Characterization of potato genotypes by chlorophyll fluorescence during plant aging in a Mediterranean environment

Field experiments were conducted in Sicily (south Italy) during two seasons to characterize by chlorophyll (Chl) fluorescence four genotypes (Spunta, Sieglinde, Daytona, and Ninfa) of potato (Solanum tuberosum L.) for off-season production during plant aging and to analyse the possible relation between Chl parameters and tuber yield. Chl fluorescence parameters [initial fluorescence (F₀), maximum fluorescence (F_m), F_v/F_m, time in which maximal fluorescence occurs (T_max)] gained from Kautsky kinetics and Chl content were measured weekly, from 5^th to 6^th leaf appearance to beginning of plant senescence in the first season and to full plant senescence in the second season. F₀ and F_v/F_m were the most reliable Chl fluorescence parameters for the definition of genotypic differences while Chl content and T_max were the most reliable Chl parameters to predict plant aging. Tuber yield was highly correlated with Chl content, T_max, F₀, and F_m.     

Leaf Gas Exchange and Chlorophyll Fluorescence Parameters in Phaseolus Vulgaris as Affected by Nitrogen and Phosphorus Deficiency

The effects of N and P deficiency, isolated or in combination, on leaf gas exchange and fast chlorophyll (Chl) fluorescence emission were studied in common bean cv. Negrito. 10-d-old plants grown in aerated nutrient solution were supplied with high N (HN, 7.5 mol m⁻³) or low N (LN, 0.5 mol m⁻³), and also with high P (HP, 0.5 mol m⁻³) or low P (LP, 0.005 mol m⁻³). Regardless of the external P supply, in LN plants the initial fluorescence (F₀) increased 12 % in parallel to a quenching of about 14 % in maximum fluorescence (F_m). As a consequence, the variable to maximum fluorescence ratio (F_v/F_m) decreased by about 7 %, and the variable to initial fluorescence ratio (F_v/F₀) was lowered by 25 % in relation to control plants. In LP plants, F_v/F_m remained unchanged whilst F_v/F₀ decreased slightly as a result of 5 % decline in F_m. Under N deficiency, the net photosynthetic rate (P _N) halved at 6 d after imposition of treatment and so remained afterwards. As compared to LN plants, P _N declined in LP plants latter and to a less extent. From 12 d of P deprivation onwards. P _N fell down progressively to display rates similar to those of LN plants only at the end of the experiment. The greater P _N in LP plants was not reflected in larger biomass accumulation in relation to LN beans. In general, P and N limitation affected photosynthesis parameters and growth without showing any synergistic or additive effect between deficiency of both nutrients. This revised version was published online in June 2006 with corrections to the Cover Date.     

Chlorophyll fluorescence and chlorophyll content in field-grown potato as affected by nitrogen supply, genotype, and plant age

Field experiments were conducted in Sicily (south Italy) to assess chlorophyll (Chl) fluorescence parameters in response of potato crop to nitrogen dose, to variation in genotype and in plant age, and to detect relationships between Chl content, fluorescence parameter F_v/F_m, and tuber yield. The experiment included five nitrogen doses (0, 10, 20, 30, and 40 g m⁻²) and four genotypes (Spunta, Sieglinde, Daytona, and Igea). Chl fluorescence parameters (initial fluorescence, F₀, maximum fluorescence, F_m, variable fluorescence, F_v, F_v/F_m, T_max (the time required to reach F_m), and Chl content were measured weekly between the appearance of the fifth and sixth leaves and the onset of plant senescence. A positive linear relationship was established between nitrogen supply and Chl content, F₀, and T_max. Nitrogen supply up to 10 g m⁻² also had a positive effect on F_m and F_v, but above this rate it reduced F_v/F_m. Spunta had the highest Chl content, F_m, F_v, and F_v/F_m, but the lowest F₀, whereas Sieglinde had the lowest Chl content, F_v, F_v/F_m, and T_max and the highest F₀. The cvs. Igea and Daytona exhibited intermediate Chl fluorescence parameters. Chl content and T_max decreased with increasing plant age, whereas F₀, F_m, and F_v increased until complete canopy development and thereafter declined until crop maturity. Tuber and plant dry matter yield were significantly correlated with Chl content, F₀, and T_max. Thus Chl fluorescence and content detect differences in the response of potato to N supply, can discriminate between genotypes, predict plant age, and yield performance under field conditions.     

