Effects of climate warming and species richness on photochemistry of grasslands

In view of the projected climatic changes and the global decrease in plant species diversity, it is critical to understand the effects of elevated air temperature (T_air) and species richness (S) on physiological processes in plant communities. Therefore, an experiment of artificially assembled grassland ecosystems, with different S (one, three or nine species), growing in sunlit climate-controlled chambers at ambient T_air and ambient T_air + 3°C was established. We investigated whether grassland species would be more affected by midday high-temperature stress during summer in a warmer climate scenario. The effect of elevated T_air was expected to differ with S. This was tested in the second and third experimental years by means of chlorophyll a fluorescence. Because acclimation to elevated T_air would affect the plant's stress response, the hypothesis of photosynthetic acclimation to elevated T_air was tested in the third year by gas exchange measurements in the monocultures. Plants in the elevated T_air chambers suffered more from midday stress on warm summer days than those in ambient chambers. In absence of severe drought, the quantum yield of PSII was not affected by elevated T_air. Our results further indicate that species had not photosynthetically acclimated to a temperature increase of 3°C after 3 years exposure to a warmer climate. Although effects of S and T_air × S interactions were mostly not significant in our study, we expect that combined effects of T_air and S would be important in conditions of severe drought events.     

Transitions in photosynthetic parameters of midvein and interveinal regions of leaves and their importance during leaf growth and development

Abstract: The areal development of photosynthetic efficiency and growth patterns in expanding leaves of two different dicotyledonous species - Coccoloba uvifera and Sanchezia nobilis - was investigated by imaging both processes repeatedly over 32 days. Measurements were performed using combined imaging systems for chlorophyll fluorescence and growth, with the same spatial resolution. Significant differences in potential quantum yield of photosynthesis (F_v/F_m), a parameter indicating the functional status of photosystem II, were found between midvein and interveinal tissue. Although base-tip gradients and spatial patchiness were observed in the distribution of relative growth rate, neither midvein nor interveinal tissue showed such patterns in F_v/F_m. In young leaves, F_v/F_m of the midvein was higher than F_v/F_m of interveinal tissue. This difference declined gradually with time, and upon cessation of growth, F_v/F_m of interveinal regions exceeded those of midvein tissue. Images of chlorophyll fluorescence quenching showed that ΔF/F_m' in the different tissues correlated with F_v/F_m, indicating that, in these uniformly illuminated leaves, transitions in photosynthetic electron transport activity follow those of predawn quantum efficiency. We explore the implications of these observations during leaf development, discuss effects of sucrose delivery from veins to interveinal areas on relative rates of photosynthetic development in these tissues, and propose that the initially higher photosynthetic activity in the midvein compared to the intervein tissues may supply carbohydrates and energy for leaf growth processes.     

The consequences of freezing temperatures followed by high irradiance on in vivo chlorophyll fluorescence and growth in Picea abies

Picea abies (L.) Karst. plants, propagated by cuttings, were subjected to one night of freezing temperatures (-5°C), high irradiance (1 200 or 1 800 μmol m⁻² s⁻¹), or freezing temperatures followed by high irradiance. The treatments were applied at bud burst, at time of shoot elongation, and when the shoots had ceased to elongate. The maximum quantum yield of photosynthesis, F_v/F_m, dry weight of branches and needles, and length and survival of shoots were measured. F_v/F_m and growth decreased after a night of freezing temperatures followed by high irradiance, at the time of bud burst and shoot elongation. High irradiance alone influenced F_v/F_m, but not growth. Freezing temperatures affected F_v/F_m, and growth at the time of shoot elongation. F₀ increased after a night of freezing temperatures and decreased with age of the current-year needles. It was concluded that the use of short-term measurements of chlorophyll fluorescence induction to predict changes in growth after a night of frost and subsequent high light was not a reliable method.     

