ABSTRACT: Vale do Rio Doce shoot dieback (VRDSD) is an anomaly whose cause seems to be associated with hypoxic conditions and their consequences (excess Mn and Fe) triggered by elevation of the water table in areas with poor drainage. Different plants have distinct survival strategies under this form of stress. The objective of this study was to understand the physiological responses involved in the differential tolerance of eucalyptus clones to VRDSD and their relationship to hypoxia and excess Mn. A hydroponic experiment was carried out using a 2 × 2 × 2 factorial arrangement, two eucalyptus clones with different levels of tolerance to VRDSD (sensitive Urograndis hybrid - 1213; and the tolerant Rio Claro hybrid - Eucalyptus grandis x unknown - 2719), two concentrations of O2 (8 and 4 mg L-1), and two Mn concentrations (1.39 and 300 mg L-1) in a randomized block design (RBD) with three replicates. Forty-day-old clones were maintained in Clark nutrient solution for 30 days. After this period, the treatments were applied for 11 days. Plant gaseous exchange shoot and root production, and the quantity of enzymes related to oxidative stress in leaves and roots were evaluated. In the tolerant clone, reactive oxygen species (ROS) were produced under hypoxic conditions, accompanied by reduction in production of dry matter, malondialdehyde (MDA), and in activity of the enzyme alcohol dehydrogenase (ADH). However, this clone had greater production of superoxide dismutase (SOD) under these conditions, an enzyme responsible for detoxification of ROS, which acts as part of the Low Oxygen Quiescence Syndrome (LOQS). In contrast, sensitive clones did not exhibit expressive reductions in growth or changes in the leaf/root ratio. These clones formed large quantities of adventitious roots and had high levels of MDA and ADH and low levels of SOD. Therefore, sensitive clones appear not to be prepared for detoxification of ROS and other toxic metabolites, but rather adopt morphological escape mechanisms, the Low Oxygen Escape Syndrome (LOES), in response to hypoxia. Thus, the period of soil waterlogging may cause the death of large numbers of roots in sensitive clones, limiting their ability to absorb water and nutrients and culminating in the death of these plants. Excess Mn seems to aggravate the damage caused by hypoxia, but it is not the causal agent of VRDSD.

ABSTRACT: Visual symptoms of Zn deficiency, induced by excessive P applications, have been observed in commercial orchid nurseries. The supply of other metallic micronutrients, such as Fe and Mn, may also be inadequate in the plant due to high application rates of P. The aim of this study was to investigate this interaction in the nutrition of Phalaenopsis plants. Experimental treatments consisted of three P rates (0.0, 0.5, and 1.0 g L-1) and three Zn rates (0.00, 0.35, and 0.70 g L-1), as well as fertilization with other basic nutrients, and a control treatment with the fertilizer B&G Orchidée(r), at 1.0 g L-1. Dry matter production was evaluated, as well as the levels of P, Zn, Fe, and Mn in both shoots and roots. Higher P rates induced higher shoot dry matter production. However, symptoms of Zn deficiency were observed in plants treated with the highest P rate in the absence of Zn. With increasing P rates, Zn concentrations in the shoots decreased more markedly than in roots, with accumulation of the element in the roots, indicating low Zn translocation to the shoot. A much higher Mn content in shoots (661 mg kg-1) than in roots (75 mg kg-1) suggested that the species is highly tolerant to this micronutrient. The Fe concentrations in the plant were much higher than those indicated in the literature as critical levels for this genus.

