Wheat cultivars selected for high Fv/Fm under heat stress maintain high photosynthesis,total chlorophyll,stomatal conductance,transpiration and dry matter

The chlorophyll fluorescence parameter F_v/F_m reflects the maximum quantum efficiency of photosystem II (PSII) photochemistry and has been widely used for early stress detection in plants. Previously, we have used a three‐tiered approach of phenotyping by F_v/F_m to identify naturally existing genetic variation for tolerance to severe heat stress (3 days at 40°C in controlled conditions) in wheat (Triticum aestivum L.). Here we investigated the performance of the previously selected cultivars (high and low group based on F_v/F_m value) in terms of growth and photosynthetic traits under moderate heat stress (1 week at 36/30°C day/night temperature in greenhouse) closer to natural heat waves in North‐Western Europe. Dry matter accumulation after 7 days of heat stress was positively correlated to F_v/F_m. The high F_v/F_m group maintained significantly higher total chlorophyll and net photosynthetic rate (P_N) than the low group, accompanied by higher stomatal conductance (g_s), transpiration rate (E) and evaporative cooling of the leaf (ΔT). The difference in P_N between the groups was not caused by differences in PSII capacity or g_s as the variation in F_v/F_m and intracellular CO₂ (C_i) was non‐significant under the given heat stress. This study validated that our three‐tiered approach of phenotyping by F_v/F_m performed under increasing severity of heat was successful in identifying wheat cultivars differing in photosynthesis under moderate and agronomically more relevant heat stress. The identified cultivars may serve as a valuable resource for further studies to understand the physiological mechanisms underlying the genetic variability in heat sensitivity of photosynthesis.     

Cultivar variation in cotton photosynthetic performance under different temperature regimes

Cotton (Gossypium hirsutum L.) yields are impacted by overall photosynthetic production. Factors that influence crop photosynthesis are the plants genetic makeup and the environmental conditions. This study investigated cultivar variation in photosynthesis in the field conditions under both ambient and higher temperature. Six diverse cotton cultivars were grown in the field at Stoneville, MS under both an ambient and a high temperature regime during the 2006–2008 growing seasons. Mid-season leaf net photosynthetic rates (P_N) and dark-adapted chlorophyll fluorescence variable to maximal ratios (F_v/F_m) were determined on two leaves per plot. Temperature regimes did not have a significant effect on either P_N or F_v/F_m. In 2006, however, there was a significant cultivar × temperature interaction for P_N caused by PeeDee 3 having a lower P_N under the high temperature regime. Other cultivars’ P_N were not affected by temperature. FM 800BR cultivar consistently had a higher P_N across the years of the study. Despite demonstrating a higher leaf F_v/F_m, ST 5599BR exhibited a lower P_N than the other cultivars. Although genetic variability was detected in photosynthesis and heat tolerance, the differences found were probably too small and inconsistent to be useful for a breeding program.     

Protection effect of nitric oxide on photosynthesis in rice under heat stress

The effect of exogenous applied nitric oxide on photosynthesis under heat stress was investigated in rice seedlings. High temperature resulted in significant reductions of the net photosynthetic rate (P _N) due to non-stomatal components. Application of nitric oxide donors, sodium nitroprusside (SNP) or S-nitrosoglutathione (GSNO), dramatically alleviated the decrease of P _N induced by high temperature. Chlorophyll fluorescence measurement revealed that high temperature caused significant increase of the initial fluorescence (F _o) and non-photochemical quenching (NPQ) whereas remarkable decrease of the maximal fluorescence (F _m), the maximal efficiency of PSII photochemistry (F _v/F _m), the actual PSII efficiency (Φ_PSII), and photochemical quenching (q _p). In the presence of SNP or GSNO pretreatment, the increase of F _o and decrease of F _m, F _v/F _m, Φ_PSII and q _p were markedly mitigated, but NPQ was further elevated. Moreover, with SNP or GSNO pretreatment, H₂O₂ accumulation and electrolyte leakage induced by heat treatment were significantly reduced, whereas zeaxanthin content and carotenoid content relative to chlorophyll were elevated. The potassium salt of 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), a specific NO scavenger, arrested NO donors mediated effects. These results suggest that NO can effectively protect photosynthesis from damage induced by heat stress. The activation effect of NO on photosynthesis may be mediated by acting as ROS scavenging, or/and alleviating oxidative stress via maintaining higher carotenoid content relative to chlorophyll or/and enhancing thermal dissipation of excess energy through keeping higher level of zeaxanthin content under heat stress.     

