Hexanoic acid 2-(diethylamino)ethyl ester enhances chilling tolerance in strawberry seedlings by impact on photosynthesis and antioxidants

Strawberry (Fragaria ananassa Duch.) seedlings were pretreated with hexanoic acid 2-(diethylamino)ethyl ester (DA-6) in concentrations of 0, 10, 20 and 40 mg dm⁻³ and then subjected to chilling and rewarming. The effects of applied DA-6 on the generation of reactive oxygen species (O₂ ⁻, H₂O₂), lipid peroxidation, proline accumulation and photosynthesis were evaluated. Pretreatment with DA-6 alleviated the inhibition of superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) activities caused by chilling stress thus reducing O₂ ⁻ and H₂O₂ production and lipid peroxidation in pretreated plants. DA-6 pretreatment also accelerated accumulation of proline and reduce the decrease in proline content after rewarming. DA-6 pretreatment increases maximum quantum yield of photosystem 2 (F_v/F_m), actual photochemical efficiency of photosystem 2 (Φ_PS2), photochemical quenching coefficient (qP) and net photosynthetic rate (P_N) and decreases non-photochemical quenching coefficient (qNP) of the seedlings under chilling stress. DA-6 pretreatment also increased the recovery rate of photosynthesis after rewarming.     

Up-regulation of cyclic electron flow and down-regulation of linear electron flow in antisense-<Emphasis Type="Italic">rca</Emphasis> mutant rice

To investigate how excess excitation energy is dissipated in a ribulose-1,5-bisphospate carboxylase/oxygenase activase antisense transgenic rice with net photosynthetic rate (P _N) half of that of wild type parent, we measured the response curve of P _N to intercellular CO₂ concentration (C _i), electron transport rate (ETR), quantum yield of open photosystem 2 (PS2) reaction centres under irradiation (F_v′/F_m′), efficiency of total PS2 centres (Φ_PS2), photochemical (q_P) and non-photochemical quenching (NPQ), post-irradiation transient increase in chlorophyll (Chl) fluorescence (PITICF), and P700⁺ re-reduction. Carboxylation efficiency dependence on C _i, ETR at saturation irradiance, and F_v′/F_m′, Φ_PS2, and q_P under the irradiation were significantly lower in the mutant. However, NPQ, energy-dependent quenching (q_E), PITICF, and P700⁺ re-reduction were significantly higher in the mutant. Hence the mutant down-regulates linear ETR and stimulates cyclic electron flow around PS1, which may generate the ΔpH to support NPQ and q_E for dissipation of excess excitation energy.     

Photosynthetic performance and light response of two olive cultivars under different water and light regimes

The olive tree (Olea europaea L.) is commonly grown in the Mediterranean area, where it is adapted to resist periods characterized by severe drought and high irradiance levels. Photosynthetic efficiency (in terms of F_v/F_m and Φ_PSII), photochemical (q_P) and nonphotochemical quenching (NPQ) were determined in two-year-old olive plants (cultivars Coratina and Biancolilla) grown under two different light levels (exposed plants, EP, and shaded plants, SP) during a 21-day controlled water deficit. After reaching the maximum level of drought stress, plants were rewatered for 23 days. During the experimental period, measurements of gas exchange and chlorophyll (Chl) fluorescence were carried out to study the photosynthetic performance of olive plants. The synergical effect of drought stress and high irradiance levels caused a reduction of gas exchange and photosynthetic efficiency and these decreases were more marked in EP. EP showed a higher degree of photoinhibition, a higher NPQ and a lower q_P if compared to SP. Coratina was more sensitive to high light and drought stress but also showed a slower recovery during rewatering, whereas Biancolilla showed a less marked photosynthesis depression during drought and a considerable resilience during rewatering. The results confirm that photoinhibition due to high light intensity and water deficit can be an important factor that affects photosynthetic productivity in this species.     

