Effect of nitrogen-deficiency on midday photoinhibition in flag leaves of different rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) cultivars

Effects of nitrogen (N)-deficiency on midday photoinhibition in flag leaves were compared between two contrastive Japanese rice cultivars, a traditional japonica cultivar with low yield, cv. Shirobeniya (SRB), and a japonica-indica intermediate type with high yield, cv. Akenohoshi (AKN). Both cultivars were grown under high-N and low-N conditions. At midday, low-N supply resulted in more intensive reductions in net photosynthetic rate, stomatal conductance, maximal quantum yield of photosystem II (PSII) and quantum yield of PSII electron transport in SRB than in AKN, indicating that SRB was more strongly photoinhibited than AKN under low-N condition. At midday, the low-N plants of two cultivars showed higher superoxide dismutase (SOD) activities than the high-N plants. However, ascorbate peroxidase (APX) activity was maintained in AKN but significantly decreased in SRB under low-N condition (N-deficiency). In contrast, hydrogen peroxide (H₂O₂) content in SRB significantly increased under low-N condition, indicating that the susceptibility to midday photoinhibition in the low-N plants of SRB is related to the increased H₂O₂ accumulation. It is suggested that the midday depression in photosynthesis may be a result of oxidative stress occurring in the low-N plants in which antioxidant capacity is not enough to cope with the generation of H₂O₂. Therefore, H₂O₂-scavenging capacity could be an important factor in determining the cultivar difference of midday photoinhibition in flag leaves of rice under low-N condition.     

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.     

Changes in leaf photosynthesis of transgenic rice with silenced <Emphasis Type="Italic">OsBP-73</Emphasis> gene

In comparison with its wild type (WT), the transgenic (TG) rice with silenced OsBP-73 gene had significantly lower plant height, grain number per panicle, and leaf net photosynthetic rate (P _N). Also, the TG rice showed significantly lower chlorophyll (Chl), ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO), RuBPCO activase, and RuBP contents, photosystem 2 (PS2) photochemical efficiency (F_v/F_m and ΔF/F_m′), apparent quantum yield of carbon assimilation (Φ_c), carboxylation efficiency (CE), photosynthetic electron transport and photophosphorylation rates as well as sucrose phosphate synthase activity, but higher intercellular CO₂ concentration, sucrose, fructose, and glycerate 3-phosphate contents, and non-photochemical quenching of Chl fluorescence (NPQ). Thus the decreased P _N in the TG rice leaves is related to both RuBP carboxylation and RuBP regeneration limitations, and the latter is a predominant limitation to photosynthesis.     

Elevated CO<Subscript>2</Subscript> and temperature differentially affect photosynthesis and resource allocation in flag and penultimate leaves of wheat

Differences in acclimation to elevated growth CO₂ (700 μmol mol⁻¹, EC) and elevated temperature (ambient +4 °C, ET) in successive leaves of wheat were investigated in field chambers. At a common measurement CO₂, EC increased photosynthesis and the quantum yield of electron transport (Φ) early on in the growth of penultimate leaves, and later decreased them. In contrast, EC did not change photosynthesis, and increased Φ at later growth stages in the flag leaf. Contents of chlorophyll (Chl), ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO), and total soluble protein were initially higher and subsequently lower in penultimate than flag leaves. EC decreased RuBPCO protein content relative to soluble protein and Chl contents throughout the development of penultimate leaves. On the other hand, EC initially increased the RuBPCO:Chl and Chl a/b ratios, but later decreased them in flag leaves. In the flag leaves but not in the penultimate leaves, ET initially decreased initial and specific RuBPCO activities at ambient CO₂ (AC) and increased them at EC. Late in leaf growth, ET decreased Chl contents under AC in both kinds of leaves, and had no effect or a positive one under EC. Thus the differences between the two kinds of leaves were due to resource availability, and to EC-increased allocation of resources to photon harvesting in the penultimate leaves, but to increased allocation to carboxylation early on in growth, and to light harvesting subsequently, in the flag leaves.     

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.     

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.     

Responses of Penultimate and Flag Leaves of Wheat to Different Nitrogen Supply

Uniculm wheat (Triticum aestivum L.) was grown to maturity at four concentrations of nitrogen corresponding to 3 (N1), 6 (N2), 9 (N3) and 12 (N4) g m^–2. Penultimate and flag leaves were examined throughout the ontogeny. Sub-optimal concentrations of N resulted in sharp decline in both area and dry mass of the leaves. Decline in leaf area was due to fewer mesophyll cells. Net photosynthetic rate (P_N) increased up to full expansion, remained constant for about a week and then declined. P_N, nitrogen, ribulose-1,5-bis-phosphate carboxylase/oxygenase (RuBPCO) amount and activity, chlorophyll and soluble protein contents were similar at all the N concentrations. Both amount and activity of RuBPCO in the flag leaf were about two fold higher as compared to penultimate leaf, but P_N was similar. This indicates the presence of an excess amount of RuBPCO in the flag leaf.     

