RuBP Carboxylase Activity Research Articles

Effects of Nitrogen Fertilization on Physiological Response of Maize to Soil Salinity

Soil salinization is a global problem that causes huge losses in agricultural production. Salt can interfere with crop absorption and metabolism of nutrients and water, affect plant physiological responses and reduce plant biomass. Maize, a very important economic crop, can adapt to a certain degree of saline-alkali soil. It is essential to understand the physiological indexes of response to soil salinity concentrations and explore the effects of different nitrogen fertilizer treatments on maize growth. In this study, three soil salinity gradients (S1, S2 and S3 were with soil salt concentration, Ssc, of 0, 0.1% and 0.25%, respectively) and two nitrogen application rates (N0 and N1 were without and with nitrogen applied (13.2 g per pot), respectively) were set up. Plant growth and photosynthetic parameters were measured. Whether nitrogen was applied or not, with the increase in Ssc, leaf area, plant height, stem diameter, SPAD, leaf water potential, RuBP carboxylase, and PEP carboxylase activities, photosynthetic rate (A), stomatal conductance (gs), the maximum stomatal conductance (gsmax), and the stomatal morphological parameters such as stomatal width and maximum stomatal area (amax), all showed a downward trend. Under the S1 and S2 treatments, compared with the N0, the N1 treatment alleviated the stress effect of the Ssc on these indicators. However, under S3 treatment, the stress degrees of leaf water potential, gs, gsmax and amax, were aggravated after nitrogen application. This indicated that under the high Ssc of S3, the interaction between nitrogen application and soil salinity should be considered. WUEin increased with the increase in Ssc. Moreover, under N1 treatments, the increase in WUEin with Ssc was greater than that with N0. With the increase in Ssc, whether nitrogen was applied or not, the dry weight of maize declined by 44.2% and 73.0%, respectively, for the S2 and S3 treatments. Under S2 treatment, N1 significantly improved the dry matter mass of maize compared with the N0 treatment. The results showed that soil salt stress can inhibit crop growth, physiology and dry matter accumulation, and that nitrogen application can alleviate this within a specific salinity range. Such results indicate that in saline-alkali areas, whether nitrogen fertilizer is applied or not should depend on the level of Ssc to improve plant growth.

Jasmonic acid and methyl jasmonate modulate growth, photosynthetic activity and expression of photosystem II subunit genes in Brassica oleracea L

The effects of jasmonic acid (JA) and methyl jasmonate (Me-JA) on photosynthetic efficiency and expression of some photosystem (PSII) related in different cultivars of Brassica oleracea L. (var. italica, capitata, and botrytis) were investigated. Plants raised from seeds subjected to a pre-sowing soaking treatment of varying concentrations of JA and Me-JA showed enhanced photosynthetic efficiency in terms of qP and chlorophyll fluorescence. Maximum quantum efficiency of PSII (Fv/Fm) was increased over that in the control seedlings. This enhancement was more pronounced in the Me-JA-treated seedlings compared to that in JA-treated ones. The expression of PSII genes was differentially regulated among the three varieties of B. oleracea. The gene PsbI up-upregulated in var. botrytis after treatment of JA and Me-JA, whereas PsbL up-regulated in capitata and botrytis after supplementation of JA. The gene PsbM showed many fold enhancements in these expressions in italica and botrytis after treatment with JA. However, the expression of the gene PsbM increased by both JA and Me-JA treatments. PsbTc(p) and PsbTc(n) were also found to be differentially expressed which revealed specificity with the variety chosen as well as JA or Me-JA treatments. The RuBP carboxylase activity remained unaffected by either JA or Me-JA supplementation in all three varieties of B. oleracea L. The data suggest that exogenous application of JA and Me-JA to seeds before germination could influence the assembly, stability, and repair of PS II in the three varieties of B. oleracea examined. Furthermore, this improvement in the PS II machinery enhanced the photosynthetic efficiency of the system and improved the photosynthetic productivity in terms of saccharides accumulation.