Leaf Gas Exchange and Chlorophyll Fluorescence Parameters in Phaseolus Vulgaris as Affected by Nitrogen and Phosphorus Deficiency

The effects of N and P deficiency, isolated or in combination, on leaf gas exchange and fast chlorophyll (Chl) fluorescence emission were studied in common bean cv. Negrito. 10-d-old plants grown in aerated nutrient solution were supplied with high N (HN, 7.5 mol m⁻³) or low N (LN, 0.5 mol m⁻³), and also with high P (HP, 0.5 mol m⁻³) or low P (LP, 0.005 mol m⁻³). Regardless of the external P supply, in LN plants the initial fluorescence (F₀) increased 12 % in parallel to a quenching of about 14 % in maximum fluorescence (F_m). As a consequence, the variable to maximum fluorescence ratio (F_v/F_m) decreased by about 7 %, and the variable to initial fluorescence ratio (F_v/F₀) was lowered by 25 % in relation to control plants. In LP plants, F_v/F_m remained unchanged whilst F_v/F₀ decreased slightly as a result of 5 % decline in F_m. Under N deficiency, the net photosynthetic rate (P _N) halved at 6 d after imposition of treatment and so remained afterwards. As compared to LN plants, P _N declined in LP plants latter and to a less extent. From 12 d of P deprivation onwards. P _N fell down progressively to display rates similar to those of LN plants only at the end of the experiment. The greater P _N in LP plants was not reflected in larger biomass accumulation in relation to LN beans. In general, P and N limitation affected photosynthesis parameters and growth without showing any synergistic or additive effect between deficiency of both nutrients. This revised version was published online in June 2006 with corrections to the Cover Date.     

Low nitrogen stress regulates chlorophyll fluorescence in coordination with photosynthesis and Rubisco efficiency of rice

Nitrogen (N) is the basis of plant growth and development and, is considered as one of the priming agents to elevate a range of stresses. Plants use solar radiations through photosynthesis, which amasses the assimilatory components of crop yield to meet the global demand for food. Nitrogen is the main regulator in the allocation of photosynthetic apparatus which changes of the photosynthesis (P_n) and quantum yield (F_v/F_m) of the plant. In the present study, dynamics of the photosynthetic establishment, N-dependent relation with chlorophyll fluorescence attributes and Rubisco efficacy was evaluated in low-N tolerant (cv. CR Dhan 311) and low-N sensitive (cv. Rasi) rice cultivars under low-N and optimum-N conditions. There was a decrease in the stored leaf N under low-N condition, resulting in the decreased P_n and F_v/F_m efficiency of the plants through depletion in the activity and content of Rubisco. The P_n and F_v/F_m followed the parallel trend of leaf N content during low-N condition along with depletion of intercellular CO₂ concentration and overall conductance under low-N condition. Photosynthetic saturation curve cleared abrupt decrease of effective quantum yield in the low-N sensitive rice cultivar than the low-N tolerant rice. Also, the rapid light curve highlighted the unacclimated regulation of photochemical and non-photochemical quenching in the low-N condition. The low-N sensitive rice cultivar triumphed non-photochemical quenching, whereas the low-N tolerant rice cultivar rose gradually during the light curve. Our study suggested that the quantum yield is the key limitation for photosynthesis in low-N condition. Regulation of Rubisco, photochemical and non-photochemical quenching may help plants to grow under low-N level.