棒叶落地生根对干旱与复水的生理响应

为探讨棒叶落地生根(Kalanchoe tubiflora)耐旱的机制,在干旱与复水条件下,对其叶片的一些生理生化指标进行了测定。结果表明,随干旱时间延长,棒叶落地生根叶片中O2-·生成速率增大,H2O2含量升高,导致脂质过氧化产物MDA含量增高;同时SOD活性升高,CAT活性降低;可溶性糖与脯氨酸含量增加,但复水后这些指标均恢复到干旱前的水平。这说明棒叶落地生根能够耐受干旱环境是通过积累渗透调节物质,提高活性氧的清除能力,从而减少氧化胁迫造成的伤害。     

Photosystem 2 photochemistry and pigment composition of a yellow mutant of rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) under different irradiances

A yellow leaf colouration mutant (named ycm) generated from rice T-DNA insertion lines was identified with less grana lamellae and low thylakoid membrane protein contents. At weak irradiance [50 μmol(photon) m⁻² s⁻¹], chlorophyll (Chl) contents of ycm were ≈20 % of those of WT and Chl a/b ratios were 3-fold that of wild type (WT). The leaf of ycm showed lower values in the actual photosystem 2 (PS2) efficiency (Φ_PS2), photochemical quenching (q_P), and the efficiency of excitation capture by open PS2 centres 1 (F_v′/F_m′) than those of WT, except no difference in the maximal efficiency of PS2 photochemistry (F_v/F_m). With progress in irradiance [100 and 200 μmol(photon) m⁻² s⁻¹], there was a change in the photosynthetic pigment stoichiometry. In ycm, the increase of total Chl contents and the decrease in Chl a/b ratio were observed. Φ_PS2, q_P, and F_v′/F_m′ of ycm increased gradually along with the increase of irradiance but still much less than in WT. The increase of xanthophyll ratio [(Z+A)/(V+A+Z)] associated with non-photochemical quenching (q_N) was found in ycm which suggested that ycm dissipated excess energy through the turnover of xanthophylls. No significant differences in pigment composition were observed in WT under various irradiances, except Chl a/b ratio that gradually decreased. Hence the ycm mutant developed much more tardily than WT, which was caused by low photon energy utilization independent of irradiance.     

Role of calcium ion in protection against heat and high irradiance stress-induced oxidative damage to photosynthesis of wheat leaves

Cross stress of heat and high irradiance (HI) resulted in the accumulation of active oxygen species and photo-oxidative damage to photosynthetic apparatus of wheat leaves during grain development. Pre-treatment with calcium ion protected the photosynthetic system from oxidative damage by reducing O^-. ₂ production, inhibiting lipid peroxidation, and retarding electrolyte leakage from cell. Therefore, high F_v/F_m [maximal photochemical efficiency of photosystem 2 (PS2) while all PS2 reaction centres are open], F_m/F₀ (another expression for the maximal photochemical efficiency of PS2), Φ_PS2 (actual quantum yield of PS2 under actinic irradiation), q_P (photochemical quenching coefficient), and P _N (net photosynthetic rate) were maintained, and lower q_NP (non-photochemical quenching coefficient) of the leaves was kept under heat and HI stress. EGTA (a chelant of calcium ion) and LaCl₃ (a blocker of Ca²⁺ channel in cytoplasmic membrane) had the opposite effect. Thus Ca ion may help protect the photosynthetic system of wheat leaves from oxidative damage induced by the cross stress of heat and HI.     

Effects of the interaction between ozone and carbon dioxide on gas exchange,photosystem II and antioxidants in rice leaves

To understand the interactive effects of O₃ and CO₂ on rice leaves; gas exchange, chlorophyll (Chl) fluorescence, ascorbic acid and glutathione were examined under acute (5 h), combined exposures of O₃ (0, 0.1, or 0.3 cm³ m⁻³, expressed as O₀, O_0.1, or O_0.3, respectively), and CO₂ (400 or 800 cm³ m⁻³, expressed as C⁴⁰⁰ or C⁸⁰⁰, respectively) in natural-light gas-exposure chambers. The net photosynthetic rate (P _N), maximum (F_v/F_m) and operating (F_q′/F_m′) quantum efficiencies of photosystem II (PSII) in young (8^th) leaves decreased during O₃ exposure. However, these were ameliorated by C⁸⁰⁰ and fully recovered within 3 d in clean air (O⁰ + C⁴⁰⁰) except for the O^0.3 + C⁴⁰⁰ plants. The maximum PSII efficiency at 1,500 μmol m⁻² s⁻¹ PPFD (F_v′/F_m′) for the O^0.3 + C₄₀₀ plants decreased for all measurement times, likely because leaves with severely inhibited P _N also had a severely damaged PSII. The P _N of the flag (16^th) leaves at heading decreased under O₃ exposure, but the decline was smaller and the recovery was faster than that of the 8^th leaves. The F_q′/F_m′ of the flag leaves in the O^0.3 + C⁴⁰⁰ and O^0.3 + C⁸⁰⁰ plants decreased just after gas exposure, but the F_v/F_m was not affected. These effects indicate that elevated CO₂ interactively ameliorated the inhibition of photosynthesis induced by O₃ exposure. However, changes in antioxidant levels did not explain the above interaction.     