Winter photoinhibition in needles of <Emphasis Type="Italic">Taxus baccata</Emphasis> seedlings acclimated to different light levels

Seasonal variability of maximum quantum yield of PSII photochemistry (F_v/F_m) was studied in needles of Taxus baccata seedlings acclimated to full light (HL, 100% solar irradiance), medium light (ML, 18% irradiance) or low light (LL, 5% irradiance). In HL plants, F_v/F_m was below 0.8 (i.e. state of photoinhibition) throughout the whole experimental period from November to May, with the greatest decline in January and February (when F_v/F_m value reached 0.37). In ML seedlings, significant declines of F_v/F_m occurred in January (with the lowest level at 0.666), whereas the decline in LL seedlings (down to 0.750) was not significant. Full recovery of F_v/F_m in HL seedlings was delayed until the end of May, in contrast to ML and LL seedlings. F_v/F_m was significantly correlated with daily mean (T _mean), maximal (T _max) and minimal (T _min) temperature and T _min was consistently the best predictor of F_v/F_m in HL and ML needles. Temperature averages obtained over 3 or 5 days prior to measurement were better predictors of F_v/F_m than 1- or 30-day averages. Thus our results indicate a strong light-dependent seasonal photoinhibition in needles of T. baccata as well as suggest a coupling of F_v/F_m to cumulative temperature from several preceding days. The dependence of sustained winter photoinhibition on light level to which the plants are acclimated was further demonstrated when plants from the three light environments were exposed to full daylight over single days in December, February and April and F_v/F_m was followed throughout the day to determine residual sensitivity of electron transport to ambient irradiance. In February, the treatment revealed a considerable midday increase in photoinhibition in ML plants, much less in HL (already downregulated) and none in LL plants. This suggested a greater capacity for photosynthetic utilization of electrons in LL plants and a readiness for rapid induction of photoinhibition in ML plants. Further differences between plants acclimated to contrasting light regimes were revealed during springtime de-acclimation, when short term regeneration dynamics of F_v/F_m and the relaxation of nonphotochemical quenching (NPQ) indicated a stronger persistent thermal mechanism for energy dissipation in HL plants. The ability of Taxus baccata to sustain winter photoinhibition from autumn until late spring can be beneficial for protection against an excessive light occurring together with frosts but may also restrict photosynthetic carbon gain by this shade-tolerant species when growing in well illuminated sites.     

Photoinhibition of photosynthesis in Minquartia guianensis and Swietenia macrophylla inferred by monitoring the initial fluorescence

We assessed the effect of the exposure to full sunlight (5, 35, and 120 min, i.e. T₅, T₃₅, and T₁₂₀) on fluorescence parameters of two young tropical trees, Swietenia macrophylla, a gap-demanding species, and Minquartia guianensis, a shade tolerant species. Fluorescence parameters (F₀, F_m, F_v/F_m) were recorded before treatments and after the transition to low irradiance (LI). Recovery from photoinhibition (measured as F_v/F_m) was monitored for 24 h at LI. In Swietenia, an almost complete restoration of the F_v/F_m values occurred in T₅ and T₃₅ plants, when a rise in F₀ was observed after the transition to LI. This was inferred as indicative of dynamic photoinhibition. T₁₂₀ led to a decline in F₀ in Minquartia, but not in Swietenia. The plants of both species were unable to recovery from photoinhibition after 24 h at LI, when F₀ declined or remained unchanged. This was interpreted as indicative of chronic photoinhibition. Compared with Swietenia, Minquartia was more susceptible to photoinhibition, as indicated by lower F_v/F_m values.     

