Showing papers in "Brazilian Journal of Plant Physiology in 2004"

The effects of salt stress on growth, nitrate reduction and proline and glycinebetaine accumulation in Prosopis alba

Abstract: Prosopis alba (algarrobo) is one of the most important salt-tolerant legumes used in the food and furniture industries. The effects of salinity on some growth and physiological parameters in algarrrobo seedlings were investigated. 17-Day-old seedlings were subjected to three salt treatments by adding NaCl to the growth medium in 50 mmol.L-1 increments every 24 h until the final concentrations of 0, 300 and 600 mmol.L-1 were reached. Only the highest NaCl concentration affected all of the considered parameters. Thus, 600 mmol.L-1 NaCl caused a significant reduction in root and shoot growth, but an increase in the root/shoot ratio. Leaf relative water content, nitrate content and nitrate reductase activity in leaves and roots were also decreased. At 300 and 600 mmol.L-1, the glycinebetaine content was significantly increased in both leaves and roots but this was not found for proline content. Total soluble carbohydrates increased only in roots. The results suggest that glycinebetaine enhancement may be important for osmotic adjustment in Prosopis alba under salinity stress.

Effects of salt stress on plant growth, stomatal response and solute accumulation of different maize genotypes

Abstract: Seeds from eight different maize genotypes (BR3123, BR5004, BR5011, BR5026, BR5033, CMS50, D766 and ICI8447) were sown in vermiculite, and after germination they were transplanted into nutrient solution or nutrient solution containing 100 mmolL-1 of NaCl and placed in a greenhouse During the experimental period plant growth (dry matter, shoot to root dry mass ratio, leaf area, relative growth rate, and net assimilation rate), leaf temperature, stomatal conductance, transpiration, predawn water potential, sodium, potassium, soluble amino acids and soluble carbohydrate contents were determined in both control and salt stressed plants of all genotypes studied Salt stress reduced plant growth of all genotypes but the genotypes BR5033 and BR5011 were characterized as the most salt-tolerant and salt-sensitive, respectively Stomatal response of the salt-tolerant genotype was not affected by salinity Among the studied parameters, shoot to root dry mass ratio, leaf sodium content and leaf soluble organic solute content showed no relation with salt tolerance, ie, they could not be considered as good morpho-physiological markers for maize salt tolerance In contrast, sodium and soluble organic solutes accumulation in the roots as a result of salt stress appeared to play an important role in the acclimation to salt stress of the maize genotypes studied, suggesting that they could be used as physiological markers during the screening for salt tolerance

Exploring drought tolerance in coffee: a physiological approach with some insights for plant breeding

Abstract: This paper briefly reviews some selected traits (leaf area, crown architecture, water-use efficiency and carbon isotope discrimination, water relations and root characteristics), which may be explored in breeding programmes to tolerance to drought stress in arabica and robusta coffee.

Interactions between leaf water potential, stomatal conductance and abscisic acid content of orange trees submitted to drought stress

Abstract: Laranjeiras 'Pera' de dois anos e meio de idade enxertadas em limoeiros 'Cravo' foram cultivadas em vasos de 100 L com solo e submetidas ao estresse hidrico, pela suspensao da irrigacao. Valores de assimilacao de CO2 (A), transpiracao (E) e condutância estomatica (gs) comecaram a declinar a partir do setimo dia de estresse, embora o potencial da agua na folha as 6 a.m. (Ypd) e as 2 p.m. (Y2) comecaram a declinar a partir do quinto dia de deficiencia hidrica. A concentracao interna de CO2 (Ci) das plantas estressadas aumentou a partir do mesmo dia, atingindo uma concentracao maxima no dia de estresse mais severo. Assim, a eficiencia de carboxilacao dessas plantas, verificada pela relacao A/Ci foi menor nesse dia, nao ocorrendo recuperacao dessa razao mesmo dez dias apos a reidratacao. Ate o quinto dia apos a reirrigacao, somente o Ypd e o Y2 se recuperaram, sendo semelhantes aos das plantas controle. Quando o potencial da agua na folha diminuiu, ocorreu uma tendencia de aumento da concentracao de acido abscisico (ABA) nas folhas; da mesma forma, foi possivel verificar que a condutância estomatica diminuiu em funcao do decrescimo em Y2. O acumulo de ABA e o fechamento estomatico ocorreram quando Y2 foi menor que -1.0 MPa. A deficiencia hidrica em laranjeiras 'Pera' aumentou o conteudo de ABA, com consequente fechamento estomatico e decrescimo em Y2.

