Grafting of <Emphasis Type="Italic">Cucumis sativus</Emphasis> onto <Emphasis Type="Italic">Cucurbita ficifolia</Emphasis> leads to improved plant growth,increased light utilization and reduced accumulation of reactive oxygen species in chilled plants

The effects of chilling at 14 and 7°C on plant growth, CO₂ assimilation, light allocation, photosynthetic electron flux and antioxidant metabolism were examined in cucumber (Cucumis sativus L. cv. Jinyan No. 4, CS) plants with figleaf gourd (Cucurbita ficifolia Bouché, CF) and cucumber as rootstocks, respectively. Growth inhibition by chilling at 7°C was characterized by irreversible inhibition of CO₂ assimilation in grafted plants with cucumber as rootstock and scion (CS/CS) but this effect was significantly alleviated by grafting onto CF roots (CS/CF). Chilled CS/CF plants exhibited a higher photosynthetic activity and lower proportion of energy dissipation than chilled CS/CS plants. Chilling resulted in a greater decrease in the electron flux in photosystem (PS) II (J _PSII) than the rate of energy dissipation either via light-dependent (J _NPQ) or via constitutive thermal dissipation and fluorescence (J _f,D) in CS/CS plants. In parallel with the reduction in J _PSII, electron flux to oxygenation (J _o) and carboxylation by Rubisco (J _c) all decreased significantly whilst alternative electron flux in PS II (J _a) increased, especially in CS/CS plants. Moreover, CS/CF plants exhibited higher activity of antioxidant enzymes, lower antioxidant content and less membrane peroxidation relative to CS/CS plants after chilling.     

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.     

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.     

Water-water cycle involved in dissipation of excess photon energy in phosphorus deficient rice leaves

The water-water cycle which may be helpful for dissipating the excitation pressure over electron transport chain and minimizing the risk of photoinhibition and photodamage was investigated in rice after 10-d P-deficient treatment. Net photosynthetic rate decreased under P-deficiency, thus the absorption of photon energy exceeded the energy required for CO₂ assimilation. A more sensitive response of effective quantum yield of photosystem 2 (Φ_PS2) to O₂ concentration was observed in plants that suffered P starvation, indicating that more electrons were transported to O₂ in the P-deficient leaves. The electron transport rate through photosystem 2 (PS 2) (J_f) was stable, and the fraction of electron transport rate required to sustain CO₂ assimilation and photorespiration (J_g/J_f) was significantly decreased accompanied by an increase in the alternative electron transport (J_a/J_f), indicating that a considerable electron amount had been transported to O₂ during the water-water cycle in the P-deficient leaves. However, the fraction of electron transport to photorespiration (J_o/J_f) was also increased in the P-deficient leaves and it was less sensitive than that of water-water cycle. Therefore, water-water cycle could serve as an efficient electron sink. The higher non-photochemical fluorescence quenching (q_N) in the P-deficient leaves depended on O₂ concentration, suggesting that the water-water cycle might also contribute to non-radiative energy dissipation. Hence, the enhanced activity of the water-water cycle is important for protecting photosynthetic apparatus under P-deficiency in rice.     

Compensatory acclimated mechanisms of photoprotection in a Xa mutant of Lycopersicon esculentum Mill.

To probe the role of xanthophylls in non-photochemical quenching (NPQ) and the compensatory acclimated photoprotection mechanisms, a tomato (Lycopersicon esculentum Mill. cv. Ailsa Craig) Xa mutant with deficit in lutein (L) and neoxanthin (N) contents was used. The Xa mutant showed lowered NPQ, an increased degree of de-epoxidation state [(A+Z)/(V+A+Z)], and decreases of photosystem 2 (PS2) antenna size. Although the Xa mutant had a CO₂ assimilation rate similar to that of Ailsa Craig, it exhibited a much larger stomatal conductance (g _s) than Ailsa Craig. Decreased electron flux in PS2 (J _PS2) for the Xa mutant was associated with electron flux for photorespiratory carbon oxidation (J _o) and alternative electron flux in PS2 (J _a) while electron flux for photosynthetic carbon reduction (J _c) was not different from Ailsa Craig. Moreover, the Xa mutant also exhibited higher activities of antioxidant enzymes, higher contents of ascorbate and glutathione, and lower contents of reactive oxygen species. Hence some compensatory acclimated mechanisms of photoprotection operated properly in the lack of NPQ and xanthophylls.     