     

Effects of light quality on growth and development,photosynthetic characteristics and content of carbohydrates in tobacco (<Emphasis Type="Italic">Nicotiana tabacum</Emphasis> L.) plants

In this study, effects of yellow (Y), purple (P), red (R), blue (B), green (G), and white (W) light on growth and development of tobacco plants were evaluated. We showed that monochromatic light reduced the growth, net photosynthetic rate (P _N), stomatal conductance, intercellular CO₂, and transpiration rate of tobacco. Such a reduction in P _N occurred probably due to the stomatal limitation contrary to plants grown under W. Photochemical quenching coefficient (qP), maximal fluorescence of dark-adapted state, effective quantum yield of PSII photochemistry (Φ_PSII), and maximal quantum yield of PSII photochemistry (F_v/F_m) of plants decreased under all monochromatic illuminations. The decline in ΦPSII occurred mostly due to the reduction in qP. The increase in minimal fluorescence of dark-adapted state and the decrease in F_v/F_m indicated the damage or inactivation of the reaction center of PSII under monochromatic light. Plants under Y and G showed the maximal nonphotochemical quenching with minimum P _N compared with the W plants. Morphogenesis of plants was also affected by light quality. Under B light, plants exhibited smaller angles between stem and petiole, and the whole plants showed a compact type, while the angles increased under Y, P, R, and G and the plants were of an unconsolidated style. The total soluble sugar content increased significantly under B. The reducing sugar content increased under B but decreased significantly under R and G compared with W. In conclusion, different monochromatic light quality inhibited plants growth by reducing the activity of photosynthetic apparatus in plants. R and B light were more effective to drive photosynthesis and promote the plant growth, while Y and G light showed an suppression effect on plants growth. LEDs could be used as optimal light resources for plant cultivation in a greenhouse.     

Effect of phosphorus deficiency on the photosynthetic characteristics of rice plants

The effects of phosphorus deficiency on the photosynthetic characteristics were studied in rice seedlings (Oryza sativa L.) every 8 days after treatment. P deficiency caused a significant reduction in the net photosynthesis rate (P _N) in rice plants. During the first 16 days of P deficiency, the maximum efficiency of PSII photochemistry (F _v/F _m), the effective PSII quantum yield (ϕ_PSII), the electron transport rate (ETR) as well as photochemical quenching (qP) in the P-limited rice plants kept close to the control, but the excitation energy capture efficiency of PSII reaction centers (F′_v/F′_m) was significantly declined in the P-deficient rice leaves. Meanwhile, in the stressed leaves, we also found a significant increase in nonphotochemical quenching (NPQ) as well as in the activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX). It was indicated that a series of photoprotective mechanisms had been initiated in rice plants in response to short-term P deficiency. Therefore, PSII functioning was not affected significantly under such stress. As P deficiency continued, the excess excitation energy was accumulated in excess of the capacity of photoprotection systems. When the rice suffered from P deficiency more than 16 days, ϕ_PSII, ETR, and qP were decreased more rapidly than that in the control plants, although NPQ still kept higher in the stressed plants. These results were also consistent with the data on the distribution of excitation energy. The excess energy induced the generation of reactive oxygen species, which might lead to the further damage to PSII functioning. This text was submitted by the authors in English.     

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.     

Silicon Alleviates Drought Stress of Rice Plants by Improving Plant Water Status,Photosynthesis and Mineral Nutrient Absorption

Drought is a major constraint for rice production in the rainfed lowlands in China. Silicon (Si) has been verified to play an important role in enhancing plant resistance to environmental stress. Two near-isogenic lines of rice (Oryza sativa L.), w-14 (drought susceptible) and w-20 (drought resistant), were selected to study the effects of exogenous Si application on the physiological traits and nutritional status of rice under drought stress. In wet conditions, Si supply had no effects on growth and physiological parameters of rice plants. Drought stress was found to reduce dry weight, root traits, water potential, photosynthetic parameters, basal quantum yield (F _v/F ₀), and maximum quantum efficiency of PSII photochemistry (F _v/F _m) in rice plants, while Si application significantly increased photosynthetic rate (Pr), transpiration rate (Tr), F _v/F ₀, and F _v/F _m of rice plants under drought stress. In addition, water stress increased K, Na, Ca, Mg, Fe content of rice plants, but Si treatment significantly reduced these nutrient level. These results suggested that silicon application was useful to increase drought resistance of rice through the enhancement of photochemical efficiency and adjustment of the mineral nutrient absorption in rice plants.     