Growth, photosynthesis, nodule nitrogen and carbon fixation in the chickpea cultivars under salt stress

Abstract: Four cultivars of chickpea, two of them of Mediterranean origin (kabuli), CSG 9651, BG 267 and two Indian (desi) types, CSG 8962, DCP 92-3, differing in their salt sensitivities were identified after screening ten genotypes in saline soils. The cultivars CSG 9651 and CSG 8962 were salt tolerant while BG 267 and DCP 92-3 were salt sensitive, respectively. The seeds of different cultivars were inoculated with Mesorhizobium ciceri, strain F: 75 and the plants were grown in the greenhouse. After the establishment of symbiosis, 15-day-old seedlings were administered doses of saline at varying concentrations (0, 4, 6, 8 dSm-1 NaCl, Na2SO4, CaCl2). Plants were harvested at 40, 70 and 100 days after sowing, for analyses. The main aim was to compare the relative salt tolerance of both desi and kabuli cultivars in terms of nitrogen fixation and carbon metabolism, as well as to ascertain whether the negative effects of saline stress on nitrogen fixation were due to a limitation of photosynthate supply to the nodule or to a limitation on the nodular metabolism that sustains nitrogenase activity. Plant growth, nodulation and nitrogenase activity was more severely affected in BG 267 and DCP 92-3 under salinity treatments (6 and 8 dSm-1) compared with CSG 9651 and CSG 8962. Nodule number as well as nodule mass increased under salt stress in CSG 9651 and CSG 8962 which might be responsible for their higher nitrogen fixation. Salinity reduced leaf chlorophyll and Rubisco activities in all cultivars. However, tolerant cultivars CSG 9651 and CSG 8962 showed smaller declines than the sensitive ones. Phosphoenolpyruvate carboxylase (PEPCase) activity increased significantly in the nodules of tolerant cultivars under salt stress at all harvests, and this was clearly related to salt concentrations. Our results suggest that in salt-affected soils tolerant cultivars have more efficient nodulation and support higher rates of symbiotic nitrogen fixation than the sensitive cultivars.

Gas exchange and yield response to foliar phosphorus application in Phaseolus vulgaris L. under drought

Abstract: This study was conducted to evaluate the effect of foliar Pi application on gas exchange and yield of bean genotypes submitted to a mild water deficit at the pre-flowering stage. In the first experiment, when extra Pi (10 g.L-1) was sprayed on leaves during water stress or during recovery, there was no effect on gas exchange or yield in the A320, Carioca and Ouro Negro genotypes. However, net CO2 assimilation (A) of A320 and Ouro Negro was less affected, but not significantly, than Carioca at the end of the stress, when Pi was supplied five days before water deficit. In the second experiment, two different doses of Pi (10 and 20 g.Pi L-1) were sprayed five days before water deficit on the Carioca genotype. During the last three days of the mild water deficit, A values were significantly higher for the Pi20 treatment when compared to the control plants without extra Pi supply. The intrinsic water use efficiency for plants receiving Pi20 was significantly higher than for the other treatments. In addition, seed dry weight per plant was higher for plants receiving Pi20 dose than for plants with Pi10 and its control.

