Paulo E. M. Silva

Silicon nutrition alleviates the negative impacts of arsenic on the photosynthetic apparatus of rice leaves: an analysis of the key limitations of photosynthesis

Silicon (Si) plays important roles in alleviating various abiotic stresses. In rice (Oryza sativa), arsenic (As) is believed to share the Si transport pathway for entry into roots, and Si has been demonstrated to decrease As concentrations. However, the physiological mechanisms through which Si might alleviate As toxicity in plants remain poorly elucidated. We combined detailed gas exchange measurements with chlorophyll fluorescence analysis to examine the effects of Si nutrition on photosynthetic performance in rice plants [a wild-type (WT) cultivar and its lsi1 mutant defective in Si uptake] challenged with As (arsenite). As treatment impaired carbon fixation (particularly in the WT genotype) that was unrelated to photochemical or biochemical limitations but, rather, was largely associated with decreased leaf conductance at the stomata and mesophyll levels. Indeed, regardless of the genotypes, in the plants challenged with As, photosynthetic rates correlated strongly with both stomatal (r(2)  = 0.90) and mesophyll (r(2)  = 0.95) conductances, and these conductances were, in turn, linearly correlated with each other. The As-related impairments to carbon fixation could be considerably reverted by Si in a time- and genotype-dependent manner. In conclusion, we identified Si nutrition as an important target in an attempt to not only decrease As concentrations but also to ameliorate the photosynthetic performance of rice plants challenged with As.

Physiological and biochemical abilities of robusta coffee leaves for acclimation to cope with temporal changes in light availability.

The effects of varying intensities of light on plants depend on when they occur, even if the total amount of light received is kept constant. We designed an experiment using two clones of robusta coffee (Coffea canephora) intercropped with shelter trees in such a way that allowed us to compare coffee bushes shaded in the morning (SM) with those shaded in the afternoon (SA), and then confronting both with bushes receiving full sunlight over the course of the day (FS). The SM bushes displayed better gas-exchange performance than their SA and FS counterparts, in which the capacity for CO2 fixation was mainly constrained by stomatal (SA bushes) and biochemical (FS bushes) factors. Physiological traits associated with light capture were more responsive to temporal fluctuations of light rather than to the amount of light received, although this behavior could be a clone-specific response. The activity of key antioxidant enzymes differed minimally when comparing the SM and SA clones, but was much larger in FS clones. No signs of photoinhibition or cell damage were found regardless of the light treatments. Acclimations to varying light supplies had no apparent additional cost for constructing and maintaining the leaves regardless of the light supply. Both the SM and SA individuals displayed higher return in terms of revenue streams (e.g. higher mass-based light-saturated photosynthetic rates, photosynthetic nitrogen use efficiencies and long-term water use efficiencies) than their FS counterparts. In conclusion, shading may improve the physiological performance of coffee bushes growing in harsh, tropical environments.

The Physiology of Abiotic Stresses

Plants are often exposed to several adverse environmental conditions that potentially generate stress and thus negatively affect their growth and productivity. Understanding the physiological responses of crops to stress conditions is essential to minimizing the deleterious impacts of stress and maximizing productivity. Therefore, there is urgent need for more scientific research to increase our understanding of the physiological behavior of crops in response not only to a specific type of stress but also to multiple interacting stressors, such as water‚ and thermal stresses. The proper assessment of this information may result in important tools for monitoring the most promising genetic material in plant breeding programs. In this chapter, the plant strategies associated with satisfactory growth and yield under abiotic stress conditions are discussed, with emphasis in tropical environments. In addition, the state of the art on the physiology of the major abiotic stresses (drought, salinity, heat, nitrogen and phosphorus deficiencies and aluminum toxicity) and possible strategies to develop cultivars with satisfactory productivity in stressful environments using a physiological approach are summarized.