Photosynthetic Characteristics of Two New Chlorophyll b-Less Rice Mutants

Two yellow rice mutants VG28-1 and VG30-5 were obtained during the tissue culture process from a rice plant (cv. Zhonghua No.11 japonica) with inserted maize Ds transposon element. Absorption spectra and pigment composition showed that two mutants had no chlorophyll (Chl) b and lower Chl a content in comparison to the wild type (WT). Net photosynthetic rate (P _N), total electron transport rate (J_F), photochemical quenching (q_p), quantum yield of PS2 dependent non-cyclic electron transport (_PS2), fraction of P_rate, and leaf area were lower but F_v/F_m and apparent quantum yield (AQY) remained at similar levels as in the WT plant. Xanthophyll cycle pool size (V+A+Z) on a Chl basis, and de-epoxidation state were enhanced in the mutants. The mutants had larger amounts of soluble protein and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO), especially the small subunit of RuBPCO, than WT. The characteristics of two rice mutants differed somewhat from the other common Chl b-less mutants originating from mutagenic agent treatments.     

Stomatal and non-stomatal limitations to photosynthesis in field-grown grapevine cultivars

Diurnal changes of photosynthesis in the leaves of grapevine (Vitis vinifera × V. labrusca) cultivars Campbell Early and Kyoho grown in the field were compared with respect to gas exchanges and actual quantum yield of photosystem 2 (Φ_PS2) in late May. Net photosynthetic rate (P_N) of the two cultivars rapidly increased in the morning, saturated at photosynthetic photon flux density (PPFD) from 1200 to 1500 μmol m⁻² s⁻¹ between 10:00 and 12:00 and slowly decreased after midday. Maximum P_N was 13.7 and 12.5 μmol m⁻² s⁻¹ in Campbell Early and Kyoho, respectively. The stomatal conductance (g_s) and transpiration rate changed in parallel with P_N, indicating that P_N was greatly affected by g_s. However, the decrease in P_N after midday under saturating PPFD was also associated with the observed depression of Φ_PS2 at high PPFD. The substantial increase in the leaf to air vapour pressure deficit after midday might also contribute to decline of g_s and P_N.     

Photosynthetic characteristics of diploid honeysuckle (<Emphasis Type="Italic">Lonicera japonica</Emphasis> Thunb.) and its autotetraploid cultivar subjected to elevated ozone exposure

In order to investigate the effect of chromosome doubling on ozone tolerance, we compared the physiological responses of a diploid honeysuckle (Lonicera japonica Thunb.) and its autotetraploid cultivar to elevated ozone (O₃) exposure (70 ng g⁻¹, 7 h d⁻¹ for 31 d). Net photosynthetic rate (P _N) of both cultivars were drastically (P<0.01) impaired by O₃. Although there were significantly positive correlation between P _N and stomatal conductance (g _s) in both cultivars under each treatment, the decreased g _s in O₃ might be the result rather than the cause of decreased P _N as indicated by stable or increasing the ratio of intercellular to ambient CO₂ concentration(C _i/C _a). P _N under saturating CO₂ concentration (P _Nsat) and carboxylation efficiency (CE) significantly decreased under O₃ fumigation, which indicated the Calvin cycle was impaired. O₃ also inhibited the maximum efficiency of photosystem II (PSII) photochemistry in the dark-adapted state (F_v/F_m), actual quantum yield of PSII photochemistry (Φ_PSII), electron transport rate (ETR), photochemical quenching coefficient (qP), non-photochemical quenching (NPQ), the maximum in vivo rate of Rubisco carboxylation (V_cmax) and the maximal photosynthetic electron transport rate (J_max) which demonstrated that the decrease in P _N of the honeysuckle exposed to elevated O₃ was probably not only due to impairment of Calvin cycle but also with respect to the light-harvesting and electron transport processes. Compared to the diploid, the tetraploid had higher relative loss in transpiration rate (E), (g _s), (P _Nsat), V_cmax and J_max. This result indicated that the Calvin cycle and electron transport in tetraploid was damaged more seriously than in diploid. A barely nonsignificant (P=0.086) interaction between O₃ and cultivar on P _N suggested a higher photosynthetic sensitivity of the tetraploid cultivar.     

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.     

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.     

