Rafael Vasconcelos Ribeiro

Photosynthetic changes and protective mechanisms against oxidative damage subjected to isolated and combined drought and heat stresses in Jatropha curcas plants

Photosynthetic changes and protective mechanisms against oxidative damage were evaluated in Jatropha curcas leaves subjected to drought and heat stresses, both individually and combined, in order to elucidate the synergistic and antagonistic mechanisms involved with these abiotic factors. Both the drought and heat stresses caused significant damage to the leaf membrane integrity and lipid peroxidation, and the combination of these stresses greatly enhanced these physiological disturbances. The leaf CO(2) assimilation rate, stomatal conductance and instantaneous carboxylation efficiency (P(N)/C(I)) were significantly decreased in all plants subjected to stressful conditions in comparison to unstressed plants (reference). In contrast, a reduction in photochemical activity was observed only in plants exposed to drought and drought+heat conditions. Catalase (CAT), ascorbate peroxidase (APX) and superoxide dismutase (SOD) activities were stimulated only under heat stress, whereas APX activity was increased in all treated plants in comparison to the references. Moreover, the leaf H(2)O(2) content was increased similarly under all studied stresses. However, the balance of reduced and oxidized ascorbate did not show significant differences between reference and stressed plants. Although J. curcas plants acclimated to the studied stresses, they did not present an efficient mechanism for protection against drought-induced oxidative stress, especially when at high temperatures. However, heat-treated plants triggered an efficient enzymatic antioxidant system of reactive oxygen species scavenging and an effective protection against photochemical damages. The combination of drought and heat most significantly impaired the photosynthetic assimilation of CO(2) and the photochemical activity. These results indicate that drought greatly disturbs photosystem II activity and oxidative metabolism and that these negative effects are strongly stimulated by heat stress. The data also evidence that the combination of heat and drought triggers an intricate response involving antagonistic and synergistic interactions.

Respostas biométricas e fisiológicas ao deficit hídrico em cana-de-açúcar em diferentes fases fenológicas

The aim of this work was to evaluate the biometric and physiological responses of sugarcane (Saccharum spp.) to water deficit (WD), during different phenological phases. Genotypes IACSP 94-2094 and IACSP 96-2042 were subjected to WD conditions during the initial, maximum and sucrose accumulation phases. The experiment was carried out in a completely randomized design. Susceptibility to WD was established by reduction in stalk dry matter and soluble solids. Water deficit reduced leaf gas exchange in all phenological phases of both genotypes. Lower plant height, less stalk dry matter and soluble solids, and reduction in number and length of internodes were only observed during the initial growth phase of the IACSP 96-2042 clone. In the initial growth phase, tolerance to WD was observed for IACSP 94-2094, with evidence of physiological acclimation, and for IACSP 96-2042 in reduction phytomass production and its soluble solid content, caused by lower stomatal conductance and lower apparent carboxylation efficiency which limit photosynthesis. Regardless of the phenological phase, genotype IACSP 94-2094 was tolerant to WD, since its phytomass production was maintained even with impairment of leaf gas exchange.

Photosynthesis and water relations of well-watered orange plants as affected by winter and summer conditions

The aim of this study was to evaluate how the summer and winter conditions affect the photosynthesis and water relations of well-watered orange trees, considering the diurnal changes in leaf gas exchange, chlorophyll (Chl) fluorescence, and leaf water potential (Ψ) of potted-plants growing in a subtropical climate. The diurnal pattern of photosynthesis in young citrus trees was not significantly affected by the environmental changes when compared the summer and winter seasons. However, citrus plants showed higher photosynthetic performance in summer, when plants fixed 2.9 times more CO2 during the diurnal period than in the winter season. Curiously, the winter conditions were more favorable to photosynthesis of citrus plants, when considering the air temperature (< 29 °C), leaf-to-air vapor pressure difference (< 2.4 kPa) and photon flux density (maximum values near light saturation) during the diurnal period. Therefore, low night temperature was the main environmental element changing the photosynthetic performance and water relations of well-watered plants during winter. Lower whole-plant hydraulic conductance, lower shoot hydration and lower stomatal conductance were noticed during winter when compared to the summer season. In winter, higher ratio between the apparent electron transport rate and leaf CO2 assimilation was verified in afternoon, indicating reduction in electron use efficiency by photosynthesis. The high radiation loading in the summer season did not impair the citrus photochemistry, being photoprotective mechanisms active. Such mechanisms were related to increases in the heat dissipation of excessive light energy at the PSII level and to other metabolic processes consuming electrons, which impede the citrus photoinhibition under high light conditions.