A simple chlorophyll fluorescence parameter that correlates with the rate coefficient of photoinactivation of Photosystem II

A method of partitioning the energy in a mixed population of active and photoinactivated Photosystem II (PS II) complexes based on chlorophyll fluorescence measurements is presented. There are four energy fluxes, each with its quantum efficiency: a flux associated with photochemical electron flow in active PS II reaction centres (J_{PS II}), thermal dissipation in photoinactivated, non-functional PS IIs (J_NF), light-regulated thermal dissipation in active PS IIs (J_NPQ) and a combined flux of fluorescence and constitutive, light-independent thermal dissipation (J_f,D). The four quantum efficiencies add up to 1.0, without the need to introduce an ‘excess’ term E, which in other studies has been claimed to be linearly correlated with the rate coefficient of photoinactivation of PS II (k_pi). We examined the correlation of k_pi with various fluxes, and found that the combined flux (J_NPQ + J_f,D= J_pi) is as well correlated with k_pi as is E. This combined flux arises from F_s/F_m^′, the ratio of steady-state to maximum fluorescence during illumination, which represents the quantum efficiency of combined non-photochemical dissipation pathways in active PS IIs. Since F_s/F_m^′ or its equivalent, J_pi, is a likely source of events leading to photoinactivation of PS II, we conclude that F_s/F_m^′ is a simple predictor of k_pi.     

The regulation of component processes of photosynthesis in transgenic tobacco with decreased phosphoribulokinase activity

Tobacco plants (Nicotiana tabacum L.) transformed with an inverted cDNA encoding ribulose 5-phosphate kinase (phosphoribulokinase,PRK; EC 2.7.1.19) were employed to study the in vivo relationship between photosynthetic electron transport and the partitioning of electron transport products to major carbon metabolism sinks under conditions of elevated ATP concentrations and limited ribulose 1,5-bisphosphate (RuBP) regeneration. Simultaneous measurements of room temperature chlorophyll fluorescence and CO₂ gas exchange were conducted on intact leaves. Under ambient CO₂ concentrations and light intensities above those at which the plants were grown, transformants with only 5% of PRK activity showed down-regulation of PS II activity and electron transport in response to a decrease in net carbon assimilation when compared to wild-type. This was manifested as a decline in the efficiency of PS II electron transport (_{PS II}), an increase in dissipation of excess absorbed light in the antennae of PS II and a decline in: total linear electron transport (J₁), electron transport dedicated to carbon assimilation (J_A) and electron transport allocated to photorespiration (J_L). The transformants showed no alteration in the Rubisco specificity factor measured in vitro and calculated in vivo but had a relatively smaller ratio of RuBP oxygenation to carboxylation rates (v_o/v_c), due to a higher CO₂ concentration at the carboxylation site (C_c). The relationship between _{PS II} and _{CO 2}was similar in transformants and wild-type under photorespiratory conditions demonstrating no change in the intrinsic relationship between PS II function and carbon assimilation, however, a novel result of this study is that this similar relationship occurred at different values of quantum flux, J₁, J_A, J_L and v_o/v_c in the transformant. For both wild-type and transformants, an assessment was made of the possible presence of a third major sink for electron transport products, beside RuBP oxygenation and carboxylation, the data provided no evidence for such a sink.Abbreviations C_c CO₂ concentration at the site of carboxylation - C_i intercellular CO₂ concentration - g_m mesophyll conductance to CO₂ - J₁ total linear electron flow - J_A linear electron flow allocated to CO₂ assimilation - J_c linear electron flow supporting carbon reduction and oxidation cycles - J_L linear electron flow allocated to photorespiration (RuBP oxygenation and fixation of released photorespiratory CO₂) - PRK phosphoribulokinase - q_P, q_N coefficients for photochemical and non-photochemical quenching of fluorescence respectively - Rubisco ribulose 1,5-bisphosphate carboxylase-oxygenase - S Rubisco specificity to CO₂/O₂ - v_c, v_o rates of RuBP carboxylation and RuBP oxygenation, respectively - _{CO 2} relative quantum yield of CO₂ assimilation - _C maximum _{CO 2} under non-photorespiratory conditions - _exc the efficiency of excitation capture by open PS II centres - _{PS II} relative quantum yield of PS II electron transport     

Chilling Sensitivity in Oryza sativa: The Role of Protein Phosphorylation in Protection against Photoinhibition