Effects and mechanisms of foliar application of silicon and selenium composite sols on diminishing cadmium and lead translocation and affiliated physiological and biochemical responses in hybrid rice (Oryza sativa L.) exposed to cadmium and lead

Currently, exploring effective measures to reduce multiple toxic metals accumulation in rice grains is an urgent issue to be tackled. Pot experiments were thus conducted to explore the effects and mechanisms of foliar spraying with composite sols of silicon (Si) and selenium (Se) during tillering to booting stage on diminishing cadmium (Cd) and lead (Pb) translocation to rice grains and affiliated physiological and biochemical responses in rice seedlings grown in Cd + Pb-polluted soils (positive control). Results showed that Cd and Pb contents in leaves or grains were distinctly below the positive control by the sols. Compared to the positive control, transcriptions of Cd transporter-related genes including OsLCT1, OsCCX2, OsHMA2 and OsPCR1 genes in leaves, and OsLCT1, OsCCX2, TaCNR2 and OSPCR1 in peduncles were downregulated by the increasing sols. Meanwhile, Se-binding protein 1 was evidently upregulated, together to retard Cd and Pb translocation to rice grains. The sols not only upregulated transcriptions of Lhcb1, RbcL, and OsBTF3 genes and production of psbA, Lhcb1 and RbcL proteins, but also increased the chlorophylls contents and RuBP carboxylase activities in the leaves, improving photosynthesis. The sols restrained ROS production from NADPH oxidases, but activated glutathione peroxidase, alleviating oxidative stress and damage. Additionally, Se was significantly enriched and was existed as selenomethionine in the rice grains. However, Pb transporter-related genes remain to be specified. Thus, the composite sols have potential to reduce Cd and Pb accumulation, mitigate oxidative damage, and promote photosynthesis and organic Se enrichment in rice plants under Cd and Pb combined pollution.

Effects of light quality ratio on photosynthetic characteristics and quality of purple lettuce

To explore the effects of different light quality ratios on the growth and development of purple lettuce, we used LED intelligent light control console as light source to study the effects of adding different proportions of red and blue light (1:1, 2:1, 4:1 and 1:2) to the white light on photosynthetic characteristics and commercial quality of purple lettuce (Lactuca sativa L. cv 'Zhongshu Purple Lettuce'). The results showed that when the ratio of red to blue light was 4:1, the chlorophyll content, RuBP carboxylase activity, electron transport efficiency of photosynthesis, net photosynthetic rate and growth were all significantly higher than that in control and other treatments, whereas the nitrate content was the lowest. When the ratio of red to blue light was 1:2, the contents of soluble protein, vitamin C, secondary metabolites such as anthocyanins, flavonoids, total phenolic compounds, and the contents of antioxidant capacity were significantly higher than those in the control and other treatments. We concluded that adding appropriate ratios of red to blue light to the white light could increase photosynthesis and improve commercial quality of purple lettuce.

Response and adaptation of photosynthesis of cucumber seedlings to high temperature stress

Cucumber seedlings (Cucumis sativus Jinyou 35) were used to study the effects of high temperature (HT: 42 ℃/32 ℃) and sub-high temperature (SHT: 35 ℃/25 ℃) on its photosynthesis and growth. The results showed that the growth of cucumber seedlings was dramatically inhibited by the high and sub-high temperature stresses. The photosynthetic rate (Pn) was gradually reduced, while intercellular CO2 concentration (Ci) was increased as heat stress lasted. Under heat stress, stomatal conductance (gs), transpiration rate (Tr), photorespiration rate (Pr) and dark respiration rate (Dr) showed a trend from rise to decline in cucumber seedlings, which implied that heat-induced decline of photosynthesis was mainly due to non-stomatal limitation. Maximal photochemical efficiency of PS2 in darkness (Fv/Fm), actual photochemical efficiency (χPS2), photochemical quenching (qP) and electron transport rate (ETR) were severely hampered, while initial fluorescence (Fo) and non-chemical quenching (NPQ) were increased as a result of high and sub-high temperature stresses. Under extended high temperature stress, the activities of RuBP carboxylase (RuBPCase) and Rubisco activase (RCA) as well as the mRNA abundance of Rubisco and RCA were in the trend of decrease, while they were reduced 3 days following the sub-high temperature treatment. The activities and mRNA expressions of sedoheptulose-1,7-bisphosphatase (SBPase) and fructose 1,6-bisphosphate aldolase (FBA) increased initially, but decreased afterwards under heat stress. Taken together, our data suggested that short-term sub-high temperature did not cause photoinhibition under optimal light conditions, however, high temperature led to severe damage to PS2 reaction center in cucumber seedlings. The photosynthetic enzymes were induced by high temperature stress and the induction was affected by temperature and stress duration.