Influence of phytohormones on morphology and chlorophyll a fluorescence parameters in embryos of Fucus vesiculosus L. (Phaeophyceae)

While a variety of plant hormones from brown algae were described, there were few studies that examined the combined effects of these hormones on morphogenesis and photosynthetic physiology in developing fucoid embryos. We evaluated the effects of phytohormones to determine the extent, to which responses were similar to those of terrestrial plants. Kinetin, IAA, ABA, GA₃, and kinetin + IAA were added to seawater at a physiological concentration (1 mg/L), and embryos of Fucus vesiculosus L. were grown for 10 days. Photosynthetic activity of single embryos or embryo cells were characterized using the following fluorescence parameters: minimum fluorescence yield (F ₀), maximum quantum yield (F _v/F _m), relative maximum rate of electron transfer to photosystem II under saturation irradiances (rETR_max), photosynthetic efficiency under non-saturating irradiances (αETR) and saturation irradiance (E _k). In addition, embryo length and diameter and apical hair length and number were determined. Morphological changes associated with hormone treatments included an increase in the embryo length in the presence of IAA, an increase in the embryo diameter in the presence of IAA, kinetin, and kinetin + IAA, an increase in the maximum hair length and number in the presence of kinetin + IAA, and a decrease in the hair length and number in the presence of ABA. With respect to fluorescence parameters, significant effects of phytohormones included an increase in the F ₀ and F _v/F _m at kinetin treatment, a synergistic effect of kinetin + IAA on F _v/F _m, rETR_max, and αETR, a promotion of F _v/F _m by GA, and a decrease of the parameters by ABA. These results are consistent with the data on responses of land plants to the same hormones and suggest that brown algae have evolved regulatory mechanisms for morphogenesis and photosynthetic regulation similar to plants.     

Gibberellin A3 stimulates adventitious rooting of cuttings from cherry (Prunus avium)

We have examined the effect of exogenous gibberellin A₃(GA₃) on adventitious rooting of Prunus avium(cherry) cultivars Stella, F12/1 and Charger. We show that GA₃pre-treatment of P. avium stock plants causes an increasein shoot growth rate and also improves the rooting of cuttings subsequentlytaken from the treated plants. Approximately 37% of cuttings from controlshootsrooted, whereas the percentage rooting could be increased to 80% or more withGA₃ pre-treatment. The number of roots per rooted cutting was alsoincreased by GA₃ pre-treatment. The stimulation of adventitiousrooting could be partially explained by the increase in shoot growth rate.Cultivar Charger responded better than the other cultivars to the lowest levelof GA₃ treatment. In vitro cultures of cultivarCharger were also treated with GA₃. However, the stimulation ofadventitious rooting was less marked than in the GA₃-treated stockplants: percentage rooting increased from 70% to 85%. The results are discussedin the context of 'rejuvenation' effects of GA₃.     

Mineral nutrient deficiency increases the sensitivity of photosynthesis to sulphur dioxide in needles of a coniferous tree,Abies nordmanniana

Summary Abies nordmanniana (Stev.) Spach was cultivated in rooting media either rich in nutrients (control) or low in magnesium (low Mg) or low in magnesium and nitrogen (low Mg-N), respectively. Intact, attached needles were exposed, in the light (460 mol photons m^-2 s^-1), to an atmosphere containing 1 ppm SO₂ for 5 h. Measurements of light- and CO₂-saturated rates of photosynthetic O₂ evolution, A _max, were performed before and after SO₂ treatments. In needles from well fertilized plants, A _max was high (about 50 mol m^-2 s^-1) and was not affected by SO₂. Needles from low-Mg and low-Mg-N plants had lower photosynthetic rates and showed a marked decline in A _max in response to the SO₂ treatment. Stomatal conductance was similar in the three groups of plants during SO₂ treatments.Abbreviations A _max photosynthetic capacity (CO₂- and light-saturated rate of O₂ evolution) - DW dry weight - F_o yield of dark level fluorescence - F_M maximum yield of fluorescence, induced in a pulse of saturating light - F_v yield of variable fluorescence (= F_M–F_O) - FW fresh weight; g, conductance to water vapor transfer     

