Decline In Net Photosynthetic Rate Research Articles

Manganese-induced oxidative stress, ultrastructural changes, and proteomics studies in rice plants.

Manganese (Mn) is an essential element for plant growth but it becomes phytotoxic at higher concentrations. The effect of Mn-excess in hydroponics medium was examined on growth, oxidative stress, and ultrastructural changes in chloroplasts and mitochondria as well proteomic alterations in rice (Oryza sativa L.) seedlings. Seedlings grown with 1mM and 2mM Mn in nutrient medium for 8days showed decline in length and fresh biomass, and decline in net photosynthetic rate, transpiration rate, and stomatal conductance. Shoots of the seedlings had higher Mn content than roots. Mn-treated seedlings showed increased production of O2·-, H2O2, and .OH, increased lipid peroxidation, increased carbonylation of proteins, and increased proteolytic activity compared to untreated seedlings. Mn-treated seedlings showed disorganization and swelling of chloroplasts with appearance of plastoglobuli in TEM images and deformity in shape of mitochondria. Using confocal microscopy depolarization of mitochondrial membrane was observed marked by green fluorescence of JC-1 dye monomers in Mn-treated roots. Proteomics studies from leaves of Mn-treated seedlings involving 2DE and PDQuest analysis showed differential expression of 23 proteins, among which MALDI-TOF/TOF mass spectrometry analysis revealed Mn-led downregulation of photosynthesis-related proteins, namely oxygen-evolving complex protein associated with PSII, PAP-3, enzyme involved in protein folding peptidyl-prolyl cis-trans isomerase (PPIase) and carbohydrate metabolizing enzymes hydrolase, fructose-bisphosphate aldolase, transketolase, and isocitrate dehydrogenase, whereas ATP-dependent Clp protease, peroxidase, and nucleic acid-binding proteins were downregulated due to Mn treatment. Results indicate that Mn-excess inhibits growth of rice plants with induction of oxidative stress, causing structural alterations in chloroplasts, mitochondria, inhibiting photosynthesis, and downregulating many photosynthesis and carbohydrate metabolism-related proteins.

Combined effects of elevated CO<sub>2</sub> concentration and drought stress on photosynthetic performance and leaf structure of cucumber (Cucumis sativus L.) seedlings

Drought stress is one of the main environmental factors limiting plant growth and productivity of many crops. Elevated carbon dioxide concentration (eCO2) can ameliorate, mitigate, or compensate for the negative impact of drought on plant growth and enable plants to remain turgid and functional for a longer period. In order to investigate the combined effects of eCO2 and drought stress on photosynthetic performance and leaf structures, we analyzed photosynthetic characteristics and structure and ultrastructure of cucumber leaves. The decline in net photosynthetic rate under moderate drought stress occurred due to stomatal limitation alone, while under severe drought stress, it was the result of stomatal and nonstomatal limitations. Conversely, eCO2 improved photosynthetic performance under moderate drought stress, increased the lengths of the palisade cells and the number of chloroplasts per palisade cell under severe drought stress, and significantly increased the grana thickness under moderate drought stress. Additionally, eCO2 significantly decreased stomatal density, stomatal widths and stomatal aperture on the abaxial surface of leaves under moderate drought stress. In conclusion, eCO2 can alleviate the negative effects of drought stress by improving the drought resistance of cucumber seedlings through stomatal modifications and leaf structure.

Growth, photosynthetic performance and antioxidative response of 'Hass' and 'Fuerte' avocado (Persea americana Mill.) plants grown under high soil moisture

‘Hass’ and ‘Fuerte’ avocado plants were grown under well-watered or waterlogged conditions. Results indicated significant effects on the majority of the allometric parameters in waterlogged plants, with ‘Fuerte’ displaying a more pronounced growth inhibition. Waterlogged conditions caused a progressive and simultaneous decline in net photosynthetic rate and stomatal conductance, earlier in ‘Fuerte’ than in ‘Hass’. Maximal potential quantum yield of PSII was unaffected by the soil water regime and/or variety and leaf water potential values in waterlogged plants were not more negative compared with control plants. ‘Fuerte’ waterlogged plants exhibited increased contents of thiobarbituric acid reactive substances, whereas oxidative injury was not detected in ‘Hass’. Finally, none of the two cultivars displayed valuable antioxidant potential, as evidenced by the decreased activities of the antioxidant enzymes superoxide dismutase, guaiacol peroxidase, glutathione peroxidase, and ascorbate peroxidase.

