Effect of Cd on growth, photosynthetic gas exchange, and chlorophyll fluorescence of wild and Cd-sensitive mutant rice

Growth, photosynthetic gas exchange, and chlorophyll fluorescence characteristics were investigated in wild type (WT) and Cd-sensitive mutant rice (Oryza sativa L.) plants using 50 μM Cd treatment for 12 d followed by a 3-d recovery. Under Cd stress, net dry mass and pigment contents were significantly lower in the mutant plants than in the WT. The mutant had lower net photosynthetic rate (P _N), transpiration rate (E), and stomatal conductance (g _s) than WT rice, however, it had higher intercellular CO₂ concentration (C _i), indicating that non-stomatal factors accounted for the inhibition of P _N. Maximal photochemical efficiency of photosystem 2 (F_v/F_m), effective quantum yield of PS2 (Φ_PS2), and photochemical quenching (q_P) decreased much in the mutant under Cd stress. Cd content in roots and leaves of the mutant was significantly higher than those in the WT. Hence Cd toxicity was associated with the marked increases in Cd contents of plant tissue. After the recovery for 3 d, the WT rice had higher capacity to recover from Cd injury than the mutant.     

Rehydration of Sugar Beet Plants after Water Stress: Effect of Cytokinins

The possibility to improve the recovery of sugar beet plants after water stress by application of synthetic cytokinins N⁶-benzyladenine (BA) or N⁶-(m-hydroxybenzyl)adenosine (HBA) was tested. Relative water content (RWC), net photosynthetic rate (P_N), transpiration rate (E), stomatal conductance (g_s), chlorophyll (Chl) a and Chl b contents, and photosystem 2 efficiency characterized by variable to maximal fluorescence ratio (F_v/F_m) were measured in control plants, in water-stressed plants, and after rehydration (4, 8, 24, and 48 h). Water stress markedly decreased parameters of gas exchange, but they started to recover soon after irrigation. Application of BA or HBA to the substrate or sprayed on leaves only slightly stimulated recovery of P_N, E, and g_s in rehydrated plants, especially during the first phases of recovery. Chl contents decreased only under severe water stress and F_v/F_m ratio was not significantly affected by water stress applied. Positive effects of BA or HBA application on Chl content and F_v/F_m ratio were mostly not observed.     

Effects of copper and cadmium on photosynthesis in cucumber cotyledons

The effects of 20 and 50 μM concentrations of Cu and Cd on photosynthesis in cucumber (Cucumis sativus L.) cotyledons were studied by the measurements of gas exchange characteristics, chlorophyll (Chl) fluorescence parameters, photosynthetic pigment contents, and two Calvin cycle enzymes activities: glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and 3-phosphoglyceric acid kinase (PGK). To minimize indirect metal action, seedlings were treated with metals in the stage of green, fully developed cotyledons. The metals reached the cotyledon tissue after 48 h of treatments, though symptoms of metal action were not visible at that time. The effect of metals on the light phase of the photosynthesis parameters such as potential efficiency of photosystem 2 (PS2; F_v/F_m), and photochemical and nonphotochemical quenching of Chl fluorescence (q_P and q_NP) was negligible. In contrast, a decrease of PS2 quantum efficiency (Φ_PS2) was much more noticeable. Changes in the pigment contents were slight, as only 50 μM Cd decreased Chl a and b contents in small extent. On the contrary, metals in both concentrations drastically decreased (50 and more % of control) the net photosynthetic rate and the stomatal conductance, but not the internal CO₂ concentration. The activities of both GAPDH and PGK were also decreased by metals, although the effect on PGK was more prominent, particularly on its potential activity (dithiothreitol in extraction and incubation media). Hence Cu and Cd affected the synthesis of enzyme proteins rather than they influenced their modifications. The effects of both metals on most of the measured photosynthesis parameters were similar, but the accumulation of Cd in the cotyledons was significantly higher than Cu accumulation. Thus Cu was more toxic for the photosynthesis of cucumber cotyledons than Cd.     

Effects of water stress and high temperature on gas exchange and chlorophyll fluorescence in Triticum aestivum L.

