PSII only occurs at high temperatures, often above 45 °C (Terzaghi et al. 1989; Thompson et al. 1989; Gombos et al. 1994; Çjánek et al. 1998; Yamane et al. 1998). The reversible effects of moderate heat stress on PSII likely represent regulation to match reductions in the capacity of downstream reactions. Reactions downstream of PSII are inhibited at lower temperature than that needed to damage PSII. Thus, damage to PSII cannot explain the heat-induced depression in photosynthesis seen at temperatures between 35 and 40 °C. This is a surprising conclusion that is counter to the prevailing wisdom. Nevertheless, it is now well-confirmed that moderate heat stress normally encountered by most plants does not damage PSII, but does substantially reduce the rate of photosynthesis.

There are some studies linking heat shock proteins (HSPs) and photosynthetic capacity (Heckathorn et al. 1998; Downs, Coleman, & Heckathorn 1999; Heckathorn et al. 2002; Barua, Downs, & Heckathorn 2003). The small chloroplast HSP has been implicated in protecting PSII, but this means HSPs may not be important to protection against moderate heat stress. Most recent studies of moderate heat stress have focused on thylakoid reactions or rubisco.

HEAT STRESS EFFECTS ON THYLAKOID REACTIONS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. HEAT STRESS EFFECTS ON THYLAKOID REACTIONS
  5. HEAT-STRESS EFFECTS ON RUBISCO ACTIVATION
  6. THERMOTOLERANCE INDUCED BY ISOPRENE
  7. CONCLUSION
  8. ACKNOWLEDGMENTS
  9. REFERENCES

When leaves that have never been heat-stressed are first exposed to moderately high temperature, photosynthetic electron flow can be stimulated (Schrader et al. 2004). However, upon returning the leaf to the pre-stress temperature, the capacity for photosynthetic electron flow can be substantially inhibited relative to the control situation (Schrader et al. 2004). Because of the frequent, wide fluctuations of individual leaf temperature, the inhibition of electron transport capacity following heat stress is physiologically relevant. As reduced electron transport capacity is sometimes seen only following heat stress, the effects of moderate heat stress on thylakoid reactions are easily missed despite their physiological relevance. Heat stress studies should include examination of the consequences of one or more heat stress episodes, not simply the rate of photosynthesis during the first heat stress episode experienced by a leaf.

Changes in the structure of the thylakoid membrane following moderate heat stress were observed by freeze fracture (Armond, Björkman, & Staehelin 1980; Gounaris et al. 1984) and these structural changes may cause changes in thylakoid function. Pastenes & Horton (1996) and Bukhov et al. (1999) proposed that moderate heat stress causes increased thylakoid proton conductance and increased cyclic electron flow around PSI (Bukhov, Samson, & Carpentier 2000; Bukhov & Carpentier 2000; Egorova & Bukhov 2002). Havaux et al. (1996) found increased thylakoid leakiness that could be counteracted by the induction of zeaxanthin synthesis; increased levels of zeaxanthin have been shown to alter the physical state of the thylakoid membrane (Havaux & Gruszeckia 1993; Havaux & Tardy 1996). The importance of the properties of the thylakoid membrane in temperature stress was also supported by the finding that mutants lacking trienoic fatty acids in their thylakoids are more tolerant of heat (Murakami et al. 2000) but such plants are also more susceptible to photoinhibition at low temperature (Vijayan & Browse 2002).

Therefore, the damage to thylakoid reactions by moderate heat stress is not a result of damage to PSII, but probably involves pathways related to cyclic electron flow and perhaps the cytochrome complex. Substantial progress in understanding cyclic electron flow and the cytochrome complex has been reported recently and the relevance of these new findings to heat stress will now be described.

Cyclic electron flow

The amount of energy consumed in cyclic and pseudocyclic (a.k.a. the water–water cycle (Asada 1999)) electron flow has been debated, with most studies reporting relatively low fluxes in either pathway under normal steady-state conditions. PSI-mediated cyclic electron flow can occur by either of two routes (Joët et al. 2001), an antimycin A-sensitive route involving ferredoxin plastoquinone reductase or a second route involving the NAD(P)H dehydrogenase complex (Thomas et al. 1986; Boucher, Harnois, & Carpentier 1989; Bukhov et al. 2000; Joët et al. 2001). Joliot & Joliot (2002) hypothesized that cyclic electron flow occurs in structurally isolated electron transport chains that include PSI, cyt b6f complex, one ferredoxin and one plastocyanin. These isolated chains would account for many PSI complexes when leaves are dark-adapted. The thylakoid membrane is hypothesized to undergo a structural rearrangement during photosynthetic induction so that linear electron flow dominates during steady-state photosynthesis.

Makino, Miyake & Yokota (2002) investigated the physiological function of PSI-mediated cyclic electron flow and the water–water cycle. They found that when dark-adapted leaves were first illuminated, there was a substantial electron flow through the water–water cycle. As carbon fixation began, linear electron flow to carbon predominated but a low level of cyclic electron flow mediated by PSI was found at high light. Upon switching to low oxygen, PSI activity increased whereas PSII activity decreased indicating a switch from the water–water cycle to PSI-mediated cyclic electron flow. The Km for oxygen of the endogenous Mehler reaction (part of the water–water cycle) is equivalent to 8–15% O2 in the atmosphere so it is expected that an atmosphere with 2% O2 will effectively eliminate the water–water cycle (Furbank & Badger 1983). Makino et al. (2002) interpreted their results to indicate that during the induction phase of photosynthesis the water–water cycle and PSI-mediated cyclic electron flow play critical roles in establishing a transthylakoid energy gradient. The disadvantage of the water–water cycle is the generation of activated oxygen species, which must be quenched before damage can occur. PSI-mediated cyclic electron flow (which would generate the transthylakoid energy gradient needed to regulate PSII (Demmig-Adams & Adams 1992) could be a mechanism for limiting PSII activity and so limit the production of activated oxygen species (Heber & Walker 1992; Heber 2002).