Effects of irradiance on the photosynthetic traits, antioxidative enzymes, and growth of Cryptotaenia japonica

Growth, photosynthetic characteristics, chlorophyll (Chl) fluorescence parameters, and peroxidation of membrane lipids of Cryptotaenia japonica were studied under differing irradiances (15, 35, 60, and 100 % of full irradiance). At full irradiance, C. japonica exhibited a typical decline in net photosynthetic rate (PN) at midday, which was not observed in the other irradiance treatments. This indicates a possible photoinhibition for C. japonica at the high irradiance. Diurnal patterns of stomatal conductance (gs) were remarkably similar to those of PN in each irradiance treatment, and the intercellular CO2 concentration (ci) had the opposite trend. C. japonica growing under 60 % of the full irradiance exhibited the highest plant height, stem diameter, leaf area, and biomass. The initial fluorescence (F0) value was lowest at 60 % of the full irradiance. Maximal fluorescence (Fm), potential activity of photosystem II (PS II) (Fv/F0), and maximal photochemical efficiency of PS II (Fv/Fm) values were highest at 60 % of full irradiance and lowest at 15 % of the full irradiance. The malondialdehyde (MDA) content in 15 % and 100 % of the full irradiance were higher than under the other irradiances. During the treatment, catalase, peroxidase, and superoxide dismutase activities firstly increased and then declined under 15 % and 100 % of the full irradiance and were steadily low under 60 % of the full irradiance, indicating a low production of reactive oxygen species. Therefore, C. japonica thrived best under 60 % of the full irradiance.

Genotypic Differences in Spectral and Photosynthetic Response of Peanut to Iron Deficiency

To assess the genetic variability of peanut (Arachis hypogaea L.) in tolerance to iron (Fe) deficiency, spectral and photosynthetic parameters of 12 peanut cultivars were determined. The results showed that peanut exhibit significant variations in spectral and photosynthetic parameters within cultivars in response to Fe deficiency. The 12 peanut cultivars were separated into three groups, which include (i) a Fe-deficient tolerant cultivar (‘Zhenghong 3’), (ii) a Fe-deficient sensitive cultivar (‘Huayu 22’), and (iii) ten intermediate cultivars. Iron deficiency caused an increase in root biomass, root/shoot ratio, structure independent pigment index and intercellular carbon dioxide (CO2) concentration, but resulted in a decrease in net photosynthetic rate (Pn), quantum yield of PS II photochemistry (Fv/Fm), effective quantum yield of PS II (ΦPS II), photochemical reflectance index, red edge point, and chlorophyll normalized difference index. Iron deficiency-induced decline in net photosynthetic rate may be resulted from the reduction of photosynthetic pigment contents and inhibition of PSII photochemistry.