Wheat plants grown in controlled growth chambers were exposed to drought stress (DS) and high temperature (HT) singly and in combination (DS+HT). The effects of these two stresses on net photosynthetic rate (P _N), stomatal conductance (g _s), intercellular CO₂ concentration (C _i), quantum efficiency of photosystem 2 (Φ_PS2), variable to maximum chlorophyll (Chl) fluorescence (F_v/F_m), photochemical (q_p) and non-photochemical (NPQ) Chl fluorescence, and yield were investigated. Grain yield was decreased by 21 % due to DS, while it was increased by 26 % due to HT. P _N, g _s, C _i, and Chl fluorescence were dramatically reduced to DS, HT, and their interaction, except NPQ which showed an increase due to HT.     

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.     

Photosynthetic responses of radish (<Emphasis Type="Italic">Raphanus sativus</Emphasis> var. <Emphasis Type="Italic">longipinnatus</Emphasis>) plants to infection by turnip mosaic virus

Plant growth, chlorophyll (Chl) content, photosynthetic gas exchange, ribulose-1,5-bisphosphate carboxylase (RuBPCO) enzyme activity, and Chl fluorescence in radish (Raphanus sativus var. longipinnatus) plants were examined after turnip mosaic virus (TuMV) infection. Plant fresh mass, dry mass, Chl content, net photosynthetic rate (P _N), transpiration rate (E), stomatal conductance (g _s), and RuBPCO activity were significantly lower in infected plants after 5 weeks of virus infection as compared to healthy plants. The 5-week virus infection did not induce significant differences in intercellular CO₂ concentration (C _i, photochemical efficiency of photosystem 2, PS2 (F_v/F_m), excitation capture efficiency of open PS2 reaction centres (F_v'/F_m'), effective quantum efficiency of photosystem 2 (ΔF/F_m'), and photochemical quenching (q_P), but non-photochemical quenching (q_N) and alternative electron sink (AES) were significantly enhanced. Thus the decreased plant biomass of TuMV-infected plants might be associated with the decreased photosynthetic activity mainly due to reduced RuBPCO activity.     

Effects of rhizobia inoculation and nitrogen fertilization on photosynthetic physiology of soybean

Plant growth, contents of photosynthetic pigments, photosynthetic gas exchange, and chlorophyll (Chl) fluorescence in soybean [Glycine max (L.) Merr. cv. Heinong37] were investigated after it was inoculated with Sinorhizobium fredii USDA191 or treated with 5 mM (NH₄)₂SO₄ (N5) and 30 mM (NH₄)₂SO₄ (N30), respectively. In the plants following N5 fertilization, not only plant biomass, leaf area, and Chl content, but also net photosynthetic rate (P _N), stomatal conductance (g _s), carboxylation efficiency (CE), maximum photochemical efficiency (F_v/F_m) of photosystem 2 (PS2), and quantum yield of PS2 (Φ_PS2) were markedly improved as compared with the control plants. There were also positive effects on plant growth and plant photosynthesis after rhizobia inoculation, but the effects were much less than those of N5 fertilization. For N30 plants there were no significant positive effects on plant growth and photosynthetic capacity. Plant biomass, P _N, and g _s were similar to those of N-limited (control) plants. Φ_PS2 and photochemical quenching (q_P) were obviously declined while content of carotenoids and non-photochemical quenching (q_N) were significantly enhanced in N30 treated plants. This indicated that excess N supply may cause some negative effects on soybean plants.     

Photosynthesis and protective mechanisms during ageing in transgenic tobacco leaves with over-expressed cytokinin oxidase/dehydrogenase and thus lowered cytokinin content