The effects of exposure to low temperature on photosynthesis and protein phosphorylation in chilling-sensitive and cold-tolerant plant species were compared. Chilling temperatures resulted in light-dependent loss of photosynthetic electron transport in chilling-sensitive rice (Oryza sativa L.) but not in cold-tolerant barley (Hordeum vulgare L.). Brief exposure to chilling temperatures (0-15°C, 10 min) did not cause a significant difference in photosynthetic O₂ evolution capacity in vivo between rice and barley. Analysis of in vivo chlorophyll fluorescence in chilling-sensitive rice suggests that low temperatures cause an increased reduction of the plastoquinone pool that could result in photoinhibitory damage to the photosystem II reaction centers. Analysis of ³²P incorporation into thylakoid proteins both in vivo and in vitro demonstrated that chilling temperature inhibited protein phosphorylation in rice, but not in barley. Low temperature (77 K) fluorescence analysis of isolated thylakoid membranes indicated that state I to state II transitions occurred in barley, but not in rice subjected to chilling temperatures. These observations suggest that protein phosphorylation may play an important role in protection against photoinhibition caused by exposure to chilling temperatures.     

Diurnal changes in photochemical efficiency,the reduction state of Q,radiationless energy dissipation,and non-photochemical fluorescence quenching in cacti exposed to natural sunlight in northern Venezuela

Summary Diurnal measurements of chlorophyll a fluorescence from cacti (Nopalea cochenillifera, Opuntia ficus-indica, and Opuntia wentiana) growing in northern Venezuela were used to determine photochemical fluorescence quenching related to the reduction state of the primary electron acceptor of PS II as well as non-photochemical fluorescence quenching which reflects the fraction of energy going primarily into radiationless deexcitation. The cladodes used in this study were oriented such that one surface received direct sunlight in the morning and the other one during the afternoon. Both surfaces exhibited large increases in radiationless energy dissipation from the photochemical system accompanied by decreases in PS II photochemical efficiency during direct exposure to natural sunlight. During exposure to sunlight in the morning, dissipation of absorbed light energy through photosynthesis and radiationless energy dissipation was sufficient to maintain Q, the primary electron acceptor for PS II, in a low reduction state. During exposure to sunlight in the afternoon, however, the reduction state of Q rose to levels greater than 50%, presumably due to a decrease in photosynthetic electron transport as the decarboxylation of the nocturnally accumulated malic acid was completed. Exposure to direct sunlight in the afternoon also led to more sustained increases in radiationless energy dissipation. Furthermore, the increases in radiationless energy dissipation during exposure of a water-stressed cladode of O. wentiana to direct sunlight were much greater than those from other well-watered cacti, presumably due to sustained stomatal closure and decreased rates of photosynthetic electron transport. These results indicate that the radiationless dissipation of absorbed light is an important process in these CAM plants under natural conditions, and may reflect a protective mechanism against the potentially damaging effects of the accumulation of excessive energy, particularly under conditions where CO₂ availability is restricted.Abbreviations CAM crassulacean acid metabolism - F _o instantaneous fluorescence emission - F _M maximum fluorescence emission - F _v variable fluorescence emission - K _D rate constant for radiationless energy dissipation in the antenna chlorophyll - PFD photon flux density - PS I photosystem I - PS II photosystem II - Q primary electron acceptor of photosystem II - q _NP non-photochemical fluorescence quenching - q _P photochemical fluorescence quenching - T _C cladode temperature     

Water stress induces different levels of photosynthesis and electron transport rate regulation in grapevines

A, net CO₂ assimilation rate
E, leaf transpiration
ETR, electron transport rate
F_s, fluorescence yield at steady state
F_m and F_m', maximal fluorescence levels when all PSII reaction centres are closed in dark- and light-acclimated leaves, respectively
F_o and F_o', initial fluorescence levels when all PSII reaction centres are closed in dark- and light-acclimated leaves, respectively
F_v/F_m, efficiency of excitation capture by open PSII in dark-adapted leaves
ΔF/F_m', actual photochemical efficiency of PSII
g, stomatal conductance
NPQ, non-photochemical quenching of chlorophyll fluorescence
PPFD, photosynthetic photon flux density
Ψ_PD and Ψ_MD, leaf water potential at pre-dawn and midday, respectively
Rl, estimated photorespiration rate
I₁ and I₂, Irrigation treatments
R, Recovery treatment
D₁ and D₂, drought treatments
HD₁ and HD₂, hard drought treatments