莳萝蒿适应盐渍环境的Na+区域化方式和生理特征研究

莳萝蒿是广泛分布在我国北方的一种特殊类型的菊科盐生植物,阐明莳萝蒿特殊的耐盐机制和生理特征有助于丰富植物抗盐性研究的内容.用0、100、200、300、400 mmol/L NaCl处理莳萝蒿7 d后,比较莳萝蒿盐处理植株与对照植株在生长和生理方面的差异,并详细分析了Na⁺在莳萝蒿体内的积累水平和区域化方式.结果显示:莳萝蒿虽然能够耐受400 mmol/L NaCl,但盐处理显著抑制了莳萝蒿的生长,整株鲜重随着盐处理浓度的升高逐渐减小.在水分生理方面,随着盐处理浓度的升高,莳萝蒿叶片细胞的渗透调节能力逐渐增强,其叶片肉质化程度却呈逐渐降低的趋势.分析盐处理对光合作用的影响发现,盐处理后莳萝蒿叶片光合速率与气孔导度显著下降,而其PSⅡ光化学活性并未受到抑制,叶绿素含量甚至逐渐增大,说明盐处理后莳萝蒿叶片光合速率的降低主要是由于气孔因素造成的,而不是由于光合结构被破坏.莳萝蒿体内的Na⁺含量随着盐处理浓度的升高显著增加,400 mmol/L NaCl条件下叶、茎、根中的Na⁺含量分别高达321.4、242.1和182.3 μmol/g鲜重;莳萝蒿体内的Na⁺ 70%以上积累在叶片内,而叶片内98%左右的Na⁺积累在叶片原生质体中,叶片原生质体中的Na⁺平均浓度是质外体1.2-1.8倍,推测其叶片细胞内存在着有效的Na⁺区域化机制.盐处理后莳萝蒿叶片液泡膜V-H⁺-ATPase的质子泵活性比对照增加了30%-50%,液泡膜Na⁺/H⁺逆向转运活性则增加至对照的4-7倍,进一步证实莳萝蒿叶片具有较强的液泡Na⁺区域化能力.随着盐处理浓度的升高,Na⁺在叶片中的分布比例相对减少,V-H⁺-ATPase的质子泵活性和Na⁺/H⁺逆向转运活性增幅也减缓.这种Na⁺区域化能力使莳萝蒿获得了较强的耐盐性,有效保护了其光系统,降低了细胞汁液渗透势.但是盐处理后这种耐盐方式并不能阻止莳萝蒿叶片肉质化程度和光合活性下降,莳萝蒿生长仍然受盐抑制,说明Na⁺区域化是莳萝蒿适应盐渍环境的必要条件而非充分条件.;<em>Artemisia anethifolia </em>(<em>Compositae</em>) is a halophyte that is widely distributed in northern areas of China. Studying the mechanisms by which this plant adapts to high levels of salt will increase our understanding of salt adaptation in vascular plants. <em>A. anethifolia</em> plants were treated with 0, 100, 200, 300, and 400 mmol/L NaCl for 7 d, respectively. Then, the differences in growth and physiology were compared between salt-treated and control <em>A. anethifolia </em>plants. In particular, the Na⁺ accumulation levels and Na⁺ compartmentation patterns in <em>A. anethifolia</em> plants were analyzed in detail. Although the <em>A. anethifolia</em> plants were able to survive under 400 mmol/L NaCl, salt-treated plants showed lower fresh weight as the NaCl concentration increased, indicating that plant growth was inhibited by salt. The leaves of <em>A. anethifolia </em>showed an increased capacity for osmotic adjustment but a decreased degree of leaf succulence with increasing NaCl concentrations. Photosynthetic analyses showed that there was a gradual decline in net photosynthetic rate and stomatal conductance of salt-treated leaves with increasing NaCl concentrations. However, the maximal efficiency of photosystem II photochemistry (<em>F</em>_v/<em>F</em>_m) in <em>A. anethifolia</em> leaves was not inhibited by salt, and the chlorophyll content even increased with increasing salt concentrations. These observations suggested that the decreased photosynthetic rate was due to stomatal factors, rather than damage to the components of the photosynthetic machinery. The Na⁺ content in <em>A. anethifolia</em> plants tended to increase with increasing salt concentrations. The Na⁺ content in leaves, stems, and roots of <em>A. anethifolia</em> was 321.4, 242.1, and 182.3 μmol/g FW, respectively, in the 400 mmol/L NaCl treatment. More than 70% of the Na⁺ absorbed by salt-treated <em>A. anethifolia</em> plants accumulated in their leaves, and approximately 98% of the Na⁺ that accumulated in leaves was localized in leaf protoplasts. The average Na⁺ concentration in protoplasts of <em>A. anethifolia </em>leaf tissue was 1.2-1.8 times that in the apoplast. These results indicate that efficient Na⁺ compartmentation occurred in <em>A. anethifolia</em> leaf cells. The V-H⁺-ATPase proton pump activity of salt-treated leaves was 30%-50% higher than that of control leaves, and the tonoplast Na⁺/H⁺ antiporter activity of salt-treated leaves was 4-7 times that in leaves of control plants. These findings suggested that <em>A. anethifolia</em> plants have a strong ability to compartmentalize Na⁺ in the vacuole. As the concentrations of salt increased, the Na⁺ distribution ratio in leaves decreased. Likewise, the range of increased V-H⁺-ATPase proton pump activity and tonoplast Na⁺/H⁺ antiporter activity decreased as the salt concentration increased. Na⁺compartmentation plays an important role in the salt tolerance of <em>A. anethifolia</em> plants, because it protects their photosystems and results in lower osmotic potential in the leaf cells. However, Na⁺ compartmentation could not prevent the decrease in the degree of leaf succulence and the photosynthetic activity of <em>A. anethifolia </em>leaves under highly saline conditions, and so their growth was inhibited by salt. These results suggest that Na⁺ compartmentation is necessary but not sufficient for adaptation of <em>A. anethifolia</em> to a saline environment.

Comparison of the photosynthetic characteristics of four Lycoris species with leaf appearing in autumn under field conditions.