A rapid, whole-tissue determination of the functional fraction of PSII after photoinhibition of leaves based on flash-induced P700 redox kinetics

Assaying the number of functional PSII complexes by the oxygen yield from leaf tissue per saturating, single-turnover flash, assuming that each functional PSII evolves one oxygen molecule after four flashes, is one of the most direct methods but time-consuming. The ratio of variable to maximum Chl fluorescence yield (F_v/F_m) in leaves can be correlated with the oxygen yield per flash during a progressive loss of PSII activity associated with high-light stress and is rapid and non-intrusive, but suffers from being representative of chloroplasts near the measured leaf surface; consequently, the exact correlation depends on the internal leaf structure and on which leaf surface is being measured. Our results show that the average F_v/F_m of the adaxial and abaxial surfaces has a reasonable linear correlation with the oxygen yield per flash after varied extents of photoinactivation of PSII. However, we obtained an even better linear correlation between (1) the integrated, transient electron flow (Σ) to P700⁺, the dimeric Chl cation in PSI, after superimposing a single-turnover flash on steady background far-red light and (2) the relative oxygen yield per flash. Leaves of C3 and C4 plants, woody and herbaceous species, wild-type and a Chl- b -less mutant, and monocot and dicot plants gave a single straight line, which seems to be a universal relation for predicting the relative oxygen yield per flash from Σ. Measurement of Σ is non-intrusive, representative of the whole leaf tissue, rapid and applicable to attached leaves; it may even be applicable in the field.     

ON-LINE MONITORING OF CHLOROPHYLL FLUORESCENCE TO ASSESS THE EXTENT OF PHOTOINHIBITION OF PHOTOSYNTHESIS INDUCED BY HIGH OXYGEN CONCENTRATION AND LOW TEMPERATURE AND ITS EFFECT ON THE PRODUCTIVITY OF OUTDOOR CULTURES OF SPIRULINA PLATENSIS (CYANOBACTERIA)

The saturating pulse fluorescence technique was applied to study photoinhibition of photosynthesis in outdoor cultures of the cyanobacterium Spirulina platensis (Nordstedt) Geitler strain M2 grown under high oxygen and low temperature stress. Diurnal changes in maximum photochemical yield (F_v/F_m), photon yield of PSII (ΔF/F 'm), and nonphotochemical quenching (qN) were measured using a portable, pulse-amplitude–modulated fluorometer. When solar irradiance reached the maximum value, the F _v/F_m and ΔF/F 'm ratios of the Spirulina cultures grown under high oxygen stress decreased by 35% and 60%, respectively, as compared with morning values. The depression of the F_v/F_m and ΔF/F 'm ratios reached 55% and 84%, respectively, when high oxygen stress was combined with low temperature (i.e. 10° C below the optimal value for growth). Photoinhibition reduced the daily productivity of the culture grown under high oxygen stress by 33% and that of the culture grown under high oxygen–low temperature stress by 60%. Changes in the biomass yield of the cultures correlated well with changes in the daily integrated value of the estimated electron transport rate through the PSII (ΔF/F 'm × photon flux density). The results indicate that on-line chlorophyll fluorescence measurement is a powerful tool for assessing the photosynthetic performance of outdoor Spirulina cultures.     

CO2 assimilation, respiration and chlorophyll fluorescence in peach leaves infected by Taphrina deformans

Photosynthesis, respiration and chlorophyll fluorescence parameters were determined in peach ( Prunus persica L. cv. Dixired) leaves naturally infected by Taphrina deformans (Berk.) Tul. and in healthy leaves (controls), in two successive springs. A drastic decrease in net photosynthesis and an evident increase in respiration in curled leaves were noted. The instantaneous PSII fluorescence yield, with no (F₀) and with (F₀) quenching component, and steady state fluorescence yield (under actinic light, F_s) were essentially unchanged. Maximal fluorescence in dark-adapted (F_m) and illuminated (F'_m) leaves and the corresponding variable fluorescence (F_v and F_v) clearly decreased. The indicators of PSII quantum yield (F_v/F_m) in dark-adapted leaves, and the potential PSII excitation capture efficiency (F'_v/F'_m) and the quantum yield of PSII (q_p [F'_v/F'_m]) in the light were also significantly lower in curled leaves. Decreasing tendencies were also noted for the PSII photochemical yield (photochemical quenching, q_p) and in the energy status of the chloroplast (non-photochemical quenching, q_N, and Stern-Vollmer value, NPQ) although the differences were not always significant. In curled leaves the main alteration documented is the imbalance between the drastic inhibition of CO₂ fixation and the moderate decrease in photochemical reactions (i.e. F_v/F_m and ΔF/F'_m), indicating changes in the energy flux.     

