Joaquim Albenisio Gomes Silveira

Photosynthetic gas exchange, chlorophyll fluorescence and some associated metabolic changes in cowpea (Vigna unguiculata) during water stress and recovery

Abstract The responses of photosynthetic gas exchange and chlorophyll fluorescence along with changes in carbohydrate and proline levels were studied in cowpea ( Vigna unguiculata ) during water stress and recovery. Three experiments were conducted under greenhouse and laboratory conditions. Decreased CO 2 assimilation rates during water stress were largely dependent on stomatal closure, which reduced available internal CO 2 and restricted water loss through transpiration. During the initial phase of stress, photochemical activity was not affected, as revealed by lack of alterations in fluorescence parameters associated with photosystem II (PSII) activity. Development of non-radiative energy dissipation mechanisms was evidenced during stress by increases in non-photochemical quenching and decreases in efficiency of excitation capture by open centers. At an advanced phase of stress, a down-regulation of PSII activity was observed along with some impairment of photochemical activity, as revealed by decreases in the maximum quantum yield of PSII (Fv/Fm). However, this impairment did not limit the overall photosynthetic process, since assimilation rates recovered, upon rewatering, independent of the still present decreased Fv/Fm values. Complete recovery of all gas exchange and fluorescence parameters occurred 3 days after rewatering. However, on the first day after water stress relief, assimilation rates only partially recovered in spite of the availability of internal CO 2 , suggesting some non-stomatal limitation of photosynthesis. Accordingly, the downregulation of PSII activity observed during stress persisted at this time. Our results on carbohydrate metabolic changes revealed an accumulation of soluble sugars in water-stressed leaves, which also persisted for 1 day after rewatering. This finding suggest a transient end-product inhibition of photosynthesis, contributing to a minor non-stomatal limitation during stress and initial phase of recovery. Increases in proline level were small and their onset was delayed after stress imposition, so that it may rather be a consequence and not a stress-induced beneficial response.

Proline accumulation and glutamine synthetase activity are increased by salt-induced proteolysis in cashew leaves

In this study cashew (Anacardium occidentale) plants were exposed to a short- and long-term exposure to NaCl in order to establish the importance of the salt-induced proteolysis and the glutamine synthetase activity on the proline accumulation. The cashew leaf showed a prominent proline accumulation in response to salt stress. In contrast, the root tissue had no significant changes in proline content even after the drastic injury caused by salinity on the whole plant. The leaf proline accumulation was correlated to protease activity, accumulation of free amino acid and ammonia, and decrease of both total protein and chlorophyll contents. The leaf GS activity was increased by the salt stress whereas in the roots it was slightly lowered. Although the several amino acids in the soluble pool of leaf tissue have showed an intense increment in its concentrations in the salt-treated plants, proline was the unique to show a proportional increment from 50 to 100 mol m-3 NaCl exposure (16.37 to 34.35 mmol kg-1 DM, respectively). Although the leaf glutamate concentration increased in the leaves of the salt-stressed cashew plants, as compared to control, its relative contribution to the total amino acid decreased significantly in stressed leaves when compared to other amino acids. In addition, when the leaf discs were incubated with NaCl in the presence of exogenous precursors (Glu, Gln, Orn or Arg) involved in the proline synthesis pathways, the glutamate was unique in inducing a significant enhancement of the proline accumulation compared to those discs with precursor in the absence of NaCl. These results, together with the salt-induced increase in the GS activity, suggest an increase in the de novo synthesis of proline probably associated with the increase of the concentration of glutamate. Moreover, the prominent salt-induced proline accumulation in the leaves was associated with the higher salt-sensitivity in terms of proteolysis and salt-induced senescence as compared to the roots. In conclusion, the leaf-proline accumulation was due, at least in part, to the increase in the salt-induced proteolysis associated with the increments in the GS activity and hence the increase in the concentration of glutamate precursor in the soluble amino acid pool.

Salinity-induced effects on nitrogen assimilation related to growth in cowpea plants

Abstract This study was carried out to establish relationships between nitrate and ammonia assimilation and growth of cowpea (Vigna unguiculata (L.) Walp) plants when exposed to NaCl-salinity. Cowpea plants were initially pre-acclimated under a mild NaCl-stress (50 mol m−3) during 8 days in order to induce salt acclimation. Subsequently, pre-acclimated and non-acclimated plants received a NaCl-osmotic shock (100 mol m−3) in the nutrient solution for 4 days. Although pre-acclimated plants exhibited lower NO3− uptake rate, in situ nitrate reduction, nitrate reductase (NR) activity, and shoot growth, they did not show alterations in these parameters, expressed as % of control, during the time-course experiment. These data suggest a salt adaptative response to a mild NaCl-salt stress. In contrast, addition of high level of NaCl (100 mol m−3) induced a steady decrease in nitrate uptake and assimilation parallel to a reduction in the shoot growth, particularly when the salt-shock was applied on the non-acclimated plants. On the other hand, the leaf glutamine synthetase activity, free amino acids and proline concentration showed a slight increase in all of salt treatments. The leaf soluble protein was slightly reduced by salt treatments during the short-term exposure. Moreover, the plants pre-acclimated with NaCl exhibited lower concentrations of protein and amino acids when compared with non-acclimated plants. Our study indicates that the regulation processes of N acquisition and growth of cowpea plants do operate at the whole plant level resulting in N homeostasis, especially in plants exposed to a mild-salt stress. Furthermore, the nitrate uptake and leaf-NR activity are more limiting for shoot growth than the primary ammonia assimilation from the glutamine synthetase.