The diurnal trends of gas exchange and chlorophyll fluorescence parameters in four Lycoris species (L. houdyshelii, L. aurea, L. radiata var. pumila and L. albiflora) were determined and compared with a portable photosynthesis analysis system. Our study revealed that L. houdyshelii had the lowest light compensation point (LCP), while the other three species had higher LCP (12.37–14.99 μmol m−2 s−1); L. aurea had the highest light saturation point (LSP) (1,189 μmol m−2 s−1), and L. houdyshelii and L. albiflora had lower LSP with the values being 322 and 345 μmol m−2 s−1, respectively, and L. radiata var. pumila showed the intermediate LSP. Both the species L. houdyshelii and L. albiflora exhibited a typical and obvious decline in net photosynthetic rate (P N) during midday, which was not observed in L. aurea. This indicated a possible photoinhibition in L. houdyshelii and L. albiflora as the ratio of variable to maximum fluorescence (Fv/Fm) values were higher in these two species. The minimal fluorescence (F0) values were lower in L. aurea and L. radiata var. pumila. The diurnal changes of transpiration rate (E) in all four species presented only one peak, appearing between 11:00 h or 13:00 h. By using simple correlation analyses, it was observed that the environmental factors affecting P N were different among four species and the main factors were photosynthetic photon flux density (PPFD) and relative humidity especially for L. aurea and L. radiata. The results of studying indicated that the four species could be divided into two groups. The species L. radiata var. pumila and L. aurea were more adapted to a relatively high irradiance, and L. houdyshelii and L. albiflora could be grown in moderate-shade environment in order to scale up their growth and productivity.

Evaluation of zinc tolerance and accumulation potential of the coastal shrub Limoniastrum monopetalum (L.) Boiss.

The coastal shrub Limoniastrum monopetalum is capable of growth in soil containing extremely high concentrations of heavy metals. A greenhouse experiment was conducted in order to investigate the effects of a range of Zn concentrations (0–130mmoll−1) on growth and photosynthetic performance, by measuring relative growth rate, total leaf area, plant height, gas exchange, chlorophyll fluorescence parameters and photosynthetic pigment concentrations. We also determined the total zinc, nitrogen, phosphorus, sulphur, calcium, magnesium, sodium, potassium, iron and copper concentrations in the plant tissues. The study species demonstrated hypertolerance to Zn stress, since survival was recorded with leaf concentrations of up to 1700mgZnkg−1 dry mass when treated with 130mmolZnl−1. L. monopetalum exhibited little overall effects on photosynthetic function at Zn levels of up to 90mmoll−1. At greater external Zn concentration, plant growth was negatively affected, due in all probability to the recorded decline in net photosynthetic rate, which may be linked to the adverse effect of the metal on photosynthetic electron transport. Growth parameters were virtually unaffected by leaf tissue concentrations as high as 1400mgZnkg−1 dry mass thus indicating that this species could play an important role in the phytoremediation of Zn-polluted areas.

Cadmium at high dose perturbs growth, photosynthesis and nitrogen metabolism while at low dose it up regulates sulfur assimilation and antioxidant machinery in garden cress ( Lepidium sativum L.)

Metal contamination of soils has become a worldwide problem and great environmental threat, as these metals accumulate in soils and plants in excess, and enter the food chain. Increased cadmium (Cd) uptake from contaminated soils leads to altered plant metabolism and limits the crop productivity. The experimental crop, Lepidium sativum L. (Garden Cress, Family: Brassicaceae) is a medicinally and economically important plant. An experiment was conducted to examine the effect of different concentrations of Cd (0, 25, 50 or 100 mg kg −1 soil) on the performance of L. sativum. Cd accumulation in roots and leaves (roots > leaves) increased with the increaseing Cd concentration in soil. High Cd concentration (100 mg Cd kg −1 soil) inhibited the leaf area and plant dry mass and significant decline in net photosynthetic rate ( P N), stomatal conductance (gs), intercellular CO 2 (C i), chlorophyll (Chl a, Chl b, total Chl) content, carbonic anhydrase (CA; E.C. 4.2.1.1) activity, nitrate reductase (NR; E.C. 1.6.6.1) activity and nitrogen (N) content was also observed. However, ATP-sulfurylase (ATP-S; EC. 2.7.7.4) activity, sulfur (S) content and activities of antioxidant enzymes such as superoxide dismutase (SOD; E.C. 1.15.1.1); catalase (CAT; E.C. 1.11.1.6); ascorbate peroxidase (APX; E.C. 1.11.1.11) and glutathione reductase (GR; E.C. 1.6.4.2) and glutathione (GSH) content were increased. Specifically, the decrease in NR activity and N content showed that Cd affects N metabolism negatively; whereas, the increase in ATP-S activity and S content suggests the up-regulation of S assimilation pathway for possible Cd tolerance in coordination with enhanced activities of antioxidant enzymes and GSH. High Cd concentration (100 mg Cd kg −1 soil) perturbs the L. sativum growth by interfering with the photosynthetic machinery and disrupting the coordination between carbon, N and S metabolism. On the other hand, at low Cd concentration (25 mg Cd kg −1 soil) co-ordination of S and N metabolism complemented to the antioxidant machinery to protect the growth and photosynthesis of L. sativum plants.