Effect of Brassinolide on Leaf Photosynthetic Function and Yield in Spring Maize Filling Stage

A field experiment was conducted to investigate the effect of plant growth regulator brassinolide(BR) on the regulation of physiological activities of ear leaf, the grain filling and yield in spring maize(Zea mays L., cv. Zhengdan 958). Husks and ear leaves were treated with BR at 15 days after anthesis by foliar spraying. The results showed that the chlorophyll content, photosynthetic rate and activities of PEP carboxylaes, Ru BP carboxylase, sucrose synthase and sucrose phosphate synthase in ear leaf as well as source content in kernel were significantly reduced, while starch accumulation and dry weight of kernel were increased during grain filling. BR significantly increased the content of chlorophyll and photosynthetic rate, enhanced the activities of PEP carboxylaes and Ru BP carboxylase, definitely resulting in enhanced CO2 assimilation. BR markedly increased the activities of sucrose synthase and sucrose phosphate synthase in ear leaf. And in kernel, the accumulation of sucrose and starch as well as the grain filling rate were accelerated by BR. BR treatment significantly reduced the length of ear barren tip, and increased the number of kernels per ear, which led to higher yield. These results suggest that BR treatment at filling stage may be a good strategy to get a high yield in maize, by enhancing activity of resource, delaying sencence and stimulating the flow of assimilate from the source to the sink.

Growth and metabolic responses of maize (C4species) and rice (C3species) genotypes to cadmium toxicity

Toxic metals such as cadmium (Cd) are detrimental for growth of the crops. The differential sensitivity of maize and rice, the two vital cereals, which also belong to C4 and C3 plant types, respectively, to a similar degree of Cd stress is not known that comprised the objective of this study. Maize and rice genotypes were grown hydroponically in the presence of 5 and 10 μM cadmium (Cd) concentrations to examine their relative sensitivity to this metal at growth and metabolic levels. The shoots of maize genotypes accumulated relatively greater cadmium than those of rice while the roots of both the plant types did not differ significantly in cadmium accumulation. The shoots and roots of maize genotypes showed higher growth than rice genotypes under both Cd levels. At similar Cd level, the rice shoots showed greater damage to membranes and chlorophyll than maize shoots. The activities of RUBP carboxylase and phosphoenolpyruvate carboxylase were significantly greater in maize than in rice under Cd stress. Maize genotypes also possessed greater sucrose content and sucrose phosphate synthase activity in their shoots than rice genotypes. The oxidative damage as malondialdehyde and hydrogen peroxide was higher in rice genotypes. The rice genotypes showed greater induction of antioxidants such as superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase and ascorbic acid than maize genotypes. On the other hand, glutathione, proline, metallothioneins, total thiols content and glutathione-s-transferase activity were significantly higher in maize genotypes under Cd stress suggesting their superior capacity to deal with Cd toxicity. The observations indicated that the maize genotypes representing C4 plant type appear to have greater Cd tolerance than the rice genotypes representing C3 plant type. The findings speculate that C4 plant species may adapt to toxic metals such as Cd in a better way than C3 ones but further study involving several representatives of these plant types need to be carried out to corroborate it.

