Chilling injury and recovery in detached and attached leaves measured by chlorophyll fluorescence

Smillie, R. M., Nott, R., Hetherington, S. E. and Öyustt, G. 1987. Chilling injury and recovery in detached and attached leaves measured by chlorophyll fluorescence Chilling injury was compared in detached and attached leaves chilled at 0 or 0.5°C by measuring the decrease in induced chlorophyll fluorescence in vivo. The fluorescence parameter measured was F_R, the maximal rate of rise of induced chlorophyll fluorescence emission after irradiating dark-adapted leaves. The plants used were bean, Phaseolus vulgaris L. cv. Pioneer, and maize, Zea mays L. cvs hybrid GH 390 and Northern Belle. Leaves were detached and placed on wet paper and covered with thin polyethylene film to prevent water loss during chilling. Leaves left attached on plants were treated similarly. When chilled in this way at 100% relative humidity, the chilling-induced decrease in F_R was the same in detached and attached leaves. For the attached leaves, the same result was obtained whether just a single leaf was chilled or the whole plant. Expression of chilling injury was greatest in fully turgid leaves and comparisons can be invalid unless the water status of the detached and attached leaves are the same. Problems arising from diurnal fluctuations in water potential of plants grown in a glasshouse were circumvented by placing leaves on the wet filter paper under polyethylene film prior to chilling, which allowed high water potentials to be regained, or mist sprays in the glasshouse were employed. Determinations of the time course for changes in F_R of maize (cv. Northern Belle) during chilling at 0°C showed that F_R decreased exponentially, at the same rate (time to 50% decrease in F_R was 9.3 h) in detached and attached leaves. Chilling injury was largely reversible for the first 20 h of chilling stress as both detached and attached leaves recovered their pre-chilling values of F_R after a further 20 h at 20°C in darkness. Leaves chilled for 48 h showed partial recovery, while those chilled for 72 h did not recover. Recovery was impeded by light. Inability to recover from chilling as indicated by measurements of F_R was paralleled by the incidence of visible symptoms of injury. It is concluded that detached and attached leaves behave similarly during chilling and short-term recovery, provided a similarity in treatments is rigorously maintained.     

Effects of Chilling Temperature on the Activity of Enzymes of Sucrose Synthesis and the Accumulation of Saccharides in Leaves of Three Sugarcane Cultivars Differing in Cold Sensitivity

The effects of short-term exposure to chilling temperature (10 °C) on sucrose synthesis in leaves of the cold-tolerant sugarcane cultivars Saccharum sinense R. cv. Yomitanzan and Saccharum sp. cv. NiF4, and the cold-sensitive cultivar S. officinarum L. cv. Badila were studied. Plants were grown at day/night temperatures of 30/25 °C, and then shifted to a constant day/night temperature of 10 °C. After 52-h exposure to the chilling temperature, sucrose content in the leaves of NiF4 and Yomitanzan showed a 2.5- to 3.5-fold increase relative to that of the control plants that had been left on day/night temperatures of 30/25 °C. No such increase was observed in Badila leaves. Similarly, starch content in the leaves of NiF4 and Yomitanzan was maintained high, but starch was depleted in Badila leaves after the 52-h exposure. During the chilling temperature, sucrose phosphate synthase (SPS; E.C.2.4.1.14) activity was relatively stable in the leaves of NiF4 and Yomitanzan, whereas in Badila leaves SPS activity significantly decreased. There was no significant change in cytosolic fructose-1,6-bisphosphatase activity for the three cultivars at the chilling temperature. This supports the hypothesis that: (1) on exposure to chilling temperature, sucrose content in sugarcane leaves is determined by the photosynthetic rate in the leaves, and is not related to SPS activity; (2) SPS activity in sugarcane leaves at chilling temperature is to be determined by sugar concentration in the leaves.     

