Changing Environmental Effects on Frost Hardiness of Scots Pine During Dehardening

The effects of raised temperature and extended photoperiod onthe dehardening of quiescent and winter-hardy Scots pine saplingswere examined in an open-top-chamber experiment. The saplingswere exposed during winter to natural, square-curve fluctuating(between 1 and 11 °C with a 14 d interval), and constant(6 °C) temperatures with a natural and an extended (17 h)photoperiod. Frost hardiness of needles was determined by controlledfreezing tests and visual damage scoring. The constant 6 °Ctemperature treatment caused a gradual dehardening of needleswhereas under fluctuating temperatures the level of frost hardinessfluctuated. Trees exposed to extended photoperiods were lesshardy than under natural photoperiods after the initiation ofshoot elongation, but before this there were no clear differencesin frost hardiness between different photoperiodic treatments.The results indicate that the frost hardening competence ofScots pine changes during quiescence. Climate change; frost hardiness; hardening competence; photoperiod; Pinus sylvestris, Scots pine; temperature     

The Influence of Photoperiod and Temperature on the Development of Frost Hardiness in Seedlings of Pinus silvestris and Picea abies

The influence of short days and low temperature on the development of frost hardiness in seedlings of Scots pine (Pinus silvestris L.) and Norway spruce [Picea abies (L.) Karst.], grown for 6 months in glasshouses and climate chambers, was investigated. The degree of hardiness was estimated by freezing the shoots of the seedlings to predetermined temperatures. After 8 weeks in a glasshouse the viability of the seedlings was determined by establishing bud flushing. The most effective climate for the development of frost hardiness was short days (SD) and low temperature (2°C); the next most effective was SD and room temperature (20°C). However, long days (LD) and low temperature also had a marked effect on the development of hardiness. A combination of 3 weeks’treatment with SD and 20°C, and 3 weeks with SD and 2°C gave the same results as 6 weeks with SD and 2°C. The results clearly demonstrate the importance of the photoperiod prior to low temperature for the development of frost hardiness. In conclusion both short days and low temperature induce frost hardiness development. Probably this occurs by initiation of different processes in the two cases. The degree of frost hardiness development appears to depend on the sum of these different processes and on the timing between them.     

The effects of long-term elevation of air temperature and CO on the frost hardiness of Scots pine

The frost hardiness of 20 to 25-year-old Scots pine (Pinus sylvestris L.) saplings was followed for 2 years in an experiment that attempted to simulate the predicted climatic conditions of the future, i.e. increased atmospheric CO₂ concentration and/or elevated air temperature. Frost hardiness was determined by an electrolyte leakage method and visual damage scoring on needles. Elevated temperatures caused needles to harden later and deharden earlier than the controls. In the first year, elevated CO₂ enhanced hardening at elevated temperatures, but this effect disappeared the next year. Dehardening was hastened by elevating CO₂ in both springs. The frost hardiness was high (相似文献   

The relation between growth cessation and frost hardening in Scots pines of different origins

The cessation of shoot elongation, diameter growth and needle elongation were compared with the initiation of frost hardening of the stems and needles in an 8-year-old provenance trial of Scots pine (Pinus sylvestris L.) established in central Finland. The saplings were of six different origins ranging from Estonia to northern Finland, forming a latitudinal gradient of ca. 10°N. The frost hardiness of the stems of current-year shoots was assessed by electrical impedance analysis and that of current-year needles by electrolyte leakage and visual scoring of damage. Artificial freezing tests were used in the assessments. The pattern of growth cessation (shoot and needle elongation, diameter growth) tended to follow the latitude of origin, i.e. growth ceased in the northernmost provenance first and in the southernmost one last. Both stems and needles of the northern provenances hardened earlier than the southern ones, but the differences in hardiness disappeared as hardening progressed. Growth cessation and initial hardening to -15°C were clearly correlated at the provenance level, indicating that growth must cease prior to hardening, and that earlier cessation of growth predicts earlier frost hardening of stems and needles. No differences in frost hardiness of stems were found at the provenance level at the end of the growing period in August. At that time, the frost hardiness of needles of the northernmost provenance was higher than that of other origins. Within the provenance, the stems were less hardy than the needles.     