Effect of Altitude on Photosynthetic and Biochemical Characteristics of Podophyllum hexandrum: An Alpine Endangered Medicinal Herb from Central Himalayas

Podophyllum hexandrum is a high value endangered medicinal plant from alpines of Central Himalaya. This study was conducted to explore its conservation and cultivation possibilities at comparatively lower altitudes. Seedlings of the same age, were selected from alpine medicinal crop nursery, Tungnath (3600m) and were planted at the nurseries located at 3600 and 550m altitudes. After well acclimatization at both the altitudes, plants were examined for gas exchange and biochemical characteristics during their active growth period. Our data shows that the foliage nitrogen content was higher and, P, K and C content were found lower in the plants grown at higher elevation as compared to lower altitude; however, these altitudinal variations were not found statistically significant. Chlorophyll a, Chl b, total Chl and Carotenoids were higher and Chl a/b ratio was lower at lower altitude as compared to plant grown at higher altitude. Level of protein and RUBP Carboxylase activity was higher and level of PEP Carboxylase activity was observed lower in the plants grown at high elevation as compared to their counterparts at lower altitude. Light optima for photosynthesis (PARopt) was observed around 2000µmolm–2s–1 at both the elevations in this species. A shift in optimum temperature for photosynthesis (Topt) was observed towards higher temperature in plants grown at lower altitude as compared to higher elevation, which indicates their potential to survive in the comparatively warmer environment. Topt was observed at 20°C in plants grown at higher altitude whereas, at lower altitude Topt was observed at 25°C for the plants of P. hexandrum. Conductance and transpiration was found slightly higher at high altitude as compared to plants grown at lower altitude. In view of these results, it can be concluded that these species can be cultivated at lower altitudes to meet their ever-increasing industrial demand.

Phenotypic properties of Sulfobacillus thermotolerans: Comparative aspects

The phenotypic characteristics of the species Sulfobacillus thermotolerans Kr1(T), as dependent on the cultivation conditions, are described in detail. High growth rates (0.22-0.30 h(-1)) and high oxidative activity were recorded under optimum mixotrophic conditions at 40 degrees C on medium with inorganic (Fe(II), S(0), or pyrite-arsenopyrite concentrate) and organic (glucose and/or yeast extract) substrates. In cells grown under optimum conditions on medium with iron, hemes a, b, and, most probably, c were present, indicating the presence of the corresponding cytochromes. Peculiar extended structures in the form of cylindrical cords, never observed previously, were revealed; a mucous matrix, likely of polysaccharide nature, occurred around the cells. In the cells of sulfobacilli grown litho-, organo-, and mixotrophically at 40 degrees C, the enzymes of the three main pathways of carbon utilization and some enzymes of the TCA cycle were revealed. The enzyme activity was maximum under mixotrophic growth conditions. The growth rate in the regions of limiting temperatures (55 degrees C and 12-14 degrees C) decreased two- and tenfold, respectively; no activity of 6-phosphogluconate dehydrogenase, one of the key enzymes of the oxidative pentose phosphate pathway, could be revealed; and a decrease in the activity of almost all enzymes of glucose metabolism and of the TCA cycle was observed. The rate of 14CO2 fixation by cells under auto-, mixo-, and heterotrophic conditions constituted 31.8, 23.3, and 10.3 nmol/(h mg protein), respectively. The activities of RuBP carboxylase (it peaked during lithotrophic growth) and of carboxylases of heterotrophic carbon dioxide fixation were recorded. The physiological and biochemical peculiarities of the thermotolerant sulfobacillus are compared versus moderately thermophilic sulfobacilli.

Carbon and nitrogen metabolism in barley plants exposed to UV-B radiation.