Diurnal time courses of chlorophyll fluorescence and gas-exchange rates were measured in young potted grapevines (Vitis vinifera L. cv. Tempranillo) subjected to different conditions of water supply under Mediterranean summer conditions. The irrigated plants exhibited typical diurnal patterns for all measured parameters, showing a correspondence between electron transport rate, net CO₂ assimilation and stomatal conductance. Mild decreases in soil-water availability led to different degrees of down-regulation of photosynthesis and increased nonphotochemical quenching of chlorophyll fluorescence. A good correspondence between electron transport rate and CO₂ assimilation was still maintained, suggesting a coregulation of both photosynthetic processes. In contrast, a severe water deficit induced a drastic down-regulation of photosynthesis and breakage of the above-mentioned link. Both midday net CO₂ assimilation and electron transport rate significantly correlated with pre-dawn water potential (Ψ_PD) (r² = 0·65 and r² = 0·92, P < 0·001, respectively). However, when field data were analysed, the relationship between electron transport rate and Ψ_PD was not maintained, although net CO₂ assimilation was similarly correlated with Ψ_PD. Interestingly, the steady-state chlorophyll fluorescence yield was a good indicator of plant water stress.     

Low temperature stress modifies the photochemical efficiency of a tropical tree species <Emphasis Type="Italic">Hevea brasiliensis</Emphasis>: effects of varying concentration of CO<Subscript>2</Subscript> and photon flux density

Two clones of Hevea brasiliensis (RRII 105 and PB 235) were grown for one year in two distinct agroclimatic locations (warmer and colder, W and C) in peninsular India. We simultaneously measured gas exchange and chlorophyll (Chl) fluorescence on fully mature intact leaves at different photosynthetic photon flux densities (PPFDs) and ambient CO₂ concentrations (C _a) and at constant ambient O₂ concentration (21 %). Net photosynthetic rate (P _N), apparent quantum yield for CO₂ assimilation (Φ_c), in vivo carboxylation efficiency (CE), and photosystem 2 quantum yield (Φ_PS2) were low in plants grown in C climate and these reductions were more predominant in RRII 105 than in PB 235 which was also reflected in their growth. We estimated in these clones the partitioning of photosynthetic electrons between CO₂ reduction (J_A) and processes other than CO₂ reduction (J^*) at low and high PPFDs and C _a. At high C _a (700 µmol mol⁻¹) most of the photosynthetic electrons were used for CO₂ assimilation and negligible amount went for other processes when PPFD was low (200–300 µmol m⁻² s⁻¹) both in the C and W climates. But at high PPFD (900-1 100 µmol m⁻² s⁻¹), J^* was appreciably high even at a high C _a. Hence at normal ambient C _a and high irradiance, electrons can be generated in the photosynthetic apparatus far in excess of what can be safely utilised for photosynthetic CO₂ reduction. However, at high C _a there was increased diversion of electrons to photosynthetic CO₂ reduction which resulted in improved photosynthetic parameters even in plants grown in C climate.     

Photosynthesis and non-photochemical excitation quenching components of chlorophyll excitation in maize and field bean during chilling at different photon flux density

The influence of chilling (8 °C, 5 d) at two photon flux densities [PFD, L = 200 and H = 500 μmol(photon) m⁻² s⁻¹] on the gas exchange and chlorophyll fluorescence was investigated in chilling-tolerant and chilling-sensitive maize hybrids (Zea mays L., K383×K130, K185×K217) and one cultivar of field bean (Vicia faba L. minor, cv. Nadwiślański). The net photosynthetic rate (P _N) for the both studied plant species was inhibited at 8 °C. P _N of both maize hybrids additionally decreased during chilling. Changes in the quantum efficiency of PS2 electron transport (Φ_PS2) as a response to chilling and PFD were similar to P _N. Measurements of Φ_PS2/Φ_CO2 ratio showed that in field bean seedlings strong alternative photochemical sinks of energy did not appear during chilling. However, the high increment in Φ_PS2/Φ_CO2 for maize hybrids can indicate reactions associated with chill damage generation. At 8 °C the non-photochemical quenching (NPQ) increased in all plants with chilling duration and PFD. The appearance of protective (q_I,p) and damage (q_I,d) components of q_I and a decrease in q_E (energy dependent quenching) took place. NPQ components of field bean and maize hybrids differed from each other. The amount of protective NPQ (q_E + q_I,p) components as part of total NPQ was higher in field bean than in maize hybrids at both PFD. On 5^th day of chilling, the sum of q_E and q_I,p was 26.7 % of NPQ in tolerant maize hybrids and 17.6 % of NPQ in the sensitive one (averages for both PFD). The increased PFD inhibited the ability of all plants to perform protective dissipation of absorbed energy. The understanding of the genotypic variation of NPQ components in maize may have implications for the future selection of plants with a high chilling tolerance.     