O extrator Mehlich-1 (M-1), assim como o Fosfato Monocálcico em ácido acético (FMCa), tem na acidez e na troca de ligantes - seja do sulfato do extrator pelo fosfato do solo ou do fosfato do extrator pelo sulfato do solo - os mecanismos de extração do P e do S disponíveis. Em solos argilosos, com maior poder de adsorção de P, ou menor valor de P-remanescente (P-rem), o que corresponde a solos com maior Poder-Tampão de Fosfato (PTF), mais tamponados para a acidez, o pH inicialmente baixo dos extratores é, durante o seu contato com o solo, aumentado em direção àquele do solo, e o sulfato do M-1 ou o fosfato do FMCa é adsorvido por sítios de adsorção ocupados ou não por esses ânions. Tal fato faz com que o extrator perca seu poder de extração, fenômeno conhecido como desgaste de extrator, objetivo de estudo deste trabalho. Escolheram-se 20 amostras de solos, de modo a cobrir a faixa do P-rem (0 a 60 mg L-1). Como medida do PTF, foi utilizado o P-rem. Os teores de P e de S disponíveis das amostras dos solos pelo M-1 e FMCa, de demais nutrientes e de matéria orgânica (MO) foram determinados. Para a determinação do desgaste dos extratores, após o período de repouso, foram medidos o pH e os teores de P e de S em ambos os extratos-solos. Houve efeito pouco expressivo, embora significativo, do desgaste da acidez dos extratores M-1 e FMCa com a diminuição do valor de P-rem. Observou-se para o M-1 um modelo linear plateau para desgaste descontínuo do teor do P, com a diminuição da concentração do P-rem ou aumento do PTF, sugerindo que se deve adotar um modelo também descontínuo para a interpretação do P-disponível de solos com diferentes valores de P-rem. Entretanto, observou-se resposta linear-contínua entre as variáveis P no extrato-solo e P-rem para o extrator FMCa, o que evidenciou desgaste crescente desse extrator com o aumento do PTF do solo, indicando a validade da relação linear entre o S disponível do solo e o PTF, estimado pelo P-rem, como atualmente adotada.

The Mehlich-1 (M-1) extractant and Monocalcium Phosphate in acetic acid (MCPa) have mechanisms for extraction of available P and S in acidity and in ligand exchange, whether of the sulfate of the extractant by the phosphate of the soil, or of the phosphate of the extractant by the sulfate of the soil. In clayey soils, with greater P adsorption capacity, or lower remaining P (Rem-P) value, which corresponds to soils with greater Phosphate Buffer Capacity (PBC), more buffered for acidity, the initially low pH of the extractants increases over their time of contact with the soil in the direction of the pH of the soil; and the sulfate of the M-1 or the phosphate of the MCPa is adsorbed by adsorption sites occupied by these anions or not. This situation makes the extractant lose its extraction capacity, a phenomenon known as loss of extraction capacity or consumption of the extractant, the object of this study. Twenty soil samples were chosen so as to cover the range of Rem-P (0 to 60 mg L-1). Rem-P was used as a measure of the PBC. The P and S contents available from the soil samples through M-1 and MCPa, and the contents of other nutrients and of organic matter were determined. For determination of loss of extraction capacity, after the rest period, the pH and the P and S contents were measured in both the extracts-soils. Although significant, the loss of extraction capacity of the acidity of the M-1 and MCPa extractants with reduction in the Rem-P value did not have a very expressive effect. A “linear plateau” model was observed for the M-1 for discontinuous loss of extraction capacity of the P content in accordance with reduction in the concentration of the Rem-P or increase in the PBC, suggesting that a discontinuous model should also be adopted for interpretation of available P of soils with different Rem-P values. In contrast, a continuous linear response was observed between the P variables in the extract-soil and Rem-P for the MCPa extractor, which shows increasing loss of extraction capacity of this extractor with an increase in the PBC of the soil, indicating the validity of the linear relationship between the available S of the soil and the PBC, estimated by Rem-P, as currently adopted.