The content of cytokinins (CKs), the plant inhibitors of the final phase of plant development, senescence, is effectively controlled by irreversible degradation catalysed by cytokinin oxidase/dehydrogenase (CKX). In transgenic tobacco, denoted as AtCKX, with over-expressed CKX causing lowered CK content, we investigated changes in the time courses of chlorophyll (Chl) and xanthophyll (violaxanthin, antheraxanthin, zeaxanthin, neoxanthin, and lutein) contents. We also determined parameters of slow Chl fluorescence kinetics such as minimum Chl fluorescence yield in the darkadapted state F₀, maximum quantum yield of PS2 photochemistry (F_v/F_m), maximum ratio of quantum yields of photochemical and concurrent non-photochemical processes in photosystem 2 (PS2), F_v/F₀, non-photochemical quenching (NPQ), and effective quantum yield of photochemical energy conversion in PS2 (Φ₂). We used three different developmental leaf stages, old, mature, and young, and compared this with time courses of these characteristics in leaves with natural CK levels. The parameters F_v/F_m, F_v/F₀, and Φ₂ were unchanged during ageing in AtCKX plants in contrast to control ones where a significant decrease in old leaves was found. In control plants F₀ increased during ageing, but in the oldest leaf a considerable decrease was observed. This could indicate progressive damage to PS2 reaction centres and then detachment and rapid degradation of Chl. This is in agreement with time course of Chl content. NPQ decreased with age and was similar in both plant types. We observed a decline of xanthophyll contents in the oldest leaves in both plant types, but the contents were enhanced in AtCKX compared to control plants, especially of neoxanthin. The higher xanthophyll contents in the transgenic plants contribute to a better photoprotection and the fluorescence parameters indicated that photosynthetic apparatus was in better condition compared to control and it consequently postponed the onset of leaf senescence.     

Photosynthesis and photosystem 2 efficiency of two salt-adapted halophytic seashore <Emphasis Type="Italic">Cakile maritima</Emphasis> ecotypes

The effects of salinity (0–400 mM NaCl, marked S₀, S₁₀₀, S₂₀₀, and S₄₀₀) on growth, photosynthesis, photosystem 2 (PS2) efficiency, ion relations, and pigment contents were studied in two seashore Cakile maritima ecotypes (Tabarka and Jerba, respectively, sampled from humid and arid bioclimatic areas). Growth of Jerba plants was improved at S₁₀₀ as compared to S₀. Tabarka growth was inhibited by salinity at all NaCl concentrations. Leaf sodium and chloride concentrations increased with medium salinity and were higher in Jerba than in Tabarka plants. Chlorophyll content, net photosynthetic rate, stomatal conductance (g _s), and intracellular CO₂ concentration were stimulated at moderate salinity (S₁₀₀) in Jerba plants and inhibited at higher salt concentrations in both ecotypes: g _s was the most reduced parameter. The maximum quantum efficiency of PS2 (F_v/F_m), quantum yield, linear electron transport rate, and efficiency of excitation energy capture by open PS2 reaction centres showed no significant changes with increasing salt concentration in Jerba plant and were decreased in Tabarka subjected to S₄₀₀. However, the efficiency of dissipation of excess photon energy in the PS2 antenna was maintained in Jerba and was increased in Tabarka plants challenged with S₄₀₀. Hence the relative salt tolerance of Jerba was associated with a better ability to use Na⁺ and Cl⁻ for osmotic adjustment, the absence of pigment degradation, and the concomitant PS2 protection from photodamage.     

Changes of photosynthetic parameters in cucumber leaves under Cu,Cd, and Pb stress

The effects of Cu, Cd, and Pb toxicity on photosynthesis in cucumber leaves (Cucumis sativus L.) were studied by the measurements of gas exchange characteristics, chlorophyll (Chl) fluorescence parameters, and Chl content. Concentrations of metals in sequence of 20 μM Cu, 20 and 50 μM Cd, and 1 000 μM Pb decreased the plant dry mass to 50–60 % after 10 d of treatment whereas 50 μM of Cu decreased it to 30 %. The content of Cd in leaves of plants treated with 50 μM Cd was three times higher than the contents of Cu and Pb after plant treatment with 50 μM Cu or 1 000 μM Pb. Hence Cd was transported to leaves much better than Cu and Pb. Nevertheless, the net photosynthetic rate and stomatal conductance in leaves treated with 50 μM Cu or Cd were similarly reduced. Thus, Cu was more toxic than Cd and Pb for photosynthesis in cucumber leaves. None of the investigated metals decreased internal CO₂ concentrations. Also the effect of metals on potential efficiency of photosystem 2, PS2 (F_v/F_m) was negligible. The metal dependent reduction of PS2 quantum efficiency (Φ_PS2) after plant adaptation in actinic irradiation was more noticeable. This could imply that reduced demand for ATP and NADPH in a dark phase of photosynthesis caused a down-regulation of PS2 photochemistry. Furthermore, in leaves of metal-treated plants the decrease in water percentage as well as lower contents of Chl and Fe were observed. Thus photosynthesis is not the main limiting factor for cucumber growth under Cu, Cd, or Pb stress.     