Stimulation of cyclic electron flow by heat stress

Heating appears to specifically engage cyclic electron flow around PSI (Havaux 1996; Bukhov, Boucher, & Carpentier 1998; Bukhov et al. 2000). The dark reduction of PSI was found to undergo ‘spectacular acceleration’ with the half-life of P700+ falling from over 500 ms to less than 50 ms between 34 and 40 °C (Havaux 1996). Havaux (1996) also reported that following heating, plastoquinone was reduced in darkness by stromal reductants, and could be oxidized by brief illumination of far red light (preferentially exciting PSI). A flow of electrons from the stroma to the plastoquinone pool in the dark at 36 °C was also reported by Yamane et al. (2000). In these studies a specific protein was hypothesized that might catalyse electron donation to plastoquinone and that would only be active above 35 °C. Yamane et al. (2000) found little effect of antimycin A, the inhibitor of ferredoxin-plastoquinone reductase-type cyclic electron flow. They also saw only minor inhibition of the high temperature electron flow to plastoquinone by feeding inhibitors of Ndh1, which catalyses the other known cyclic electron transport path. Haldimann and Feller report in this issue that the dark reduction of plastoquinone following high temperature stress is one of the processes adjusted by growing pea plants at high temperature. Low-temperature-grown pea plants exhibited dark reduction of plastoquinone following a 38 °C leaf temperature treatment but plants grown at 35 °C did not. Apparently, in the dark, plastoquinone is normally not in redox equilibrium with the stroma, but heat stress opens a path for electrons from the stroma to plastoquinone. This results in reduction of plastoquinone in the dark and stimulation of cyclic electron flow in the light.

The increase in cyclic electron flow could require movement of cyt b6/f complexes from appressed regions of the thylakoid to non-appressed regions (Joliot & Joliot 2002). In addition, recent crystallography studies indicate that cyt b6/f complexes normally exist as dimers with a lipid-rich cavity between the two monomers (Kurisu et al. 2003; Cramer et al. 2004). This cavity is the site of electron arrival from plastoquinone. The nature of the lipid in the cavity, and the effects of heat on the overall structure is not known. It has been suggested that the cytochrome complex can exist in different states (Schreiber, Heimann, & Klughammer 1998), but the effect of heat on the cyt b6 /f complex location or structure has not been examined.

During a 39 °C heat pulse, a stimulation of PSI with little concurrent effect on PSII was observed (Schrader et al. 2004) confirming that cyclic electron flow is dramatically accelerated during heat stress. We also found a significant decline in stromal oxidation status in moderately high temperature (Schrader et al. 2004) consistent with a heat-stress-induced pathway for electrons to flow from the stroma to plastoquinone. Lowered stromal redox status can lead to significant stimulation of PSI-mediated cyclic electron flow (Joët et al. 2002), which raises the question – is cyclic electron flow stimulated by high temperature directly or does stromal oxidation cause the stimulation of cyclic electron flow?

The involvement of protein phosphorylation with cyclic electron flow and heat stress

One of the mechanisms involved in stimulating cyclic electron flow is phosphorylation of light-harvesting chlorophyll complex of PSII (LHCII). Phosphorylated LHCII moves from the appressed thylakoid regions, where PSII is located, to the unappressed regions, where PSI is located (Chow, Miller, & Anderson 1991). The phosphorylated LHCII becomes energetically disconnected from PSII core complex (slowing its turnover rate) and energetically coupled to a PSI core (increasing its turnover rate). This process requires a specific polypeptide within PSI (Lunde et al. 2000). This is the well-known state transition (Allen 1992) and is known to accompany an increase in cyclic electron flow around PSI, including in response to heat (Wise et al. 2004). However, heat has been reported to stimulate dephosphorylation of a number of PSII core proteins namely D1, D2 and CP43 (Rokka et al. 2000; Vener et al. 2001). Therefore, the regulation of phosphorylation of LHCII may well be quite different from the regulation of phosphorylation of the PSII core proteins (Harrison & Allen 1991; Pursiheimo et al. 2003). The regulation of phosphorylation of thylakoid proteins interacts with redox status (Vener et al. 1995). Likewise, dephosphorylation of LHCII appears to be catalysed by a different phosphatase than dephosphorylation of other thylakoid-associated proteins (Hammer, Markwell, & Sarath 1997; Vener et al. 1999). There are several thylakoid associated kinases (TAKs) (Snyders & Kohorn 1999, 2001) but also another kinase that appears unrelated to TAKs that is necessary for state transitions and phosphorylation of LHCII (Depège, Bellafiore, & Rochaix 2003).

Thus, there may be two different regulatory systems that control thylakoid protein phosphorylation/dephosphorylation; one that controls phosphorylation/dephosphorylation of LHCII and state transitions and a second system that controls phosphorylation/dephosphorylation of other thylakoid proteins. A large number of thylakoid proteins undergo reversible phosphorylation (Hansson & Vener 2003) and heat stress is one of the most effective ways of modulating the phosphorylation status of many of them (Vener et al. 2001). For some thylakoid proteins whose phosphorylation status varies, the function of the protein is not known (Carlberg et al. 2003; Hansson & Vener 2003).

In summary, the physical state and structure of the thylakoid membrane could be critically important for shifting between linear and cyclic electron flow. It is possible that structures found at high temperature (e.g. unstacked thylakoids) optimize its function at high temperature. However, it may be that the structural rearrangements that allow photosynthesis to tolerate moderate heat stress leads to damage when the leaves cool back to non-stressful temperatures. Repeated episodes of moderately high temperature may result in very different types of damage than occurs during the first heating episode.