The role of thylakoid lipids in the photodamage of photosynthetic activity

The effect of excess light at 10 or 30°C under aerobic or low O₂ condition on peroxidation of thylakoid lipids and primary photochemistry of photoinsynthesis was studied in wheat ( Triticum aestivum L. cv. HD 2329). Photoinhibitory damage to photosythesis was directly proportional to the peroxidation of thylakoid lipids. Photoinhibitory treatment given under low O₂ conditions resulted in significantly less peroxidation of the primary photochemistry of photosythesis measured using chlorophyll fluorescence and photosythetic electron trasport. Short term recovery of F_v/F_m ratio was fast while thylakoid lipids did not show much recovry. However, recovery (of F_v/F_m ratio and thylakoid lipids) was almost complete within 12 h after photoinhibition treatment. A possible relationship between peroxidation of thylakoid lipids and photodamage to photosynthesis is discussed.     

Light and oxygen stress in Spirulina platensis (cyanobacteria) grown outdoors in tubular reactors

As the effects of light and oxygen stress in algae on mass culture has not been intensively studied, we investigated them in Spirulina platensis under outdoor conditions in controlled tubular reactors where the respective roles of each stress can be distinguished. It was observed that exposure of this cyanobacterium at two oxygen concentrations (ca 20 and 53 mg 1⁻¹) caused very little change in the ratio between variable and maximum fluorescence (F_v/F_m) during the day even when the culture was grown at higher oxygen concentration (about 7% lower in the evening than in the morning). Vice-versa, when the photochemical efficiency of PSII (photon yield, Φ_c) was measured, a reduction of about 20% was observed. Neither the F_v/F_m ratio nor the Φ_c of the culture grown at the lower oxygen concentration changed significantly during the day. The daily productivity of the culture exposed to the higher oxygen concentration was reduced by about 20%. Laboratory cultures bubbled with air or pure oxygen under continuous light showed a similar response; i.e., a smaller decrease in F_v/F_m (17%) than in the Φ_c (56%) after 4 h. After 32 h of culture in pure oxygen, a total lysis of the cells occurred. Our results support the hypothesis that photoinhibition and photooxidation, two traditionally linked terms, although often closely associated under similar environmental conditions, may comprise two types of stress with different sites of inhibition.     

Photochemical response to drought acclimation in two sunflower genotypes

The effects of drought acclimation on CO₂ assimilation and light utilization were investigated in two sunflower genotypes ( Helianthus annuus L., T32 and Viki) in relation to water deficit and/or high light conditions. Drought interaction with PSII efficiency was observed in the genotype T32 with a sustained decrease in the potential photochemical efficiency of PSII, F_n/F_m. In response to drought acclimation, T32 displayed some tendency to accumulate closed PSII traps (higher value of 1-qp) without an enhancement of thermal deactivation (Stem-Volmer non-photochemical quenching, NPQ). Irrespective of the growth conditions (growth chamber or greenhouse), only Viki was responsive to drought acclimation, with (1) increased net photosynthesis in well-watered plants, (2) higher maintenance of photochemical electron transfer under water deficit and/or high light, (3) limited PSII inactivation (lower value of 1-qp) through increased non-photochemical energy dissipation (Stern-Volmer NPQ) which was readily reversible even at low leaf water potentials, and (4) higher F_v/F_m recovery after high light treatment. Additionally, drought acclimation delayed turgor loss during subsequent water stress in Viki. Thus, the response to drought acclimation, with an adjustment of water relations and of energy utilization by PSII, was observed under both growth conditions and was mainly genotype dependent.     