The effect of UV-B radiation on FW, leaf and stem length, photosynthetic O2 evolution, levels of carbohydrates and nitrates, and extractable activities of some of the enzymes involved in C and N metabolism was evaluated in barley (Hordeum vulgare L. cv. Express) seedlings during the 9 days following transfer to an UV-B enriched environment. The results show that under our experimental conditions UV-B radiation scarcely affects the photosynthetic competence of barley leaves, expressed as RuBP carboxylase (EC 4.1.1.39) activity, O2 evolution rate and chlorophyll content. Nevertheless, this treatment induced significant alterations of the enzyme activity of nitrate reductase (EC 1.6.6.1) and glutamine synthetase (EC 6.3.1.2), although only after a few days of treatment. The effects were not confined to the exposed tissue, but were detectable also at the root level. In fact, nitrate reductase decreased in response to UV-B in both leaf and root tissue, whereas glutamine synthetase was affected only in the root. In contrast, nitrate content was not influenced by the treatment, neither in root nor in leaf tissue, whilst leaf sucrose diminished in exposed plants only on the last day of treatment.

Changes in Mulberry Leaf Metabolism in Response to Water Stress

A series of experiments were conducted to characterize the water stress-induced changes in the activities of RuBP carboxylase (RuBPCO) and sucrose phosphate synthase (SPS), photosystem 2 activity, and contents of chlorophylls, carotenoids, starch, sucrose, amino acids, free proline, proteins and nucleic acids in mulberry (Morus alba L. cv. K-2) leaves. Water stress progressively reduced the activities of RuBPCO and SPS in the leaf extracts, the chlorophyll content, and PS2 activity in isolated chloroplasts. Plants exposed to drought showed lower content of starch and sucrose but higher total sugar content than control plants. While the soluble protein content decreased under water stress, the amino acid content increased. Proline accumulation (2.5-fold) was noticed in stressed leaves. A reduction in the contents of DNA and RNA was observed. Reduced nitrogen content was associated with the reduction in nitrate reductase activity. SDS-PAGE protein profile showed few additional proteins (78 and 92 kDa) in the water stressed plants compared to control plants.

Morphological and Physiological Characteristics Associated with Heat Tolerance in Creeping Bentgrass

Growth of creeping bentgrass (Agrostis palustris Huds.) is limited by heat stress during summer. Understanding the morphological and physiological characteristics associated with heat tolerance of creeping bentgrass would facilitate breeding programs in improving those characteristics. The objectives of this experiment were to study responses of single‐leaf photosynthetic capacity, tillering, and root growth to heat stress for two creeping bentgrass cultivars differing in performance under heat stress and to determine the relative importance of these factors in heat tolerance. The cultivars L‐93 (heat tolerant) and Penncross (heat sensitive) were exposed to day/night temperatures of 20/15°C (control) and 35/30°C (heat stress) in growth chambers. Canopy net photosynthetic rate (Pn), single‐leaf Pn, and RuBP carboxylase (Rubisco) activity were reduced by heat stress for both cultivars. Canopy Pn of L‐93 was significantly higher (more than double at 64 d of treatment) than that of Penncross under heat stress. Cultivars were not different in single‐leaf Pn and Rubisco activity under either control or heat stress conditions. High temperature reduced plant density, tiller density, root number, and root fresh weight for both cultivars. L‐93 had higher plant and tiller densities, greater root to tiller ratio, and more and finer roots than Penncross under control and high temperature conditions. The better performance of L‐93 under heat stress was due largely to its morphological characteristics, including tillering and root growth, but was not related to single‐leaf photosynthetic capacity. Narrow leaves, small plants, dense tillers, big root system, and high root‐to‐shoot ratio could be used to select heat tolerant cultivars.

Enzymes of Ammonia Assimilation, Photosynthesis, and Respiration in Alfalfa Leaves of Different Ages

The activities of enzymes involved in ammonia metabolism ferredoxin-dependent glutamate synthase (Fd-GOGAT), glutamine synthetase (GS) and glutamate dehydrogenase (GDH), the rates of photosynthetic oxygen evolution, dark respiration, and the activity of RuBP carboxylase (RuBPC) were determined in alfalfa (Medicago sativa L.) leaves taken from the apex (apical leaves), from the second to the fourth internode (mature leaves) and from the bottom of the canopy (basal leaves). Photosynthetic rate and the activities of RuBPC, GS and Fd-GOGAT showed their maximum in the mature leaves. The respiration rate together with amino acid and ammonium contents decreased with leaf age, whereas the opposite was true for GDH activity. Basal leaves still maintained substantial levels of chlorophylls, GS and Fd-GOGAT activities and oxygen evolution rate, thus suggesting that photosynthesis has some role in the reassimilation of the nitrogen liberated during protein degradation.