HEAT-STRESS EFFECTS ON RUBISCO ACTIVATION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. HEAT STRESS EFFECTS ON THYLAKOID REACTIONS
  5. HEAT-STRESS EFFECTS ON RUBISCO ACTIVATION
  6. THERMOTOLERANCE INDUCED BY ISOPRENE
  7. CONCLUSION
  8. ACKNOWLEDGMENTS
  9. REFERENCES

Moderately high leaf temperature leads to deactivation of rubisco (Kobza & Edwards 1987) confirming the findings of Weis (1980) that rubisco activity is reduced by heat. Rubisco activase is heat labile (Feller, Crafts-Brandner, & Salvucci 1998; Salvucci et al. 2001) and deactivation of rubisco correlates with the decline in photosynthesis at moderately high temperature (Law & Crafts-Brandner 1999). These authors propose that the heat-induced deactivation of rubisco is the primary constraint on photosynthesis at moderately high temperature and show that chlorophyll fluorescence signals from PSII are not affected by temperatures that cause significant deactivation of rubisco (Crafts-Brandner & Salvucci 2000; Haldimann & Feller 2004).

In my opinion there is no doubt that rubisco deactivates at moderately high temperature and three hypotheses for this behaviour of plants can be put forward.

  • 1
    Rubisco activase is inherently heat sensitive and cannot keep up with the demands on its activity at high temperature.
  • 2
    Natural selection has favoured plants in which rubisco deactivates at moderately high temperature because of the unfavourable ratio of photorespiration to photosynthesis at these temperatures. If the cost of metabolizing phosphoglycolate is high and the carbon gain is low at moderately high temperature it could be advantageous to deactivate rubisco to avoid the generation of phosphoglycolate and forgo the carbon that could be fixed.
  • 3
    Natural selection has favoured plants in which rubisco deactivates at moderately high temperature because this prevents more severe damage, for example to thylakoid structure or function, that might otherwise occur in the absence of deactivation.

The first hypothesis is the easiest to understand. If the observation is that rubisco deactivates, and in some cases rubisco activase becomes less soluble as a result of moderately high temperature (Feller et al. 1998; Haldimann & Feller 2005), it is logical to assume that rubisco deactivation is the lesion caused by heat stress.

The second hypothesis is best illustrated with a graph. Figure 1 shows how net CO2 assimilation declines with temperature according to models of photosynthesis. This decline comes about because the ratio of photorespiration to photosynthesis increases with temperature. The rate of oxygenation, the first step of photorespiration, is shown in Fig. 1 as circles. In today's atmosphere net CO2 assimilation is high, even at 40 °C. However, if the model is run for 190 p.p.m. CO2 (open symbols), as atmospheric modellers say was probably the CO2 level for many thousands of years, then CO2 assimilation is much reduced and the velocity of oxygenation is much higher. The cost of photorespiratory metabolism relative to the benefit in terms of CO2 assimilation may be unfavourable to plants when the temperature is above 35 °C, or perhaps this was the case when the CO2 concentration was 190 p.p.m.

image

Figure 1. Modelled rates of CO2 assimilation and oxygenation. CO2 assimilation and vO were modelled as a function of temperature using the Farquhar model (Farquhar, Von Caemmerer, & Berry 1980) and parameterizations of Bernacchi et al. (Bernacchi et al. 2001; Bernacchi, Pimentel, & Long 2003). Open symbols are data assuming 190 p.p.m. CO2, and filled symbols are data assuming 370 p.p.m. CO2.

Download figure to PowerPoint

Support for the third hypothesis comes from studies of tobacco plants in which rubisco activase was reduced by antisense technology leading to a constant 20% activation state of rubisco and a lack of response of activation state to temperature. Photosynthesis of these plants was just as sensitive to temperature as was photosynthesis of plants with functional activase in which rubisco deactivated at high temperature. However, the heat-induced reduction of photosynthesis in plants unable to deactivate rubisco was irreversible while that in plants able to deactivate rubisco was reversible (Sharkey et al. 2001a). Some of the dangers at high temperature which could be avoided by deactivation of rubisco include build-up of products of photorespiration and H2O2. Kim & Portis (2004) reported that H2O2 is made as a side reaction of oxygenation of RuBP and that this side reaction occurs more frequently at higher temperature, both because it is inherently temperature sensitive and because oxygenation goes faster at high temperature. Another danger at high temperature could be deleterious effects of changes in thylakoid structure and function as described above.

If either hypothesis 1 or 2 are correct, then engineering rubisco to stay more active at high temperature should improve thermotolerance. However, if hypothesis 3 is correct, engineering a rubisco that does not deactivate could result in a plant that is less able to tolerate heat. My opinion is influenced by the fact that rubisco deactivation is regulatory in several other circumstances. These include low light (Perchorowicz, Raynes, & Jensen 1981) and when starch and sucrose synthesis limit photosynthesis (feedback-limited photosynthesis, as can be induced in low oxygen) (Sharkey 1985, 1989). Rubisco deactivation in low light or under feedback conditions appears to be regulatory; is the same true for deactivation at moderately high temperature?

THERMOTOLERANCE INDUCED BY ISOPRENE

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. HEAT STRESS EFFECTS ON THYLAKOID REACTIONS
  5. HEAT-STRESS EFFECTS ON RUBISCO ACTIVATION
  6. THERMOTOLERANCE INDUCED BY ISOPRENE
  7. CONCLUSION
  8. ACKNOWLEDGMENTS
  9. REFERENCES

An important reason for studying how moderate heat reduces the photosynthetic capacity of leaves is to determine how photosynthesis can be made more tolerant of heat. Studies of isoprene emission resulted in the hypothesis that isoprene inside leaves can improve thermotolerance of photosynthesis (Sharkey & Singsaas 1995). The ultimate proof of isoprene providing thermotolerance requires knowing the mechanism. However, given the uncertainty of how heat damages photosynthesis, it is difficult to hypothesize how isoprene protects against it. On the other hand, if isoprene provides substantial thermotolerance, then the damage caused by heat must have some relationship to things that isoprene can influence.

