Selenium (Se) is microelement beneficial to plants and essential to animals and humans. Supply of Se shows positive effects on plant growth, but high concentrations affect its growth. This study is based on the hypothesis that nutritional disorder and oxidative stress induced by toxic levels of selenium maintain a close correlation with inhibition of photosynthesis net and reduced growth in lettuce plants. In this study, we investigated impact of Se toxicity on gas exchange, oxidative stress indicators, nutritional status, and growth of lettuce plants. Two sources and ten selenium concentrations were evaluated in lettuce according to a completely randomized experimental design in a factorial scheme with two selenium sources (selenite and selenate) and ten selenium concentration (0, 2, 4, 6, 8, 16, 32, 64, 96, 128 μM). Results show that reduction in leaf area and shoot dry matter was high when selenite was supplied to plants. It was achieved due to oxidative stress and nutritional disorder that affected photosynthesis, which resulted in low photosynthesis net. These results were reinforced by strong correlation of photosynthesis with essential nutrient contents and indicators of oxidative stress in plants treated with selenite. However, photosynthesis net was increased with 8 μM concentration of selenate. Lettuce growth was reduced due to oxidative stress and nutritional disorder. The results of this study contribute to clarifying negative modulation of photosynthesis net by higher selenate or selenite concentrations in lettuce plants through growth analysis, nutritional composition, oxidative stress indicators, and gas exchange. The strong or very strong negative correlation between photosynthesis net and oxidative stress indicators (superoxide, peroxide and malondialdehyde), photosynthesis net and chlorophyll a, photosynthesis net, and selenium content support the hypothesis of this study in which selenium-induced damage to the photosynthetic apparatus reduces the growth of lettuce. These results show new evidence on the mechanism of action of selenium toxicity on the photosynthetic machinery of lettuce plants. In addition, the results found show that lettuce plants respond differently to the source and concentration of selenium, with symptoms of toxicity manifesting even in the short exposure time of lettuce plants.

In areas cultivated with oil palm, typically mechanized field operations using heavy vehicles may negatively affect soil physical properties and productivity. The aim of this study was to evaluate soil physical quality in an area cultivated with oil palm by monitoring the temporal variation of the soil water content and relating it to the critical limits of the least limiting water range. Soil bulk density (Bd), soil penetration resistance (SR), least limiting water range (LLWR), and water stress days (WSD) were used to assess soil physical quality in planting rows (PR) and the traffic zone (TZ) at depths 0-20, 20-40, and 40-60 cm. The Bd was higher and the LLWR was reduced in TZ only at the surface layer. The effect of temporal variation in soil water content on the soil physical quality was higher in TZ, mainly in subsurface layers. Bd and LLWR did not affect the fresh fruit bunch production; however, WSD in TZ at 20-40 and 40-60 cm layers provided evidence of effects of temporal variation of soil water content on oil palm productivity.

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Copper (Cu) is an essential micronutrient for plants because it participates in several redox reactions and the structural constitution of the Fe–Cu cluster. Although it is required in small concentrations at toxic levels, Cu triggers physiological and biochemical disorders that reduce plant growth. In higher plants, the normal range of Cu concentration is in the range of 2–20 mg Cu kg−1 DW. Above the upper limit of this range, Cu toxicity may occur if the plants are not tolerant to the stress caused by toxic levels of Cu. In view of the growing agricultural and industrial activity that are the main sources of Cu addition in nature, understanding the physiological and biochemical mechanisms of Cu toxicity in plants constitutes an important tool for the selection of more tolerant genotypes based on biochemical and physiological indicators to heavy metal stresses. In this chapter, we propose a systematic review of plants grown under toxic levels of Cu, based on the responses of physiological, biochemical, and nutritional variables. Understanding these responses will contribute to improving the understanding of the basic mechanisms of stress tolerance by toxic levels of Cu in higher plants, providing valuable information for the improvement of genotypes resistant to toxic levels of Cu in the plant culture medium.

RESUMO: Embora a palma de óleo seja cultivada em grande escala e em diferentes tipos de solos no estado do Pará, mudanças nas propriedades físicas do solo nestas áreas não vêm sendo monitoradas. Assim, o objetivo do trabalho foi avaliar a qualidade física de um Latossolo Amarelo cultivado com palma de óleo após 30 anos de implantação. Amostras de solo com estrutura deformada e indeformada foram coletadas nas profundidades de 0-20 e 20-40 cm, na zona de tráfego. O grau de compactação (GC) foi calculado a partir da densidade máxima do solo obtida pelo teste de Proctor e sua resistência à penetração (RP) determinada em amostras equilibradas em dez potenciais matriciais. Influência das mudanças temporais da umidade do solo na RP também foram avaliadas. Os parâmetros GC e RP demonstraram que a área avaliada apresenta compactação. O GC nas duas profundidades ficou acima de 90% sendo mais intenso na camada de 0-20 cm. Porém, quando avaliado pela RP, o estado de compactação foi mais evidente na camada de 20-40 cm e, as variações na umidade do solo ao longo de um ano mostraram que nos meses mais secos o solo apresenta valores limitantes de RP nesta camada. Desta forma, este estudo indica que o crescimento e desenvolvimento da palma de óleo pode estar sendo restringido pela compactação e sobretudo pela elevada resistência à penetração nos meses de menor disponibilidade de água. ABSTRACT:Although oil palm is cultivated on a large scale and in different soil types in the state of Pará, changes in the soil's physical properties in these areas have not been evaluated. Thus, the objective of this study was to evaluate the physical quality of a Yellow Latosol cultivated with oil palm, 30 years after implantation. Soil samples with disturbed and undisturbed structure were collected at 0-20 and 20-40 cm depths, in the machinery traffic zone. The degree of compaction (DC) was calculated from the maximum soil bulk density obtained by the Proctor test, and soil penetration resistance (PR) was determined in soil samples equilibrated in ten matric potentials. Temporal changes in soil moisture in PR were also evaluated. Both parameters showed that the evaluated area has an indication of compaction. The DC in both depths was above 90% and was more severe in the 0-20 cm layer. However, when evaluated by PR, the compaction was more evident in the 20-40 cm layer, and variations in soil moisture over a year showed that in the drier months, the soil showed high values of PR in this layer. Thus, this study indicates that the growth and development of oil palm may be restricted by the soil's compaction and mainly by the high penetration resistance in the months of reduced water availability.

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