Cytosolic APx knockdown indicates an ambiguous redox responses in rice

Ascorbate peroxidases (APX, EC 1.1.11.1) are class I heme-peroxidases, which catalyze the conversion of H(2)O(2) into H(2)O, using ascorbate as a specific electron donor. Previously, the presence of eight Apx genes was identified in the nuclear genome of rice (Oryza sativa), encoding isoforms that are located in different sub-cellular compartments. Herein, the generation of rice transgenic plants silenced for either both or each one of the cytosolic Apx1 and Apx2 genes was carried out in order to investigate the importance of cytosolic Apx isoforms on plant development and on plant stress responses. Transgenic double Apx1/2-silenced plants exhibited normal development, even though these plants showed a global reduction of Apx activity which strongly impacts the whole antioxidant system regulation. Apx1/2-silenced plants also showed increased H(2)O(2) accumulation under control and stress situations and presented higher tolerance to toxic concentration of aluminum when compared to wild type plants. On the other hand, silencing OsApx1 and OsApx2 genes individually resulted in strong effect on plant development producing semi-dwarf phenotype. These results suggested that the double silencing of cytosolic OsApx genes induced compensatory antioxidant mechanisms in rice while single knockdown of these genes did not, which resulted in the impairing of normal plant development.

Salt stress induced damages on the photosynthesis of physic nut young plants

A salinidade e um dos principais fatores que limitam a produtividade das culturas no mundo principalmente em regioes semiaridas. Avaliou-se a resistencia da fotossintese de plantas jovens de pinhaomanso (Jatropha curcas L.) submetidas ao estresse salino. O experimento foi realizado em delineamento inteiramente casualizado com tratamentos em fatorial 2 x 3: duas concentracoes de NaCl (0 e 100 mmol L-1) e tres tempos de avaliacao (7 e 14 dias de exposicao e tres dias de recuperacao). As concentracoes de Na+ e Cl- e a relacao K+/Na+ nas folhas, apos sete dias de exposicao ao sal, nao indicaram niveis toxicos, sugerindo os efeitos osmoticos induzidos pelo NaCl prevaleceram sobre as causas ionicas. Sob essas condicoes, o estresse salino causou reducao nos parâmetros de trocas gasosas, como fixacao de CO2, condutância estomatica e transpiracao, mas ao contrario, nao alterou a eficiencia fotoquimica do fotossistema II. Apos 14 dias de tratamento, os ions salinos atingiram concentracoes muito elevadas nas folhas, provavelmente atingindo niveis toxicos. Em tais condicoes, as trocas gasosas e a atividade fotoquimica sofreram forte reducao causada pelo estresse ionico. O tratamento de recuperacao nao induziu queda intensa nas concentracoes dos ions salinos nas folhas e nenhuma melhoria foi observada no desempenho fotossintetico. Plantas jovens de pinhao manso sao sensiveis a condicoes de salinidade elevada por NaCl, mostrando altas concentracoes de Na+ e Cl-, baixa razao K+/Na+ e danos fotossinteticos intensos causados tanto por limitacoes estomaticas como por limitacoes bioquimicas.

Superoxide dismutase and ascorbate peroxidase improve the recovery of photosynthesis in sugarcane plants subjected to water deficit and low substrate temperature.

The physiological responses of C4 species to simultaneous water deficit and low substrate temperature are poorly understood, as well as the recovery capacity. This study investigated whether the effect of these abiotic stressors is cultivar-dependent. The differential responses of drought-resistant (IACSP94-2094) and drought-sensitive (IACSP97-7065) sugarcane cultivars were characterized to assess the relationship between photosynthesis and antioxidant protection by APX and SOD isoforms under stress conditions. Our results show that drought alone or combined with low root temperature led to excessive energetic pressure at the PSII level. Heat dissipation was increased in both genotypes, but the high antioxidant capacity due to higher SOD and APX activities was genotype-dependent and it operated better in the drought-resistant genotype. High SOD and APX activities were associated with a rapid recovery of photosynthesis in IACSP94-2094 plants after drought and low substrate temperature alone or simultaneously.