Interactive effects of elevated carbon dioxide and growth temperature on photosynthesis in cotton leaves

Cotton (Gossypium hirsutum L., cv DPL 5415) plants were grown in naturally lit environment chambers at day/night temperature regimes of 26/18 (T-26/18), 31/23 (T-31/23) and 36/28 °C (T-36/28) and CO2 concentrations of 350 (C-350), 450 (C-450) and 700 μL L-1 (C-700). Net photosynthesis rates, stomatal conductance, transpiration, RuBP carboxylase activity and the foliar contents of starch and sucrose were measured during different growth stages. Net CO2 assimilation rates increased with increasing CO2 and temperature regimes. The enhancement of photosynthesis was from 24 μmol CO2 m-2 s-1 (with C-350 and T-26/18) to 41 μmol m-2 s-1 (with C-700 and T-36/28). Stomatal conductance decreased with increasing CO2 while it increased up to T-31/23 and then declined. The interactive effects of CO2 and temperature resulted in a 30% decrease in transpiration. Although the leaves grown in elevated CO2 had high starch and sucrose concentrations, their content decreased with increasing temperature. Increasing temperature from T-26/18 to 36/28 increased RuBP carboxylase activity in the order of 121, 172 and 190 μmol mg-1 chl h-1 at C-350, C-450 and C-700 respectively. Our data suggest that leaf photosynthesis in cotton benefited more from CO_2 enrichment at warm temperatures than at low growth temperature regimes.

A comparative anatomical and biochemical analysis in salsola (Chenopodiaceae) species with and without a kranz type leaf anatomy: a possible reversion of c4 to c3 photosynthesis

Leaf anatomy was studied by light and electron microscopy and the leaf activities of RUBP carboxylase, PEP carboxylase, and malic enzyme were assayed in: Salsola australis and S. oreophila grown on the West Pamirs at 1800 m altitude; in S. australis grown on the East Pamirs at 3860 m; and in S. arbusculiformis grown in the Kisil-Kum desert in Middle Asia near 500 m. Carbon isotope fractionation ratio values also were measured on whole leaf tissue for 18 Salsola species field collected in these and other regions of the former USSR. S. australis leaves are cylindrical and in cross section exhibit a peripheral ring of mesophyll and then an inner ring of bundle sheath type cells; and its biochemical characteristics and deltaC values are typical of a C4 species of the NADP-malic enzyme malate-forming group. These traits were expressed independent of the plant growth altitude up to 4000 m. C4 type deltaC values were obtained in 14 of the Salsola species. Anatomical, structural, and biochemical features typical of the C4 syndrome were absent in S. oreophila and S. arbusculiformis. Four Salsola species, including these two, had C3-type deltaC values. Their cylindrical leaves in cross section exhibited two to three peripheral rings as layers of palisade parenchyma. Although their vascular bundles were surrounded by green bundle sheath cells, their organelle numbers were comparable to those in mesophyll cells. Neither bundle sheath cell wall thickenings nor dimorphic chloroplasts in two leaf cell types were observed. In S. oreophila, there was a high activity of RuBP carboxylase, but a low activity of C4 cycle enzymes. Interpretation of these data lends evidence to the hypothesis that a small group of C3 Salsola species, including S. oreophila, S. arbusculiformis, S. montana, and S. pachyphylla, arose as the result of a reversion of a C4 to a C3 type of photosynthetic CO2 fixation in the cooler climates of Middle Asia.