The effect of additional red irradiation on the photosynthetic apparatus of <Emphasis Type="Italic">Pisum sativum</Emphasis>

Pisum sativum (L.) plants were grown under “white” luminescent lamps, W [45 μ mol(quantum) m⁻² s⁻¹] or under the same irradiation supplemented with narrow spectrum red light-emitting diodes (LEDs), RE [λ_max = 660 nm, Δλ = 20 nm, 40 μmol(quantum) m⁻² s⁻¹]. Significant differences in the chlorophyll (Chl) a fluorescence parameters, degree of State 1–State 2 transition, and the pigment-protein contents were found in plants grown under differing spectral composition. Addition of red LEDs to the “white light” resulted in higher effective quantum yield of photosystem 2 (PS2), i.e. F′_v/F′_m, linear electron transport (ϕ_PS2), photochemical quenching (q_P), and lower non-photochemical quenching (q_N as well as NPQ). The RE plants were characterised by higher degree State 1–State 2 transition, i.e. they were more effective in radiant energy utilisation. Judging from the data of “green” electrophoresis of Chl containing pigment-protein complexes of plants grown under various irradiation qualities, the percentage of Chl in photosystem 2 (PS2) reaction centre complexes in RE plants was higher and there was no difference in the total Chl bound with Chl-proteins of light-harvesting complexes (LHC2). Because the ratio between oligomeric and monomeric LHC2 forms was higher in RE plants, we suggest higher LHC2 stability in these ones.     

Diurnal Changes of Gas Exchange,Chlorophyll Fluorescence,and Stomatal Aperture of Hybrid Poplar Clones Subjected to Midday Light Stress

Diurnal changes in net photosynthetic rate (P _N), chlorophyll (Chl) fluorescence, and stomatal aperture of several hybrid poplar clones subjected to midday light stress were measured in July and August of 1996. Midday depression of P _N, photosystem 2 (PS2) efficiency, stomatal conductance (g _s), and stomatal aperture was observed in all clones, though at differing rates among them. Non-uniform stomatal closure occurred at noon and at other times, requiring a modification of intercellular CO₂ concentration (C ₁). A linear relationship was found between g _s and stomatal aperture. More than half of the photons absorbed by PS2 centre dissipated thermally when subjected to light stress at noon. There was a linear relationship between the rate of PS2 photochemical electron transport (PxPFD) and P _N. There was a consensus for two fluorescence indicators (1 – q_P/q_N and (F_m' – F)/F_m') in assessment of susceptibility of photoinhibition in the clones. According to P _N, Chl fluorescence, and stomatal aperture, we conclude that midday depression of photosynthesis can be attributed to both stomatal and non-stomatal limitations.     

Effects of different excitation and detection spectral regions on room temperature chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence parameters

The effects of different spectral region of excitation and detection of chlorophyll (Chl) a fluorescence at room temperature on the estimation of excitation energy utilization within photosystem (PS) 2 were studied in wild-type barley (Hordeum vulgare L. cv. Bonus) and its Chl b-less mutant chlorina f2 grown under low and high irradiances [100 and 1 000 μmol(photon) m⁻² s⁻¹]. Three measuring spectral regimes were applied using a PAM 101 fluorometer: (1) excitation in the red region (maximum at the wavelength of 649 nm) and detection in the far-red region beyond 710 nm, (2) excitation in the blue region (maximum at the wavelength of 461 nm) and detection beyond 710 nm, and (3) excitation in the blue region and detection in the red region (660– 710 nm). Non-photochemical quenching of maximal (NPQ) and minimal fluorescence (SV₀), determined by detecting Chl a fluorescence beyond 710 nm, were significantly higher for blue excitation as compared to red excitation. We suggest that this results from higher non-radiative dissipation of absorbed excitation energy within light-harvesting complexes of PS2 (LHC2) due to preferential excitation of LHC2 by blue radiation and from the lower contribution of PS1 emission to the detected fluorescence in the case of blue excitation. Detection of Chl a fluorescence originating preferentially from PS2 (i.e. in the range of 660–710 nm) led to pronounced increase of NPQ, SV₀, and the PS2 photochemical efficiencies (F_V/F_M and F_V′/F_M′), indicating considerable underestimation of these parameters using the standard set-up of PAM 101. Hence PS1 contribution to the minimal fluorescence level in the irradiance-adapted state may reach up to about 80 %.     

