The thermal stability of photosystem II was examined under different (light and water deficit) treatments in the moss H. lutescens. The decrease in water content under continuous light effected a heat-tolerance increase, further intensified by increasing excitation energy levels. The breakpoints (T, T, F1/2) of the F, vs. T curves significantly shifted towards higher temperatures even under a 30-minute moderate (-1.3 MPa) osmotic treatment, and this was partially inhibited by DTT. Both moderate and higher water deficit (-2.5 MPa) resulted in an increase in thermal stability, independent of the excitation energy level. This effect of water deficit remained observable over a fairly long period. Since in a dark-adapted state the critical values of the F0 vs. T curves did not shift towards significantly higher temperatures with an increase in water deficit, it seems likely that rapid thermal stability increase of PS II induced by water deficit occurs only in energized photosynthetic membranes

Dulai, Sándor

English

Dulai Sándor

University of Szeged

Volume 46(3-4):159-160, 2002Acta Biologica Szegediensishttp://www.sci.u-szeged.hu/ABS1,3Departments of Plant Physiology and of Botany, Eszterházy College, Eger, 2Department of Plant Physiology, University ofSzeged, 4Agricultural Research Institute of the Hungarian Academy of Sciences, Martonvásár, HungarySándor Dulai1*, Ferenc Horváth2, Sándor Orbán3, Éva Darkó4, Katalin Csizi1, István Molnár4ABSTRACT                         The thermal stability of photosystem II was examined under different (light and water deficit) treatments inthe moss H. lutescens. The decrease in water content under continuous light effected a heat-tolerance increase, furtherintensified by increasing excitation energy levels. The breakpoints (Tc, Tp, F1/2) of the Fs vs. T curves significantly shifted towardshigher temperatures even under a 30-minute moderate (-1.3 MPa) osmotic treatment, and this was partially inhibited byDTT. Both moderate and higher water deficit (-2.5 MPa) resulted in an increase in thermal stability, independent of theexcitation energy level. This effect of water deficit remained observable over a fairly long period. Since in a dark-adaptedstate the critical values of the F0 vs. T curves did not shift towards significantly higher temperatures with an increase in waterdeficit, it seems likely that rapid thermal stability increase of PS II induced by water deficit occurs only in energizedphotosynthetic membranes. Acta Biol Szeged 46(3-4):159-160 (2002)*Corresponding author. E-mail: ds@ektf.hu159Water deficit under continuous light enhances the thermal stabilityof photosystem II in Homalothecium lutescens mossProceedings of the 7th Hungarian Congress on Plant Physiology, 2002S4-P01The heat sensitivity of plants is closely connected to thethermal stability of PS II. It is more or less clear that the heatsensitivity of the photosynthetic apparatus, and the thermalstability of PS II, can change rapidly (within tens of minutes)as a result of heat pre-treatment (Havaux and Tardy 1996).However, it is still not widely recognized how these short-term responses to heat are influenced by other stress factorslike light and water deficit, or dessication. The study of theseproblems is further justified by the fact that under naturalconditions high light intensity, heat stress, and water deficitoccur in combination with each other. A good example of thisis that the presence or absence of light can significantlymodify the heat-induced damage: the photosynthetic ap-paratus is probably more stable in the light than in the dark(Molnár et al. 1998). In addition, the artificially generatedintrathylakoid pH gradient effects an accumulation of zea-xanthin and a parallel increase in the thermal stability ofthylakoid membranes (Havaux and Gruszecki 1993).There are observations to the effect that in higher plantsthe slow dehydration of removed leaves resulted in anincrease in the thermal stability, detected on the basis of thetemperature dependence of the initial level (F0) ofchlorophyll-α fluorescence (Havaux 1992). Since the watercontent rapidly decreases in poikilohydric plants parallel withthe increase of irradiation and leaf