In situ estimation of net CO2 assimilation, photosynthetic electron flow and photorespiration in Turkey oak (Q. cerris L.) leaves: diurnal cycles under different levels of water supply

Diurnal time courses of net CO₂ assimilation rates, stomatal conductance and light-driven electron fluxes were measured in situ on attached leaves of 30-year-old Turkey oak trees (Quercus cerris L.) under natural summer conditions in central Italy. Combined measurements of gas exchange and chlorophyll a fluorescence under low O₂ concentrations allowed the demonstration of a linear relationship between the photochemical efficiency of PSII (fluorescence measurements) and the apparent quantum yield of gross photosynthesis (gas exchange). This relationship was used under normal O₂ to compute total light-driven electron fluxes, and to partition them into fractions used for RuBP carboxylation or RuBP oxygenation. This procedure also yielded an indirect estimate of the rate of photorespiration in vivo. The time courses of light-driven electron flow, net CO₂ assimilation and photorespiration paralleled that of photosynthetic photon flux density, with important afternoon deviations as soon as a severe drought stress occurred, whereas photochemical efficiency and maximal fluorescence underwent large but reversible diurnal decreases. The latter observation indicated the occurrence of a large non-photochemical energy dissipation at PSII. We estimated that less than 60% of the total photosynthetic electron flow was used for carbon assimilation at midday, while about 40% was devoted to photorespiration. The rate of carbon loss by photorespiration (R₁) reached mean levels of 56% of net assimilation rates. The potential application of this technique to analysis of the relative contributions of thermal de-excitation at PSII and photorespiratory carbon recycling in the protection of photosynthesis against stress effects is discussed.     

Leaf photosynthetic parameters of two maize (Zea mays) cultivars in response to various patterns of chilling temperatures and photon flux densities

Chilling‐induced photosynthetic impairment was examined in leaves of maize (Zea mays L.) seedlings of two cultivars, one adapted to western Europe and one adapted to Mexican highlands. Three experiments were performed in a controlled environment. The effects of chilling night temperatures, of chilling at high light intensity and of variable chilling day temperatures on photosynthetic parameters, were evaluated. Chilling in the dark period resulted in stomatal limitation of net photosynthesis. Chilling at moderate to high light intensities caused chilling‐dependent photoinhibition of CO₂ uptake. Photobleached maize leaves did not resume normal photosynthetic function. Maize cv. Batan 8686 from the highlands of Mexico was less susceptible to photosynthetic damage than maize cv. Bastion adapted for cultivation in W. Europe, when exposed to chilling night temperatures, or to mild chilling photoinhibitory conditions.     

光强在低温弱光胁迫后番茄叶片光合作用恢复中的作用

为了研究光强在低温弱光胁迫后番茄叶片光合作用恢复中的作用,以番茄品种浙粉202为材料,研究了低温弱光后恢复期全光照与遮荫对光合作用和叶绿素荧光参数的影响。结果表明：低温弱光（8℃/12℃,PFD 80 μmol·m^-2·s^-1）导致番茄叶片P_n、Φ_PSⅡ、qP和F_v′/F_m′的下降,但诱导了NPQ的上升,未引起F_v/F_m的变化;全光照（100%光照）下恢复1 使得植株叶片P_n、F_v/F_m、Φ_PSⅡ、qP、NPQ和F_v′/F_m′均大幅下降,随后光合和荧光参数可缓慢恢复至对照水平;遮荫（40%光照）恢复植株F_v/F_m、Φ_PSⅡ和F_v′/F_m′仅在第一天稍有下降,而P_n和qP还略有上升,NPQ虽有所降低但仍显著高于对照水平,随后光合和荧光参数均可迅速恢复到对照水平。说明低温弱光虽抑制了光合作用的进行,但并未引起光抑制的发生;全光照恢复加剧了叶片光抑制的发生,而遮荫恢复可通过叶片PSⅡ光化学活性的快速恢复和天线色素热耗散能力的增强以保护光合机构免受伤害,有利于光合作用的迅速恢复。     