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.     

Effect of day/night temperature difference on chlorophyll content,photosynthesis and fluorescence parameters of tomato at fruit stage

In order to investigate the effect of day/night temperature difference (DIF) on photosynthetic characteristics of tomato plants (Solanum lycopersicum, cv. Jinguan 5) at fruit stage, an experiment was carried out in climate chambers. Five day/night temperature regimes (16/34, 19/31, 25/25, 31/19, and 34/16°C) with respective DIFs of -18, -12, 0, +12, and +18 were used and measured at mean daily temperature of 25°C. The results showed that chlorophyll (Chl) a, Chl b, net photosynthetic rate (P_N), stomatal conductance (g_s), maximum quantum yield of PSII photochemistry (F_v/F_m), effective quantum yield of PSII photochemistry (?_PSII), and photochemical quenching (qp) significantly increased under positive DIF, while they decreased with negative DIF. In contrast, the Chl a/b ratio and nonphotochemical quenching (NPQ) decreased under positive DIF, while increased with negative DIF. Chl a, Chl b, P_N, g_s, F_v/F_m, ?_PSII, and qp were larger under +12 DIF than those at +18 DIF, while Chl a/b and NPQ showed an opposite trend.     

Effects of salicylic acid pre-treatment on cadmium and/or UV-B stress in soybean seedlings

The present study examined the effect of salicylic acid (SA) pre-treatment on soybean seedlings exposed to cadmium and/or UV-B stress. Dry mass, pigment content, net photosynthetic rate (P_N), stomatal conductance (g_s), and transpiration rate (E) were decreased by the Cd and/or UV-B stress. SA alleviated the adverse effects of Cd and/or UV-B on growth, pigment content, PN, and gs, but did not mitigate the inhibitory effect of Cd/UV-B on E, or that of Cd on chlorophyll fluorescence parameters. Cd and/or UV-B induced oxidative stress and increased lipid peroxidation that was significantly decreased by SA pre-treatment. The Cd and/or UV-B increased superoxide dismutase (SOD) activity, decreased peroxidase (POD) activity, and catalase (CAT) activity was mostly unaltered. SA might act as one of the potential antioxidants as well as a stabilizer of membrane integrity to improve plant resistance to the Cd and/or UV-B stress.     

Effect of gamma radiation on mutant induction of <Emphasis Type="Italic">Fagopyrum dibotrys</Emphasis> Hara

Agronomic traits, photosynthetic pigments, gas exchange, and chlorophyll (Chl) fluorescence parameters of red stem buckwheat (Fagopyrum dibotrys Hara) mutants induced by γ-radiation were compared with green control at seedling stage. Plant height, number of first-class branches, and rhizome biomass were inhibited significantly (p<0.01). Chl a, Chl b, and Chl a+b contents decreased with elevated dose of γ-rays, while increasing carotenoid content indicated that buckwheat was capable of adjusting to the radiation damage. Decrease in net photosynthetic rate was the result of both stomatal and non-stomatal limitations. Fluorescence parameters, such as F₀, F_m, F_v/F_m, F_v/F₀, Φ_PS2, electron transport rate, and photochemical quenching declined significantly (p<0.01) as compared with control due to photoinhibition, while non-photochemical quenching increased to enhance thermal dissipation. Lower parameters implied that leaf tissue was damaged significantly by high dose of γ-radiation and therefore leaf senescence was accelerated.     