Effects of long-term chilling on growth and photosynthesis of the C4 gramineae Paspalum dilatatum

Dallis grass (Paspalum dilatatum Poir.) is a C₄/NADP‐ME gramineae, previously classified as semi‐tolerant to cold, although a complete study on this species acclimation process under a long‐term chilling and controlled environmental conditions has never been conducted. In the present work, plants of the variety Raki maintained at 25/18°C (day/night) (control) were compared with plants under a long‐term chilling at 10/8°C (day/night) (cold‐acclimated) in order to investigate how growth and carbon assimilation mechanisms are engaged in P. dilatatum chilling tolerance. Although whole plant mean relative growth rate (mean RGR) and leaf growth were significantly decreased by cold exposure, chilling did not impair plant development nor favour the investment in biomass below ground. Cold‐acclimated P. dilatatum cv. Raki had a lower leaf chlorophyll content, but a higher photosynthetic capacity at optimal temperatures, its range being shifted to lower values. Associated with this higher capacity to use the reducing power in CO₂ assimilation, cold‐acclimated plants further showed a higher capacity to oxidize the primary stable quinone electron acceptor of PSII, Q_A. The activity and activation of phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) and ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) were not significantly affected by the long‐term chilling. Cold‐acclimated P. dilatatum cv. Raki apparently showed a lower transfer of excitation energy from the light‐harvesting complex of photosystem II to the respective reaction centre and enhancement of radiationless energy‐dissipating mechanisms at suboptimal temperatures. Overall, long‐term chilling resulted in several effects that comprise responses with an intermediate character of both chilling‐tolerant and –sensitive plants, which seem to play a significant role in the survival and acclimation of P. dilatatum cv. Raki at low temperature.     

The effects of development at sub-optimal growth temperatures on photosynthetic capacity and susceptibility to chilling-dependent photoinhibition in Zea mays

When plants of Zea mays L. cv. LG11 that have been grown at optimal temperatures are transferred to chilling temperatures (0–12°C) photoinhibition of photosynthetic CO₂ assimilation can occur. This study examines how growth at sub-optimal temperatures alters both photosynthetic capacity and resistance to chilling-dependent photoinhibition. Plants of Z. mays cv. LG11 were grown in controlled environments at 14, 17, 20 and 25°C. As a measure of the capacity for photosynthesis under light limiting conditions, the maximum quantum yields of CO₂ assimilation (φ^a.c) and O₂ evolution (φ^a.o) were determined for the laminae of the second leaves at photon fluxes of 50–150 μmol m^-2s^-1. To determine photosynthetic capacity at photon fluxes approaching light saturation, rates of CO₂ uptake (A₁₅₀₀) and O₂ evolution (A₁₅₀₀) were determined in a photon flux of 1500 μmol m^-2s^-1. In leaves developed at 14°C, φ and φ were 26 and 43%, respectively, of the values for leaves grown at 25°C. Leaves grown at 17°C showed intermediate reductions in φ and φ, whilst leaves developed at 20°C showed no significant differences from those grown at 25°C. Similar patterns of decrease were observed for A₁₅₀₀ and A_1500.0 with decreasing growth temperature. Leaves developed at 25°C showed higher rates of CO₂ assimilation at all light levels and measurement temperatures in comparison to leaves developed at 17 and 14°C. A greater reduction in A₁₅₀₀ relative to A_1500.0 with decreasing growth temperature was attributed to increased stomatal limitation. Exposure of leaves to 800–1000 μmol m^-2 s^-1 when plant temperature was depressed to ca 6.5°C produced a photoinhibition of photosynthetic CO₂ assimilation in all leaves. However, in leaves developed at 17°C the decrease in A₁₅₀₀ following this chilling treatment was only 25% compared to 90% in leaves developed at 25°C. Recovery following chilling was completed earlier in leaves developed at 17°C. The results suggest that growth at sub-optimal temperatures induces increased tolerance to exposure to high light at chilling temperatures. This is offset by the large loss in photosynthetic capacity imposed by leaf development at sub-optimal temperatures.     

A Comparison of the Effects of Chilling on Leaf Gas Exchange in Pea (Pisum sativum L.) and Cucumber (Cucumis sativus L.)