Use of intraspecific variation in thermal responses for estimating an elevational cline in the timing of cold hardening in a sub‐boreal conifer

To avoid winter frost damage, evergreen coniferous species develop cold hardiness with suitable phenology for the local climate regime. Along the elevational gradient, a genetic cline in autumn phenology is often recognised among coniferous populations, but further quantification of evolutionary adaptation related to the local environment and its responsible signals generating the phenological variation are poorly understood. We evaluated the timing of cold hardening among populations of Abies sachalinensis, based on time series freezing tests using trees derived from four seed source populations × three planting sites. Furthermore, we constructed a model to estimate the development of hardening from field temperatures and the intraspecific variations occurring during this process. An elevational cline was detected such that high‐elevation populations developed cold hardiness earlier than low‐elevation populations, representing significant genetic control. Because development occurred earlier at high‐elevation planting sites, the genetic trend across elevation overlapped with the environmental trend. Based on the trade‐off between later hardening to lengthen the active growth period and earlier hardening to avoid frost damage, this genetic cline would be adaptive to the local climate. Our modelling approach estimated intraspecific variation in two model components: the threshold temperature, which was the criterion for determining whether the trees accumulated the thermal value, and the chilling requirement for trees to achieve adequate cold hardiness. A higher threshold temperature and a lower chilling requirement could be responsible for the earlier phenology of the high‐elevation population. These thermal responses may be one of the important factors driving the elevation‐dependent adaptation of A. sachalinensis.     

Are budburst dates,dormancy and cold acclimation in walnut trees (<Emphasis Type="Italic">Juglans regia</Emphasis> L.) under mainly genotypic or environmental control?

As observed for most stresses, tree frost resistance can be split into two main processes: avoidance and tolerance. Avoidance of freezing is achieved by introducing species only in the climatic context in which the probability of freezing events is very low for the sensitive stages of buds or stems; i.e., when good synchronism exists between the annual cycle and the critical climatic periods. Buds become able to grow only after chilling requirements have been satisfied (endodormancy released) during winter; they subsequently break after heat requirements have been completed (end of ecodormancy) in early spring. Actually, this period is often subject to more or less severe freezing events. Trees are also able to adjust their freezing tolerance by increasing their capacity of extracellular freezing and decreasing the possibility of intracellular freezing through the process of frost acclimation. Both freezing resistance processes (avoidance and tolerance) are environmentally driven (by photoperiod and temperature), but there are also genotypic effects among species or cultivars. Here, we evaluated the degree to which differences in dormancy release and frost acclimation were related to environmental and genetic influences by comparing trees growing in common garden conditions. This investigation was carried out for two winters in lowland and mountain locations on different walnut genotypes differing significantly for budburst dates. Chilling requirement for endodormancy release and heat requirement during ecodormancy were evaluated in all situations. In addition, frost acclimation was assessed by the electrolyte leakage method on stems from the same trees before leaf fall through budburst. No significant differences were observed in chilling requirements among genotypes. Moreover, frost acclimation dynamics were similar between genotypes or locations when expressed depending on chilling units accumulated since 15 September as a time basis instead of Julian day. The only exception was for maximal frost hardiness observed during winter with the timber-oriented being significantly more resistant than fruit-oriented genotypes. Heat requirement was significantly different among genotypes. Thus, growth was significantly faster in fruit-oriented than in wood-oriented genotypes. Furthermore, among wood-oriented genotypes, differences in growth rate were observed only at cold temperatures. Frost acclimation changes differed significantly between fruit- and wood- walnuts from January through budburst. In conclusion, from September through January, the acclimation dynamic was driven mainly by environmental factors whereas from January through budburst a significant genotype effect was identified in both frost tolerance and avoidance processes.     