Seasonal leaf gas exchange and water potential in a woody cerrado species community

Abstract: Predawn leaf water potential (Ypd) and morning values of leaf gas exchange, as net photosynthesis (A), stomatal conductance (gs), transpiration (E), and morning leaf water potential (Ymn) were determined seasonally in 22 woody cerrado species growing under natural conditions. Despite the lower mean values of Ypd in the dry season (-0.35 ± 0.23 MPa) compared to the wet season (-0.08 ± 0.03 MPa), the lowest Ypd in the dry season (-0.90 ± 0.00 MPa) still showed a good nocturnal leaf water status recovery for all species studied through out the year. Mean gs values dropped 78 % in the dry season, when the vapor pressure of the air was 80% greater than in the wet season. This reduction in gs led to an average reduction of 33% in both A and E, enabling the maintainance of water use efficiency (WUE) during the dry season. Network connectance analysis detected a change in the relationship between leaf gas exchange and Ymn in the dry season, mainly between gs-E and E-WUE. A slight global connectance value increase (7.25 %) suggested there was no severe water stress during the dry season. Multivariate analysis showed no link between seasonal response and species deciduousness, suggesting similar behavior in remaining leaves for most of the studied species concerning leaf gas exchange and Ymn under natural drought.

Ascorbic acid biosynthesis: a precursor study on plants

Abstract: leaves, proposed what can be considered the main pathway of biosynthesis of AsA, in which L-galactose (L-GAL) is a keyprecursor. This paper reports the effectiveness of some precursors (cold or radiolabeled) in the biosynthesis of AsA in differentplants: green sweet pepper, white-pulp guava, red-pulp guava, papaya and strawberry at two ripening stages (mature green andripe for papaya and mature green and half red for strawberry) and broccoli. The ‘Smirnoff-Wheeler’ pathway was functioningand active in all sources studied, as demonstrated by the increase in AsA contents and incorporation of labeled precursors intoAsA. In papaya, the AsA content in the ripe fruit was higher than in the mature green, indicating the synthesis of AsA duringripening. On the other hand, the AsA content in the mature green strawberry was similar to that of the half red fruits. Our datademonstrate that L-GAL and L-Galactono-1,4-lactone (L-GL) are effective precursors for the biosynthesis of AsA in fruits andalso provided additional evidence for the participation of D-mannose (D-MAN) and D-glucose-1P in the biosynthesis of AsAin plants.

Changes in growth and superoxide dismutase activity in Hydrilla verticillata L. under abiotic stress

Abstract: A study was made of the role of various heavy metals (chromium, zinc, copper and cadmium) together with the effects of NaCl-salinity, PEG-mediated water stress and temperature on growth and superoxide dismutase (SOD) activity in hydrilla (Hydrilla verticillata L.). Almost all metals studied reduced growth but presented a modulatory effect at lower concentrations. NaCl-salinity, water stress and temperature treatments also reduced growth. SOD activity was inhibited by both chromium and zinc but increased by copper and cadmium. NaCl-salinity increased SOD activity while water stress decreased it. SOD showed thermostability with a higher activity being recorded at 45oC.

EgLFY, the Eucalyptus grandis homolog of the Arabidopsis gene LEAFY is expressed in reproductive and vegetative tissues

Abstract: The EgLFY gene cloned from Eucalyptus grandis has sequence homology to the floral meristem identity gene LEAFY (LFY) from Arabidopsis and FLORICAULA (FLO) from Antirrhinum. EgLFY is preferentially expressed in the developing eucalypt floral organs in a pattern similar to that described previously for the Arabidopsis LFY. In situ hybridization experiments have shown that EgLFY is strongly expressed in the early floral meristem and then successively in the primordia of sepals, petals, stamens and carpels. It is also expressed in the leaf primordia of adult trees. The expression of the EgLFY coding region under control of the Arabidopsis LFY promoter could complement strong lfy mutations in transgenic Arabidopsis plants. These data suggest that EgLFY plays a similar role to LFY in flower development and that the basic mechanisms involved in flower initiation and development in Eucalyptus may be similar to those occurring in Arabidopsis.