Changes in in vivo chlorophyll fluorescence quenching in lichen thalli as a function of water content and suggestion of zeaxanthin-associated photoprotection

The function of photosystem II (PSII) during desiccation was investigated via analysis of Chl a fluorescence emission in thalli from Parmelia quercina (Willd.) Vainio, Parmelia acetabulum (Necker) Duby, Ramalina farinacea (L.) Ach., Pseudevernia furfuracea (L.) Zopf., and Evernia prunastri (L.) Ach. Water loss followed the same exponential pattern in all these species, the half time being dependent on species. Desiccation affected the fluorescence parameters. Dark-adapted maximum fluorescence (F_m), instantaneous fluorescence (F_o) and the ratio of variable (F_m–F_o) to F_m were dependent on water content and decreased in two distinct phases: a slow and apparently linear phase, followed by a more steep decline at low water content. Actual PSII photochemical yield (φ_PSII), non-photochemical quenching (NPQ), efficiency of photon capture (φ_exc), and photochemical quenching (q_p) remained nearly constant until 30% relative water content (RWC), decreasing rapidly thereafter. In contrast, increased NPQ appeared to occur only at water content values lower than 20%. Treatment of thalli with dithiothreitol (DTT) effectively reduced NPQ during desiccation and increased susceptibility to photoinhibition caused by exposure to high light as measured by dark recovery of the F_vF_m ratio. HPLC analysis showed that the level of the de-epoxidized xanthophyll cycle pigments antheraxanthin (Anth) and zeaxanthin (Zea) increased during lichen desiccation. The results point towards the existence of a photoprotective mechanism with the involvement of Zea and Anth in non-radiative dissipation of the desiccation-induced excess of energy.     

Photoinhibition, xanthophyll cycle and in vivo chlorophyll fluorescence quenching of chilling-tolerant Oxyria digyna and chilling-sensitive Zea mays

The relationship between susceptibility to photoinhibition, zeaxanthin formation and chlorophyll fluorescence quenching at suboptimal temperatures was studied in chilling-sensitive maize and in non-acclimated and cold-acclimated Oxyria digyna , a chilling-tolerant plant of arctic and alpine habitats. In maize, zeaxanthin formation was strongly suppressed by chilling. Zeaxanthin formed during preillumination at 20°C did not protect maize leaves from photoinhibition during a subsequent high-light, low-temperature treatment, as judged from the ratios of variable to maximal fluorescence, F_v/F_m. However, such preillumination significantly increased non-photochemical quenching (q_N) at low temperatures, mainly due to an enhancement of the fast-relaxing q_N component (i.e., of energy-dependent quenching. q_E). In O. digyna , cold-acclimation resulted in an increased zeaxanthin formation in the temperature range of 2.5–20°C. Cold-acclimation substantially decreased the susceptibility towards photoinhibition at 4°C, but q_N remained nearly unchanged between 2 and 38°C, as compared to control plants. Effects of cold acclimation on photosynthesis, photochemical quenching and quantum efficiency of photosystem II were small and indicated a slight amelioration only of the function of the photosynthetic apparatus at suboptimal temperatures (2–20°Ct. I) is concluded, that the xanthophyll cycle is strongly influenced by cold acclimation, while effects on the photosynthetic carbon assimilation only play a minor role in O. digyna.     

Growth and physiological responses of canola (Brassica napus) to three components of global climate change: temperature, carbon dioxide and drought