A Second-order Dynamic Model for the Frost Hardiness of Trees

The development of frost hardiness in forest trees is describedby a dynamic model in which the input variables are the prevailingenvironmental conditions and the developmental stage of trees.The assumption of the model is that for each temperature andphotoperiod there is a discrete stationary level of frost hardiness,which is attained if these environmental factors remain constant.The dependence of the stationary level on temperature and photoperiodis assumed to be piece-wise linear and additive. The rate ofacclimation, i.e. frost hardening or dehardening, is describedas a second-order dynamic process with two time constants, thesecond of which changes depending on the stage of the annualdevelopment of the trees. The frost hardiness model was calibratedand tested using experimental data from Douglas fir [Pseudotsugamenziesii var. glauca (Beissn.) Franco] seedlings. The resultssuggest that the second-order model describes the changes infrost hardiness better than the first-order model with onlyone time constant.Copyright 1995, 1999 Academic Press Acclimation, developmental stage, Douglas fir, dynamic model, frost hardiness, photoperiod, Pseudotsuga menziesii, temperature     

Risk of spring frost to apple production under future climate scenarios: the role of phenological acclimation

In the context of global warming, the general trend towards earlier flowering dates of many temperate tree species is likely to result in an increased risk of damage from exposure to frost. To test this hypothesis, a phenological model of apple flowering was applied to a temperature series from two locations in an important area for apple production in Europe (Trentino, Italy). Two simulated 50-year climatic projections (A2 and B2 of the Intergovernmental Panel on Climate Change - Special Report on Emission Scenarios) from the HadCM3 general circulation model were statistically downscaled to the two sites. Hourly temperature records over a 40-year period were used as the reference for past climate. In the phenological model, the heat requirement (degree hours) for flowering was parameterized using two approaches; static (constant over time) and dynamic (climate dependent). Parameterisation took into account the trees’ adaptation to changing temperatures based on either past instrumental records or the downscaled outputs from the climatic simulations. Flowering dates for the past 40 years and simulated flowering dates for the next 50 years were used in the model. A significant trend towards earlier flowering was clearly detected in the past. This negative trend was also apparent in the simulated data. However, the significance was less apparent when the “dynamic” setting for the degree hours requirement was used in the model. The number of frost episodes and flowering dates, on an annual basis, were graphed to assess the risk of spring frost. Risk analysis confirmed a lower risk of exposure to frost at present than in the past, and probably either constant or a slightly lower risk in future, especially given that physiological processes are expected to acclimate to higher temperatures. An erratum to this article can be found at     

Connections between climatic variables and the growth and needle dynamics of Scots pine (Pinus sylvestris L.) in Estonia and Lapland

The relationships between climatic variables and Scots pine (Pinus sylvestris L.) growth and needle dynamics were studied in three stands in Estonia and in four stands located near the northern timberline in Lapland. The trees sampled in Estonia had low correlations with the analysed climatic variables (air temperature, precipitation and indices of atmospheric circulation). Moreover, the weak cross-correlation of the time-series of the Estonian sample trees indicated that Scots pine is affected mainly by local factors in that region. In Lapland, however, height increment and needle production correlated strongly among trees within a stand (mean r=0.45 and 0.46, respectively) and between stands (r=0.32 and 0.37). Radial increment also showed a high inter-correlation among the trees within a stand in Lapland (r=0.45). Both height increment and needle production were strongly influenced by the temperature regime of the previous summer in Lapland (mean r=0.64 and 0.64, respectively). Radial increment was correlated with the mean July temperature of the current year (mean r=0.29). The correlations between the indices of atmospheric circulation and tree attributes were weak, while the strongest correlation was between the Ponta Delgada NAO index (PD–NAO) and height increment and needle production in Lapland. Height increment, needle production and radial increment have increased since the 1990s in the trees growing in Lapland. This may indicate a positive effect of climate warming on tree growth in Lapland. In Estonia, where climatic conditions do not limit tree growth, the climate warming seems not to directly influence the growth and needle dynamics of Scots pine.     