The effects of chilling on the photosynthesis of a chilling-resistant species, pea (Pisum sativum L. cv Alaska) and a chilling-sensitive species, cucumber (Cucumis sativus L. cv Ashley) were compared in order to determine the differences in the photosynthetic chilling sensitivity of these two species. For these experiments, plants were chilled (5°C) for different lengths of time in the dark or light. Following a 1 hour recovery period at 25°C, photosynthetic activity was measured by gas exchange (CO₂ uptake and H₂O release), quantum yield, and induced chlorophyll fluorescence. The results show that pea photosynthesis was largely unaffected by two consecutive nights of chilling in the dark, or by chilling during a complete light and dark cycle (15 hours/9 hours). Cucumber gas exchange was reduced by one night of chilling, but its quantum yield and variable fluorescence were unaffected by dark chilling. However, chilling cucumber in the light led to reduced CO₂ fixation, increased internal leaf CO₂ concentration, decreased quantum yield, and loss of variable fluorescence. These results indicate that chilling temperatures in conjunction with light damaged the light reactions of photosynthesis, while chilling in the dark did not.     

The relationship between CO2 assimilation, photosynthetic electron transport and water–water cycle in chill-exposed cucumber leaves under low light and subsequent recovery

The effects of chilling under low light (9/7 °C, 100 µmol m^?2 s^?1) on the photosynthetic and antioxidant capacities and subsequent recovery were examined in two (one tolerant and one sensitive) cucumber genotypes. Chilling resulted in an irreversible inhibition of net CO₂ assimilation and growth for the sensitive genotype, which was accompanied by decreases in the maximum velocity of RuBP carboxylation by Rubisco (V_cmax), the capacity for ribulose‐1,5‐bisphosphate regeneration (J_max), Rubisco content and activity, and the quantum efficiency of photosystem II, in the absence of any stomatal limitation of CO₂ supply or inorganic phosphate limitation. In contrast, CO₂ assimilation for the tolerant genotype fully recovered after chill. The chill‐induced decrease in the proportion of electron flux for photosynthetic carbon reduction was mostly compensated by an O₂‐dependent alternative electron flux driven by the water–water cycle, especially in the sensitive genotype. Compared with the tolerant genotype, the sensitive genotype after chill showed reduced capacity for scavenging reactive oxygen species and increased accumulation of reactive oxygen species. The balance between O₂‐dependent alternative electron flux and the capacity for scavenging reactive oxygen species in response to chill plays a major role in determining the tolerance of cucumber leaves to this stress factor. It is concluded that the water–water cycle operates at high rates when CO₂ assimilation is restricted in cucumber leaves subjected to chill and low light conditions.     

The recovery of photosynthesis in tomato subsequent to chilling exposure

The overall success of a plant in coping with low temperature sensitivity of photosynthesis is dependent not only on the maximum extent of inhibition suffered for a given time of low temperature exposure but also on the persistence of the inhibition after normal growth temperatures are restored. Thus the capacity of recovery and the speed with which a plant can recover from the effects of chilling exposure are important parameters in determining how devastating the chilling event will be on season-long growth and yields. We have studied the recovery of CO₂-saturated photosynthesis from the injury caused by exposing intact tomato plants (Lycopersicon esculentum Mill. cv. Floramerica) or detached tomato leaves to a temperature of 1°C in the dark for varying periods of time. We found that net photosynthesis was fully recovered within 12 h after returning the plants to 25°C in the dark, even after chilling exposures as long as 45 h. This was true for intact plants as well as for detached leaves that were supplied with water. When chilling took place in the light (4°C, 1000 E · m^-2 · s^-1, PAR) inhibition of photosynthesis was more severe and appeared more quickly and the recovery was slower and incomplete. A 12 h chilling exposure in the light resulted in injury to net photosynthesis that was not fully recovered even after 50 h. Chilling damage to photosynthesis developing in the light was distinguished from chilling in the dark by the decreased photosynthetic quantum yield. Not only did high intensity illumination enhance chilling damage of photosynthesis but bright light subsequent to the chilling exposure also delayed the recovery of photosynthesis. At none of the three ambient CO₂ concentrations investigated (300, 1500 and 5000 1.1^-1) did the recovery of photosynthesis depend on stomatal conductance.     