Stomatal and chlorophyll fluorescence characteristics in European beech cultivars during leaf development

Changes in stomatal and chlorophyll fluorescence characteristics were analyzed in the course of leaf expansion in European beech (Fagus sylvatica L.) cultivars Aurea Pendula, Cristata, Rohanii, Rotundifolia and Viridivariegata. Stomatal length increased gradually from the second to the fifth phenological stage. Rotundifolia reached the highest mean stomatal length whereas Aurea Pendula and Cristata had the lowest values. Stomatal density for all cultivars decreased from the second to the fifth stage. Aurea Pendula reached the highest stomatal density in all phenological stages. The highest values of variable to maximum fluorescence ratio (F_v/F_m) were recorded in Rotundifolia, Rohanii, and the wild type, whereas Viridivariegata showed the lowest F_v/_Fm. Similar trend was found in maximum to initial fluorescence ratio (F_m/F₀), but extremely low F_m/F₀ values were recorded in Viridivariegata in the last phenological stage. The highest potential electron capacity was found in Rohanii, Viridivariegata and the wild type and lowest in Cristata. This parameter increased in the course of early leaf development.     

Source-to-sink relationship between green leaves and green pseudobulbs of C<Subscript>3</Subscript> orchid in regulation of photosynthesis

In this paper, photosynthetic characteristics of green leaves (GL) and green pseudobulbs (GPSB) of C₃ orchid Oncidium Golden Wish were first studied. Light saturation for photosynthesis and maximum photosynthetic rates (P _max) were significantly higher in GL than in GPSB. The results of the optimal PSII quantum yield (F_v/F_m ratio), electron transport rate (ETR), the effective photochemical quantum yield (ΔF/F_m′) and nonphotochemical quenching (NPQ) of Chl fluorescence revealed that GPSB had lower light utilization than that of GL. Significantly higher photosynthetic pigments were found in GL than in GPSB. Alteration of source/sink ratio had no impact on all photosynthetic parameters for both GL and GPSB after a short term of 3 days or even a long term of 2 weeks of treatments although there were significant decreases in GL carbohydrate concentration of GL-darkened plants by the end of the day. However, decreases of all photosynthetic parameters of GL were observed in GL-darkened plants after 4 weeks of treatment compared to those of fully illuminated (FI) and GPSB-darkened plants. These results indicate that the level of carbohydrates in GL plays an important role in regulating their photosynthesis. Due to their lower photosynthetic capacities, GPSB function mainly as sinks. Darkening GPSB up to 2 weeks did not affect their own P _max and the P _max of GL and thus, did not result in significant decreases of total carbohydrate concentration of GPSB. As GPSB store a large amount of carbohydrates, it could also act as a source when the level of carbohydrates decreased. Thus, GL could depend on GPSB carbohydrates to regulate their photosynthesis when their source capacity was removed. However, 4 weeks after treatments, photosynthetic capacities of GL were significantly lower in GL- and GPSB-darkened plants than in FI plants, which could be due to the lower total soluble and insoluble sugar concentrations of both GL and GPSB in these plants.     

Chlorophyll fluorescence as a tool for evaluation of viability in freeze-stressed grapevine buds

This study examined the utility of the ratio of variable fluorescence to maximum fluorescence (F_v/F_m) to detect freezing injury on buds of two Vitis vinifera cultivars: Pinot noir and Pinot gris. Freezing treatments on buds caused a decrease both in F_v/F_m and percentage of budburst, more severely on Pinot gris than Pinot noir, specifically at the lower temperature (−20°C). F_v/F_m ratio showed a close correlation with percentage of budburst, and a threshold of the lethal F_v/F_m was proposed as an indicator of bud mortality.     