Mepiquat chloride (PIX)-induced changes in photosynthesis and growth of cotton

Mepiquat chloride (N, N-dimethylpiperidinium chloride), well known as PIX, is a potential systemic plant growth regulator. The effects of PIX on plant height, stem elongation, leaf area, net photosynthetic rates, chlorophyll content, sucrose and starch levels, and RuBP carboxylase activity in cotton (Gossypium hirsutum L. cv. DES 119) plants were measured. PIX was sprayed (0, 7.65, 15.3, 30.6 or 61.2 g active ingredient ha−1) on the plants at first square (25 days after emergence) and measurements were made at frequent intervals. Plant height was clearly reduced by PIX. The total length of vegetative branches and fruiting branches was 40% and 50% less than the control. Total leaf area in PIX treated plants was 16% less than the control. Net photosynthetic rates were 25% less in PIX-treated leaves. PIX treated leaves had more chlorophyll content. The activity of RuBP carboxylase was decreased in PIX treated plants. Starch accumulation was noticed in PIX treated leaves while sucrose content was not changed. The data reported here suggest that reduced growth responses induced by PIX results in partial loss of photosynthetic capacity in cotton at least up to 20 days after application of the growth regulator.

Effects of salicylic acid on growth and photosynthesis in barley plants

Summary Hordeum vulgare L./cv. Alfa /was grown for 7 days over a range of salicylic acid (SA) concentrations (100 μM – 1 mM), and the effects on growth and photosynthetic performance were examined. Leaf and root growth, and chlorophyll and protein contents were reduced by SA. Gas exchange measurements and analysis of carboxylating enzyme activities were used to study the reactions of photosynthesis to SA. Longterm treatment of barley seedlings with SA led to a noticeable decrease in both the initial slope of the curves representing net photosynthetic rate versus intercellular CO 2 concentration and the maximal rate of photosynthesis. The calculated values of the intercellular CO 2 concentration, CO 2 -compensation point and the maximal carboxylating efficiency of ribulose-l,5-bisphosphate carboxylase (RuBP carboxylase, EC 4.1.1.39) support the suggestion that biochemical factors are involved in the response of photosynthesis to SA. RuBP carboxylase activity decreased with increasing SA concentrations, while the activity of phosphoenolpyruvate carboxylase (PEP carboxylase, EC 4.1.1.31) increased. When 7-day-old barley seedlings were supplied with SA through the transpiration stream for 2 h no changes in photosynthetic reactions were observed. Six and 24 h after treatment of barley seedlings with SA the rate of photosynthesis declined in a concentration-dependent manner. Possible reasons for the responses of photosynthesis to SA are discussed.

An examination of the advantages of C3‐C4 intermediate photosynthesis in warm environments

Abstract. The photosynthetic responses to temperature in C3, C3‐C4 intermediate, and C4 species in the genus Flaveria were examined in an effort to identify whether the reduced photorespiration rates characteristic of C3‐C4 intermediate photosynthesis result in adaptive advantages at warm leaf temperatures. Reduced photorespiration rates were reflected in lower CO2 compensation points at all temperatures examined in the C3‐C4 intermediate, Flaveria floridana, compared to the C3 species, F. cronquistii. The C3‐C4 intermediate, F. floridana, exhibited a C3‐like photosynthetic temperature dependence, except for relatively higher photosynthesis rates at warm leaf temperatures compared to the C3 species, F. cronquistii. Using models of C3 and C3‐C4 intermediate photosynthesis, it was predicted that by recycling photorespired CO2 in bundle‐sheath cells, as occurs in many C3‐C4 intermediates, photosynthesis rates at 35°C could be increased by 28%, compared to a C3 plant. Without recycling photorespired CO2, it was calculated that in order to improve photosynthesis rates at 35°C by this amount in C3 plants, (1) intercellular CO2 partial pressures would have to be increased from 25 to 31 Pa, resulting in a 57% decrease in water‐use efficiency, or (2) the activity of RuBP carboxylase would have to be increased by 32%, resulting in a 22% decrease in nitrogen‐use efficiency. In addition to the recycling of photorespired CO2, leaves of F. floridana appear to effectively concentrate CO2 at the active site of RuBP carboxylase, increasing the apparent carboxylation efficiency per unit of in vitro RuBP carboxylase activity. The CO2‐concentrating activity also appears to reduce the temperature sensitivity of the carboxylation efficiency in F. floridana compared to F. cronquistii. The carboxylation efficiency per unit of RuBP carboxylase activity decreased by only 38% in F. floridana, compared to 50% in F. cronquistii, as leaf temperature was raised from 25 to 35°C. The C3‐C4 intermediate, F. ramosissima, exhibited a photosynthetic temperature temperature response curve that was more similar to the C4 species, F. trinervia, than the C3 species, F. cronquistii. The C4‐like pattern is probably related to the advanced nature of C4‐like biochemical traits in F. ramosissima The results demonstrate that reductions in photorespiration rates in C3‐C4 intermediate plants create photosynthetic advantages at warm leaf temperatures that in C3 plants could only be achieved through substantial costs to water‐use efficiency and/or nitrogen‐use efficiency.