Different photosynthetic responses to night chilling among twelve populations of Jatropha curcas

Jatropha curcas, one of the most important energy plant resources, is vulnerable to chilling. To evaluate the effects of chilling on photosynthesis of J. curcas and intraspecific differences in chilling tolerance, seedlings of twelve populations were treated with the temperature of 4–6°C for five consecutive nights with normal environmental temperature during the day. Night chilling treatment decreased light-saturated photosynthetic rate (P _max) significantly for all populations. Stomatal limitation could not explain the decreased P _max because intracellular CO₂ concentration was not significantly reduced by night chilling in all populations (with only one exception). The decreased soluble-protein content, which may be related to the increased malondialdehyde (MDA) content, contributed to the decreased P _max. The increased MDA content indicated that oxidative stress occurred after night chilling, which was associated with the larger decrease in P _max compared with the decrease in actual photochemical efficiency of photosystem II, and the slight increase in thermal dissipation of excessive energy. After five-day recovery, MDA (with two exceptions) and P _max still did not recover to the levels as those before night chilling treatment for all populations, indicating that J. curcas was vulnerable to chilling. Chilling tolerance was significantly different among populations. Populations originating from high elevations had greater chilling-tolerant abilities than populations originating from low elevations, showing a local adaptation to environmental temperatures of origins. Our study shed light on the possibility to find or breed chilling-tolerant genotypes of J. curcas.     

Chilling and photosynthetic productivity of field grown maize (Zea mays); changes in the parameters of the light-response curve,canopy leaf CO2 assimilation rate and crop radiation-use efficiency

The effect of growth temperatures on the photosynthetic performance of field grown maize (Zea mays cv. LG11) was examined for crops sown on 1 May and 28 June 1991. During the period of growth, 2 May to 10 August, the early-sown crop experienced temperatures below 10 °C on 33 occasions compared with only one for the crop sown on 28 June. The prolonged period of low temperatures throughout May and beginning of June were associated with a marked depression in CO₂ assimilation rates at all light levels in the early-sown treatment. Chill-induced depression of the photosynthetic light-response curve reflected a sustained reduction in canopy leaf photosynthesis and crop radiation-use efficiency (RUE). During the early stages of growth, RUE was 65% lower in the early- than late-sown treatment, with no marked recovery observed in the former treatment until approximately three weeks after chilling conditions had ceased. Data show a close correlation between chill-induced depression of quantum yield () and RUE, with corresponding reductions in the light-saturated rates of CO₂ assimilation (P_max). The convexity of the light-response curve recovered most rapidly from chilling temperatures, and at least three weeks before any improvement in RUE. It is concluded that photosynthetic productivity of immature maize stands is less sensitive to changes in the convexity of the light response, than to changes in either or P_max.     

Light-dependent damage to photosynthesis in olive leaves during chilling and high temperature stress

Abstract The leaves of olive are long lived and likely to experience both chilling and high temperature stress during their life. Changes in photosynthetic CO₂ assimilation resulting from chilling and high temperature stress, in both dim and high light, are investigated. The quantum yield (φ) of photosynthesis at limiting light levels was reduced following chilling (at 5°C for 12 h), in dim light by approximately 10%, and in high light by 75%; the difference being attributed to photoinhibition. Similar reductions were observed in the light-saturated rate of CO₂ uptake (A_max). Decrease in A_max correlated with a halving of the leaf internal CO₂ concentration (c_i), suggesting an increased limitation by stomata following photoinhibition. Leaves were apparently more susceptible to photoinhibitory damage if the whole plant, rather than the leaf alone, was chilled. On return to 26 °C, I he photosynthetic capacity recovered to pre-stress levels within a few hours if leaves had been chilled in high light for 8 h or less, but did not fully recover from longer periods of chilling when loss of chlorophyll occurred. Leaves which were recovering from chilling in high light showed far more damage on being chilled a second time in high light. Three hours in high light at 38 °C reduced φ by 80%, but φ recovered within 4h of return to 26 °C. Although leaves of Olive are apparently less susceptible to photoinhibitory damage during chilling stress than the short-lived leaves of chilling-sensitive annual? crops, the results nevertheless show that photoinhibition during temperature stress is potentially a major factor influencing the photosynthetic productivity of Olive in the field.     