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.

Some aspects of citrus ecophysiology in subtropical climates: re-visiting photosynthesis under natural conditions

Received: 14 November 2007; Accepted: 12 December 2007In this review we re-visit and discuss the current knowledge on ecophysiology of citrus trees, addressing the influenceof environmental conditions on citrus photosynthesis. Knowledge of physiological responses of citrus trees to theirsurrounding environment is essential in order to improve crop production and plant development, both beingconsequences of appropriate horticultural management in citrus orchards. In this context, citrus photosynthesis isaddressed as the primary source of carbon and energy for plant growth and development. The photosynthetic activityon both a daily and a seasonal scale is reviewed, taking into consideration the physiological aspects related to seasonalvariation of photochemical and biochemical activities, stomatal conductance and leaf water potential. These aspects aretreated for citrus plants growing in subtropical climates with varying environmental conditions, such as moderate tosevere drought during the winter season. In addition, the possible inhibitory/stimulatory effects of carbohydratemetabolism on citrus photosynthesis are discussed with regard to the source-sink relationship. Field experimentationthat enhances knowledge concerning citrus ecophysiology in subtropical climates is highlighted. Among interestingsubjects to be unraveled by future research, we may point out the effects of low temperatures on citrus photosynthesisand water relations, the nature of the relationship between leaf carbohydrate content and photosynthesis, and thesignificance of photosynthesis in different canopy layers and positions in relation to the total carbon gain in maturecitrus trees.

Photosynthetic responses of tropical tree species from different successional groups under contrasting irradiance conditions

This study evaluated the photosynthetic responses of seven tropical trees of different successional groups under contrasting irradiance conditions, taking into account changes in gas exchange and chlorophyll a fluorescence. Although early successional species have shown higher values of CO2 assimilation (A) and transpiration (E), there was not a defined pattern of the daily gas exchange responses to high irradiance (FSL) among evaluated species. Cariniana legalis (Mart.) Kuntze (late secondary) and Astronium graveolens Jacq. (early secondary) exhibited larger reductions in daily-integrated CO2 assimilation (DIA) when transferred from medium light (ML) to FSL. On the other hand, the pioneer species Guazuma ulmifolia Lam. had significant DIA increase when exposed to FSL. The pioneers Croton spp. trended to show a DIA decrease around 19%, while Cytharexyllum myrianthum Cham. (pioneer) and Rhamnidium elaeocarpum Reiss. (early secondary) trended to increase DIA when transferred to FSL. Under this condition, all species showed dynamic photoinhibition, except for C. legalis that presented chronic photoinhibition of photosynthesis. Considering daily photosynthetic processes, our results supported the hypothesis of more flexible responses of early successional species (pioneer and early secondary species). The principal component analysis indicated that the photochemical parameters effective quantum efficiency of photosystem II and apparent electron transport rate were more suitable to separate the successional groups under ML condition, whereas A and E play a major role to this task under FSL condition.

REVEALING DROUGHT-RESISTANCE AND PRODUCTIVE PATTERNS IN SUGARCANE GENOTYPES BY EVALUATING BOTH PHYSIOLOGICAL RESPONSES AND STALK YIELD

This study was conducted to investigate the physiological response of sugarcane genotypes to drought and its consequence for stalk yield. Sugarcane genotypes IACSP94-2094, IACSP96-2042 and SP87-365 were subjected to water deficit during the initial growth phase by withholding water. Resistance and sensitivity patterns were defined by the impact of drought on the stalk yield and content of soluble solids in the stalk juice. IACSP94-2094 and SP87-365 were considered drought-resistant genotypes, as the stalk dry matter production and yield of soluble solids were not reduced by the water deficit. Although drought caused reductions in leaf gas exchange in all the genotypes, IACSP96-2042 was most affected when considering the cumulative reduction in photosynthesis throughout the experimental period. This photosynthetic impairment of IACSP96-2042 was related to both non-stomatal and stomatal limitations, whereas photosynthesis in SP87-365 and IACSP94-2094 were only stomatally limited under drought. In general, a reduced photosynthetic sensitivity to water deficit was an important physiological trait for dry matter production in sugarcane plants, and the concentrations of soluble carbohydrates, sucrose, starch and proline in the leaves did not reveal consistent differences between the patterns of resistance and sensitivity. Even though IACSP96-2042 was severely affected by water shortage, this genotype presented a similar stalk yield under drought and the highest stalk yield under well-watered conditions when compared to the other genotypes. This response to variable water conditions is interesting for regions with seasonal drought, whereas the pattern of drought resistance is more appropriate for regions in which drought occurs for long periods during the crop season. Our findings are also discussed from the point of view that increases in sugarcane yield and sustainable agriculture may be reached by choosing the best genotype for each specific environmental condition.