Elevated CO₂ appears to be a significant factor in global warming, which will likely lead to drought conditions in many areas. Few studies have considered the interactive effects of higher CO₂, temperature and drought on plant growth and physiology. We grew canola ( Brassica napus cv. 45H72) plants under lower (22/18°C) and higher (28/24°C) temperature regimes in controlled-environment chambers at ambient (370 μmol mol⁻¹) and elevated (740 μmol mol⁻¹) CO₂ levels. One half of the plants were watered to field capacity and the other half at wilting point. In three separate experiments, we determined growth, various physiological parameters and content of abscisic acid (ABA), indole-3-acetic acid and ethylene. Drought-stressed plants grown under higher temperature at ambient CO₂ had decreased stem height and diameter, leaf number and area, dry matter, leaf area ratio, shoot/root weight ratio, net CO₂ assimilation and chlorophyll fluorescence. However, these plants had increased specific leaf weight, leaf weight ratio and chlorophyll concentration. Elevated CO₂ generally had the opposite effect, and partially reversed the inhibitory effects of higher temperature and drought on leaf dry weight accumulation. This study showed that higher temperature and drought inhibit many processes but elevated CO₂ partially mitigate some adverse effects. As expected, drought stress increased ABA but higher temperature inhibited the ability of plants to produce ABA in response to drought.     

Photoinhibition of photosynthesis in intact willow leaves in response to moderate changes in light and temperature

When willow leaves were transferred from 270 to 650 μmol m^-2 s^-1 photosynthetic photon flux density (PPFD), partial photoinhibition developed over the next hours. This was manifested as roughly parallel inhibitions of the ratio of variable over maximal chlorophyll fluorescence (F_v/F_M), and of the maximal quantum yield and the capacity of photosynthesis. This occurred even though photosynthesis was operating well below its capacity and only about one fourth of the reaction centres of photosystem (PS) II were in the closed state. When the air temperature was lowered from 25 to 15°C (18°C leaf temperature) photoinhibition was markedly accelerated. This temperature effect is suggested to be mediated largely by a decrease in the rate of energy dissipation through photosynthesis and indicated by a 50% increase in the number of closed PSII reaction centres. The pool size of the carotcnoid zeaxanthin and the extent of inhibition of the F_v/F_M ratio were positively correlated during the treatment. However, the relaxation following imposition of darkness was much faster for zeaxanthin than for the F_v/F_M ratio, ruling out the possibility of a direct causal relationship. The energy distribution between PSII and PSI was unaltered upon photoinhibition. However, the functioning of the PSII reaction centres was altered, as indicated by a rise in the minimal fluorescence, Fa.     

Latent manganese deficiency increases transpiration in barley (Hordeum vulgare)

To investigate if latent manganese (Mn) deficiency leads to increased transpiration, barley plants were grown for 10 weeks in hydroponics with daily additions of Mn in the low n M range. The Mn-starved plants did not exhibit visual leaf symptoms of Mn deficiency, but Chl a fluorescence measurements revealed that the quantum yield efficiency of PSII (F_v/F_m) was reduced from 0.83 in Mn-sufficient control plants to below 0.5 in Mn-starved plants. Leaf Mn concentrations declined from 30 to 7 μg Mn g⁻¹ dry weight in control and Mn-starved plants, respectively. Mn-starved plants had up to four-fold higher transpiration than control plants. Stomatal closure and opening upon light/dark transitions took place at the same rate in both Mn treatments, but the nocturnal leaf conductance for water vapour was still twice as high in Mn-starved plants compared with the control. The observed increase in transpiration was substantiated by ¹³C-isotope discrimination analysis and gravimetric measurement of the water consumption, showing significantly lower water use efficiency in Mn-starved plants. The extractable wax content of leaves of Mn-starved plants was approximately 40% lower than that in control plants, and it is concluded that the increased leaf conductance and higher transpirational water loss are correlated with a reduction in the epicuticular wax layer under Mn deficiency.     

Role of light in changes in free amino acid and polyamine contents at chilling temperature in maize (Zea mays)

The effect of irradiance on changes in photosynthesis, free amino acids and polyamines was investigated. Two-week-old maize ( Zea mays L.) plants were chilled at 5°C in the light (250 μmol m⁻² s⁻¹ PAR) or dark. The chlorophyll fluorescence ratio, F_v/F_m, decreased in the light by ca 50% but did not change in the dark. Similarly to the F_v/F_m, there was no change in the transpiration rate or the stomatal conductance in the dark, while these parameters decreased by ca 55% in the light. The net photosynthesis rate declined in both cases, but to a far greater extent in the light (73%) than in the dark (40%). The intercellular CO₂ partial pressure increased by ca 50% in all cases. The free amino acid contents increased compared to the control during the cold treatment. In most cases this increase was more pronounced in the light than in the dark. There was a continuous increase in the putrescine level, which was more pronounced in the light than in the dark. The spermidine content increased one and a half times after one day in the light but decreased by 70% in the dark compared to the control values. From the second day a 50% decline in the spermidine content was observed in the light and an 80% decline in the dark. These results suggest that light has an influence not only on the photosynthetic processes during chilling stress but also on other stress markers such as polyamines and free amino acids.     