Effects of climatic warming on cold hardiness of some northern woody plants assessed from simulation experiments

Effects of climatic warming on cold hardiness were investigated for some northern woody plants. In the first experiment, seedlings of Norway spruce ( Picea abies [L.] Karst.), Scots pine ( Pinus sylvestris L.) and lodgepole pine ( Pinus contorta Dougl. var. latifolia Engelm.) were exposed to naturally fluctuating temperatures averaging −6°C (ambient) and 0°C (elevated) for 16 weeks in midwinter before they were thawed and re-saturated with water. In lodgepole pine, needle sugar concentrations had decreased by 15%, and the temperature needed to induce 10% injury to needles in terms of electrolyte leakage had increased by 6°C following treatment to elevated as compared with control temperatures. In contrast, Norway spruce and Scots pine showed no effects. The lack of an effect for Scots pine was ascribed to seedlings containing unusually large energy reserves that buffered respiratory expenditure of sugars. A strong, linear relationship between levels of cold hardiness, assessed by the electrolyte leakage method, and sugars was found when combining data from this and previous, similar experiments. In the second experiment, the evergreen dwarf shrub Empetrum hermaphroditum Hagerup was analysed for leaf cold hardiness, using the electrolyte leakage method, and sugar concentrations in late spring and late autumn during the third year of a warming experiment in a subarctic dwarf shrub community. The objective was to test the hypothesis that warming in the growing season alters hardening/dehardening cycles by increasing soil nitrogen mineralization and plant growth. Data found, however, suggested that cold hardening/dehardening cycles were unaffected by warming.     

Inheritance of autumn frost hardiness in Pinus sylvestris L. seedlings

Summary Inheritance of frost hardiness was analysed making use of a 12×12 incomplete factorial mating design. Owing to space limitations only 59 families could be tested in four experiments. To link the four experiments, some families were common to two or more experiments. The seedlings were grown in climate chambers under conditions inducing autumn hardening. The plants were exposed to a freezing temperature of –10 °C for three hours at night lengths of 11–13 h. A statistical model was developed for analyses of variance of our data. The genetic variation and the variation due to the cultivation regimes during autumn hardening were of the same magnitude. The additive effects were the most important ones for induction of frost damage. No interaction following long-distance crossing was noted. Mixed model equations were used for ranking of the parents. The results obtained support a polygenic inheritance of frost hardiness. The large within-population variation offers good opportunities for hardiness breeding.     

Application of Impedance Spectroscopy for Selecting Frost Hardy Varieties of English Ryegrass

The frost hardening and frost damage of 12 varieties of Englishryegrass (Lolium perenne) was studied by electrical impedancespectroscopy. For the measurement of the impedance spectrum(80 Hz to 1 MHz) a 10 mm length sample was cut from the stemabove the growing point, but the growing point was included.The impedance spectra were analysed by an asymmetric distributedcircuit model. The impedance spectra were measured at two phasesof hardiness and after freezing, i.e. (a) before hardening,(b) after hardening in controlled conditions, and (c) aftercontrolled frost exposure at -16 °C after hardening. Twomodel parameters, i.e. intra- and extracellular resistance,increased with hardening. The intracellular resistance and theskewness factor before hardening, and the ratio between thosetwo parameters before and after hardening, strongly correlatedwith hardening of different varieties of English ryegrass. Theextracellular resistance and the relaxation time decreased asa result of the frost exposure at -16 °C. Cold acclimation; electrical impedance; English ryegrass; frost hardiness; impedance spectroscopy; Lolium perenne     

Does ice crystal formation in buds explain growth disturbances in boron-deficient Norway spruce?