Chlorophyll fluorescence and leaf chlorophyll content of bean leaves injured by spider mites (Acari: Tetranychidae)

The use of chlorophyll fluorescence as a method for detecting and monitoring plant stress arising from Tetranychus urticae (Koch) feeding injury was investigated. The effect of mite density (1–32 mites per 1.5 cm² of leaf) and the duration of the feeding period (1–5 days) on the chlorophyll fluorescence parameters of bean (Phaseolus vulgaris) leaves were examined. Changes in chlorophyll fluorescence parameters were dependent both on mite density and duration of feeding. Decreases in F _o, the initial fluorescence and F _m, the maximum fluorescence led to a decrease in the ratio of variable to maximum fluorescence, F _v/F _m. The decrease in F _v/F _m is typical of the response of many plants to a wide range of environmental stresses and indicates a reduced efficiency of photosystem II (PSII) photochemistry. T _1/2, which is proportional to the pool size of electron acceptors on the reducing side of PSII, was also reduced in response to mite-feeding injury. The leaf chlorophyll content decreased with increasing mite density and duration of feeding but did not appear to contribute to the decrease in F _v/F _m. Chlorophyll fluorescence is an effective method for detecting and monitoring stress in T. urticae-injured bean leaves.     

Induction of cyclic electron flow around photosystem 1 and state transition are correlated with salt tolerance in soybean

We investigated the role of cyclic electron flow around photosystem 1 (CEF1) and state transition (ST) in two soybean cultivars that differed in salt tolerance. The CEF1 and maximum photochemical efficiency (F_v/F_m) were determined under control and NaCl (50 mM) stress and the NaCl-induced light-harvesting complex 2 (LHC2) phosphorylation in vitro was analysed in light and dark. NaCl induced the increase of CEF1 more greatly in wild soybean Glycine cyrtoloba (cv. ACC547) than in cultivated soybean Glycine max (cv. Melrose). The F_v/F_m was reduced less in G. cyrtoloba than in G. max after 10-d NaCl stress. In G. cyrtoloba, the increase of CEF1 was associated with enhancement of LHC2 phosphorylation in thylakoid membrane under both dark and light. However, in G. max the NaCl treatment decreased the LHC2 phosphorylation. Treatment with photosynthetic electron flow inhibitors (DCMU, DBMIB) inhibited LHC2 phosphorylation more in G. max than in G. cyrtoloba. Thus the NaCl-induced up-regulation in CEF1 and ST might contribute to salt resistance of G. cyrtoloba.     

Electron Transport-Dependent Chlorophyll-a Fluorescence Quenching by O(2) in Various Algae and Higher Plants

A comparison of chlorophyll-a fluorescence in brown algae (Macrocystis integrifolia, Fucus vesiculosis), green algae (Scenedesmus obliquus, Ulva sp.) and higher plants (bean, corn) show differences in the relative fluorescence intensities and induction time courses which characterize each type of plant. These differences are not reflected in either the maximum fluorescence emission in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (F_max) or the nonvariable fluorescence (F_o). Constancy of F_o and F_max suggests functional similarities of photosystem II and associated antennae pigments in the various classes of plants. The time course differences are observed only in the absence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea and appear, therefore, to be electron transport dependent. During induction, the peak in fluorescence (F_p) is much lower in all of the algae studied than in the higher plants. Exogenous O₂ strongly quenches F_p in all plants studied and our data indicate that the low F_p in the algae can be partially accounted for by endogenous O₂ quenching.     