Effects of different irradiances on the photosynthetic process during ex-vitro acclimation of Anoectochilus plantlets

Six months old in vitro-grown Anoectochilus formosanus plantlets were transferred to ex-vitro acclimation under low irradiance, LI [60 μmol(photon) m⁻² s⁻¹], intermediate irradiance, II [180 μmol(photon) m⁻² s⁻¹], and high irradiance, HI [300 μmol(photon) m⁻² s⁻¹] for 30 d. Imposition of II led to a significant increase of chlorophyll (Chl) b content, rates of net photosynthesis (P _N) and transpiration (E), stomatal conductance (g _s), electron transfer rate (ETR), quantum yield of electron transport from water through photosystem 2 (Φ_PS2), and activity of ribulose-1,5-bisphosphate carboxylase/ oxygenase (RuBPCO, EC 4.1.1.39). This indicates that Anoectochilus was better acclimated at II compared to LI treatment. On the other hand, HI acclimation led to a significant reduction of Chl a and b, P _N, E, g _s, photochemical quenching, dark-adapted quantum efficiency of open PS2 centres (F_v/F_m), probability of an absorbed photon reaching an open PS2 reaction centre (F_v′/F_m′), ETR, Φ_PS2, and energy efficiency of CO₂ fixation (Φ_CO2/Φ_PS2). This indicates that HI treatment considerably exceeded the photo-protective capacity and Anoectochilus suffered HI induced damage to the photosynthetic apparatus. Imposition of HI significantly increased the contents of antheraxanthin and zeaxanthin (ZEA), non-photochemical quenching, and conversion of violaxanthin to ZEA. Thus Anoectochilus modifies its system to dissipate excess excitation energy and to protect the photosynthetic machinery.     

Photosystem 2 activity of <Emphasis Type="Italic">Citrus volkameriana</Emphasis> (L.) leaves as affected by Mn nutrition and irradiance

Citrus volkameriana (L.) plants were grown for 43 d in nutrient solutions containing 0, 2, 14, 98, or 686 μM Mn (Mn₀, Mn₂, Mn₁₄, Mn₉₈, and Mn₆₈₆, respectively). To adequately investigate the combined effects of Mn nutrition and irradiance on photosystem 2 (PS2) activity, irradiance response curves for electron transport rate (ETR), nonphotochemical quenching (q_N), photochemical quenching (q_P), and real photochemical efficiency of PS2 (Φ_PS2) were recorded under 10 different irradiances (66, 96, 136, 226, 336, 536, 811, 1 211, 1 911, and 3 111 μmol m⁻² s⁻¹, I₆₆ to I₃₁₁₁, respectively) generated with the PAM-2000 fluorometer. Leaf chlorophyll content was significantly lower under Mn excess (Mn₆₈₆) compared to Mn₀; its highest values were recorded in the treatments Mn₂-Mn₉₈. However, ETR and Φ_PS2 values were significantly lower under Mn₀ compared to the other Mn treatments, when plants were exposed to irradiances ≥96 μmol m⁻² s⁻¹. Furthermore, Mn₀ plants had significantly higher values of q_N and lower values of q_P at irradiances ≤226 and ≥336 μmol m⁻² s⁻¹, respectively, than those grown under Mn₂-Mn₆₈₆. Irrespective of Mn treatment, the values of Φ_PS2 and q_N decreased, while those of q_P increased progressively by increasing irradiance from I₁₃₆ to I₃₁₁₁. Finally, Mn₂-Mn₉₈ plants were less sensitive to photoinhibition of photosynthesis (≥811 μmol m⁻² s⁻¹) than the Mn₆₈₆ (≥536 μmol m⁻² s⁻¹) and Mn₀ (≥336 μmol m⁻² s⁻¹) ones.     