Photosynthetic performance of Lycoris radiata var. radiata to shade treatments

The effects of shade on the growth, leaf photosynthetic characteristics, and chlorophyll (Chl) fluorescence parameters of Lycoris radiata var. radiata were determined under differing irradiances (15, 65, and 100% of full irradiance) within pots. The HI plants exhibited a typical decline in net photosynthetic rate (P N) during midday, which was not observed in MI- and LI plants. This indicated a possible photoinhibition in HI plants as the ratio of variable to maximum fluorescence (Fv/Fm) value was higher and the minimal fluorescence (F0) was lower in the, and LI plants. Diurnal patterns of stomatal conductance (g s) and transpiration rate (E) were remarkably similar to those of P N at each shade treatments, and the intercellular CO2 concentration (C i) had the opposite change trend. Under both shading conditions, the light saturation point, light compensation point and photon-saturated photosynthetic rate (P max) became lower than those under full sunlight, and it was the opposite for the apparent quantum yield (AQY). The higher the level of shade, the lower the integrated daytime carbon gain, stomatal and epidermis cell densities, specific leaf mass (SLM), bulb mass ratio (BMR), leaf thickness, and Chl a/b ratio. In contrast, contents of Chls per dry mass (DM), leaf area ratio (LAR), leaf mass ratio (LMR), leaf length, leaf area and total leaf area per plant increased under the same shade levels to promote photon absorption and to compensate for the lower radiant energy. Therefore, when the integrated daytime carbon gain, leaf area and total leaf area per plant, which are the main factors determining the productivity of L. radiata var. radiata plant, were taken into account together, this species may be cultivated at about 60∼70% of ambient irradiance to promote its growth.

Effects of intraspecific competition on growth and photosynthesis of Atriplex prostrata

We investigated the effect of intraspecific competition on growth parameters and photosynthesis of the salt marsh species Atriplex prostrata Boucher in order to distinguish the effects of density-dependent growth inhibition from salt stress. High plant density caused a reduction of 30% in height, 82% in stem dry mass, 80% in leaf dry mass, and 95% in root dry mass. High density also induced a pronounced 72% reduction in leaf area, 29% decrease in length of mature internodes and 50% decline in net photosynthetic rate. The alteration of net photosynthesis paralleled growth inhibition, decreasing from 7.6 ± 0.9 μmol CO 2 m −2 s −1 at low density to 3.5 ± 0.4 μmol CO 2 m −2 s −1 at high density, indicating growth inhibition caused by intraspecific competition is mainly due to a decline in net photosynthesis rate. Plants grown at high density also exhibited a reduction in stomatal conductance from 0.7 ± 0.1 mol H 2O m −2 s −1 at low density to 0.3 ± 0.1 mol H 2O m −2 s −1 at high density and a reduction in transpiration rate from 6.0 ± 0.3 mmol H 2O m −2 s −1 at low density to 4.3 ± 0.3 mmol H 2O m −2 s −1 at high density. Biomass production was inhibited by an increase in plant density, which reduced the rate of photosynthesis, stomatal conductance and leaf area of plants.