Photoinhibition in Vitis californica: interactive effects of sunlight, temperature and water status

Abstract. In a series of factorial experiments with cultivated Vitis californica Benth. (California wild grape) growth outdoors in full sun, we examined the effects of sunlight, temperature and water status on net CO₂ uptake and PSH chlorophyll fluorescence at 77K. Exposure to either high light or high temperature caused reductions in PSH activity followed by partial or complete overnight recovery. Upon simulataneous exposure to high light and high temperature, PSH inhibition was severe and persistent. The maximum chlorophyll fluorescence (F_M) and the ratio of variable to maximum fluorescence (F_v/F_M) differed in their responses to combinations of light and temperature. At both low and high light. F_M declined with increasing temperature over a wide temperature range, while F_v/F_M exhibited a similar sensitivity to temperature only at high light. Net CO₂ uptake declined by mid-afternoon and recovered by the next morning in most leaves, regardless of incident light or temperature. However, high-light leaves exhibited severe and lasting declines if temperatures exceeded 45°C. Water-stressed leaves exposed to high light exhibited greater reductions of net CO₂ uptake than water-stressed leaves exposed to low light. However, the degree of light-dependent decline in PSH fluorescence (F_M and F_v/F_M) did not vary with water status, indicating that reduced PSH activity was not a primary cause of reduced carbon gain during water stress.     

Relative contributions of photochemical and non-photochemical routes to excitation energy dissipation in rice and barley illuminated at a chilling temperature

The mechanistic basis for differential sensitivities to chilling-induced photoinhibition among two rice ( Oryza sativa L.) cultivars (an Indica and a Japonica type) and one barley cultivar ( Hordeum vulgare L. cv. Albori) was examined. When leaf segments were exposed to moderate illumination at 4°C, a sustained decrease in the photochemical efficiency of photosystem (PS) II measured as the ratio of variable to maximal fluorescence (F_v/F_m) was observed for several hours. An analysis of fluorescence quenching revealed a sudden drop in PSII-driven electron transport rate (ETR) and a rapid rise in the reduction state of the primary electron acceptor Q_A upon exposure to chilling in moderate light. There was no appreciable difference in the level of non-photochemical quenching (NPQ) nor in the xanthophyll cycle activity between Japonica rice and barley. However, barley was capable of sustaining a higher ETR, thereby keeping a lower reduction state of Q_A throughout the chilling for 6 h. The Indica rice was characterized by the lowest ability to develop the xanthophyll cycle-associated NPQ, particularly the fast relaxing NPQ component (qf), accompanied by the highest reduction state of Q_A and photoinhibitory quenching (qI). It is concluded that the lower susceptibility of barley to chilling-induced photoinhibition than Japonica rice is attributable to its higher potential to dissipate excess light energy via a photochemical mechanism, whereas Indica rice is more sensitive to photoinhibition at a chilling temperature than Japonica rice, due primarily to its lower capacity to develop an efficient NPQ pathway.     