Chill-induced inhibition of photosynthesis was alleviated by 24-epibrassinolide pretreatment in cucumber during chilling and subsequent recovery

To investigate whether brassinosteroids (BRs) could be used to alleviate chill-induced inhibition of photosynthesis in cucumber (Cucumis sativus L) during chilling and subsequent recovery, the effects of exogenously applied 24-epibrassinolide (EBR) on gas exchange, chlorophyll fluorescence parameters, and antioxidant enzyme activity were studied. Cucumber plants were exposed to chilling under low light (12/8°C and 100 μmol m⁻² s⁻¹ PPFD) for 3 days and then recovered under normal temperature and high irradiance (28/18°C and 600 μmol m⁻² s⁻¹ PPFD) for 6 days. Chilling significantly decreased the net photosynthetic rate (P _N) and stomatal conductance (g _s), and increased rate of O₂ ^·− formation and H₂O₂ and malondialdehyde (MDA) content in cucumber leaves, but did not influence the optimal quantum yield of PSII (F_v/F_m). Chilling also decreased the effective quantum yield of PSII photochemistry (Φ_PSII) and photochemical quenching (q_P), but induced an increase in nonphotochemical quenching (NPQ), and the activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX). High irradiance (600 μmol m⁻² s⁻¹) further aggravated the decrease in P _N, g _s, Φ_PSII and q_P, and enhanced the increase in reactive oxygen species (ROS) generation and accumulation in the first day of recovery after chilling. However, high irradiance induced a sharp decrease in F_v/F_m and NPQ, as well as the activities of SOD and APX on the first day of recovery. EBR pretreatment significantly alleviated chill-induced inhibition of photosynthesis during chilling stress and subsequent recovery period, which was mainly due to significant increases in g _s, Φ_PSII, q_P and NPQ. EBR pretreatment also reduced ROS generation and accumulation, and increased the activities of SOD and APX during chilling and subsequent recovery. Those results suggest that EBR pretreatment alleviates the chill reduction in photosynthesis and accelerated the recovery rate mainly by increasing of the stomatal conductance, the efficiency of utilization and dissipation of leaf absorbed light, and the activity of the ROS scavenging system during chilling and subsequent recovery period.     

Chlorophyll fluorescence as a tool for evaluation of drought stress in strawberry

The effect of water deficit on chlorophyll fluorescence, sugar content, and growth parameters of strawberry (Fragaria×ananassa Duch. cv. Elsanta) was studied. Drought stress caused significant reductions in leaf water potential, fresh and dry masses, leaf area, and leaf number. A gradual reduction of photochemical quenching (q_P) and quantum efficiency (Φ_PS2) was observed under drought stress while non-photochemical quenching (q_N) increased. Maximum efficiency of photosystem 2 (F_v/F_m) was not affected by drought stress.     

Effect of 24-epibrassinolide on drought stress-induced changes in Chorispora bungeana

Brassinosteroids (BRs) have been proposed to increase the resistance of plants to drought stress. The effect of foliar application of 0.1 μM 24-epibrassinolide (EBR) on chlorophyll (Chl) content, photosystem 2 (PS 2) photochemistry, membrane permeability, lipid peroxidation, relative water content (RWC), proline content, and the antioxidant system in drought-stressed Chorispora bungeana plants was investigated. The results showed that polyethylene glycol (PEG) induced water stress decreased RWC, Chl content and variable to maximum Chl fluorescence ratio (F_v/F_m) less in plants pretreated with EBR than in non-pretreated plants. In addition, lipid peroxidation, measured in terms of malondialdehyde content, membrane permeability and proline content in drought-stressed plants were less increased in EBR pretreated plants, while antioxidative enzyme activities and reduced ascorbate and glutathione contents were more increased in EBR pretreated than in non-pretreated plants. These results suggested that EBR could improve plant growth under drought stress     