Involvement of ASR genes in aluminium tolerance mechanisms in rice

Among cereal crops, rice is considered the most tolerant to aluminium (Al). However, variability among rice genotypes leads to remarkable differences in the degree of Al tolerance for distinct cultivars. A number of studies have demonstrated that rice plants achieve Al tolerance through an unknown mechanism that is independent of root tip Al exclusion. We have analysed expression changes of the rice ASR gene family as a function of Al treatment. The gene ASR5 was differentially regulated in the Al-tolerant rice ssp. Japonica cv. Nipponbare. However, ASR5 expression did not respond to Al exposure in Indica cv. Taim rice roots, which are highly Al sensitive. Transgenic plants carrying RNAi constructs that targeted the ASR genes were obtained, and increased Al susceptibility was observed in T1 plants. Embryogenic calli of transgenic rice carrying an ASR5-green fluorescent protein fusion revealed that ASR5 was localized in both the nucleus and cytoplasm. Using a proteomic approach to compare non-transformed and ASR-RNAi plants, a total of 41 proteins with contrasting expression patterns were identified. We suggest that the ASR5 protein acts as a transcription factor to regulate the expression of different genes that collectively protect rice cells from Al-induced stress responses.

Source–sink regulation of cotyledonary reserve mobilization during cashew (Anacardium occidentale) seedling establishment under NaCl salinity

Seedling establishment is a critical process to crop productivity, especially under saline conditions. This work was carried out to investigate the hypothesis that reserve mobilization is coordinated with salt-induced inhibition of seedling growth due to changes in source-sink relations. To test this hypothesis, cashew nuts (Anacardium occidentale) were sown in vermiculite irrigated daily with distilled water (control) or 50mM NaCl and they were evaluated at discrete developmental stages from the seed germination until the whole seedling establishment. The salt treatment coordinately delayed the seedling growth and the cotyledonary reserve mobilization. However, these effects were more pronounced at late seedling establishment than in earlier stages. The storage protein mobilization was affected by salt stress before the lipid and starch breakdown. The globulin fraction represented the most important storage proteins of cashew cotyledons, and its mobilization was markedly delayed by NaCl along the seedling establishment. Free amino acids were mostly retained in the cotyledons of salt-treated seedlings when the mobilization of storage proteins, lipids and starch was strongly delayed. Proline was not considerably accumulated in the cotyledons of cashew seedlings as a response to NaCl salinity. According to these results it is noteworthy that the salt-induced inhibition of seedling growth is narrowly coordinated with the delay of reserve mobilization and the accumulation of hydrolysis products in cotyledons. Also, it was evidenced that free amino acids, especially those related to nitrogen transport, are potential signals involved in the regulation of storage protein hydrolysis during cashew seedling establishment under NaCl salinity.

Effects of NaCl-salinity on growth and inorganic solute accumulation in young cashew plants

Os efeitos da salinidade sobre o crescimento e acumulacao de solutos inorgânicos foram avaliados em plantas de cajueiro (Anacardium occidentale L.), com 30 dias de idade, cultivadas em diferentes doses de NaCl, por 8 dias (curta duracao) e 40 dias (longa duracao). A producao de massa fresca (MF) da parte aerea, apos 40 dias, foi reduzida, aproximadamente, 25 e 75% sob 50 e 100 mol m-3 de NaCl, respectivamente. Nas raizes, a producao de MF nao foi afetada em 50 mol m-3 de NaCl, entretanto decresceu 30% em 100 mol m-3 de NaCl. No experimento de curta duracao, a concentracao de K+ nos diferentes tecidos foi similar aquela das plantas controle, enquanto que, no experimento de longa duracao, a concentracao de K+ foi fortemente reduzida, principalmente nas raizes. Em resposta ao tempo e a salinidade crescente, as concentracoes de Na+ e Cl- atingiram niveis toxicos nas folhas o que levou, a partir do quarto dia do estresse salino, ao surgimento de sintomas tipicos de toxicidade por estes ions. Em consequencia, as plantas de cajueiro nao foram habeis em regular suas funcoes metabolicas e fisiologicas nessas condicoes adversas de crescimento.

Salinity tolerance of halophyte Atriplex nummularia L. grown under increasing NaCl levels

The current study reports effects of salt stress on growth, K+ nutrition and organic composition of Atriplex nummularia. The upper limit of the NaCl gradient imposed on the plants was close to seawater salinity (600 mM). An external NaCl of 150 mM improved the growth of this species, which corroborates its halophytic nature. Evidence show that Atriplex nummularia was responsive to NaCl, but the mechanisms of this response are still not known. In such stress condition, Na+ and Cl- accumulation in leaves was far greater than that in roots; therefore salinity tolerance of Atriplex nummularia is not due to ion exclusion mechanism. In spite of a reduction of K+ content of tissues under salinity conditions, no corresponding physiological relevance of this in terms of growth was found. The high affinity of root cells for Na+ during uptake and, probably, its subsequent sequestration into cell vacuoles and structures of protection seems to contribute to osmotic adjustment as an increase in relative water content (RWC) of leaves was observed. NaCl caused a decrease in total soluble proteins and chlorophylls; the relevance of this is discussed in terms of mechanisms of salinity tolerance. The ability of Atriplex nummularia plants to keep Na+ and Cl- away from enzymes and cytosolic structures, together with an effective osmotic adjustment, is suggested as having a role in its tolerance to high salinity.