temperature, efficientphotosynthesis is necessary even under such unfavourableconditions to achieve adequate dry matter production andgrowth rate: the effect of the three stress factors needs to betolerated at the same time. This short study reports on theeffects of increasing light intensity and decreasing watercontent on the thermal stability of the photosynthetic ap-paratus in the dessication-tolerant moss H. lutescens.Materials and MethodsAll experiments were performed on green segments ofHomalothecium lutescens (Hedw.) Robins moss. Before themeasurements the samples were rehydrated and transferredto a growth chamber for two days (25 oC, 100 µmol m-2 s-1PFD, 100 % RH). Short-term (30 min) osmotic treatmentswere performed using polyethylene glycol solutions with –1.3 and -2.5 MPa osmotic potential. Longer-term treatmentswere carried out using exsicators at given air humidity for atleast 24 hours but for no more than 48.The responses of the in vivo chlorophyll-a fluorescenceto heat were measured in dark-adapted leaves with a pulseamplitude modulation fluorometer (PAM 101-103, Walz,Effeltrich, Germany). For the determination of the break-points (Tc, F1/2 and Tp) of the F0 vs. T or Fs vs. T curves theheat induction of fluorescence method was applied as des-cribed by Schreiber and Berry (1977).Results and DiscussionIn the original habitat of H. lutescens the temperature oftenrises to about 45-50 oC, which is always coupled with highirradiation. The data in Table 1 show that the heat toleranceof PSII determined on the basis of the F0 vs. T curves is notsufficient for the toleration of such high temperatures.However, since the temperature dependence of F0 is recordedin the dark, it is inadequate for determining the thermaltolerance of samples in a light-adapted state at a steady-statephotosynthesis level. Similarly to F0, the temperature depen-dence of Fs is also biphasic, and the breakpoints of the curve– according to recent results – provide a satisfactory illustra-tion of the thermal stability of samples with a steady-statephotosynthesis level (Molnár et al. 1998). In the energizedstate of the thylakoids an increase in the intensity of theactinic light caused Tc, and Tp values of the Fs vs.T curvesto shift towards significantly higher temperatures, comparedwith the same values of the F0 vs. T curves (recorded indarkness) indicating the higher thermal tolerance of PS II(Table 1). This shift was inhibited by DTT, and in theuntreated samples, parallel with the upwards shift of the160critical values, the effective quantum yield values alsoindicated decreased heat sensitivity (data not shown). In thisconnection, a close correlation was found in higher plantsbetween the activity of photoprotective mechanisms and thethermal stability of PSII (Molnár et al. 1998).The critical values of the F0 vs. T curves of untreatedleaves and of leaves which received a 30-min osmotictreatment at -1.3 or -2.5 MPa did not show a significantdifference. Similarly, in the dark-adapted state a longer waterdeficit did not effect an increase in thermal stability (Table1). As a result of short osmotic treatments, in samples withsteady-state photosynthesis at 100 µmol m-2 s-1 AL intensity,the critical values of the Fs vs. T curves shifted to signifi-cantly higher values, compared to the 100% relative watercontent control (Table 1), and, contrary to some resultsconcerning peas (Dulai et al. unpublished data), this increasein heat tolerance prevailed under longer water deficit. Thisenhanced thermal tolerance was further increased by theincrease in AL. In addition, at given AL intensity greaterwater deficit also shifted the Tc and Tp values of the Fs vs. Tcurves upwards, indicating the increased thermal stability ofPS II (Table 1). This is also manifested by the temperaturedependence of the effective quantum yield of PSII: with anincrease in the water deficit, the DF/Fm’ values starteddecreasing drastically at higher temperatures (data notshown). Previous studies also showed that the slow de-hydration of intact leaves caused an increase in the thermalstability of PS II, based on the F0 vs. T curves (in darkness),which was intensified by strong pre-illumination (Havaux1992). Contrary to these