Response of tomato plants to chilling stress in association with nutrient or phosphorus starvation

The experiments were conducted on two tomato cultivars: Garbo and Robin. Mineral starvation due to plant growth in 20-fold diluted nutrient solution (DNS) combined with chilling reduced the rate of photosynthesis (P _N) and stomatal conductance (g) to a greater extent than in plants grown in full nutrient solution (FNS). In phosphate-starved tomato plants the P _N rate and stomatal conductance decreased more after chilling than in plants grown on FNS. In low-P plants even 2 days after chilling the recovery of CO₂ assimilation rate and stomatal conductance was low. A resupply of phosphorus to low-P plants (low P + P) did not improve the rate of photosynthesis in non-chilled plants (NCh) but prevented P_N inhibition in chilled (Ch) plants. The greatest effect of P resupply was expressed as a better recovery of photosynthesis and stomatal conductance, especially in non-chilled low P + P plants. The F _v/F _m (ratio of variable to maximal chlorophyll fluorescence) decreased more during P starvation than as an effect of chilling. Supplying phosphorus to low-P plants caused the slight increase in the F _v/F _mratio. In conclusion, after a short-term chilling in darkness a much more drastic inhibition of photosynthesis was observed in nutrient-starved or P-insufficient tomato plants than in plants from FNS. This inhibition was caused by the decrease in both photochemical efficiency of photosystems and the reduction of stomatal conductance. The presented results support the hypothesis that tomato plants with limited supply of mineral nutrients or phosphorus are more susceptible to chilling.     

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.     

Lanthanum improves salt tolerance of maize seedlings

In this study, the effects of lanthanum were investigated on contents of pigments, chlorophyll (Chl) fluorescence, antioxidative enzymes, and biomass of maize seedlings under salt stress. The results showed that salt stress significantly decreased the contents of Chl and carotenoids, maximum photochemical efficiency of PSII (F_v/F_m), photochemical quenching (q_P), and quantum efficiency of PSII photochemistry (Φ_PSII), net photosynthetic rate (P_N), and biomass. Salt stress increased nonphotochemical quenching (q_N), the activities of ascorbate peroxidase, catalase, superoxide dismutase, glutathione peroxidase, and the contents of malondialdehyde and hydrogen peroxide compared with control. Pretreatment with lanthanum prior to salt stress significantly enhanced the contents of Chl and carotenoids, F_v/F_m, q_P, q_N, Φ_PSII, P_N, biomass, and activities of the above antioxidant enzymes compared with the salt-stressed plants. Pretreatment with lanthanum also significantly reduced the contents of malondialdehyde and hydrogen peroxide induced by salt stress. Our results suggested that lanthanum can improve salt tolerance of maize seedlings by enhancing the function of photosynthetic apparatus and antioxidant capacity.     

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.     

Chronic mild salinity affects source leaves physiology and productivity parameters of rice plants (Oryza sativa L., cv. Taipei 309)

Aim

In rice, the top two leaves are the major carbohydrate source during grain filling. Physiological performance of these leaves under salinity may allow estimate stress-induced yield loss.

Methods

Greenhouse grown rice plants (cv. Taipei 309) were subjected to 10 and 20 mM NaCl stress levels from germination till maturity. Plant development was measured at the flowering stage and yield parameters were quantified after complete ripening of panicles.