Diurnal and Seasonal Photosynthesis in Two Asparagus Cultivars with Contrasting Yield

To investigate physiological characters underpinning spear yield in asparagus (Asparagus officinalis L.), diurnal and seasonal changes in photosynthetic parameters were measured under field conditions in two cultivars with contrasting yield. Seasonal patterns in photosynthetic parameters were strongly dependent on cladophyll developmental stage in both cultivars. The greatest photosynthetic rates of 8.94 ± 0.54 μmol m−2 s−1 for the high‐yielding cultivar (ASP‐69) and 6.50 ± 0.38 μmol m−2 s−1 for the low‐yielding cultivar (ASP‐03) were observed in fully expanded cladophyll tissue measured in mid‐summer (February) when both photon flux density (PFD) and temperature were at a maximum. A significant decline in net photosynthetic rate (A) was measured in April, when plants experienced colder night temperatures and shorter day lengths. A close correlation between A and stomatal conductance (gs) (r = 0.84) was observed. Timing of cladophyll initiation and duration did not appear to be significant factors contributing to cultivar difference in photosynthesis. Variation in photosynthetic capacity between the two cultivars was related to significant differences in cladophyll thickness and specific leaf weight (SLW). The results substantiate the conclusion that both metabolic and anatomical factors play significant roles in determining differences in photosynthetic capacity between the two asparagus cultivars studied.

Diurnal and Seasonal Photosynthesis in Two Asparagus Cultivars with Contrasting Yield

To investigate physiological characters underpinning spear yield in asparagus (Asparagus officinalis L.), diurnal and seasonal changes in photosynthetic parameters were measured under field conditions in two cultivars with contrasting yield. Seasonal patterns in photosynthetic parameters were strongly dependent on cladophyll developmental stage in both cultivars. The greatest photosynthetic rates of 8.94 ± 0.54 μmol m−2 s−1 for the high-yielding cultivar (ASP-69) and 6.50 ± 0.38 μmol m−2 s−1 for the low-yielding cultivar (ASP-03) were observed in fully expanded cladophyll tissue measured in mid-summer (February) when both photon flux density (PFD) and temperature were at a maximum. A significant decline in net photosynthetic rate (A) was measured in April, when plants experienced colder night temperatures and shorter day lengths. A close correlation between A and stomatal conductance (gs) (r = 0.84) was observed. Timing of cladophyll initiation and duration did not appear to be significant factors contributing to cultivar difference in photosynthesis. Variation in photosynthetic capacity between the two cultivars was related to significant differences in cladophyll thickness and specific leaf weight (SLW). The results substantiate the conclusion that both metabolic and anatomical factors play significant roles in determining differences in photosynthetic capacity between the two asparagus cultivars studied.

Interactive Effects of Sulphur Dioxide and Mineral Nutrient Supply on Photosynthetic Characteristics and Yield in Four Wheat Cultivars

Field experiments were conducted on four cultivars of wheat (Triticum aestivum L.) to examine the variability in cultivar response to sulphur dioxide (SO2) under different concentrations of mineral nutrients. Thirty-days-old plants were exposed for 8 weeks to 390&plusmn;20 &micro;g m-3 (0.15 ppm) SO2 for 4 h per day, 5 d per week. Decline in net photosynthetic rate, contents of pigments and nitrogen, biomass and grain yield of each cultivars were due to SO2 at all the nutrient concentrations studied. However, the magnitude of reduction was higher in plants grown without nutrient application. On the basis of the reductions in photosynthesis and yield, the susceptibility of wheat cultivars to SO2 was in the order of Malviya 213 &gt; Malviya 37 &gt; Malviya 206 &gt; Malviya 234 at recommended dose of NPK, whereas the same without the nutrients was Malviya 206 &gt; Malviya 234 &gt; Malviya 213 &gt; Malviya 37.

Seasonal Changes in Photosynthesis and Stomatal Conductance of Five Plant Species from a Semiarid Ecosystem

In order to determine whether stomatal closure alone regulates photosynthesis during drought under natural conditions, seasonal changes in leaf gas exchange were studied in plants of five species differing in life form and carbon fixation pathway growing in a thorn scrub in Venezuela. The species were: Ipomoea carnea, Jatropha gossypifolia, (C3 deciduous shrubs), Alternanthera crucis (C4 deciduous herb), and Prosopis juliflora and Capparis odoratissima (evergreen phreatophytic trees). Xylem water potential (Ψ) of all species followed very roughly the precipitation pattern, being more closely governed by soil water content in I. carnea and A. crucis. Maximum rate of photosynthesis, Pmax, decreased with Ψ in I. carnea, J. gossypifolia, and A. crucis. In I. carnea and J. gossypifolia stomatal closure was responsible for a 90 % decline in net photosynthetic rate (PN) as Ψ decreased from -0.3 to -2.0 MPa, since stomatal conductance (gs) was sensitive to water stress, and stomatal limitation on PN increased with drought. In A. crucis, PN decreased by 90 % at a much lower Ψ (-9.3 MPa), and gs was relatively less sensitive to Ψ. In P. juliflora and C. odoratissima, Pmax, gs, and intercellular CO2 concentration (Ci) were independent of soil water content. In the C3 shrubs stomatal closure was apparently the main constraint on photosynthesis during drought, Ci declining with Ψ in I. carnea. In the C4 herb, Ci was constant along the range of Ψ values, which suggested a coordinated decrease in both gs and mesophyll capacity. In P. juliflora Ci showed a slow decrease with Ψ which may have been due to seasonal leaf developmental changes, rather than to soil water availability.