Loss of apical dominance in boron-deficient trees has been suggested to be due to frost damage of terminal buds and leaders. Excessive nitrogen (N) supply can exacerbate boron (B) deficiency by the dilution-effect. N may also have direct effects on winter hardiness. We studied frost hardening of buds of Norway spruce (Picea abies L. Karst.) in healthy-looking trees and in trees with growth disturbances. The effect of B and N on frost hardiness was studied in a factorial fertilisation experiment during cold acclimation. Frost hardiness was determined by differential temperature analysis (DTA) and scoring of visual damage. In a DTA profile of apical buds with a piece of stem, low-temperature exotherm (LTE) predicted bud injury, while two of the observed high-temperature exotherms and two of the observed intermediate-temperature exotherms were non injurious. Appearance of LTE followed changes in air temperature. The risk of frost damage was not affected by fertilisation treatments or previously observed growth disturbances. However, when the bud structure was deformed by severe B deficiency, the supercooling ability disappeared. Such buds are probably killed by freezing in nature and therefore, frost damage may play a secondary role in the development of growth disturbances.     

Effects of freeze-thaw injury on parameters of distributed electrical circuits of stems and needles of Scots pine seedlings at different stages of acclimation

Frost hardening and frost injury of both the stems and needlesof Scots pine (Pinus sylvestris L.) seedlings were studied byelectrical impedance analysis. This impedance analysis was basedon equivalent circuits with distributed circuit elements (DCE).A double-DCE model for stems and two single-DCE models for needlesprovided excellent fit to the experimental impedance data. However,the single-DCE model for needles which takes into account theasymmetry of the impedance arc proved more appropriate thanthe symmetric model. Several parameters changed in proportionto frost injury. In stems, extracellular resistance and onerelaxation time decreased with increasing damage, whereas intracellularresistance remained relatively unchanged. In needles, the overallpattern for extracellular resistance and relaxation time wassimilar to that in the stem. Intracellular resistance remainedapproximately constant in the case of the symmetric DCE model.During frost hardening, both extracellular and intracellularresistance increased in stems. In needles, extracellular resistanceincreased but relaxation time decreased with hardening. Theskewness of the impedance spectra in the Cole-Cole plot forneedles increased with hardening. The coefficient for distributionof the relaxation time(s) did not change in either stems orneedles with frost hardening but some changes were found withfrost damage. Key words: Pinus sylvestris L, electrical impedance, equivalent circuit, frost hardening, frost injury     

Seasonal Variation of Western White Pine (Pinus monticola D. Don) Foliage Proteins

Recently, a western white pine protein, Pin m III, was shownto be associated with overwintering and frost hardiness of westernwhite pine foliage. To examine whether Pin m III is directlyinvolved in frost hardiness by functioning as an antifreezeprotein, work is underway to clone the gene encoding this proteinand to assess the function of this gene in freezing toleranceby incorporating the gene in a test plant, such as tobacco.Here, we examined in more detail, by SDS-PAGE and also by twodimensional gel electrophoresis, the seasonal variation of additionalproteins in western pine foliage. SDS-PAGE analysis of threeseedlots showed that different proteins reached a maximum levelin different months, although most proteins (5 to 11) reacheda maximum level in winter months (December, January and February).The 2-D gel analysis of foliage sampled on three harvest dates(October, January and April) of one seedlot revealed a seasonalvariation of a large number proteins (76 to 184). Of the seasonallyvaried proteins, the amino terminal sequence of several proteinsincluding Pin m III was determined. One of the sequences wasidentified by homology to that of the small subunit of ribulosebiphosphate carboxylase, whose level increased substantiallyfrom fall to spring. The amino terminal sequence of Pin m IIIhad 89% homology to a sugar pine protein, Pin 11. The anti-photosystemII antibody was used to monitor the annual variation of theextrinsic 23-kDa photosystem II protein. The level of the extrinsic23-kDa photosystem II protein decreased slowly as fall progressedand reached its lowest level in December and then increasedin early spring indicating that this variation is due to photosyntheticactivity of the foliage during the season. (Received July 29, 1995; Accepted March 5, 1996)     