Narrow band spectral indexes for chlorophyll determination in soybean canopies [Glycine max (L.) Merril]

Abstract: Photosynthetic pigments are essential for plant development. Quantifying these pigments in great extensions of agricultural crops is an important objective in remote sensing for agricultural purposes. This information can be used to produce a more accurate estimation of the physiological state of the vegetation, for species discrimination and productivity estimation. The aim of the present study was to (a) evaluate the potential for estimating chlorophyll content of crop canopies, using narrow band spectral indexes, and (b) in this respect compare the performances of NDVI (a multispectral wide band index) and two narrow band vegetation indexes (R750/700 and R750/550). Experiments were carried out under greenhouse conditions whereby soybean [Glycine max (L.), Merril] was monitored with a high-resolution spectroradiometer (10 nm at 365-1,126 nm range) during the phenological cycle of the crop. Chlorophyll (a, b and total) contents were determined weekly in the laboratory. A statistical correlation analysis was performed between narrow band spectral indexes against chlorophyll content and r2 coefficients near 0.84 were obtained. For NDVI r2 was around 0.51. These analyses showed that R750/700 and R750/550 ratios are very useful indexes for chlorophyll determination and very effective compared with NDVI (one of the wide band indexes widely used). Thus, it can be stated that hyperspectral remote sensing has great potential for providing a reliable estimate of photosynthetic pigment content at the canopy level through evaluated indexes and other such indexes that might arise. Thus, further studies should be carried out for evaluating other indexes at the canopy level, both in the laboratory and under field conditions, using spectroradiometers and hyperspectral images, aimed at providing information for agricultural purposes.

Transpiration, leaf diffusive conductance, and atmospheric water demand relationship in an irrigated acid lime orchard

Abstract: Leaf vapor diffusive conductance (gl) and transpiration (T) measurements in an irrigated orchard of the acid lime "Tahiti" were carried out in a subtropical climatic condition in the State of Sao Paulo, Brazil. Measurements were made using steady-state null-balance porometers and were taken throughout the day, on several occasions from February to November, 1998. During September and October, measurements were performed on exposed and shaded young and fully expanded leaves, as well as on old leaves inside the tree canopy. The old leaves showed lower values of gl and T when compared to the other groups. In the other months, measurements were taken only with exposed and shaded fully expanded leaves. The highest values of gl and T were obtained from February to April. For exposed leaves gl was higher from early morning to midday, decreasing thereafter with some oscillation in the afternoon. Shaded leaves produced a pattern of increasing gl and T from 8:00-9:00 a.m. to midday, followed by a decrease when values matched those of the exposed leaves in the afternoon. In April, the first measurement in midmorning showed low gl and T values, increasing sharply in the exposed leaves until midday and then decreasing until the end of the afternoon. Between May and November, gl and T decreased sharply compared to the previous months. Boundary line and regression analysis were used to find the mathematical relationships between mean values of gl for the tree and photosynthetic photons flux density, temperature, and vapor pressure deficit of the air measured in the orchard. Based on this analysis, the reasons for the gl and T diurnal and annual variation patterns are discussed, with emphasis on the effects of atmospheric variables upon stomatal regulation.

Growth and total-N content of Prosopis juliflora (SW) D. C. are stimulated by low NaCl levels

Abstract: In this study the early effects of salinity on growth, total N and ion accumulation and partitioning in Prosopis juliflora (common name algaroba) were studied with 30-day-old plants grown for 8 days in aerated nutrient solution (NS) free of (control) or supplied with 25, 50, 75 and 100 mmol.L-1 NaCl. Plants grown in 25 and 50 mmol.L-1 NaCl accumulated a total dry mass (DM) and shoot N content greater than the control. However, at 75 and 100 mmol.L-1 NaCl such parameters were diminished. Whole plant transpiration rates were higher with 25 and 50 mmol.L-1 NaCl and lower with 75 and 100 mmol.L-1 NaCl. Salinity did not influence shoot (80%) and root (70%) water content (WC) but reduced the K+ content of shoot while that of root was not affected. Increasing external NaCl concentrations increased K+/Na+ ratios of both plant parts. The data support the hypothesis that increase in transpiration rates and in shoot total N may account for growth stimulation of algaroba under mild salinity.

Effect of maleic hydrazide on respiratory parameters of stored onion bulbs (Allium cepa L.)

Abstract: -treated bulbs was 75%, 38% and33% at 20oC respectively, and 50%, 22% and 17% at 10oC respectively. At 4oC, the sprouting level of control bulbs did notexceed 7% and was 5% for MH-treated bulbs. No difference was observed in rotting of control and MH-treated bulbs, and thelow rotting observed at 4oC could be attributed to low temperature rather than maleic hydrazide treatment.