WATER DEFICIENCY AT DIFFERENT DEVELOPMENTAL STAGES OF GLYCINE MAX CAN IMPROVE DROUGHT TOLERANCE ( 1 )

Developmental windows are specific periods of sensitivity during normal plant development in which a perturbation may be adaptively integrated. In these periods, sub-lethal environmental perturbations may improve the capacity to grow at lethal conditions. The aim of this study was to test the hypothesis that previous non-lethal water deficit applied in different developmental stages in soybean plants could enables them to improve the tolerance to environmental perturbations. In order to test this hypothesis we carried out an experiment with soybean plants submitted to water deficit in different stages of plant development, evaluating yield and physiological aspects. Our results indicated that water deficit experienced on V4 stage (vegetative) induces more suitable response, enabling plants to develop a process of tolerance improvement to a further water shortage period, probably through a reduction of growth, which maintains a conservative strategy of energy use. On the other hand, water deficit in R1 stage (reproductive), increased the plant susceptibility to posterior water withholding. This “strategy” was the opposite of the one employed by plants on V4 stage, i.e., to maintain growth rate probably at the expense of a higher energetic cost.

Ocorrência de condições ambientais para a indução do florescimento de laranjeiras no Estado de São Paulo

O presente estudo objetivou avaliar a ocorrencia de condicoes ambientais propicias para a inducao do florescimento de laranjeiras no Estado de Sao Paulo, considerando como fatores de inducao a baixa temperatura, dada pelo numero de horas de frio abaixo de 13oC (NHF), e a deficiencia hidrica acumulada nos meses de junho, julho e agosto (DEFI), dada pelo balanco hidrico climatologico. Os calculos e as estimativas foram realizados a partir dos dados de temperatura maxima e minima diaria e precipitacao diaria dos ultimos 5 a 14 anos, dependendo da localidade. Foram consideradas areas representativas das principais regioes produtoras de citros, onde os plantios estao em expansao ou areas com potencial para exploracao citricola, sendo: Barretos, Bauru, Botucatu, Catanduva, Itapetininga, Itapeva, Jaboticabal, Jau, Limeira, Lins, Matao, Mococa, Ourinhos, Piracicaba, Sao Jose do Rio Preto e Votuporanga. Com excecao de Botucatu, Itapetininga, Itapeva e Ourinhos, onde a inducao do florescimento ocorre por baixa temperatura, a deficiencia hidrica e o principal fator de inducao nas demais regioes. Ja nas regioes de Jau, Limeira e Piracicaba, a inducao do florescimento e ocasionada pelos dois fatores. A influencia de NHF na inducao do florescimento e mais variavel se comparada a DEFI. Em 1996, 2000 e 2004, NHF foi superior a 300 h na maioria das localidades estudadas, mesmo em areas onde a baixa temperatura nao e comum, ex. Barretos, S. J. Rio Preto e Votuporanga. Em relacao a deficiencia hidrica, a maioria das localidades apresentou esse tipo de influencia ambiental no periodo analisado, sendo as menores ocorrencias observadas em Itapeva e Itapetininga (54,5 e 72,7% dos anos, respectivamente). Situacao atipica ocorreu em 2004, quando a deficiencia hidrica variou de fraca (10<DEFI<30 mm) a moderada (31<DEFI<69 mm) na maioria das localidades. Concluindo, a deficiencia hidrica e a principal variavel ambiental durante o periodo de inducao do florescimento de laranjeiras na regiao centro-norte do Estado de Sao Paulo, enquanto a baixa temperatura e predominante na regiao centro-sul do Estado, havendo influencia desses dois fatores ambientais na regiao central.