Results

Gas exchange in the main source leaves were not significantly affected by any of the stress levels. However, growth parameters as well as total metabolizable carbohydrates content, chlorophyll content (CCI), maximal efficiency of PSII photochemistry in dark-adapted state (F _v/F _m) and lipid peroxidation were significantly affected. Rice yield, measured as total panicle production, declined to 78 and 21 % of controls in 10 and 20 mM NaCl stress, respectively. Stress-induced yield loss was positively related with the decline in CCI, F _v/F _m and K⁺/Na⁺ ratio as well as with the increase in lipid peroxidation and total soluble carbohydrate contents.

Conclusions

Though the stress levels used in this work are below what is considered the minimal critical threshold of toxicity for rice, they induce significant negative effects on plant development and yield, when present along the whole plant life cycle.     

Photosynthesis is improved by exogenous calcium in heat-stressed tobacco plants

Effects of exogenous calcium chloride (CaCl₂) (20 mM) on photosynthetic gas exchange, photosystem II photochemistry, and the activities of antioxidant enzymes in tobacco plants under high temperature stress (43 °C for 2 h) were investigated. Heat stress resulted in a decrease in net photosynthetic rate (P_n), stomatal conductance as well as the apparent quantum yield (AQY) and carboxylation efficiency (CE) of photosynthesis. Heat stress also caused a decrease of the maximal photochemical efficiency of primary photochemistry (F_v/F_m). On the other hand, CaCl₂ application improved P_n, AQY, and CE as well as F_v/F_m under high temperature stress. Heat stress reduced the activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), peroxidase (POD), whereas the activities of these enzymes either decreased less or increased in plants pretreated with CaCl₂; glutathione reductase (GR) activity increased under high temperature, and it increased more in plants pretreated with CaCl₂. There was an obvious accumulation of H₂O₂ and O₂⁻ under high temperature, but CaCl₂ application decreased the contents of H₂O₂ and O₂⁻ under heat stress conditions. Heat stress induced the level of heat shock protein 70 (HSP70), while CaCl₂ pretreatment enhanced it. These results suggested that photosynthesis was improved by CaCl₂ application in heat-stressed plants and such an improvement was associated with an improvement in stomatal conductance and the thermostability of oxygen-evolving complex (OEC), which might be due to less accumulation of reactive oxygen species.     

Bioregulators protected photosynthetic machinery by inducing expression of photorespiratory genes under water stress in chickpea

Globally, water deficit is one of the major constraints in chickpea (Cicer arietinum L.) production due to substantial reduction in photosynthesis. Photorespiration often enhances under stress thereby protecting the photosynthetic apparatus from photoinhibition. Application of bioregulators is an alternative to counter adverse effects of water stress. Thus, in order to analyze the role of bioregulators in protecting the photosynthetic machinery under water stress, we performed an experiment with two contrasting chickpea varieties, i.e., Pusa 362 (Desi type) and Pusa 1108 (Kabuli type). Water deficit stress was imposed at the vegetative stage by withholding water. Just prior to exposure to water stress, plants were pretreated with thiourea (1,000 mg L^?1), benzyladenine (40 mg L^?1), and thidiazuron (10 mg L^?1). Imposed water deficit decreased relative water content (RWC), photosynthetic rate (P_N), quantum efficiency of PSII (F_v/F_m), and enhanced lipid peroxidation (LPO). However, bioregulator application maintained higher RWC, P_N, F_v/F_m, and lowered LPO under water stress. Expression of Rubisco large subunit gene (RbcL) was low under water stress both in the Kabuli and Desi type. However, bioregulators strongly induced its expression. Although poor expression of two important photorespiratory genes, i.e., glycolate oxidase and glycine decarboxylase H subunit, was observed in Desi chickpea under imposed stress, bioregulators in general and cytokinins in particular strongly induced their expression. This depicts that the application of bioregulators protected the photosynthetic machinery by inducing the expression of RbcL and photorespiratory genes during water deficit stress.     