Photoprotection processes under water stress and recovery in Mediterranean plants with different growth forms and leaf habits

The response of photoprotection mechanisms to a short-term water stress period followed by rewatering, to simulate common episodic water stress periods occurring in Mediterranean areas, was studied in 10 potted plants representative of different growth forms and leaf habits. During water stress and recovery, relative water content, stomatal conductance, leaf pigment composition, electron transport rates, maximum quantum efficiency of PSII photochemistry (F_v/F_m), thermal energy dissipation and photorespiration rates (P_r) were determined. All the species analyzed proved to be strongly resistant to photoinactivation of PSII under the imposed water stress conditions. The responses of the analyzed parameters did not differ largely among species, suggesting that Mediterranean plants have similar needs and capacity for photoprotection under episodic water stress periods regardless of their growth form and leaf habit. A general pattern of photoprotection emerged, consisting in maintenance or increase of P_r at mild stress and the increase of the thermal energy dissipation at more severe stress. Adjustments in pigment pool sizes were not an important short-term response to water stress. The increase of thermal energy dissipation because of water stress depended mostly on the de-epoxidation state of xanthophylls, although the slope and kinetics of such relationship strongly differed among species, suggesting species-dependent additional roles of de-epoxidated xanthophylls. Also, small decreases in F_v/F_m at predawn during water stress were strongly correlated with maintained de-epoxidation of the xanthophylls cycle, suggesting that a form of xanthophyll-dependent sustained photoprotection was developed during short-term water stress not only in evergreen but also in semideciduous and annual species.     

Beech trees exposed to high CO2 and to simulated summer ozone levels: Effects on photosynthesis, chloroplast components and leaf enzyme activity

Young trees of European beech ( Fagus sylvatica L.) were exposed in a phytotron to different levels of ozone and CO₂ under the climatic simulation of one vegetation period. High ozone levels were simulated similar to high ozone concentration in the field (up to 110 ppb), while CO₂ was added as 300 ppm to the present level of ca 380 ppm. Our study describes different aspects of photosynthesis from the leaf level to the reactions of selected thylakoid components at different harvest times during growth of the beech trees under the different fumigation regimes. Ozone effects appeared in the first weeks of the treatment as a stimulation of chlorophyll fluorescence (F_v/F_m), in oxygen production and in ribulose-1,5-bisphosphate carboxylase/oxygenase activity, while the summer and early autumn harvests showed strong reductions in these parameters. Only phosphoenolpyruvate carboxylase (PEPcase) activity remained higher under high ozone. The effects of high CO₂ appeared in general as a small stimulation in enzyme activity like PEPcase in spring. However, with increasing time of fumigation, reductions of all parameters were observed. Especially chlorophylls showed strong reductions under high CO₂. The combined treatment with high ozone plus high CO₂ resulted mostly in an amelioration of the negative ozone effects, although control levels were not reached.     

Chilling sensitivity of the frost-tolerant potato Solanum commersonii

Frost tolerance has been reported in the shoots of wild, tuberiferous potato species such as Solanum commersonii when the plants are grown in either field or controlled conditions. However, these plants can survive as underground tubers and avoid unfavorable environmental conditions altogether. As such, leaf growth and photosynthesis at low temperature may not be required for survival of the plants. In order to determine the temperature sensitivity of S. commersonii shoots, we examined leaf growth, development and photosynthesis in plants raised at 20/16°C (day/night). 12/9°C and 5/2°C. S. commersonii leaves grown at 5°C exhibited a marked decrease in leaf area and in total chlorophyll (Chl) content per leaf area when compared with leaves grown at 20°C. Furthermore, leaves grown at 5°C did not exhibit the expected decrease in either water content or susceptibility to low-temperature-induced photoinhibition that normally characterizes cold acclimation in frost-tolerant plants. Measurements of CO₂-saturated O₂ evolution showed that the photosynthetic apparatus of 5°C plants was functional, even though the efficiency of photosystem II photochemistry was reduced by growth at 5°C. A decrease in the resolution of the M-peak in the slow transients for Chl a fluorescence in leaves grown at 12 and 5°C and in all leaves exposed to high light at 5°C indicated that low temperature significantly affected processes on the reducing side of Q_A, the primary quinone electron acceptor in photosystem II. Thus S. commarsonii exhibits the characteristics of a plant that is limited by chilling temperatures. Although S. commersonii can tolerate light frosts, its sensitivity to chilling temperatures may result in shoot dieback in winter in its native habitat. The plants may avoid both chilling and freezing temperatures by overwintering as underground tubers.