Direct tests of the thermotolerance hypothesis became possible when it was discovered that fosmidomycin, an inhibitor of the isoprene synthesis pathway (Zeidler et al. 1998) was highly specific and did not inhibit photosynthesis (Sharkey, Chen, & Yeh 2001b). Following three heat spikes (2 min at 42 °C), photosynthesis at 30 °C was inhibited by one-third when leaves were able to make isoprene but by two-thirds when isoprene production was inhibited. Exogenous isoprene given to fosmidomycin-fed leaves could restore thermotolerance (Sharkey et al. 2001b).

Peñulas et al. report in this issue that isoprene fumigation of Quercus ilex, which normally does not emit isoprene, resulted in substantial thermotolerance between 35 and 45 °C. Photosynthesis at 50 °C was depressed to the same degree in both treatments (Peñuelas et al. 2005). Photosystem II chlorophyll fluorescence was not affected by heat below 50 °C. Isoprene fumigation interacted with some antioxidant enzymes and suppressed the endogenous monoterpene emission. Also in this issue, Velikova and Loreto report that exposing leaves of Phragmites australis to 38 °C for 90 min caused a greater reduction in photosynthesis when isoprene synthesis was inhibited by fosmidomycin, than when it was not (Velikova & Loreto 2005). In addition, photosynthesis recovered more when the leaf temperature was returned to the pre-stress level of 30 °C when isoprene was present than when it was inhibited. The effect was substantial, with about twice as much photosynthetic capacity found in leaves making isoprene than in leaves not making isoprene. Thermotolerance is much easier to see during recovery from heat stress than during the initial heat stress episode (Sharkey et al. 2001b; Velikova & Loreto 2005). Heat damage to photosynthetic electron flow was apparent after heat stress but not during an initial heat stress episode (Schrader et al. 2004), and so in this way, thermotolerance induced by isoprene appears similar to heat effects on thylakoid reactions. Specifically, much of the protection occurs during recovery of leaf temperature to non-stressful levels rather than during the heat stress.

It is my opinion that the thermotolerance hypothesis for isoprene function now has substantial support, given three independent reports using different species and methods (Sharkey et al. 2001b; Peñuelas et al. 2005; Velikova & Loreto 2005). The effect is strongest between 25 and 45 °C and is not related to photosystem II damage. At this time, the effect of isoprene on heat-induced dark reduction of plastoquinone has not been tested. Isoprene has been shown to protect against reactive oxygen species (Loreto et al. 2001; Loreto & Velikova 2001; Affek & Yakir 2002). Both Peñuelas et al. and Velikova and Loreto report in this issue isoprene protection against both high temperature and reactive oxygen species, or increased levels of antioxidant enzymes. However, in the experiments of Velokova and Loreto, reactive oxygen species were reduced by isoprene at all temperatures and the increase in reactive oxygen species with an increase in temperature was similar whether or not isoprene was present. It is not clear whether the antioxidant activity of isoprene is parallel with the effect on thermotolerance or indicates how temperature in the range of 35–45 °C inhibits photosynthesis. Knowing the mode of action of isoprene requires knowing the mode of action of moderate heat on photosynthesis. Isoprene induced thermotolerance is very fast (Singsaas & Sharkey 1998; Singsaas et al. 1999) compared with changes in xanthophylls (Havaux et al. 1996) or fatty acid composition of thylakoid membranes (Murakami et al. 2000).

What of an alternative explanation for isoprene emission, that it is a way of freeing phosphate that has inadvertently become stuck on DMAPP (Logan, Monson, & Potosnak 2000)? The need for isoprene production to release phosphate from DMAPP depends on there being no mechanism for regulating the early steps of DMAPP synthesis. If the MEP pathway has significant feedback control so that DMAPP levels can be regulated, then a phosphate release mechanism is not needed and is a futile cycle. If there is little feedback control, then the amount of DMAPP could increase and become a significant sink for phosphate, and isoprene production could release this phosphate for recycling back into ATP. Wolfertz et al. (2004) tested the regulation of the early steps of the MEP pathway by feeding deuterium-labelled deoxyxylulose (DOX) to leaves. This compound is taken up by leaves and quickly converted to deoxyxylulose 5-phosphate, and then isoprene. The isoprene coming from the deuterated DOX is deuterated and can be distinguished from undeuterated isoprene using laser photo-acoustics (Kühnemann et al. 2002). There was essentially a one-for-one reduction in unlabelled isoprene as labelled isoprene started to be detected during feeding of deuterated DOX. In other words, exogenous sources of carbon fed into the MEP pathway past deoxyxylulose-5-phosphate synthase caused the synthase activity to be reduced so that the flux through the pathway (total isoprene emission) stayed remarkably constant (Wolfertz et al. 2004). This indicates that there is no need to invoke a futile cycle to get rid of DMAPP inadvertently made by the MEP pathway, the pathway is sufficiently regulated to prevent this problem.