Characteristics of Chlorophyll Fluorescence and Membrane-Lipid Peroxidation During Senescence of Flag Leaf in Different Cultivars of Rice

With japonica rice 98-08, indica hybrids Shanyou 63, Gangyou 881, and X07S/Zihui 100, and sub-species hybrid Peiai 64S/9311 as materials, chlorophyll (Chl) content, Chl a fluorescence parameters, and membrane lipid peroxidation in flag leaf were measured at late developmental stages under natural conditions. F_v/F_m, q_P, _PS2, and electron transport rate gradually decreased while q_N increased conversely. Excessive photon energy led to the accumulation of active oxygen (O₂ ^–), H₂O, malonyldialdehyde, and products of membrane lipid peroxidation, and resulted in reduced Chl content and early ageing subsequent to the photooxidation during flag leaf senescence. There was obvious diversification of these parameters among rice cultivars. In comparison with japonica cv. 98-08 (tolerant to photooxidation), F_v/F_m decreased in indica cv. Shanyou 63 (susceptible to photooxidation) with greater accumulation of active oxygen and a sharp drop in Chl content, which resulted in yellowish early ageing, and affected the filling and setting of rice grains. The mechanism for premature ageing in indica rice was related to irradiance and temperature at filling stages. On a sunny day at above 25 °C, the reaction centre of photosystem 2 (PS2) exhibited a dynamic change on reversible inactivation. Under the intense irradiance at noon, PS2 function in indica rice exhibited obvious down-regulation and photoinhibition. Under intense irradiance with lowered temperatures, PS2 resulted in photo-damage and early ageing, related to the degradation of PS2-D1 protein and the inhibition of endogenous protection systems such as the xanthophyll cycle and enzymes scavenging active oxygen. Hence for high-yield breeding, based on a good plant-type and utilising heterosis and tolerance of photooxidation, the selection of japonica rice or a sterile line with the japonica genotype as female is a strategy worthy of consideration.     

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.     

Chilling stress and chilling tolerance of sweet potato as sensed by chlorophyll fluorescence

We studied changes in the chlorophyll (Chl) fluorescence components in chilling-stressed sweet potato (Ipomoea batatas L. Lam) cv. Tainung 57 (TN57, chilling-tolerant) and cv. Tainung 66 (TN66, chilling-susceptible). Plants under 12-h photoperiod and 400 μmol m⁻² s⁻¹ irradiance at 24/20 °C (day/night) were treated by a 5-d chilling period at 7/7 °C. Compared to TN66, TN57 exhibited a significantly greater basic Chl fluorescence (F₀), maximum fluorescence (F_m), maximum fluorescence yield during actinic irradiation (F_m′ ), and the quantum efficiency of electron transport through photosystem 2, PS2 (Φ_PS2). Chilling stress resulted in decrease in the potential efficiency of PS2 (F_v/F_m), Φ_PS2, non-photochemical fluorescence quenching (NPQ), non-photochemical quenching (q_N), and the occurrence of chilling injury in TN66. Chilling increased the likelihood of photoinhibition, characterized by a decline in the Chl fluorescence of both cultivars, and photoinhibition during low temperature stress generally occurred more rapidly in TN66.     

Peanut Photosynthesis Under Drought and Re-Watering

The photosynthetic response of three Arachis hypogaea L. cultivars (57-422, 73-30, and GC 8-35) grown for two months was measured under water available conditions, severe water stress, and 24, 72, and 93 h following re-watering. At the end of the drying cycle, all the cultivars reached dehydration, relative water content (RWC) ranging between 40 and 50 %. During dehydration, leaf stomatal conductance (g _s), transpiration rate (E), and net photosynthetic rate (P _N) decreased more in cvs. 57-422 and GC 8-35 than in 73-30. Instantaneous water use efficiency (WUE_i) and photosynthetic capacity (P _max) decreased mostly in cv. GC 8-35. Except in cv. GC 8-35, the activity of photosystem 1 (PS1) was only slightly affected. PS2 and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) were the main targets of water stress. After re-watering, cvs. 73-30 and GC 8-35 rapidly regained g _s, E, and P _N activities. Twenty-four hours after re-watering, the electron transport rates and RuBPCO activity strongly increased. P _N and P _max fully recovered later. Considering the different photosynthetic responses of the studied genotype, a general characterisation of the interaction between water stress and this metabolism is presented.     

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.     

High photosynthetic efficiency of a rice (Oryza sativa L.) xantha mutant

Comparative analysis revealed that a xantha rice mutant (cv. Huangyu B) had higher ratios of chlorophyll (Chl) a/b and carotenoids/Chl, and higher photosynthetic efficiency than its wild type parent (cv. II32 B). Unexpectedly, the mutant had higher net photosynthetic rate (P _N) than II32 B. This might have resulted from its lower non-photochemical quenching (q_N) but higher maximal photochemical efficiency (F_V/F_M), higher excitation energy capture efficiency of photosystem 2 (PS2) reaction centres (F_V′/F_M′), higher photochemical quenching (q_P), higher effective PS2 quantum yield (Φ_PS2), and higher non-cyclic electron transport rate (ETR). This is the first report of a chlorophyll mutant that has higher photosynthetic efficiency and main Chl fluorescence parameters than its wild type. This mutant could become a unique material both for the basic research on photosynthesis and for the development of high yielding rice cultivars.     

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.