Leaf Photosynthesis of the Mangrove Avicennia Germinans as Affected by NaCl

In leaves of the mangrove species Avicennia germinans (L.) L. grown in salinities from 0 to 40 ‰, fluorescence, gas exchange, and δ¹³C analyses were done. Predawn values of F_v/F_m were about 0.75 in all the treatments suggesting that leaves did not suffer chronic photoinhibition. Conversely, midday F_v/F_m values decreased to about 0.55-0.60 which indicated strong down-regulation of photosynthesis in all treatments. Maximum photosynthetic rate (P _max) was 14.58 ± 0.22 μmol m^-2 s^-1 at 0 ‰ it decreased by 21 and 37 % in plants at salinities of 10 and 40 ‰, respectively. Stomatal conductance (g _s) was profoundly responsive in comparison to P _max which resulted in a high water use efficiency. This was further confirmed by δ¹³C values, which increased with salinity. From day 3, after salt was removed from the soil solution, P _max and g _s increased up to 13 and 30 %, respectively. However, the values were still considerably lower than those measured in plants grown without salt addition. This revised version was published online in June 2006 with corrections to the Cover Date.     

低温胁迫下丛枝菌根真菌对玉米光合特性的影响

利用盆栽试验,在15 ℃和5 ℃低温胁迫下研究了丛枝菌根（AM）真菌对玉米生长、叶绿素含量、叶绿素荧光和光合作用的影响.结果表明：低温胁迫抑制了AM真菌的侵染;接种AM真菌的玉米地上部和地下部干物质量、相对叶绿素含量高于不接种植株.与非菌根玉米相比,菌根玉米具有较高的最大荧光（F_m）、可变荧光（F_v）、最大光化学效率（F_v/F_m）和潜在光化学效率（F_v/F_o）及较低的初始荧光（F_o）,并且在5 ℃处理中差异显著.接种AM真菌使玉米叶片的净光合速率（P_n）和蒸腾速率（T_r）显著增强;低温胁迫下,菌根植株的气孔导度（G_s）显著高于非菌根植株;而胞间CO₂浓度（C_i）显著低于非菌根植株.表明AM真菌可通过提高叶绿素含量及改善叶片叶绿素荧光和光合作用来减轻低温胁迫对玉米植株造成的伤害,提高玉米耐受低温的能力,进而提高玉米的生物量,促进玉米生长.     

Wheat cultivars selected for high Fv/Fm under heat stress maintain high photosynthesis,total chlorophyll,stomatal conductance,transpiration and dry matter

The chlorophyll fluorescence parameter F_v/F_m reflects the maximum quantum efficiency of photosystem II (PSII) photochemistry and has been widely used for early stress detection in plants. Previously, we have used a three‐tiered approach of phenotyping by F_v/F_m to identify naturally existing genetic variation for tolerance to severe heat stress (3 days at 40°C in controlled conditions) in wheat (Triticum aestivum L.). Here we investigated the performance of the previously selected cultivars (high and low group based on F_v/F_m value) in terms of growth and photosynthetic traits under moderate heat stress (1 week at 36/30°C day/night temperature in greenhouse) closer to natural heat waves in North‐Western Europe. Dry matter accumulation after 7 days of heat stress was positively correlated to F_v/F_m. The high F_v/F_m group maintained significantly higher total chlorophyll and net photosynthetic rate (P_N) than the low group, accompanied by higher stomatal conductance (g_s), transpiration rate (E) and evaporative cooling of the leaf (ΔT). The difference in P_N between the groups was not caused by differences in PSII capacity or g_s as the variation in F_v/F_m and intracellular CO₂ (C_i) was non‐significant under the given heat stress. This study validated that our three‐tiered approach of phenotyping by F_v/F_m performed under increasing severity of heat was successful in identifying wheat cultivars differing in photosynthesis under moderate and agronomically more relevant heat stress. The identified cultivars may serve as a valuable resource for further studies to understand the physiological mechanisms underlying the genetic variability in heat sensitivity of photosynthesis.     