CONCLUSION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. HEAT STRESS EFFECTS ON THYLAKOID REACTIONS
  5. HEAT-STRESS EFFECTS ON RUBISCO ACTIVATION
  6. THERMOTOLERANCE INDUCED BY ISOPRENE
  7. CONCLUSION
  8. ACKNOWLEDGMENTS
  9. REFERENCES

Substantial progress has been made recently in understanding how photosynthesis is affected by heat. There are substantial effects of moderate heat on photosystem I and cytochrome complex reactions. There is no effect of these moderately stressful heat treatments on photosystem II. In addition, rubisco normally deactivates at these moderately stressful temperatures. The rubisco deactivation can occur in parallel with the effects on thylakoid reactions or can mask the damage to thylakoid reactions, especially during the first exposure of a leaf to heat stress. Because many of the things that improve thermotolerance are known or presumed to work on thylakoid function, my opinion is that the rubisco deactivation is a regulatory response and is not the underlying problem that needs to be fixed to make plants more thermotolerant. Three areas remain as important areas of research: (1) what is the specific nature of the effects of moderate heat stress on thylakoid reactions other than PSII? (2) what is the role of rubisco deactivation, is it regulatory or simply a deleterious response to elevated temperature? and (3) what are the roles of reactive oxygen species at high temperature?

ACKNOWLEDGMENTS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. HEAT STRESS EFFECTS ON THYLAKOID REACTIONS
  5. HEAT-STRESS EFFECTS ON RUBISCO ACTIVATION
  6. THERMOTOLERANCE INDUCED BY ISOPRENE
  7. CONCLUSION
  8. ACKNOWLEDGMENTS
  9. REFERENCES

Research in my laboratory on heat stress effects on photosynthesis is supported by the National Research Initiative, CSREES, USDA grant 2004-35100-14860. Research on isoprene is supported by the National Science Foundation grant IBN-0212204. I thank Mike Salvucci for comments on the manuscript. I first heard of the idea of rubisco deactivation protecting against high rates of photorespiration at high temperature from T. John Andrews.