Apesar do grande número de publicações voltadas para a avaliação da disponibilidade de fósforo (P) por diferentes métodos de extração, persistem ainda muitas indagações sobre o assunto. O objetivo deste trabalho foi comparar os extratores Mehlich-1, Resina Aniônica (RA) e Resina Mista (RM), quanto à avaliação da disponibilidade de P, em diferentes condições experimentais. O trabalho foi conduzido em condições de laboratório, em blocos casualizados, no fatorial [(3 x 3 x 2) + 3] x 4, com quatro repetições, empregando-se três solos com diferentes constituições texturais: um Latossolo Vermelho (LV), muito argiloso; um Latossolo Vermelho-Amarelo (LVA), franco-argiloarenoso; e um Latossolo Amarelo (LA), franco-arenoso, três fontes de P (superfosfato triplo, fosfato reativo de Gafsa e fosfato natural de Araxá), em duas doses de P total da fonte (75 e 150 mg dm-3) mais três solos sem adição de P e quatro tempos (15, 30, 60 e 120 dias) de contato das fontes de P com os solos. A acidez dos solos LV e LVA foi corrigida pelo método da saturação por bases, elevando-a para 60 % com a aplicação de CaCO3 e MgCO3 (4:1); para o LA não houve necessidade de correção. Essas amostras foram mantidas por 30 dias com a umidade na capacidade de campo. Foram coletadas amostras nos tempos de contato estabelecidos e determinadas as concentrações do P-disponível pelos três extratores. Verificou-se, de modo geral, redução no teor do P-disponível pelos extratores Mehlich-1, RM e RA, com o aumento do tempo de contato das fontes de P com os solos. Considerando-se as três fontes de P, essas reduções dos teores foram mais expressivas quando se utilizaram o superfosfato triplo com o fosfato reativo em posição intermediária e o fosfato de Araxá, o menos sensível ao efeito do tempo. Observou-se que a RA extraiu menores teores de P, nos três solos, quando as fontes foram os fosfatos naturais, ao passo que a RM aproximou-se dos valores obtidos pelo Mehlich-1 no LV (argiloso) e LVA (textura média), para o fosfato reativo. Para o fosfato de Araxá, os valores de P foram maiores para o Mehlich-1 do que para as Resinas, em razão do caráter ácido desse extrator. Para o superfosfato triplo, ambas as Resinas extraíram maiores teores de P do que o Mehlich-1, em decorrência do desgaste desse extrator, de modo particular no LV e LVA.

Despite the large number of studies addressing the quantification of phosphorus (P) availability by different extraction methods, many questions remain unanswered. The aim of this paper was to compare the effectiveness of the extractors Mehlich-1, Anionic Resin (AR) and Mixed Resin (MR), to determine the availability of P under different experimental conditions. The laboratory study was arranged in randomized blocks in a [(3 x 3 x 2) + 3] x 4 factorial design, with four replications, testing the response of three soils with different texture: a very clayey Red Latosol (LV), a sandy clay loam Red Yellow Latosol (LVA), and a sandy loam Yellow Latosol (LA), to three sources (triple superphosphate, reactive phosphate rock from Gafsa-Tunisia; and natural phosphate from Araxá-Minas Gerais) at two P rates (75 and 150 mg dm-3), plus three control treatments (each soil without P application) after four contact periods (15, 30, 60, and 120 days) of the P sources with soil. The soil acidity of LV and LVA was adjusted by raising base saturation to 60 % with the application of CaCO3 and MgCO3 at a 4:1 molar ratio (LA required no correction). These samples were maintained at field moisture capacity for 30 days. After the contact periods, the samples were collected to quantify the available P concentrations by the three extractants. In general, all three indicated that the available P-content in soils was reduced after longer contact periods with the P sources. Of the three sources, this reduction was most pronounced for triple superphosphate, intermediate for reactive phosphate, while Araxá phosphate was least sensitive to the effect of time. It was observed that AR extracted lower P levels from all three soils when the sources were phosphate rocks, while MR extracted values close to Mehlich-1 in LV (clay) and LVA (medium texture) for reactive phosphate. For Araxá phosphate, much higher P values were determined by Mehlich-1 than by the resins, because of the acidity of the extractor. For triple superphosphate, both resins extracted higher P levels than Mehlich-1, due to the consumption of this extractor, particularly when used for LV and LVA.