Influence of enhanced temperature on photosynthesis,photooxidative damage,and antioxidant strategies in <Emphasis Type="Italic">Ceratonia siliqua</Emphasis> L. seedlings subjected to water deficit and rewatering

Predicted future climatic changes for the Mediterranean region give additional importance to the study of photooxidative stress in local economic species subjected to combined drought and high-temperature conditions. Under this context, the impact of these stresses on photosynthesis, energy partitioning, and membrane lipids, as well as the potential ability to attenuate oxidative damage, were investigated in Ceratonia siliqua L. Two thermal regimes (LT: 25/18°C; HT: 32/21°C) and three soil water conditions (control, water stress, and rewetting) were considered. HT exacerbated the adverse effects of water shortage on photosynthetic rates (P _N) and PSII function. The decrease in P _N was 33% at LT whereas at HT it was 84%. In spite of this, the electron transport rate (ETR) was not affected, which points to an increased allocation of reductants to sinks other than CO₂ assimilation. Under LT conditions, water stress had no significant effects on yield of PSII photochemistry (Φ_PSII) and yields of regulated (Φ_NPQ) and nonregulated (Φ_NO) energy dissipation. Conversely, drought induced a significant decrease of Φ_PSII and a concomitant increase of Φ_NO in HT plants, thereby favouring the overproduction of reactive oxygen species (ROS). Moreover, signs of lipid peroxidation damage were detected in HT plants, in which drought caused an increase of 40% in malondialdehyde (MDA) content. Concurrently, a marked increase in proline content was observed, while the activities of catalase (CAT) and ascorbate peroxidase (APX) were unaffected. Despite the generation of a moderate oxidative stress response, C. siliqua revealed a great capability for photosynthetic recovery 36 h after rewatering, which suggests that the species can cope with predicted climate change.     

Photosynthesis,chlorophyll fluorescence,and antioxidant enzyme responses of invasive weed <Emphasis Type="Italic">Mikania micrantha</Emphasis> to <Emphasis Type="Italic">Bemisia tabaci</Emphasis> infestation

In a glasshouse, Bemisia tabaci infestation largely reduced response of photosynthesis to irradiance and CO₂ concentration of Mikania micrantha compared with the non-infested control (C) ones. The maximum irradiance-saturated photosynthetic rate (P _max) and saturation irradiance (SI) of the infested M. micrantha were only 21.3 % and 6.5 % of the C-plants, respectively. B. tabaci infestation led to the reduction of contents of chlorophyll and carotenoids in M. micrantha, which was accompanied with the decrease of actual photosystem 2 (PS2) efficiency (Φ_PS2), efficiency of excitation energy capture by open PS2 reaction centres (F_v′/F_m′), electron transport rate (ETR), and photochemical quenching (q_P). Moreover, superoxide dismutase and catalase activities significantly decreased while proline and glutathione contents significantly increased in infested M. micrantha. Hence B. tabaci infestation not only induced direct damage of photosynthetic apparatus but also altered the antioxidant enzymes activities in M. micrantha, which might as consequences accelerate senescence of this weed.     

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

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

Comparison of thermostability of PSII between the chromatic and green leaf cultivars of <Emphasis Type="Italic">Amaranthus tricolor</Emphasis> L.