Changes in network connectance and temporal dynamics of gas exchange in Citrus sinensis under different evaporative demands

Abstract: Stomatal aperture is an essential factor both in regulation of transpiration and net photosynthesis. This regulation is especially important in the response of plants to drought or to an increase in leaf-to-air vapor pressure difference (VPD); however, such a regulation is part of a complex dynamical environment, associated with multiple regulatory pathways. Accordingly, we studied the effects of VPD on gas exchange of Citrus sinensis via the evaluation of two complementary analytic approaches, to approach an understanding of the full scope of the system interactions. First, we used classical statistical methodologies, e.g., means, coefficient of variation, and linear correlation. Second, we used measures developed for more model-independent applications, Approximate Entropy (ApEn) to evaluate the irregularity or complexity of gas exchange time-series, and network connectance to evaluate changes in the extent of linkage among specified gas exchange parameters. The analyses of experiments carried out under constant environmental conditions in each VPD treatment (1.0, 2.0 and 3.0 kPa) showed a number of relatively subtle results of physiological consequence, such as differences in network connectance during the period of measurements at the same condition showing different patterns of gas exchange regulation. Additionally, VPD changes affect the dynamics of gas exchange by alterations in the irregularity of the time-series. These experiments highlight the endogenous and self-organized mechanisms that underlie the gas exchange process with further theoretical findings and possible practical applications.

Activity of alternative oxidase and plant uncoupling mitochondrial protein in potato tubers stored at low temperature or submitted to artificial aging

Abstract: Potato tubers (Solanum tuberosum L. cv. Binje) were stored at 18 and 4oC in order to verify changes in mitochondrial metabolism, especially in the activities of the alternative oxidase (AOX) and plant uncoupling mitochondria protein (PUMP). Tubers were also sliced and submitted to artificial aging. The oxygen consumption rate (OCR), respiratory control (RC) and ADP/O ratio for mitochondria obtained after storage at 18oC were not altered, regardless of the time of storage. Considerable increment in OCR, for both respiratory states 3 and 4, was observed after storing the tubers for five and ten days at 4oC; and it was accomplished by reductions in RC and ADP/O. The AOX activity was evident after five days at 4oC, and it was enhanced after ten days of storage at that temperature. No significant changes in PUMP activities were observed after one day at 18oC or at 4oC, neither after artificial aging. However, increased swelling of potato mitochondria was observed in the presence of valinomycin and linoleic acid when the tubers were kept at 4oC for five and ten days. Besides the changes in AOX and PUMP activities, slight alterations in ascorbate peroxidase (APX) and catalase (CAT) activities were observed after storing the tubers at 4oC for 5 days. The activities of these enzymes were higher after storage at 4oC for 10 days. The results suggest the combined effect of low temperature and time of exposure in promoting AOX activity, probably as a protective mechanism against cell damage in response to mitochondrial oxidative stress.

Photosynthetic performance in jack bean [Canavalia ensiformis (L.) D.C.] under drought and after rehydration

Abstract: Os efeitos do deficit hidrico e posterior reidratacao foram avaliados em Canavalia ensiformis (L.) D.C., mediante parâmetros de trocas gasosas: fotossintese (A), condutância estomatica (gs), razao Ci/Ca e transpiracao (E); fluorescencia da clorofila a: razao Fv/Fm e Fv/F0. As plantas cresceram em casa de vegetacao e apos apos 30 dias da emergencia, suspendeu-se a irrigacao naquelas submetidas ao deficit hidrico, obtendo-se os seguintes potenciais hidricos na antemanha (Yam): -0,40 MPa (controle), -1,00 MPa (deficit hidrico moderado) e -2,30 MPa (deficit hidrico severo). Apos, foram realizadas as analises de trocas gasosas e de fluorescencia. As plantas foram reidradatas e, apos um periodo de 24 h, as mesmas analises foram repetidas. Os regimes de deficit hidrico provocaram decrescimos significativos em A, E, gs e na razao Ci/Ca; contudo, nao alteraram as razoes Fv/Fm e Fv/F0. Apos a reidratacao houve o restabelecimento das trocas gasosas. Conclui-se que o deficit hidrico afetou negativamente a fotossintese, mediante uma limitacao estomatica, o que se confirma pelos decrescimos em gs, na razao Ci/Ca e na manutencao da eficiencia fotoquimica do FS II. Provavelmente, mecanismos de protecao tenham sido responsaveis pelo restabelecimento das trocas gasosas apos a reidratacao.