Short term effects of bisulphite on pollution‐tolerant and pollution sensitive populations of Sphagnum cuspidatum Ehrh. (ex. Hoffm.)

summaryMoss tissue from two distinct populations of Sphagnum cuspidatum from N, Wales (‘unpolluted’) and the S. Pennines (‘polluted’) was exposed to 0.2 mM HSO3− over a 24–26 h period. The rates of HSO2−, disappearance in the bathing solution were monitored in conjunction with tissue dark respiration, net photosynthetic rate and chlorophyll a fluorescence induction kinetics. Slow disappearance of HSO3− in the presence of moss tissue from N. Wales (40% of initial concentration remaining after 24 h) resulted in a rapid (&lt; 2 h) loss of photosynthetic oxygen evolution, and a gradual marked decline in tissue dark respiration and chlorophyll fluorescence quenching. In the S. Pennine tissue, rapid HSO3− disappearance in the growth medium (complete after &lt; 4 h) resulted in a marked recovery of photosynthetic and respiratory pathways after the initial inhibitory effects of HSO3− on these processes. Pretreating S. Pennine moss tissue with EDTA prior to exposure increased HSO3− residence time with consequent rapid decline in net photosynthetic rate, rendering the response of the moss similar to that from N. Wales (‘unpolluted’ site).In a second experiment, identical moss samples from the two study sites were exposed to 0.2 mM HSO3− for a period of 24 d. One set of plants from each population was prewashed in 5mM EDTA to remove extracellularly‐bound cations prior to HSO3− exposure. Growth decrease was greater in the unpretreated moss N. Wales (74% with respect to tissue not exposed to HSO3−) than that from the S. Pennines (45.5%). After 10 days the pigments of the former were visibly bleached. No bleaching was observed in the latter.In the moss from N. Wales, removal of the extracellular cations with EDTA only slightly enhanced the effect of HSO3− on growth, whilst in the S. Pennine plants EDTA pretreatment resulted in a further growth reduction of 13%.The differential response of the moss from the two sites is discussed as a function of the extracellular transition metal concentration (Notably Fe) associated with the plants.

Soluble carbohydrate allocation to roots, photosynthetic rate of leaves, and nitrate assimilation as affected by nitrogen stress and irradiance.

Upon resupply of exogenous nitrogen to nitrogen-stressed plants, uptake rate of nitrogen is enhanced relative to nonstressed plants. Absorption of nitrogen presumably is dependent on availability of carbohydrates in the roots. A buildup in soluble carbohydrates thus should occur in roots of nitrogen-stressed plants, and upon resupply of exogenous nitrogen the increased uptake rate should be accompanied by a rapid decline in carbohydrates to prestress levels. To evaluate this relationship, three sets of tobacco plants growing in a complete hydroponic solution containing 1.0 mM NO3- were either continued in the complete solution for 21 d, transferred to a minus-nitrogen solution for 21 d, or transferred to a minus-nitrogen solution for 8-9 d and then returned to the 1.0 mM NO3- solution. These nitrogen treatments were imposed upon plants growing at photosynthetic photon flux densities of 700 and 350 micromoles m-2 s-1. Soluble carbohydrate levels in roots increased during onset of nitrogen stress to levels that were fourfold greater than in roots of non-stressed plants. Following resupply of external nitrogen, a rapid resumption of nitrogen uptake was accompanied by a decline in soluble carbohydrates in roots to levels characteristic of nonstressed plants. This pattern of soluble carbohydrate levels in roots during onset of and recovery from nitrogen stress occurred at both irradiance levels. The response of net photosynthetic rate to nitrogen stress could be expressed as a nonlinear function of concentration of reduced nitrogen in leaves. The net photosynthetic rate at a given concentration of reduced nitrogen, however, averaged 10% less at the lower than at the higher irradiance. The decline in net photosynthetic rate per unit of reduced nitrogen in leaves at the lower irradiance was accompanied by an increase in the nitrate fraction of total nitrogen in leaves from 20% at the higher irradiance to 38% at the lower irradiance.