Effects of day and night temperature and temperature variation on photosynthetic characteristics

Net photosynthetic rates and mesophyll conductances were measured under standardized conditions for leaves of two C₃ and one C₄ annual species grown at temperatures of 20 to 32°C. Plants were grown with varying day and night temperatures, and also at constant temperatures equal to all the day and night temperatures used. Plants were grown with 8, 12, and 16 hours of light per day. This design allowed determination of whether photosynthetic characteristics were best correlated with day, night, mean, or time-weighted mean temperatures, The results showed that for Glycine max (L.) Merr. (C₃) night temperature was most important in determining photosynthetic characteristics, while in Helianthus annuus L. (C₃) and Amaranthus hypochondriacus L. (C₄) the time-weighted mean temperature was most important. The results for all species were consistent with the hypothesis that development of photosynthetic characteristics is related to a balance between the rate of leaf expansion and the rate of photosynthesis under the growth conditions.     

Role of thermal dissipation in the photoprotection in cucumber plants after exposure to a chill stress

Experiments were carried out to investigate the changes in CO₂ assimilation, photon allocation, and photosynthetic electron flux in leaves of cucumber (Cucumis sativus L.) plants after chilling stress. Chilling significantly decreased CO₂ assimilation, the energy flux via linear electron transport (J _PS2) and non-constitutive thermal dissipation (J _NPQ) but increased fluorescence and constitutive thermal dissipation (J _f,D) in chilling-sensitive genotype Jinyan No. 4. In contrast, chilling had little effects on J _NPQ and J _f,D although CO₂ assimilation and J _PS2 were inhibited in chilling-tolerant genotype Jinchun No. 3. In parallel with the reduction in J _PS2, electron flux to oxygenation and carboxylation by ribulose-1,5-bisphosphate carboxylase/oxygenase all significantly decreased while electron flux to O₂ significantly increased, especially in chilling-sensitive genotype. Thermal and fluorescence dissipation were the main energy dissipation pathways whilst water-water cycle was an important electron sink when photosynthetic carbon reduction was suppressed after chilling. Chilling sensitivity of the photosynthetic apparatus was related to the operation of different photoprotection mechanisms.     

Regulation of photosynthesis and antioxidant metabolism in maize leaves at optimal and chilling temperatures: review

Maize (Zea mays L.) is a chilling (below 15 °C) sensitive plant that shows little capacity to acclimate to low growth temperatures. Maize leaves are extremely sensitive to chilling injury, which usually results in premature leaf senescence. Leaves exposed to temperatures below 10 °C in the light show substantial inhibition of CO₂ assimilation and down-regulation of photosynthetic electron transport. However, the intrinsic relationships between the quantum efficiencies of photosystems I and II are not modified by chilling. Moreover, the integral relationships between non-cyclic electron transport and CO₂ fixation are similar in chilled and unchilled leaves. In this review we examine the roles and importance of photosynthetic regulation, carbon metabolism and antioxidant metabolism in determining the sensitivity of maize leaf photosynthesis to chilling. The distinct cellular localisation patterns of antioxidant enzymes such as glutathione reductase (EC 1.6.4.2) and dehydroascorbate reductase (EC 1.8.5.1) can restrict the recycling of antioxidants associated with photosynthesis during chilling. Disruption of circadian regulation of metabolism and insufficient antioxidant defence are postulated to cause chilling sensitivity.     