Lack of regeneration and climatic vulnerability to fire of Scots pine may induce vegetation shifts at the southern edge of its distribution

Aim Forest ecosystems dominated by fire‐sensitive species could suffer shifts in composition under altered crown fire regimes mediated by climate change. The aims of this study were to: (1) study the spatio‐temporal patterns and the climatic distribution of fires in Scots pine (Pinus sylvestris) forests during the last 31 years in north‐eastern Spain, (2) evaluate the climatic vulnerability to fire of these forests in Spain, (3) analyse the regeneration of Scots pine after fire, and (4) predict the mid‐term maintenance or replacement of Scots pine in burned areas. Location Catalonia (north‐eastern Spain): the southern distribution limit of Scots pine. Methods We characterized the spatio‐temporal and the climatic distribution of fires that occurred in Catalonia between 1979 and 2009. We used a generalized linear model to characterize the climatic vulnerability to fire of Scots pine in the whole of Spain. We assessed the regeneration of the species after crown fires in nine burned areas in Catalonia. The resulting data were integrated into a stochastic matrix model to predict the mid‐term maintenance or replacement of Scots pine in burned areas. Results During the last three decades, Scots pine forests distributed in dry sites were most affected by fire. Our assessment of the vulnerability to fire of Scots pine forests in Spain as a whole, based on climatic and topographical variables, showed that 32% of these forests are vulnerable to fire, and that this proportion could increase to 66% under a conservative climate change scenario. Field data showed almost no regeneration of Scots pine after crown fires, and a limited capacity to recolonize from unburned edges, even in relatively old fires, with 90% of recruits located in the first 25 m from the edge. This process could be delayed by the elapsed time for new recruits to achieve reproductive maturity, which we estimated to be c. 15 years. Finally, our matrix model predicted the replacement of burned Scots pine forests by oak (Quercus sp.) forests, shrublands or mixed resprouter forests. Main conclusions Increased vulnerability to fire of Scots pine forests under future, warmer conditions may result in vegetation shifts at the southern edge of the distribution of the species.     

The electrical impedance spectroscopy of Scots pine (Pinus sylvestris L.) shoots in relation to cold acclimation

Electrical impedance spectroscopy (EIS) was applied to stems of Scots pine (Pinus sylvestris L.) in a provenance field trial during frost hardening to find an EIS parameter for assessing frost hardiness (FH) without a controlled freezing test. The FH of stems and needles assessed by controlled freezing tests was compared with the equivalent circuit EIS parameters of a distributed model of stems (not exposed to controlled freezing treatment) and with dry matter (DM) content of stems. Significant differences in the equivalent circuit parameters, FH and DM content were found between provenances. The relaxation time (tau(1)), describing the peak of the high frequency arc of the impedance spectrum, and the intracellular resistance (r(i)) of stems increased with increasing FH. According to the linear regression, the coefficient of determination (R(2)) between the FH of stems and needles with tau(1) of the stem was 0.87 and 0.89, and with r(i) of the stem 0.74 and 0.85, respectively. The relation between FH and tau(1) changed with the degree of hardiness. The highest coefficient of determination was 0.95 in September when the FH of needles, ranging from -10 degrees C to -25 degrees C, was predicted with an accuracy of +/-2.0 degrees C. The resistance parameter r(2), describing the width of the low frequency arc of the impedance spectrum, decreased prior to and during the initial hardening: significant differences were found between provenances. This indicates that r(2) was not related to frost hardening per se. It is concluded that it is possible to distinguish the hardening patterns of different provenances by tau(1) in the rapid phase of hardening without controlled freezing tests.     