EFFECTS OF A SIMULATED ACID RAIN EPISODE ON PHOTOSYNTHESIS AND RECOVERY IN THE CARIBOU‐FORAGE LICHENS, CLADINA STELLARIS (OPIZ.) BRODO AND CLADINA RANGIFERINA (L.) WIGG.

SummaryThe effect of simulated acid rain on net photosynthesis of two common boreal forest lichens, Cladina stellaris and Cladina rangiferina, was investigated. Repeated measurements of net photosynthetic rate (NPR) were conducted on podetial tips over a week‐long period to assess both immediate response to an acid spray treatment (2:1 H2SO4:HNO3; pH range 2.5 to 3.5) and subsequent photosynthetic recovery after the solution of artificial rain had been rinsed out of the cells. A control group, which was treated with pH 5.6 episodes, was used to monitor natural fluctuations in NPR over time in a laboratory environment. Although lowered photosynthetic rates were observed for both species immediately following a low pH treatment, differences between groups were not significant. Three days following treatment, however, significant decreases in photosynthesis of up to 40% of pretreatment values were found for pH 2.5 and 3.0‐sprayed C. stellaris, and a decline in NPR of 25% was recorded for pH 3.0‐treated C. rangiferina. Six days after the simulated rain episode, a 20% recovery of photosynthesis had occurred in pH 2.5‐treated C. stellaris, while pH 3‐0‐sprayed C. rangiferina underwent a 20% stimulation of NPR compared to the pretreatment value. In spite of the demonstrated ability to recover partially following a single low pH event, it is hypothesized that even temporary declines in NPR could restrict growth potential and biomass accumulation of boreal cyptogamic species which have a short growing season and photosynthesize only when moisture is available. Results of the study indicate that C. stellaris is more sensitive to single low pH events than is C. rangiferina.

Response of Oats to Water Deficit. I. Physiological Characteristics1

AbstractAlthough an insufficient supply of water is commonly associated with reduced crop yields, the manner in which water deficits affect the functioning of many common field crops is not well established. The purpose of this study was to quantify the effects of a water deficit on some physiological processes of spring oats (Avena sativa L. ‘Jaycee’). The plants were subjected to water deficits under a field environment for 9 to 11 days at the boot stage, at anthesis through early grain formation stage, and at both stages in combination. Relative leaf water content (RLWC) as determined by the relative turgidity method was used as a measure of water deficit. Leaf diffusion resistance (LDR) was monitored using a diffusion porometer. The net photosynthetic rate (NPR) of the oat plants was determined using an infrared gas analyzer. Severe water stress at all stages of development studied caused about an 80% decline in NPR. LDR increased with a decrease in RLWC. Stress during boot stage reduced the photosynthetic sensivity of the plants to subsequent stress at the anthesis through early grain formation stage. On rewatering, LDR and RLWC recovered completely within 1 to 6 days; however, the net photosynthetic rate of plants stressed at the anthesis through grain formation stage showed a more gradual and incomplete recovery than plants stressed at other stages. The plants stressed during boot stage alone, but adequately watered thereafter, exhibited significantly higher NPR than the control plants. The results indicate that leaf diffusion resistance and net photosynthetic rate of oats show acute sensitivity to plant water deficits. Hence, these parameters hold promise as aids in determining permissible water deficit limits and in irrigation scheduling.

Diurnal Trends in Net Photosynthetic Rate and Carbohydrate Levels of Soybean Leaves

A study was made of diurnal trends in net photosynthetic rate and carbohydrate levels of unifoliolate leaves of soybean (Glycine max L. Merrill) under constant environmental conditions (50,000-lux light intensity, 24.5 C air temperature, 60% relative humidity, and 300 microliters of CO(2) per liter of air).Net photosynthetic rate remained relatively constant between 4 and 10 hours after the lights were turned on but then gradually declined to 85% of this rate by the end of the 16-hour photoperiod. The decline in net photosynthetic rate was due to increases in both stomatal diffusion resistance and residual resistance to CO(2).The decline in net photosynthetic rate began when the rate of starch accumulation began to decline rapidly. At this time, there also appeared to be an increase in soluble carbohydrate level. The results suggest that when a high starch level was reached, further starch synthesis was impaired, leading to an increase in soluble carbohydrate level and, consequently, a reduction in net photosynthetic rate.