Chilling‐induced Ca2+ overload enhances production of active oxygen species in maize (Zea mays L.) cultured cells: the effect of abscisic acid treatment

Chilling (4 °C) induced a prolonged high level of intracellular Ca²⁺ (Ca²⁺ overload) and lipid peroxidation in maize (Zea mays L. cv Black Mexican Sweet) cultured cells. However, such Ca²⁺ overload and enhanced lipid peroxidation were not seen in abscisic acid (ABA)‐treated cells, which had an improved chilling tolerance. A Ca²⁺ ionophore, A23187, caused Ca²⁺ overload in both ABA‐treated maize cells and the untreated control, whereas an enhanced lipid peroxidation was detected only in the control. The high level of active oxygen species (AOS) in the control during chilling at 4 °C could be reduced by the presence of lanthanum (La³⁺), a Ca²⁺ channel blocker, in the medium. Moreover, both the A23187‐induced lipid peroxidation and AOS production in the control could be reduced by extracellular EGTA, a Ca²⁺ chelator. Laser‐scanning confocal microscopy revealed that mitochondria were one of the major AOS sources under chilling and during A23187 treatment. In vitro assays showed that superoxide production in isolated maize mitochondria was enhanced by the presence of Ca²⁺. Findings suggest that chilling‐induced Ca²⁺ influx in the control triggers a marked generation of AOS, which in turn results in the enhanced lipid peroxidation. The ability of ABA‐treated cells to avoid the chilling‐induced Ca²⁺ influx may serve as a mechanism that prevents the chilling‐induced oxidative stress and thus results in less chilling injury.     

Cold Acclimation Ability and Photosynthesis among Species of the Tropical Coffea Genus

Abstract: Three Coffea species (C. arabica cv. Icatu, C. canephora cv. Apoatã and C. dewevrei) were tested in order to identify and study the mechanisms of tolerance to low, non‐freezing temperatures. Several photosynthesis‐related parameters were monitored during a 20‐day period of gradual temperature decrease, from 25/20 °C (day/night) down to 15/10 °C, during chilling treatments (15/4 °C), and upon rewarming (25/20 °C). Differences were found among species, both during low temperature exposure and during rewarming. In general, Coffea species showed cold‐induced photoinhibition of photosynthesis, which was attributable to biochemical (in vivo ribulose‐1,5‐bisphosphate carboxylase/oxygenase activity and carbohydrate synthesis) and biophysical (antennae functioning, photosystem II efficiency and linear electron transport) inactivation, rather than to stomatal constraints. The moderately low temperature of 15/10 °C was enough to cause a negative impact on net photosynthesis (A), mostly due to low (initial) rubisco activity in all species. However, C. arabica cv. Icatu showed a higher tolerance to chilling and recovered quickly and completely upon rewarming, as assessed from the impacts on the photosynthetic machinery (e.g. A_max, F_o, F_v/F_m, F_v′/F_m′, q_P, ?_e, rubisco activity) and on carbohydrate metabolism. Such lesser effects are likely to be related to the strong increases and higher contents of zeaxanthin, lutein and β‐carotene that presumably increased the ability to dissipate excitation energy and contributed to protect the photosynthetic apparatus. During cold exposure, a significant reduction of the α/β carotene ratio, which is considered an acclimation feature, was observed solely in C. arabica cv. Icatu. However, C. canephora cv. Apoatã and, especially, C. dewevrei showed to be highly cold‐sensitive. In these latter species, the photoinhibitory impairments to photosynthesis were stronger, probably due to the lower contents of protecting pigments during chilling conditions that lead to a higher vulnerability to excess excitation energy. Moreover, the mesophyll impairments (e.g. A_max, F_v/F_m, ?_e) became significant even at moderately low temperatures of 15/10 °C, and a lower ability to recover after chilling exposure was observed. The limitation of in vivo rubisco activity and A_max may have been due to substrate limitation, but disturbances in sugar metabolism could also play an important role in the expression of chilling sensitivity in C. canephora cv. Apoatã and C. dewevrei.     