Comparative stomatal conductance and chlorophyll a fluorescence in leaves vs. fruits of the cerrado legume tree, Dalbergia miscolobium

Abstract: The aim of this study was to compare water vapor conductance and chlorophyll a fluorescence between leaflets and fruits of Dalbergia miscolobium, the Jacaranda tree. The frequency of stomata on the leaflets was 20 times higher than that observed on the fruits, and this was related with the lower conductance of the fruits in comparison with the leaflets. The potential quantum yield of PSII (Fv /Fmax) was significantly lower in fruits than in leaflets. The Fv /Fmax values for leaflets increased to over 0.8 during the afternoon, indicating the occurrence of dynamic photoinhibition. In contrast, Fv /Fmax values for fruits remained low even at early morning, indicating the occurrence of chronic photoinhibition. The maximum values of effective quantum yield (DF/F'm), and of the apparent electron transport rate (ETRmax) were higher in leaflets than in fruits. It was concluded that, like other green tissues, the pericarp of D. miscolobium was photosynthetically active, and therefore can contribute to the maintenance of the fruits and/or to the development of the seeds.

Apoplasm hydrostatic pressure on growth of cylindrical cells

Abstract: According to the model frequently used as reference, plant cell growth occurs only when turgor surpasses a threshold. This model was proposed considering a cylindrical cell of constant wall thickness immerged in a water solution, with viscoelastic behaviour, hydraulic conductivity, variable extensibility, and unidirectional elongation. The author, Lockhart (1965), did not consider the effects of apoplasm hydrostatic potential, a subject treated later by Calbo and Pessoa (1994) who argued that this component of cell potential would interfere with cell growth rate. To evaluate this effect, where possible the same deductive procedures as those employed by Lockhart were used here, which resulted in a set of equivalent equations for cell growth, turgor and water potential, developed with respect to physico-chemical variables. Relationships were derived from the conductivity equation, the definition of extensibility, Hook's law, and considering that the tension on cell wall transversal section is proportional to turgor and apoplasm hydrostatic potential. The numeric solutions for the equations showed that suction increased extension rate at the beginning of cell growth. Some experiments on plant growth and structural models of cell walls are discussed to point out the role of suction on wall tensioning and cell hydration during cell elongation.

Inhibition of the water splitting system by sodium chloride stress in the green alga Chlorella vulgari

Abstract: . It was found that the rate of oxygen evolution under steady state and flash light conditions diminished byincreasing salt concentrations. Salt treatment of cells also decreased the fluorescence and delayed luminescence yields. However,the fluorescence yield was almost completely restored by the addition of an artificial electron donor to the NaCl-treated cells.The oscillation pattern of the thermoluminescence B band as a function of flash number indicated that the S2➔S3 transition ofthe water splitting system is inhibited by NaCl treatment.

Germination of dormant seeds of Stylosanthes humilis as promoted by ethylene accumulation in closed environments

Abstract: Germination of scarified dormant seeds of Townsville stylo (Stylosanthes humilis) is very low in Petri dishes, but may be appreciable in sealed Erlenmeyer flasks if ethylene accumulates in their atmospheres. When ethylene in the flask atmosphere was fixed by a mercuric perchlorate solution, emanation of the gaseous regulator and germination of dormant seeds were drastically decreased. On the other hand, ethylene produced by dormant seeds and accumulated in the flask atmosphere led to a further enhancement of ethylene accumulation and a corresponding increased seed germination. Germination of both dormant and non-dormant seeds did not occur when ethylene biosynthesis was completely blocked.