REFERENCES

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. HEAT STRESS EFFECTS ON THYLAKOID REACTIONS
  5. HEAT-STRESS EFFECTS ON RUBISCO ACTIVATION
  6. THERMOTOLERANCE INDUCED BY ISOPRENE
  7. CONCLUSION
  8. ACKNOWLEDGMENTS
  9. REFERENCES
  • Affek H.P. & Yakir D. (2002) Protection by isoprene against singlet oxygen in leaves. Plant Physiology 129, 269277.
  • Allen J.F. (1992) Protein phosphorylation in regulation of photosynthesis. Biochimica et Biophysica Acta 1098, 275335.
  • Armond P.A., Björkman O. & Staehelin H.A. (1980) Disociation of supramolecular complexes in chloroplast membranes. A manifestation of heat damage to the photosynthetic apparatus. Biochimica et Biophysica Acta 601, 433442.
  • Asada K. (1999) The water–water cycle in chloroplasts: scavenging of active oxygens and dissipation of excess photons. Annual Review of Plant Physiology and Plant Molecular Biology 50, 601639.
  • Barua D., Downs C.A. & Heckathorn S.A. (2003) Variation in chloroplast small heat-shock protein function is a major determinant of variation in thermotolerance of photosynthetic electron transport among ecotypes of Chenopodium album. Functional Plant Biology 30, 10711079.
  • Bernacchi C.J., Pimentel C. & Long S.P. (2003) In vivo temperature response functions of parameters required to model RuBP-limited photosynthesis. Plant, Cell and Environment 26, 14191430.
  • Bernacchi C.J., Singsaas E.L., Pimentel C., Portis A.R. Jr & Long S.P. (2001) Improved temperature response functions for models of Rubisco-limited photosynthesis. Plant, Cell and Environment 24, 253259.
  • Berry J.A. & Björkman O. (1980) Photosynthetic response and adaptation to temperature in higher plants. Annual Review of Plant Physiology 31, 491543.
  • Boucher N., Harnois J. & Carpentier R. (1989) Heat-stress stimulation of electron flow in a photosystem I submembrane fraction. Biochemistry and Cell Biology 68, 9991004.
  • Bukhov N.G. & Carpentier R. (2000) Heterogeneity of photosystem II reaction centers as influenced by heat treatment of barley leaves. Physiologia Plantarum 110, 279285.
  • Bukhov N.G., Boucher N. & Carpentier R. (1998) Loss of the precise control of photosynthesis and increased yield of non-radiative dissipation of excitation energy after mild heat treatment of barley leaves. Physiologia Plantarum 104, 563570.
  • Bukhov N.G., Samson G. & Carpentier R. (2000) Nonphotosynthetic reduction of the intersystem electron transport chain of chloroplasts following heat stress. Steady-state rate. Photochemistry and Photobiology 72, 351357.
  • Bukhov N.G., Wiese C., Neimanis S. & Heber U. (1999) Heat sensitivity of chloroplasts and leaves: leakage of protons from thylakoids and reversible activation of cyclic electron transport. Photosynthesis Research 59, 8193.
  • Carlberg I., Hansson M., Kieselbach T., Schröder W.P., Andersson B. & Vener A.V. (2003) A novel plant protein undergoing light-induced phosphorylation and release from the photosynthetic thylakoid membranes. Proceedings of the National Academy of Sciences of the USA 100, 757762.
  • Chow W.S., Miller C. & Anderson J.M. (1991) Surface charges, the heterogeneous lateral distribution of the two photosystems, and thylakoid stacking. Biochimica et Biophysica Acta 1057, 6977.
  • Çjánek M., Štroch M., Lachetová K., Kalina J. & Špunda V. (1998) Characterization of the photosystem II inactivation of heat-stressed barley leaves as monitored by the various parameters of chlorophyll a fluorescence and delayed fluorescence. Journal of Photochemistry and Photobiology 47, 3945.
  • Crafts-Brandner S.J. & Salvucci M.E. (2000) Rubisco activase constrains the photosynthetic potential of leaves at high temperature and CO2. Proceedings of the National Academy of Sciences of the USA 97, 1343013435.
  • Cramer W.A., Zhang H.M., Yan J.S., Kurisu G. & Smith J.L. (2004) Evolution of photosynthesis: Time-independent structure of the cytochrome b6f complex. Biochemistry 43, 59215929.
  • Demmig-Adams B. & Adams W.W. III (1992) Photoprotection and other responses of plants to high light stress. Annual Review of Plant Physiology and Plant Molecular Biology 43, 599626.
  • Depège N., Bellafiore S. & Rochaix J.D. (2003) Role of chloroplast protein kinase Stt7 in LHCII phosphorylation and state transition in Chlamydomonas. Science 299, 15721575.
  • Downs C.A., Coleman J.S. & Heckathorn S.A. (1999) The chloroplast 22-Ku heat-shock protein: a lumenal protein that associates with the oxygen evolving complex and protects photosystem II during heat stress. Journal of Plant Physiology 155, 477487.
  • Egorova E.A. & Bukhov N.G. (2002) Effects of elevated temperatures on the activity of alternative pathways of photosynthetic electron transport in intact barley and maize leaves. Russian Journal of Plant Physiology (Abstract Only) 49, 575584.
  • Enami I., Kitamura M., Tomo T., Isokawa Y., Ohta H. & Katoh S. (1994) Is the primary cause of thermal inactivation of oxygen evolution in spinach PSII membranes release of the extrinsic 33 kDa protein or Mn? Biochimica et Biophysica Acta 1186, 5258.
  • Farquhar G.D., Von Caemmerer S. & Berry J.A. (1980) A biochemical model of photosynthetic CO2 assimilation in leaves of C3 species. Planta 149, 7890.
  • Feller U., Crafts-Brandner S.J. & Salvucci M.E. (1998) Moderately high temperatures inhibit ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activase-mediated activation of Rubisco. Plant Physiology 116, 539546.
  • Furbank R.T. & Badger M.R. (1983) Oxygen exchange associated with electron transport and photophosphorylation in spinach thylakoids. Biochimica et Biophysica Acta 723, 400409.
  • Gombos Z., Wada H., Hideg E. & Murata N. (1994) The unsaturation of membrane lipids stabilizes photosynthesis against heat stress. Plant Physiology 104, 563567.
  • Gounaris K., Brain A.P.R., Quinn P.J. & Williams W.P. (1984) Structural reorganization of chloroplast thylakoid membranes in response to heat stress. Biochimica et Biophysica Acta 766, 198208.
  • Haldimann P. & Feller U. (2004) Inhibition of photosynthesis by high temperature in oak (Quercus pubescens L.) leaves grown under natural conditions closely correlates with a reversible heat-dependent reduction of the activation state of ribulose-1,5-bisphosphate carboxylase/oxygenase. Plant, Cell and Environment 27, 11691183.
  • Haldimann P. & Feller U. (2005) Growth at moderately elevated temperature alters the physiological response of the photosynthetic apparatus to heat stress in pea (Pisum sativum L.) leaves. Plant, Cell and Environment 28, 0000.