THE EFFECTS OF PRUNING AT DIFFERENT TIMES ON THE GROWTH, PHOTOSYNTHESIS AND YIELD OF CONILON COFFEE ( COFFEA CANEPHORA ) CLONES WITH VARYING PATTERNS OF FRUIT MATURATION IN SOUTHEASTERN BRAZIL

The economics of coffee plantations is intrinsically linked to pruning, which can improve the canopy architecture and thereby increase productivity. However, recommended pruning times on conilon coffee plantations have been made on an entirely empirical basis. In this study, by evaluating growth, photosynthetic gas exchanges, starch accumulation and crop productivity, the effects of pruning at different times between harvest and flowering were investigated for six conilon coffee clones with distinct stages of fruit maturation (early, intermediate and late). Clones with an early maturation stage were pruned at four different times: 0, 30, 60 and 90 days after harvest (DAH). Intermediate clones were pruned at 0, 30 and 60 DAH, and late clones were pruned at 0 and 30 DAH. Overall, the rates of shoot growth and net photosynthesis, the stomatal conductance and the crop yield were not affected by the pruning treatments in any of the clones. In addition, pruning times did not affect the concentrations of starch or the photochemical efficiency of photosystem II. The carbon isotope composition ratio was marginally affected by the treatments. These results suggest that the pruning time after harvests is relatively unimportant and pruning operations can be scheduled to optimise the use of labour, which directly impacts the production costs of coffee.

Quebra de dormência em sementes de Sesbania virgata (Cav.) Pers

Forest species with hard seeds often pose considerable problems to nursery managers because their hard and impermeable seed coats hinder and delay germination. Therefore, this work aimed to determine the most efficient, practical and low cost methodology to overcome dormancy in Sesbania virgata (Cav.) Pers. Seed, a specie with potential for recovery of degraded areas. The seeds were submitted to chemical scarification by immersion in sulphuric acid during a period of 5, 10, 20 and 30 minutes, mechanical scarification and heat treatment. After the scarification the seeds were submitted to germ test in germination chambers at 25 oC and photoperiod of 14 h. Treatment with mechanical scarification showed the highest germination percentage (98%), followed by chemical scarification for 30 minutes (57%). The heat treatment in spite of obtaining a higher germination percentage than the controls had abnormal development of seedlings and is not therefore recommended for seeds of S. virgata. From the parameters utilized in this study, we recommend the use of the mechanical scarification to overcome dormancy of S. virgata seeds

Fluorescência da clorofila-a e variação da simetria como ferramentas de investigação de plantas sob estresse

Perturbacoes microambientais em diferentes sistemas e individuos podem resultar numa tendencia para a assimetria flutuante, que consiste no desvio nao direcional da simetria de uma estrutura bilateral normalmente distribuida numa populacao. Neste sentido, individuos incapazes de exibirem respostas plasticas aos disturbios do desenvolvimento acontecidos ao acaso, seja de origem genetica ou ambiental, podem exibir pequenos desvios de simetria em orgaos com estrutura bilateral, como as folhas. Dessa forma, analisar a escala morfologica de plantas via padrao de simetria das folhas, vem se tornando uma ferramenta eficaz para descrever a estabilidade do desenvolvimento de plantas sob condicoes de estresse. Assim, este estudo objetivou analisar o padrao de simetria bem como a fluorescencia da clorofila-a em foliolos de Glycine max (L.) Merrill. (var. BRS 239) submetidos ao estresse por radiacao ultravioleta. Determinou-se os desvios de simetria (ds), o comprimento medio (CML) e a largura maxima do limbo (LML) assim como os parâmetros de fluorescencia da clorofila-a em foliolos de plantas jovens de G. max submetidas a 0, 30, 60, 120 ou 240 minutos semanais de radiacao ultravioleta-C (UV-C) durante quatro semanas, submetendo os valores a analise de variância e ao teste de Tukey (p<0.05). Os resultados demonstraram reducoes significativas em CML e LML, aumento significativo no ds nos foliolos irradiados, bem como queda no rendimento quântico potencial do fotossistema II (Fv/Fm) e no indice de performance (IP). As analises de simetria foliar demonstraram alta correlacao com o nivel de estresse aos quais as plantas foram submetidas, sendo uma importante analise para averiguar o fitness da planta por se tratar de uma metodologia nao-invasiva e de facil manipulacao.