Plants under Climatic Stress: I. Low Temperature, High Light Effects on Photosynthesis

Photosynthetic rates of both C₄- and C₃-pathway plants grown at 25 C were measured before and during a period of chilling stress at 10 C, and then again at 25 C following various periods at 10 C. When temperatures are first lowered photosynthetic rates drop immediately, then undergo a further reduction which is quite rapid in species such as Sorghum, maize, and Pennisetum; slower in soybean; and very slow in Paspalum and ryegrass. Visible light causes progressive permanent damage to the photosynthetic capacity of leaves during this period of lowered photosynthesis. The extent of damage increases with light intensity and the length of time leaves are held at 10 C but varies greatly between species, being roughly correlated with the extent to which chilling initially and subsequently lowers photosynthesis. Three days of chilling (10 C) at 170 w·m⁻² reduces the photosynthetic capacity of youngest-mature Paspalum leaves only 30 to 40% while Sorghum leaves are essentially inoperative when returned to 25 C after the same stress. Root temperature has a substantial rapid effect on photosynthesis of soybean and little immediate effect on Sorghum. Photosynthesis of stress-intolerant species (Sorghum) is reduced only slightly more than that of semitolerant species (Paspalum) when temperatures are lowered at mid-photo-period, but to a far greater extent if temperatures are reduced at the commencement of a photoperiod.     

Changes in electron transport,superoxide dismutase and ascorbate peroxidase isoenzymes in chloroplasts and mitochondria of cucumber leaves as influenced by chilling

In order to clarify the relationship between chill-induced disturbance in photosynthetic, respiratory electron transport and the metabolism of reactive oxygen species (ROS), leaf gas exchange, chlorophyll fluorescence quenching, respiration, and activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX) were investigated in chloroplasts and mitochondria of cucumber (Cucumis sativus) leaves subjected to a chill (8 °C) for 4 d. Chilling decreased net photosynthetic rate (P _N) and quantum efficiency of photosystem 2 (Φ_PS2), but increased the ratio of Φ_PS2 to the quantum efficiency of CO₂ fixation (ΦCO₂) and non-photochemical quenching (NPQ) in cucumber leaves. While chilling inhibited the activity of cytochrome respiration pathway, it induced an increase of alternative respiration pathway activity and the reduction level of Q-pool. Chilling also significantly increased O₂ ^• production rate, H₂O₂ content, and SOD and APX activities in chloroplasts and mitochondria. There was a more significant increase in SOD and APX activities in chloroplasts than in mitochondria with the increase of membrane-bound Fe-SOD and tAPX in chloroplasts being more significant than other isoenzymes. Taken together, chilling inhibited P _N and cytochrome respiratory pathway but enhanced the photosynthetic electron flux to O₂ and over-reduction of respiratory electron transport chain, resulting in ROS accumulation in cucumber leaves. Meanwhile, chilling resulted in an enhancement of the protective mechanisms such as thermal dissipation, alternative respiratory pathway, and ROS-scavenging mechanisms (SODs and APXs) in chloroplasts and mitochondria.     

The relative impacts of daytime and night-time warming on photosynthetic capacity in Populus deltoides

In order to investigate the relative impacts of increases in day and night temperature on tree carbon relations, we measured night‐time respiration and daytime photosynthesis of leaves in canopies of 4‐m‐tall cottonwood (Populus deltoides Bartr. ex Marsh) trees experiencing three daytime temperatures (25, 28 or 31 °C) and either (i) a constant nocturnal temperature of 20 °C or (ii) increasing nocturnal temperatures (15, 20 or 25 °C). In the first (day warming only) experiment, rates of night‐time leaf dark respiration (R_dark) remained constant and leaves displayed a modest increase (11%) in light‐saturated photosynthetic capacity (A_max) during the day (1000–1300 h) over the 6 °C range. In the second (dual night and day warming) experiment, R_dark increased by 77% when nocturnal temperatures were increased from 15 °C (0·36 µmol m^?2 s^?1) to 25 °C (0·64 µmol m^?2 s^?1). A_max responded positively to the additional nocturnal warming, and increased by 38 and 64% in the 20/28 and 25/31 °C treatments, respectively, compared with the 15/25 °C treatment. These increases in photosynthetic capacity were associated with strong increases in the maximum carboxylation rate of rubisco (V_cmax) and ribulose‐1,5‐bisphosphate (RuBP) regeneration capacity mediated by maximum electron transport rate (J_max). Leaf soluble sugar and starch concentration, measured at sunrise, declined significantly as nocturnal temperature increased. The nocturnal temperature manipulation resulted in a significant inverse relationship between A_max and pre‐dawn leaf carbohydrate status. Independent measurements of the temperature response of photosynthesis indicated that the optimum temperature (T_opt) acclimated fully to the 6 °C range of temperature imposed in the daytime warming. Our findings are consistent with the hypothesis that elevated night‐time temperature increases photosynthetic capacity during the following light period through a respiratory‐driven reduction in leaf carbohydrate concentration. These responses indicate that predicted increases in night‐time minimum temperatures may have a significant influence on net plant carbon uptake.     