  • Hammer M.F., Markwell J. & Sarath G. (1997) Purification of a protein phosphatase from chloroplast stroma capable of dephosphorylating the light-harvesting complex-II. Plant Physiology 113, 227233.
  • Hanson D.T., Swanson S., Graham L.E. & Sharkey T.D. (1999) Evolutionary significance of isoprene emission from mosses. American Journal of Botany 86, 634639.
  • Hansson M. & Vener A.V. (2003) Identification of three previously unknown in vivo protein phosphorylation sites in thylakoid membranes of Arabidopsis thaliana. Molecular and Cellular Proteomics 2, 550559.
  • Harrison M.A. & Allen J.F. (1991) Light-dependent phosphorylation of Photosystem II polypeptides maintains electron transport at high light intensity: Separation from effects of phosphorylation of LHC-II. Biochimica et Biophysica Acta 1058, 289296.
  • Havaux M. (1996) Short-term responses of photosystem I to heat stress – Induction of a PS II-independent electron transport through PS I fed by stromal components. Photosynthesis Research 47, 8597.
  • Havaux M. & Gruszecki W.I. (1993) Heat- and light-induced chlorophyll a fluorescence changes in potato leaves containing high or low levels of the carotenoid zeaxanthin: indications of a regulatory effect of zeaxanthin on membrane fluidity. Photochemistry and Photobiology 58, 607617.
  • Havaux M. & Tardy F. (1996) Temperature-dependent adjustment of the thermal stability of Photosystem II in vivo: possible involvement of xanthophyl-cycle pigments. Planta 198, 324333.
  • Havaux M., Tardy F., Ravenel J., Chanu D. & Parot P. (1996) Thylakoid membrane stability to heat stress studied by flash spectroscopic measurements of the electrochromic shift in intact potato leaves: influence of the xanthophyll content. Plant, Cell and Environment 19, 13591368.
  • Heber U. (2002) Irrungen, Wirrungen? The Mehler reaction in relation to cyclic electron transport in C3 plants. Photosynthesis Research 73, 223231.
  • Heber U. & Walker D. (1992) Concerning a dual function of coupled cyclic electron transport in leaves. Plant Physiology 100, 16211626.
  • Heckathorn S.A., Downs C.A., Sharkey T.D. & Coleman J.S. (1998) The small, methionine-rich chloroplast heat-shock protein protects photosystem II electron transport during heat stress. Plant Physiology 116, 439444.
  • Heckathorn S.A., Ryan S.L., Baylis J.A., Wang D.F., Hamilton E.W. III, Cundiff L. & Luthe D.S. (2002) In vivo evidence from an Agrostis stolonifera selection genotype that chloroplast small heat-shock proteins can protect photosystem II during heat stress. Functional Plant Biology 29, 933944.
  • Houghton J.T., Ding Y., Griggs D.J et al. (2001) Climate Change 2001: the Scientific Basis, pp. 1881. Cambridge University Press, Cambridge, UK. (www.ipcc.ch)
  • IPCC (2001) Climate Change 2001: Impacts, Adaptation, and Vulnerability. Summary for Policy Makers. p. 50. IPCC, Geneva, Switzerland.
  • Joët T., Cournac L., Horvath E.M., Medgyesy P. & Peltier G. (2001) Increased sensitivity of photosynthesis to antimycin A induced by inactivation of the chloroplast ndhB gene. Evidence for a participation of the NADH-dehydrogenase complex to cyclic electron flow around photosystem I. Plant Physiology 125, 19191929.
  • Joët T., Cournac L., Peltier G. & Havaux M. (2002) Cyclic electron flow around photosystem I in C3 plants. In vivo control by the redox state of chloroplasts and involvement of the NADH-dehydrogenase complex. Plant Physiology 128, 760769.
  • Joliot P. & Joliot A. (2002) Cyclic electron transfer in plant leaf. Proceedings of the National Academy of Sciences of the USA 99, 1020910214.
  • Kim K. & Portis J. (2004) Oxygen-dependent H2O2 production by Rubisco. FEBS Letters 571, 124128.
  • Kobza J. & Edwards G.E. (1987) Influences of leaf temperature on photosynthetic carbon metabolism in wheat. Plant Physiology 83, 6974.
  • Kühnemann F., Wolfertz M., Arnold A., Lagemann M., Popp A., Schüler G., Jux A. & Boland W. (2002) Simultaneous online detection of isoprene and isoprene-d2 using infrared photoacoustic spectroscopy. Applied Physical B 75, 397403.
  • Kurisu G., Zhang H.M., Smith J.L. & Cramer W.A. (2003) Structure of the cytochrome b6f complex of oxygenic photosynthesis: tuning the cavity. Science 302, 10091014.
  • Law R. & Crafts-Brandner S.J. (1999) Inhibition and acclimation of photosynthesis to heat stress is closely correlated with activation of ribulose-1,5-bisphosphate carboxylase/oxygenase. Plant Physiology 120, 173182.
  • Leakey A.D.B., Press M.C. & Scholes J.D. (2003) High-temperature inhibition of photosynthesis is greater under sunflecks than uniform irradiance in a tropical rain forest tree seedling. Plant, Cell and Environment 26, 16811690.
  • Lobell D.B. & Asner G.P. (2003) Climate and management contributions to recent trends in U.S. agricultural yields. Science 299, 1032.
  • Logan B.A., Monson R.K. & Potosnak M.J. (2000) Biochemistry and physiology of foliar isoprene production. Trends in Plant Science 5, 477481.
  • Loreto F. & Velikova V. (2001) Isoprene produced by leaves protects the photosynthetic apparatus against ozone damage, quenches ozone products, and reduces lipid peroxidation of cellular membranes. Plant Physiology 127, 17811787.
  • Loreto F., Mannozzi M., Maris C., Nascetti P., Ferranti F. & Pasqualini S. (2001) Ozone quenching properties of isoprene and its antioxidant role in leaves. Plant Physiology 126, 9931000.
  • Lu Z.M., Chen J.W., Percy R.G. & Zeiger E. (1997) Photosynthetic rate, stomatal conductance and leaf area in two cotton species (Gossypium barbadense and Gossypium hirsutum) and their relation with heat resistance and yield. Australian Journal of Plant Physiology 24, 693700.
  • Lunde C., Jensen P.E., Haldrup A., Knoetzel J. & Scheller H.V. (2000) The PSI-H subunit of photosystem I is essential for state transitions in plant photosynthesis. Nature 408, 613615.
  • Makino A., Miyake C. & Yokota A. (2002) Physiological functions of the water-water cycle (Mehler Reaction) and the cyclic electron flow around PSI in rice leaves. Plant and Cell Physiology 43, 10171026.
  • Murakami Y., Tsuyama M., Kobayashi Y., Kodama H. & Iba K. (2000) Trienoic fatty acids and plant tolerance of high temperature. Science 287, 476479.
  • Pastenes C. & Horton P. (1996) Effect of high temperature on photosynthesis in beans.2. CO2 assimilation and metabolite contents. Plant Physiology 112, 12531260.