Effects of low irradiation on photosynthesis and antioxidant enzyme activities in cucumber during ripening stage

In order to investigate the effects of low irradiation (LI) on cucumber (Cucumis sativus L. cv. Jinyou 35) during a ripening stage, our experiment was carried out in a climate chamber. Two levels of PAR were set for plants: normal irradiation [NI, 600 μmol(photon) m^?2 s^?1] and low irradiation [LI, 100 μmol(photon) m^?2 s^?1], respectively. The experiments lasted for 9 d; then both groups of plants were transferred under NI to recover for 16 d. The plants showed severe chlorosis after the LI treatment. Chlorophyll (Chl) a, initial slope, photosynthetic rate at saturating irradiation (P_max), light saturation point, maximal photochemical efficiency of PSII (F_v/F_m), electron transport rate of PSII (ETR), soluble protein content, and catalase (CAT) activity in cucumber leaves decreased under LI stress, while Chl b, carotenoids, light compensation point, nonphotochemical quenching (q_N), superoxide dismutase (SOD), and malondialdehyde (MDA) exhibited an increasing trend under LI. After 16 d of recovery, values of P_max, F_v/F_m, ETR, q_N, SOD, CAT, MDA, and soluble protein were close to those of the control after one, three, and five days of the LI treatment, while those kept under LI for 7 and 9 d could not return to the control level. Therefore, 7 d of LI stress was a meteorological disaster index for LI in cucumber at the fruit stage.     

Significance of mesophyll conductance for photosynthetic capacity and water-use efficiency in response to alkaline stress in Populus cathayana seedlings

Cuttings of Populus cathayana were exposed to three different alkaline regimes (0, 75, and 150 mM Na₂CO₃) in a semicontrolled environment. The net photosynthesis rate (P _N), mesophyll conductance (g _m), the relative limitations posed by stomatal conductance (L _s) and by mesophyll conductance (L _m), photosynthetic nitrogen-use efficiency (PNUE), carbon isotope composition (δ¹³C), as well as specific leaf area (SLA) were measured. P _N decreased due to alkaline stress by an average of 25% and g _m decreased by an average of 57%. Alkaline stress caused an increase of L _m but not L _s, with average L _s of 26%, and L _m average of 38% under stress conditions. Our results suggested reduced assimilation rate under alkaline stress through decreased mesophyll conductance in P. cathayana. Moreover, alkaline stress increased significantly δ¹³C and it drew down CO₂ concentration from the substomatal cavities to the sites of carboxylation (C _i-C _c), but decreased PNUE. Furthermore, a relationship was found between PNUE and C _i-C _c. Meanwhile, no correlation was found between δ¹³C and C _i/C _a, but a strong correlation was proved between δ¹³C and C _c/C _a, indicating that mesophyll conductance was also influencing the ¹³C/¹²C ratio of leaf under alkaline stress.     

Influence of Manganese Toxicity on Photosynthesis in Ricebean (Vigna Umbellata) Seedlings

Influence of manganese (Mn) toxicity on photosynthesis in ricebean (Vigna umbellata) was studied by the measurement of gas exchange characteristics and chlorophyll fluorescence parameters. The net photosynthetic rate (P _N), transpiration rate (E), and stomatal conductance (g _s) were reduced with increasing Mn concentration in nutrient solution. The reduction in g _s and E was more pronounced at 6 d of Mn treatment. However, P _N declined at 2 d of Mn treatment implying that the reduction in photosynthesis was not due to the direct effect of Mn on stomatal regulation. Mn did not affect the maximum efficiency of photosystem 2 (PS2) photochemistry (F_v/F_m). A reduction in photochemical quenching (q_P) and excitation capture efficiency of open PS2 (F_v′/F_m′) with a concomitant increase in q_N was observed. This implies that reduced demand for ATP and NADPH due to the reduction in photosynthesis causes a down-regulation of PS2 photochemistry and thus a high pH gradient (increase in q_N) and limited electron transport (decreased q_P). This revised version was published online in August 2006 with corrections to the Cover Date.