Siberian Miscanthus sacchariflorus accessions surpass the exceptional chilling tolerance of the most widely cultivated clone of Miscanthus x giganteus

Chilling temperatures (0–15°C) inhibit photosynthesis in most C₄ grasses, yet photosynthesis is chilling tolerant in the ‘Illinois’ clone of the C₄ grass Miscanthus x giganteus, a candidate cellulosic bioenergy crop. M. x giganteus is a hybrid between Miscanthus sacchariflorus and Miscanthus sinensis; therefore chilling‐tolerant parent lines might produce hybrids superior to the current clone. Recently a collection of M. sacchariflorus from Siberia, the apparent low temperature limit of natural distribution, became available, which may be a source for chilling tolerance. The collection was screened for chilling tolerance of photosynthesis by measuring dark‐adapted maximum quantum yield of PSII photochemistry (F_v/F_m) on plants in the field in cool weather. Superior accessions were selected for further phenotyping: plants were grown at 25°C, transferred to 10°C (chilling) for 15 days, and returned to 25°C for 7 days (recovery). Two experiments assessed: (a) light‐saturated net photosynthetic rate (A_sat) and operating quantum yield of PSII photochemistry (Φ_PSII), (b) response of net leaf CO₂ uptake (A) to intercellular [CO₂] (c_i). Three accessions showed superior chilling tolerance: RU2012‐069 and RU2012‐114 achieved A_sat up to double that of M. x giganteus prior to and during chilling, due to increased c_i ‐ saturated photosynthesis (V_max). RU2012‐069 and RU2012‐114 also maintained greater levels of Φ_PSII during chilling, indicating reduced photodamage. Additionally, accession RU2012‐112 maintained a stable A_sat throughout the 15‐day chilling period, while A_sat continuously declined in other accessions; this suggests RU2012‐112 could outperform others in lengthy chilling periods. Plants were returned to 25°C after the chilling period; M. x giganteus showed the weakest recovery after 1 day, but a strong recovery after 1 week. This study has therefore identified important genetic resources for the synthesis of improved lines of M. x giganteus, which could facilitate the displacement of fossil fuels by cellulosic bioenergy.     

Insensitivity of Water-Oxidation and Photosystem II Activity in Tomato to Chilling Temperatures

Chilling tomato plants (Lycopersicon esculentum Mill. cv. Rutgers and cv. Floramerica) in the dark resulted in a sizable inhibition in the rate of light- and CO₂-saturated photosynthesis. However, at low light intensity, the inhibition disappeared and the absolute quantum yield of CO₂ reduction was diminished only slightly. The quantum yield of photosystem II (PSII) electron flow was 18% lower when measured in chloroplasts isolated from chilled leaves than in chloroplasts isolated from unchilled leaves. Even though the maximum rate of PSII turnover in these chloroplasts was 12% lower subsequent to chilling, it was in all cases two or more times that required to support the light- and CO₂-saturated rate of photosynthesis measured in the attached leaf. The concentration of active PSII centers in chloroplasts isolated from leaves either before or after chilling was determined by measurement of the products of water oxidation from a series of saturating flashes short enough to turnover the electron transport carriers only a single time. There was no significant change in the concentration of active PSII centers due to dark chilling.

It was concluded that PSII activity and water oxidation capacity are not significantly impaired in tomato by chilling in the dark and therefore are not primary aspects of the inhibition of CO₂ reduction observed in attached leaves.