Plant resistance to cold stress: Mechanisms and environmental signals triggering frost hardening and dehardening

This introductory overview shows that cold, in particular frost, stresses a plant in manifold ways and that the plant’s response, being injurious or adaptive, must be considered a syndrome rather than a single reaction. In the course of the year perennial plants of the temperate climate zones undergo frost hardening in autumn and dehardening in spring. Using Scots pine (Pinus sylvestris L.) as a model plant the environmental signals inducing frost hardening and dehardening, respectively, were investigated. Over 2 years the changes in frost resistance of Scots pine needles were recorded together with the annual courses of day-length and ambient temperature. Both act as environmental signals for frost hardening and dehardening. Climate chamber experiments showed that short day-length as a signal triggering frost hardening could be replaced by irradiation with far red light, while red light inhibited hardening. The involvement of phytochrome as a signal receptor could be corroborated by respective night-break experiments. More rapid frost hardening than by short day or far red treatment was achieved by applying a short period (6 h) of mild frost which did not exceed the plant’s cold resistance. Both types of signals were independently effective but the rates of frost hardening were not additive. The maximal rate of hardening was − 0.93°C per day and frost tolerance of < − 72°C was achieved. For dehardening, temperature was an even more effective signal than day-length.     

The uptake of and structural changes induced by trichloroacetic acid in the needles of Scots pine seedlings

The uptake of trichloroacetic acid (TCA) and structural changesinduced in the needles of Scots pine (Pinus sylvestris L.) seedlingswere studied. Two exposure set-ups, a root route and an atmosphericroute through the surfaces simulating the wet deposition offog, were used. Both set-ups included two dose levels and correspondingcontrol treatments. The temperature and the relative humidityin the climate chambers were adjusted to represent the conditionsof June–July in a subarctic area in central Finland. Theseedlings were exposed three times a week for two months. Theresults showed that the uptake of TCA in needles occurred bothvia roots and via needle surface. However, most of the TCA viathe atmospheric route was absorbed on the surface of the needles.The structural responses in pine needles depended partly onthe treatment method: TCA applied via the atmospheric routedisintegrated the structures of the epicuticular waxes and thatof the stomatal cells, which was not seen in the exposures viaroots. A common feature was the decrease in size of the chloroplastsin concert with the increasing TCA concentrations inside theneedles. Key words: Pinus sylvestris L., climate chamber, effects, microscopic structure, secondary pollutant, TCA, trichloroacetic acid     

Recovery of photosynthesis in winter-stressed Scots pine

Abstract. . Winter-induced inhibition of photosynthesis in Scots pine (Pinns sylvestris L.) is caused by the combined effects of light and freezing temperatures; light causes photoinhibition of photosystem II (Strand & Oquist, 1985b, Physiologic Plantarum, 65 , 117–123), whereas frost causes inhibition of enzymatic steps of photosynthesis (Strand & Öquist, 1988, Plant, Cell & Environment, 11 , 231–238). To reveal limiting steps during recovery from winter stress, the potential of photosynthesis to recover and the actual recovery outdoors during spring, were studied in Scots pine. Studies of light dependent O₂-evolution under saturating CO₂ and recordings of room temperature fluorescence induction kinetics were used. When branches of pine, in February and March, were brought into the laboratory and kept at 18°Cand 100μmol m^?2 s^?1, light saturated rates and apparent quantum yields of photo-synthetic O₂-evolution recovered fully within approximately 48h. The photochemical efficiency of photosystem II, as measured by Fv/Fm ratios, recovered fully within 24h after an initial lag-phase of 2-3 h. Under natural winter conditions, the Fv/Fm ratio decreased more in exposed than in shaded pine, whereas the efficiency of photosynthesis was similarly inhibited in exposed and shadedpine. However, when recovery from winter stress occurred during spring, the Fv/Fm ratios of both shaded and exposed pine recovered well before photosynthesis. It is concluded that the light-induced photoinhibition component of winter stress in photosynthesis of pine recovers well before the frost induced component(s) of winter stress. In this context, reversible photoinhibition of photosynthesis in evergreen conifers is considered as a dynamic down-regulation of photosystem II to prevent more severe photodynamic damage of the thylakoid membrane when photosynthesis is inhibited by frost.