A biochemical model of photosynthetic CO2 assimilation in leaves of C3 species

Various aspects of the biochemistry of photosynthetic carbon assimilation in C₃ plants are integrated into a form compatible with studies of gas exchange in leaves. These aspects include the kinetic properties of ribulose bisphosphate carboxylase-oxygenase; the requirements of the photosynthetic carbon reduction and photorespiratory carbon oxidation cycles for reduced pyridine nucleotides; the dependence of electron transport on photon flux and the presence of a temperature dependent upper limit to electron transport. The measurements of gas exchange with which the model outputs may be compared include those of the temperature and partial pressure of CO₂(p(CO₂)) dependencies of quantum yield, the variation of compensation point with temperature and partial pressure of O₂(p(O₂)), the dependence of net CO₂ assimilation rate on p(CO₂) and irradiance, and the influence of p(CO₂) and irradiance on the temperature dependence of assimilation rate.Abbreviations RuP₂ ribulose bisphosphate - PGA 3-phosphoglycerate - C=p(CO₂) partial pressure of CO₂ - O=p(O₂) partial pressure of O₂ - PCR photosynthetic carbon reduction - PCO photorespiratory carbon oxidation     

Factors Limiting Photosynthetic Recovery in Sweet Pepper Leaves After Short-Term Chilling Stress Under Low Irradiance

The effects of chilling treatment (4 °C) under low irradiance, LI (100 mol m^–2 s^–1) and in the dark on subsequent recovery of photosynthesis in chilling-sensitive sweet pepper leaves were investigated by comparing the ratio of quantum yields of photosystem (PS) 2 and CO₂ assimilation, _PS2/_CO2, measured in normal air (21 % O₂, NA) and low O₂-air (2% O₂, LOA), and by analyzing chlorophyll (Chl) a fluorescence parameters. Chilling treatment in the dark had little effect on F_v/F_m and _PS2/_CO2, but it caused the decrease of net photosynthetic rate (P _N) under saturating irradiance after 6-h chilling treatment, indicating that short-term chilling alone did not induce PS2 photoinhibition. Furthermore, photorespiration and Mehler reaction also did not obviously change during subsequent recovery after chilling stress in the dark. During chilling treatment under LI, there were obvious changes in F_v/F_m and _PS2/_CO2, determined in NA or LOA. F_v/F_m could recover fully in 4 h at 25 °C, and _PS2/_CO2 increased at the end of the treatment, as determined in both NA and LOA. During subsequent recovery, _PS2/_CO2 in LOA decreased faster than in NA. Thus the Mehler reaction might play an important role during chilling treatment under LI, and photorespiration was an important process during the subsequent recovery. The recovery of PN under saturating irradiance determined in NA and LOA took about 50 h, implying that there were some factors besides CO₂ assimilation limiting the recovery of photosynthesis. From the progress of reduced P700 and the increase of the Mehler reaction during chilling under LI we propose that active oxygen species were the factors inducing PS1 photoinhibition, which prevented the recovery of photosynthesis in optimal conditions because of the slow recovery of the oxidizable P700.     

Chilling-Induced Photoinhibition in Two Oak Species: Are Evergreen Leaves Inherently Better Protected Than Deciduous Leaves?

We compared the sensitivity to cold stress, in terms of photosynthetic capacity and changes in chlorophyll fluorescence of photosystem 2 (PS2), of an evergreen and a deciduous oak species, which co-occur in the southeastern United States. We predicted that the evergreen species, Quercus virginiana, which must endure winter, is likely to have an inherently greater capacity for energy dissipation and to be less susceptible to chilling stress than the deciduous species, Quercus michauxii. Short-term cold stress in both species lead to greater than 50 % reduction in maximum photosynthetic rates, 60-70 % reduction in electron transport, and irreversible quenching of PS2 fluorescence. The kinetics of recovery in the dark after exposure to 1 h high irradiance (1000 μmol m^-2 s^-1) and chilling (5 °C) showed that the evergreen Q. virginiana exhibited more protective q_E and less irreversible quenching (q_I) than the deciduous Q. michauxii. The large q_E which we observed in Q. virginiana suggests that the capacity for photoprotection at low temperatures is not induced by a long-term acclimation to cold but preexists in evergreen leaves. This capacity may contribute to the ability of this species to maintain leaves during the winter. This revised version was published online in June 2006 with corrections to the Cover Date.