results, rapid osmotic treatment ora longer water deficit at growth light intensity did not causea significant change in the temperature dependence of theinitial fluorescence level of H. lutescens. The increase inthermal stability under water deficit was detected only undercontinuous light (at steady-state the photosynthesis level).The non-radiative dissipation of excess light depends onboth the intrathylakoid pH gradient and the activity of thexanthophyll cycle (Demming-Adams 1990). According tosome studies, at low lumen pH, besides its role in photo-protection, the activity of the xanthophyll cycle may have arole to play in the heat tolerance of PS II (Havaux and Tardy1996; Molnár et al. 1998), since zeaxanthin accumulation,besides intensifying high energy quenching, may increase thethermal stability of PS II through the rigidification of thethylakoids (Havaux and Gruszeczki 1993). The NPQ valuesof treated plants at higher temperatures significantly surpas-sed the control, with maxima close to Tc. DTT treatment,however, resulted in a decrease in the NPQ values, andinhibited the increase in thermal stability under both light andosmotic treatment (data not shown). On the other hand, in thedark-adapted state a water deficit did not result in an increasein the thermal stability of PS II (Table 1). All this makes itlikely that the thermal tolerance increase caused by waterdeficit only takes place in energized thylakoids. Therefore,it is possible that the protective processes (in the early stages)against the effects of excess light, high temperatures andwater deficit, at least in this cryptogamic species, sharecertain characteristics which may be related to the low lumenpH as well.ReferencesDemming-Adams B (1990) Carotenoids and photoprotection in plants: arole for xanthophyll zeaxanthin. Biocim Biophym Acta 1020:1-24.Havaux M (1992) Stress tolerance of photosystem II in vivo: antagonisticeffects of water, heat, and photoinhibition stresses. Plant Physiol100:424-432.Havaux M, Gruszecki WI (1993) Heat- and light-induced chlorophyll afluorescence changes in potato leaves containing high or low levels ofthe carotenoid zeaxanthin: indications of a regulatory effect ofzeaxanthin on thylakoid membrane fluidity. Photochem Photobiol58:607-614.Havaux M, Tardy F (1996) Temperature-dependent adjustment of thethermal stability of photosystem II in vivo: possible involvement ofxanthophyll-cycle pigments. Planta 198:324-333.Molnár I, Csízi K, Dulai S, Darkó É, Lehoczki E (1998) Light dependenceof thermostability of photosynthetic apparatus. In Garab G ed.,Photosynthesis: Mechanisms and Effects. Kluwer, Dordrecht, 2241-2244.Schreiber U, Berry J (1977) Heat-induced changes of chlorophyllfluorescence in intact leaves correlated with damage of the photo-synthetic apparatus. Planta 136:233-238.Table 1 Effects of different water contents (RWC) and of short osmotic treatments on the breakpoints (Tc, Tp, F1/2) of the F0 vs. T (indarkness) and of the Fs vs. T curves at different actinic light (AL) intensities in green segments of Homalothecium lutescens.Treatment TcTpF1/2Control (dark, F0-T, at 100 % RWC) 41.2±0.86 46.1±2.06 43.2±1.6100 µmol m-2 s-1 AL (at 100 % RWC) 45.2±0.52 52.1±1.47 47.3±0.87400 µmol m-2 s-1 AL (at 100 % RWC) 46.3±0.39 52.3±1.16 48.5±0.941000 µmol m-2 s-1 AL (at 100 % RWC) 46.6±0.49 54.3±0.94 49.3±1.07-1.3 MPa 30 min (dark, F0-T) 41.9±1.23 46.3±1.53 43.5±1.16-2.5 Mpa 30 min (dark, F0-T) 40.9±1.62 47.1±2.12 44.0±1.82~98 % RWC 24-48 hours (dark, F0-T) 41.5±1.63 46.9±1.87 43.6±1.82~90 % RWC 24-48 hours (dark, F0-T) 41.8±1.44 46.0±1.18 42.9±0.97-1.3 MPa 30 min (at 100 µmol m-2 s-1 AL) 46.9±1.23 52.9±1.18 49.0±0.72-2.5 MPa 30 min (at 100 µmol m-2 s-1 AL) 48.1±1.44 54.1±1.01 50.4±0.89~98 % RWC 24-48 h (at 100 µmol m-2 s-1 AL) 47.1±0.78 55.6±1.87 50.1±1.32~90 % RWC 24-48 h (at 100 µmol m-2 s-1 AL) 48.0±0.51 56.8±0.94 51.3±1.07

Water deficit under continuous light enhances the thermal stability of photosystem II in Homalothecium lutescens moss

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Water deficit under continuous light enhances the thermal stability of photosystem II in Homalothecium lutescens moss

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