  • Peñuelas J., Llusià J., Asensio D. & Munné-Bosch S. (2005) Linking isoprene with plant thermotolerance, antioxidants, and monoterpene emissions. Plant, Cell and Environment 28, 0000.
  • Perchorowicz J.T., Raynes D.A. & Jensen R.G. (1981) Light limitation of photosynthesis and activation of ribulose bisphosphate carboxylase in wheat seedlings. Proceedings of the National Academy of Sciences of the USA 78, 29852989.
  • Pursiheimo S., Martinsuo P., Rintamaki E. & Aro E.M. (2003) Photosystem II protein phosphorylation follows four distinctly different regulatory patterns induced by environmental cues. Plant, Cell and Environment 26, 19952003.
  • Radin J.W., Lu Z., Percy R.G. & Zeiger E. (1994) Genetic variability for stomatal conductance in Pima cotton and its relation to improvements of heat adaptation. Proceedings of the National Academy of Sciences of the USA 91, 72177221.
  • Rokka A., Aro E.M., Herrmann R.G., Andersson B. & Vener A.V. (2000) Dephosphorylation of photosystem II reaction center proteins in plant photosynthetic membranes as an immediate response to abrupt elevation of temperature. Plant Physiology 123, 15251535.
  • Salvucci M.E., Osteryoung K.W., Crafts-Brandner S.J. & Vierling E. (2001) Exceptional sensitivity of rubisco activase to thermal denaturation in vitro and in vivo. Plant Physiology 127, 10531064.
  • Santarius K.A. (1975) Sites of heat sensitivity in chloroplasts and differential inactivation of cyclic and noncyclic photophosphorylation by heating. Journal of Thermal Biology 1, 101107.
  • Santarius K.A. & Müller M. (1979) Investigations on heat resistance of spinach leaves. Planta 146, 529538.
  • Schrader S.M., Wise R.R., Wacholtz W.F., Ort D.R. & Sharkey T.D. (2004) Thylakoid membrane responses to moderately high leaf temperature in Pima cotton. Plant, Cell and Environment 27, 725735.
  • Schreiber U., Heimann S. & Klughammer C. (1998) Two distinct states of the thylakoid bf complex. FEBS Letters 426, 126130.
  • Schuster W.S. & Monson R.K. (1990) An examination of the advantages of C3-C4 intermediate photosynthesis in warm environments. Plant, Cell and Environment 13, 903912.
  • Sharkey T.D. (1985) O2-insensitive photosynthesis in C3 plants. Its occurrence and a possible explanation. Plant Physiology 78, 7175.
  • Sharkey T.D. (1989) Evaluating the role of rubisco regulation in C3 photosynthesis. Philosophical Transactions of the Royal Society of London B 323, 435448.
  • Sharkey T.D. & Singsaas E.L. (1995) Why plants emit isoprene. Nature 374, 769.
  • Sharkey T.D., Badger M.R., Von Caemmerer S. & Andrews T.J. (2001a) Increased heat sensitivity of photosynthesis in tobacco plants with reduced Rubisco activase. Photosynthesis Research 67, 147156.
  • Sharkey T.D., Chen X.Y. & Yeh S. (2001b) Isoprene increases thermotolerance of fosmidomycin-fed leaves. Plant Physiology 125, 20012006.
  • Singsaas E.L., Laporte M.M., Shi J.-Z., Monson R.K., Bowling D.R., Johnson K., Lerdau M., Jasentuliyana A. & Sharkey T.D. (1999) Leaf temperature fluctuation affects isoprene emission from red oak (Quercus rubra) leaves. Tree Physiology 19, 917924.
  • Singsaas E.L. & Sharkey T.D. (1998) The regulation of isoprene emission responses to rapid leaf temperature fluctuations. Plant, Cell and Environment 21, 11811188.
  • Snyders S. & Kohorn B.D. (1999) TAKs, thylakoid membrane protein kinases associated with energy transduction. Journal of Biological Chemistry 274, 91379140.
  • Snyders S. & Kohorn B.D. (2001) Disruption of thylakoid-associated kinase 1 leads to alteration of light harvesting in Arabidopsis. Journal of Biological Chemistry 276, 32169.
  • Terzaghi W.B., Fork D.C., Berry J.A. & Field C.B. (1989) Low and high temperature limits to PSII. A survey using trans-parinaric acid, delayed light emission, and Fo chlorophyll fluorescence. Plant Physiology 91, 14941500.
  • Thomas P.G., Dominy P.J., Vigh L., Mansourian A.R., Quinn P.J. & Williams W.P. (1986) Increased thermal stability of pigment-protein complexes of pea thylakoids following catalytic hydrogenation of membrane lipids. Biochimica et Biophysica Acta 849, 131140.
  • Thompson L.K., Blaylock R., Sturtevant J.M. & Brudvig G.W. (1989) Molecular basis of the heat denaturation of photosystem II. Biochemistry 28, 66866695.
  • Velikova V. & Loreto F. (2005) On the relationship between isoprene emission and thermotolerance in Phragmites australis leaves exposed to high temperatures and during the recovery from heat stress. Plant, Cell and Environment 28, 0000.
  • Vener A.V., Harms A., Sussman M.R. & Vierstra R.D. (2001) Mass spectrometric resolution of reversible protein phosphorylation in photosynthetic membranes of Arabidopsis thaliana. Journal of Biology Chemistry 276, 69596966.
  • Vener A.V., Rokka A., Fulgosi H., Andersson B. & Herrmann R.G. (1999) A cyclophilin-regulated PP2A-like protein phosphatase in thylakoid membranes of plant chloroplasts. Biochemistry 38, 1495514965.
  • Vener A.V., Van Kan P.J.M., Gal A., Andersson B. & Ohad I. (1995) Activation/deactivation cycle of redox-controlled thylakoid protein phosphorylation. Journal of Biological Chemistry 270, 2522525232.
  • Vijayan P. & Browse J. (2002) Photoinhibition in mutants of Arabidopsis deficient in thylakoid unsaturation. Plant Physiology 129, 876885.
  • Weis E. (1980) Reversible heat-inactivation of the Calvin cycle: a possible mechanism of the temperature regulation of photosynthesis. Planta 151, 3339.
  • Wise R.R., Olson A.J., Schrader S.M. & Sharkey T.D. (2004) Electron transport is the functional limitation of photosynthesis in field-grown Pima cotton plants at high temperature. Plant, Cell and Environment 27, 717724.
  • Wolfertz M., Sharkey T.D., Boland W. & Kühnemann F. (2004) Rapid regulation of the methylerythritol 4-phosphate pathway during isoprene synthesis. Plant Physiology 135, 19391945.
  • Yamane Y., Kashino Y., Koike H. & Satoh K. (1998) Effects of high temperatures on the photosynthetic systems in spinach: Oxygen-evolving activities, fluorescence characteristics and the denaturation process. Photosynthesis Research 57, 5159.
  • Yamane Y., Shikanai T., Kashino Y., Koike H. & Satoh K. (2000) Reduction of QA in the dark: Another cause of fluorescence Fo increases by high temperatures in higher plants. Photosynthesis Research 63, 2334.
  • Zeidler J., Schwender J., Müller C., Wiesner J., Weidemeyer C., Beck E., Jomaa H. & Lichtenthaler H.K. (1998) Inhibition of the non-mevalonate 1-deoxy-d-xylulose-5-phosphate pathway of plant isoprenoid biosynthesis by fosmidomycin. Zeitschrift Fur Naturforschung C 53, 980986.