In the present study, we investigated the antioxidative potential in leaves of the chromatic (CC) versus green (GC) Amaranthus tricolor L. under moderate high-temperature stress at 45°C. Before heat stress, CC had significantly higher levels of betacyanins [about 3.2 mg g⁻¹(FM)] than the green [1.8 mg g⁻¹(FM) (p<0.01), while similar chlorophyll (Chl) content [about 2 mg g⁻¹(FM)] was observed between both cultivars. After exposure to high temperature (45°C) for 6 days, betacyanins in leaves of CC were remarkably increased (about 2 times of that in control samples grown at 30°C). In contrast, betacyanins in GC significantly decreased by 56% in comparison with that of the control. Chl level in CC was higher than that in GC after heat stress for 6 days. Flavonoids and total phenolics in both cultivars were increased, but much more in CC. Significantly less H₂O₂ accumulation was observed in the leaves and stems of CC than in those of GC under heat stress. Interestingly, much stronger circadian oscillation in fluorescence was observed in both cultivars after treatment at 45°C, which suggested that heat stress stimulates endogenous rhythms of photosystem II (PSII). Under moderate high-temperature stress, Chl fluorescence parameters F_v/F_m (maximum quantum yield of PSII), q_P (coefficient of photochemical quenching), Φ_PSII (effective PSII quantum yield), and ETR (electron transport rate) exhibited a gradual decrease, NPQ (nonphotochemical quenching) showed a slight increase followed by a gradual decline, whereas F_o (minimum fluorescence of a dark-adapted leaf) increased continuously. In contrast to GC, after 120 h of high-temperature treatment, CC exhibited significantly lower Fo level, and higher levels of F_v/F_m and NPQ. It is clear that PSII in CC was more stable than that in GC. The results indicate that betacyanins are an effective antioxidant, and probably contribute greatly to the higher thermal stability of PSII and higher tolerance to heat stress.     

Effects of elevated ozone on chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence in symptomatic and asymptomatic leaves of two tomato genotypes

Two different genotypes of Lycopersicon esculentum Mill. (cv. Cuor di Bue, O₃-sensitive and line 93.1033/1, O₃-resistant) were treated with a single dose of ozone (150 mm³ m⁻³ for 3 h). The PS 2 activity was examined by measurements of chlorophyll a fluorescence on symptomatic and asymptomatic leaves. Symptoms were evident on the 4^th leaves from the bottom, in both genotypes, while the 2^nd leaves of the line 93.1033/1 were asymptomatic. In these leaves, the net photosynthetic rate (P_N) did not change even if the F_v/F_m ratio significantly decreased. A strong reduction in P_N, mostly due to the stomatal closure, was observed in Cuor di Bue. The non photochemical quenching coefficient (q_NP) and the degree of PS 2 reaction centres closure (1-q_P) were higher, while the quantum efficiency of PS 2 photochemistry (Φ_PS2) and quantum efficiency of excitation energy capture (Φ_exc.) were lower in O₃ treated leaves of both genotypes. The limitation of photosynthesis was shown also by a decrease in the parameter %P, which diminished compared to controls in both genotypes. The response of the two genotypes for the energy fraction dissipated as thermal energy in the PS 2 antennae (%D) was similar. The fraction of %P remained lower during the recovery in symptomatic leaves of the resistant line as compared to the controls, whereas %X, which represents the amount of light energy that is not utilized in photochemistry or dissipated in the PS 2 antennae, significantly rose in the asymptomatic leaves of this line and in both the leaves of Cuor di Bue. From data obtained we concluded that ozone affected the plants independently on the appearance of visible symptoms of injury because the leaves without visible symptoms of both the genotypes were negatively influenced.     

Photoprotective Function of Photorespiration in Several Grapevine Cultivars Under Drought Stress

Four grapevine cultivars, i.e. Cabernet Sauvignon (a member of the Western Europe cultivar group), Rizamat (a member of the East cultivar group), Red Double Taste (a hybridized cultivar from Vitis vinifera L. and V. labrusca L.), and 1103Paulsen (a hybridized rootstock), were treated by three severity orders of drought stress for 25 d. Then net photosynthetic rate (P _N), maximal photochemical efficiency (F_v/F_m), actual photochemical efficiency (_PS2) of photosystem 2, total electron transport rate (J_T), and electron transport flows used in carboxylation (J_C) and in oxygenation (J_O) reactions catalysed by ribulose-1,5-bisphosphate carboxylase/oxygenase were determined. P _N was determined again after re-watering for 2 d by gas exchange measurement. Along with the increase in severity of drought stress, P _N, F_v/F_m, _PS2, J_T, and J_C in all four cultivars decreased. The range of decrease differed among cultivars. J_O expressed various trends from cultivar to cultivar. In Rizamat that received slight and moderate drought stress, P _N evidently decreased, but J_O markedly increased, thus maintaining high values of J_T and _PS2. Prior to the moderate drought stress, the F_v/F_m was high in Rizamat, indicating that the photodamage had not happened ahead of the moderate drought stress given. Under the severe drought stress, the photorespiration rate in Rizamat decreased by 70 %, and J_T, _PS2, and F_v/F_m also dropped to very low values, i.e. the photodamage of photosynthetic apparatus has taken place. This suggested that the photorespiration has consumed the excessive assimilatory power and the photo-protective function of photorespiration is very important for Rizamat. When Cabernet Sauvignon grew under drought stress, its J_O decreased in a small range, thus maintaining higher values of J_C, J_T, _PS2, and F_v/F_m; hence no serious photodamage occurred. Despite of the fact that P _N of cv. Red Double Taste decreased markedly under the slight drought stress, J_O still increased under the severe drought stress. This suggests that photorespiration is important in photoprotection under drought stress. J_O in cv. 1103Paulsen markedly decreased under slight stress. Accordingly, P _N, F_v/F_m, _PS2, J_T, and J_C decreased to extremely low values. Thus photorespiration effectively protects the photosynthetic apparatus from photo-damage under drought, assists in maintaining a relatively high _PS2, and helps P _N to be rapidly recovered after re-watering.     