Leaf gas exchange and grain yield of common bean exposed to spermidine under water stress

Three prevalent aliphatic polyamines (PAs) include putrescine, spermidine, and spermine; they are low-molecular-mass polycations involved in many physiological processes in plants, especially, under stressful conditions. In this experiment, three bean (Phaseolus vulgaris L.) genotypes were subjected to well-watered conditions and two moderate and severe water-stressed conditions with and without spermidine foliar application. Water stress reduced leaf relative water content (RWC), chlorophyll contents, stomatal conductance (gs), intercellular CO₂ concentration (C_i), transpiration rate, maximal quantum yield of PSII (F_v/F_m), net photosynthetic rate (P_N), and finally grain yield of bean plants. However, spermidine application elevated RWC, gs, C_i, F_v/F_m, and P_N, which caused an increase in the grain yield and harvest index of bean plants under water stress. Overall, exogenous spermidine could be utilized to alleviate water stress through protection of photosynthetic pigments, increase of proline and carotenoid contents, and reduction of malondialdehyde content.

     

Effects of Nitrogen Deficiency on Gas Exchange,Chlorophyll Fluorescence,and Antioxidant Enzymes in Leaves of Rice Plants

Gas exchange, chlorophyll (Chl) fluorescence, and contents of photosynthetic pigments, soluble proteins (ribulose-1,5-bisphosphate carboxylase/oxygenase, RuBPCO), and antioxidant enzymes were characterized in the fully expanded 6^th leaves in rice seedlings grown on either complete (CK) or on nitrogen-deficient nutrient (N-deficiency) solutions during a 20-chase period. Compared with the control plants, the lower photosynthetic capacity at saturation irradiance (P _max) was accompanied by an increase in intercellular CO₂ concentration (C_i), indicating that in N-deficient plants the decline in P _max was not due to stomatal limitation but due to the reduced carboxylation efficiency. The fluorescence parameters _PS2, F_v/F_m, electron transport rate (ETR), and q_P showed the same tendency as P _max in N-deficient plants. Correspondingly, a higher q_N paralleled the rise of the ratio of carotenoid (Car) to Chl contents. However, F_v/F_m was still diminished, suggesting that photoinhibition did occur in the photosystem 2 (PS2) reaction centres. In addition, the activities of antioxidant enzymes on a fresh mass basis were gradually lowered, leading to the aggravation of membrane lipid peroxidation with the proceeding N-deficiency. The accumulation of malonyldialdehyde resulted in the lessening of Chl and soluble protein content. Analyses of regression showed PS2 excitation pressure (1 - q_P) was linearly correlated with the content of Chl and inversely with soluble protein (particularly RuBPCO) content. There was a lag phase in the increase of PS2 excitation pressure compared to the decrease of RuBPCO content. Therefore, the increased excitation pressure under N-deficiency is probably the result of saturation of the electron transport chain due to the limitation of the use of reductants by the Calvin cycle. Rice plants responded to N-deficiency and high irradiance by decreasing light-harvesting capacity and by increasing thermal dissipation of absorbed energy.     

Photosynthesis and chlorophyll fluorescence response to low sink demand of tubers and roots in <Emphasis Type="Italic">Dahlia pinnata</Emphasis> source leaves

Photosynthetic rate (P_N) and chlorophyll (Chl) fluorescence induction of source leaves in response to a low sink demand created by girdling the branch (GB) between the root-tuber-system and the leaves were studied in Dahlia pinnata L. cv. Rigolet during the stage of rapid tuber growth in the greenhouse. GB resulted in significantly lower values of P_N, stomatal conductance (g_s), and transpiration rate (E), but in higher leaf temperature (T_l) compared with those of controls. With exception of maximum quantum yield of photosystem 2 (PS 2) photochemistry (F_v/F_m) and maximum ratio of quantum yields of photochemical and concurrent non-photochemical processes in PS 2 (F_v/F₀), no significant differences were observed in Chl fluorescence parameters between girdled and control leaves on days 1 and 2 after GB, indicating no apparent damage in the photosynthetic apparatus. However, longer girdling duration resulted in higher non-photochemical Chl fluorescence quenching (NPQ), but lower F_v/F₀, actual efficiency of energy conversion in PS 2 under steady-state conditions (Φ_PS2), and photochemical quenching coefficient (qP) in comparison with controls from 10:00 to 16:00 or 15:00 on days 4 and 5, respectively, indicating reversible injury in the photosynthetic apparatus.