Gas Exchange in Populus maximowiczii in Relation to Potassium and Phosphorus Nutrition

Summary Cuttings of Populus maximowiczii Henry were grown in half-strength Hoagland nutrient solution with varying levels of K (0, 0.3 and 3 mM) and P (0, 0.05 and 0.5 mM). The effects of K and P deficiency on gas exchange and biomass of the plants was investigated. The absence of K markedly suppressed plant growth and reduced biomass of the roots more than that of the shoots. The 0.3 mM K and zero K (0 mM) treatments produced a significant increase in stomatal resistance and a decrease in net photosynthesis rate and RuBP carboxylase activity. The transpiration rate in all treatments was closely correlated with stomatal resistance (r = 0.94). The CO 2 compensation point increased with K supply. Biomass accumulation at 0.05 mM P was equal to that of the control, but was greatly reduced in the zero P treatment where the growth of shoots was much more affected than that of the roots. In the 0.05 mM P treatment stomatal resistance also increased. There was, however, a lower correlation between stomatal resistance and transpiration rate. Net photosynthesis and RuBP carboxylase activity markedly declined with the zero P treatment but not in the treatment with 0.05 mM P. The data show that K and P deficiency affect gas exchange and growth of poplar in different ways. Their influence on gas exchange and growth of the plants is discussed.

Metabolism and distribution of grasses in tropical flooded savannas in Venezuela

ABSTRACTFlooded savannas in south-west Venezuela consist of seasonal, marshy and flooded grassland communities, the distribution of which depends on soil level. Seasonal grasslands are dominated by tuft-forming, rhizomatous C4-grasses (exemplified byPaspalum chaffanjoniiin this study), while flooded grasslands are dominated by stoloniferous grasses which develop rooted, floating culms during the rainy season(Leersia hexandraandHymenachne amplexi-caulis).The interface between these two communities is a marshy grassland dominated by stoloniferous grass species which tolerate flooding to a depth of 10 to 25 cm(Panicum laxumandLeersia hexandra).All perennial grass species in flooded grasslands behave as typical C3 plants, while marshy grasslands are dominated byPanicum laxum,a species with reduced photo-respiration, low RuBP/PEP-carboxylase ratios, and low absolute RuBP-carboxylase activity compared to C3 grasses. It also has lower photosynthetic rates than the other grass species.Hymenachne amplexicaulisappears to be the least drought tolerant from the species selected, in accordance with its distribution in the wettest side of the flooding gradient studied. The other species showed marked reduction in relative water content and pronounced increase in leaf proline content in drought experiments lasting 6 to 30 days. Alcohol dehydrogenase increased markedly in response to anaerobiosis in the root environment in the tuft forming grasses, while the stoloniferous species with ascending culms were least affected by this treatment, probably as a result of better aeration inside the culms and also to the production of adventitious roots in upper nodes. Nitrate reductase activity increased as a result of anaerobiosis in the roots but not in the leaves, of all species exceptLeersia hexandra.No difference among the species was found in malate accumulation, or the activity of malic enzyme.