Short-term responses of photosynthetic membrane lipids and photochemical efficiency in plants of <Emphasis Type="Italic">Phaseolus vulgaris</Emphasis> and <Emphasis Type="Italic">Vigna unguiculata</Emphasis> submitted to high irradiance

Primary leaves of young plants of common bean (Phaseolus vulgaris cv. Carioca and Negro Huasteco) and cowpea (Vigna unguiculata Walp cv. Epace 10) were exposed to high irradiance (HI) of 2 000 μmol m⁻² s⁻¹ for 10, 20, and 30 min. The initial fluorescence (F₀) was nearly constant in response to HI in each genotype except for Carioca. A distinct reduction of maximum fluorescence (F_m) was clearly observed in stressed genotypes of beans after 20 min followed by a slight recovery for the longer stress times. In common bean, the maximum quantum yield (F_v/F_m) was reduced slowly from 10 to 30 min of HI. In cowpea, only a slight reduction of F_v/F_m was observed at 20 min followed by recovery to normal values at 30 min. HI resulted in changes in the photochemical (q_P) and non-photochemical (q_N) quenching in both species, but to a different extent. In cowpea plants, more efficiency in the use of the absorbed energy under photoinhibitory conditions was related to increase in q_P and decrease in q_N. In addition, lipid peroxidation changed significantly in common bean genotypes with an evident increase after 20 min of HI. Hence the photosynthetic apparatus of cowpea was more tolerant to HI than that of common bean and the integrity of cowpea cell membranes was apparently maintained under HI.     

Photosynthetic response of different pea cultivars to low and high temperature treatments

The thermo-sensitivity of two new pea (Pisum sativum L.) cultivars—Afila (mutant in the gene transforming leaves into mustaches) and Ranen (mutant for early ripening)—as compared to the control cultivar Pleven-4 to either low (4 °C, T₄) or high temperature (38 °C, T₃₈) was investigated by means of chlorophyll (Chl) fluorescence kinetics. The low temperature treatment decreased the photosynthetic activity, measured via a decline of the Chl fluorescence decrease ratios R_Fd690 and R_Fd735, and this was mainly due to a decline of the Chl fluorescence decrease parameter F_d and maximum Chl fluorescence F_m. In the new cv. Ranen the R_Fd ratios at first decreased and increased again after 24-h exposure to 4 °C, indicating its good acclimation ability to low temperature. The cold-induced changes in the photosynthetic performance of all cultivars were reversed after transferring plants back to 23 °C for 48 h. In the Chl and carotenoid (Car) contents no or little changes occurred during the T₄ treatment, except for a slight but clear increase of the ratio Chl a/b and a decrease in the ratio Chl/Car. In contrast to this, the T₃₈ treatment for 72 h decreased the R_Fd ratios more strongly than the T₄ exposure did. In fact, an irreversible injury of the photosynthetic apparatus was caused in the control pea cv. Pleven-4 by a 48-h T₃₈ exposure and for the new cv. Afila after a 72-h T₃₈ exposure. In contrast, the cv. Ranen was less and little sensitive to the T₃₈ exposure. In the heat-sensitive cvs. Pleven-4 and Afila, the decrease in R_Fd values at T₃₈ was associated with a strong decline of the Chl a+b and total Car contents. The Chl a+b decline could also be followed via an increase of the Chl fluorescence ratio F₆₉₀/F₇₃₅. Parallel to this, a strong decline of Chl a/b from ca. 3.0 (range 2.85–3.15) to ca. 1.9 (range 1.85–1.95) occurred indicating a preferential decline of the Chl a-pigment proteins but not of the Chl a/b-pigment protein LHC2. In the relatively heat-tolerant cv. Ranen, however, the ratio Chl a/b declined only partially. After the T₄ treatment the stress adaptation index Ap was higher in cv. Ranen than in controls and reached in heat-treated Ranen plants almost the starting value indicating a cold and heat stress hardening of the treated plants. The Chl fluorescence parameters and pigment contents were influenced by T₃₈ and T₄ treatments in various ways indicating that the mechanisms of low and high temperature injury of the photosynthetic apparatus are different. The new cv. Ranen exhibited a cross tolerance showing a fairly good acclimation ability to both T₄ and T₃₈, hence it is a very suitable plant for outdoor growth and for clarification of the acclimation mechanisms to unfavourable temperatures.