Ozone sensitivity and ethylenediurea protection in ash trees assessed by JIP chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence transient analysis

The effect of ethylenediurea (EDU) was tested using the chlorophyll (Chl) a fluorescence transient analysis, performed with JIP-test, to assess ambient ozone (O₃) effects on photosynthesis of adult trees under natural conditions. Twelve adult European ash (Fraxinus excelsior L.) trees, known to be sensitive or tolerant to O₃, determined by presence symptomatic (S) or absence asymptomatic (AS) trees of foliar symptoms in previous years, were treated either with distilled water containing 450 g m⁻³ EDU or with distilled water. Once a month across the growing season [the accumulated exposure over a threshold of 40 nmol(O₃) mol⁻¹ was 32.49 μmol mol⁻¹ h⁻¹], Chl a fluorescence transients were measured in vivo on dark-adapted leaves of 1-year-old labeled shoots, from the lower crown part. Twenty-five parameters were calculated. The maximum quantum yield of primary photochemistry (ϕ_Po or F_v/F_m) did not differentiate between S-and AS-trees, while increased Chl content and de-excitation rates suggested compensation of O₃ injury in S-trees. Seasonal reductions in absorbing fluxes and increase in heat and fluorescence dissipation processes was due to leaf ageing and drought, the latter suggesting water deficit influenced Chl a fluorescence stronger than ambient O₃ exposure. AS-trees showed elevated probability of connectivity among photosystem 2 units, a mechanism to stimulate energy dissipation and reduce photo-oxidative injury. EDU prevented the inactivation of reaction centers. This slight effect does not warrant EDU as a tool to assess O₃ effects on photosynthesis, while the JIP-test is suggested for a quantitative assessment in adult trees.     

Interactions between nitrogen fertilization and ozone in watermelon cultivar Reina de Corazones in open-top chambers. Effects on chlorophyll a fluorescence, lipid peroxidation, and yield

Watermelon (Citrillus lanatus) plants were grown for two consecutive years in open-top chambers with three different ozone concentrations (O₃-free air, O₃ ambient, and air with additional O₃; CFA, NFA, and NFA+O₃) and three nitrogen fertilizer concentrations [0, 14.0, and 29.6 g N per pot; N0, N1, and N2). There was an interaction between ozone and N fertilizer for the major parameters studied. O₃ and N2 treatments led to a significant decrease in maximum efficiency of photosystem 2 (PS2) photochemistry (F_v/F_m), and induced a significant decrease in the actual quantum yield of PS2 (Φ_PS2), due mainly to the increased closure of PS2 reaction centres (q_P) and to an increase in the non-photochemical quenching (NPQ). On the other hand, these plants exhibited an increased susceptibility to photoinhibition, which could be associated with an increased fraction of reduced Q_A. An increase in lipid peroxidation indicated that damage was occurring at the membrane levels. High N concentration enhanced the detrimental effects of ozone on the fluorescence parameter induction and lipid peroxidation. All these negative alterations led to a decreased yield.