Effects of changes in the photosynthetic photon flux density on net gas exchange of Citrus limon and Nicotiana tabacum

Abstract: O objetivo desse estudo foi avaliar o efeito da variacao da densidade de fluxo de fotons fotossinteticos (DFFF) nas trocas gasosas de Citrus limon, uma especie lenhosa, e Nicotiana tabacum, uma especie herbacea. Quando a DFFF foi aumentada de 50 para 350 mmol.m-2.s-1 durante 60 min, retornando em seguida a 50 mmol.m-2.s-1, a taxa de assimilacao de CO2 (A) aumentou e estabilizou apos 15 min, mas a condutância estomatica (gs) continuou crescendo, em ambas as especies. Quando se retornou a baixa DFFF, A diminuiu instantaneamente nas duas especies, mas a gs do limoeiro diminuiu lentamente durante 60 min, enquanto em tabaco, a gs levou apenas 15 min para decrescer. Essa diferenca nas respostas e provavelmente devida a hipersensibilidade dos estomatos de especies lenhosas, como Citrus, quando submetidas a alta DFFF, retardando o seu fechamento, quando de volta a baixa DFFF. Alem disso, quando o limoeiro, crescendo sob DFFF de 300 mmol.m-2.s-1, foi submetido subitamente a 600 mmol.m-2.s-1, iniciou-se uma flutuacao de A e gs, levando 150 min para se estabilizar. As causas dessa oscilacao sao discutidas em funcao do ajuste da eficiencia intrinseca no uso de agua, devido ao controle estomatico.

Isolation of enzymes involved in threonine biosynthesis from sorghum seeds

Abstract: Cereal seeds are poor in essential amino acids, particularly lysine, tryptophan and threonine. The amino acids lysine and threonine are synthesized in the aspartate pathway. Although most of the enzymes of the aspartate pathway have been isolated and characterized in higher plant species, the metabolism of lysine and threonine is totally unknown in sorghum. We have isolated two enzymes, aspartate kinase (AK) and homoserine dehydrogenase (HSDH) from sorghum. Optimum assay conditions were established for the determination of AK and HSDH activities. The highest level of activity was observed in immature seeds. AK was shown to be inhibited by threonine and lysine indicating the existence of at least two isoenzymes, one sensitive to threonine inhibition and the other sensitive to lysine inhibition with the latter being predominant in sorghum seeds. HSDH was shown to be inhibited by threonine indicating the existence of a threonine-sensitive HSDH, however, most of the activity was not inhibited by threonine, suggesting the existence of a second predominant isoenzyme of HSDH resistant to threonine inhibition.

Optimizing exponential growth of Triticum aestivum by application of the relative addition rate (RAR) technique utilizing a computer-controlled nutrient delivery system

Abstract: We conducted experiments designed to assess whether plants can be grown under conditions of low ionic strength simulating natural soil conditions. Wheat (Triticum aestivum, cv. Atlas 66) plants were grown using ten different relative nutrient addition rates (RAR) of nutrients (0.00, 0.06, 0.09, 0.12, 0.15, 0.18, 0.21, 0.24, 0.27, and 0.30 day-1) with a background solution containing 50 µmol.L-1 N and all other essential nutrients in optimal proportions relative to N. The relative growth rate (RGR) of plants over a 19-day experimental period responded directly to RAR (from 0.06 to 0.21 day-1) with a linear slope of 0.56 and an intercept of 0.085 day-1. Solution electrical conductivity (EC) values remained relatively stable over the experimental period. We examined the effect of varying nutrient concentrations in background solutions on growth of wheat plants at a RAR of 0.20 day-1. Plant dry mass production in the 50 µmol.L-1 N background solution was higher than the theoretical regression. On the other hand, plant dry mass production in the 0 µmol.L-1 N background solution was close to theoretical regression (considering a relative growth rate of 0.20 g.g-1.day-1) throughout most of the experimental period. Minimal fluctuations in solution EC values were observed over the experimental period. With the use of the computer-controlled nutrient delivery system and the RAR technique, it was possible to grow plants under conditions of low electrical conductivity simulating natural soil conditions.