Relationships of fatal yellowing in oil palm with environmental variables were verified in this work. The incidence of FY increased over the years evaluated, and its progress was evident in all plots of the two farms, in different percentages depending on the plot. In addition, incidence class intervals above 10% increased in 2014 and 2015 in the analysis of new cases of the disease. This result is similar to those from studies on the progression of the incidence of FY found in the literature. Bergamin Filho et at. 12 analyzed cumulative FY incidence curves constructed with monthly and annual data, noting an increase in the incidences in the proportions of 0.40, 0.15 and 0.016 for plantations of 19, 14 and 9 years, respectively. As in the present study, there were plots with up to 50% accumulated incidence, and the economic viability of planting was impaired, so clarifying the cause of this disease is essential to establish management practices. Therefore, an analysis of environmental variables in Pará State, a region where the occurrence of this disease is common, and the plant's nutrition was performed to determine their correlation with FY.
The northern region of Brazil has the ideal climatic conditions for oil palm cultivation, as it requires some specific edaphoclimatic variables for satisfactory production. Among them, the availability of water is important for plant growth and development. The ideal precipitation regime for this crop is an average annual rainfall of 1.800 mm. Another requirement is solar radiation. The necessary insolation to reach the productive potential of the oil palm is approximately 1.800/h/year, with a minimum of 5 h/day. In addition, greater productivity occurs in regions with small variations in temperature and annual averages between 25 and 27°C 40. However, these same factors promote several phytosanitary problems. According to the results obtained, higher precipitation intensity results in greater relative humidity of the air (83 to 89%), less evapotranspiration, and greater accumulation of water in the soil; these conditions and lower wind speeds provide a greater probability of FY occurrence. Thus, although oil palm requires 150 mm of precipitation per month, in this region, it can receive to 500 mm/month in the first half of the year. In months of the year with strong wind conditions (2 to 2.5 m/s), which is associated with high insolation values (220 to 250 h/month), i.e., reduced cloud cover and reduced rainfall and temperature (28°C), there was less progress of the disease. Consequently, with greater insolation and presence of winds, greater evaporation and reduction in the amount of water in the soil occurs. The importance of water balance has also been studied by Venturieri et al. 9, who showed that it has a negative correlation (p = 0.0002) with fatal yellow. In regions with mild water deficiency, no cases of this disease were observed.
In this work, the correlation of climatic variables with the incidence of FY was verified. There was a correlation between only actual evapotranspiration and wind speed in 2013 and 2014. However, when the data were lagged by thirty days immediately prior to the assessment of the disease, all climatic variables correlated with FY in 2014, indicating the importance of this approach. The lag in climatic data shows that a time interval is necessary for the plant to develop symptoms. Therefore, the manifestations of physiological imbalance in the field occurred due to intense rain and high relative air humidity thirty days immediately before the evaluations. To verify the morphological changes of water stress in the physiology of oil palm seedlings, Rivera-Mendes et al. 41 conducted experiments with four water conditions for 60 days: moderate deficit, field capacity, and partial and continuous flooding. Seedlings under permanent flooding had a 22% reduction in biomass when compared to that of the control, due to the higher rates of leaf respiration and limitations in the absorption and transport of macronutrients. Plants under partial waterlogging showed growth similar to that observed under optimal soil moisture conditions.
In addition to the relationship with climatic variables, the correlation of fatal yellow with soil fertility and plant nutrition was studied. The disease had a positive correlation with Fe, K and Mn found in the soil. Thus, there was an increased incidence in places with higher levels of these minerals in the soil. This positive relationship may have occurred due to deficiency in the absorption of the roots or the imbalance of these cations when compared to other cations in the soil. The nutrients P, Ca, Mg and B had a significant negative correlation, i.e., the disease decreased with increases in these elements in the soil. Seeking to understand the role of other nutrients in soil with FA, Silveira et al. 42 carried out an experiment with the omission of the macronutrients nitrogen, phosphorus, potassium, calcium, magnesium and sulfur and the micronutrients boron, iron, manganese, copper, zinc and molybdenum. These authors observed an increase in symptoms when there was omission of micronutrients other than zinc and less incidence in treatments with omission of macronutrients other than Ca and S. Several studies on nutrition and fertilization have been carried out to verify the relationship between mineral elements and FY. Research has not verified the simultaneous effects of soil class, soil fertility and plant nutrition. When fertilization was studied, foliar analysis of the nutrients was not performed. Therefore, it is not possible to determine if the nutrients were absorbed or if their lack was associated with the disease. In studies analyzing the mineral nutrients in the plant, soil analysis was not carried out to determine whether deficiencies in a certain nutrient in plants was caused by low concentrations in the soil or due to inadequate absorption, including that possibly due to excess water in the soil, which would cause anaerobic conditions for the roots. Therefore, studying plant nutrition and soil fertility simultaneously is essential to understand the plant's physiological processes.
As for nutrition, palm trees showed deficiency in the nutrients N, P, Ca, Mg, S, Zn and Cu and exhibited excesses of K and B. According to Guzmán 37, AUDPC had significant negative correlations with the nutrients N, Ca, Si, B and Fe and a positive correlation with K. The minerals P, Ca and Mg also had low levels in the soil, which is a possible explanation for their deficiency in plants. The N, S, Si, Fe, and Mn were present in adequate quantities in the soil. However, in the sampled plants, they were below the nutritional levels suitable for the crop. Among these elements, N and S are structural constituents of plant cell components. including chlorophyll, amino acids, nucleic acids, coenzyme A, S-adenosylmethionine, biotin, vitamin B1 and pantothenic acid. Therefore, their deficiencies inhibit plant growth, causing chlorosis and yellowing of the leaves 23. Iron deficiency is common when water saturation occurs in the soil because this causes anaerobiosis in the root system. Its characteristic symptoms are chlorosis between the ribs. The relationship between iron and FY was investigated by Viégas et al. 43 with four treatments of different doses of ferrous sulfate. Afterwards, the iron contents in the oil palm leaves were measured, they caused no reduction or increase in symptoms in the analyzed plants.
Silicon, on the other hand, is not an essential nutrient for many plant families. However, some species accumulate substantial amounts of this element in their tissues and exhibit enhanced growth, fertility and stress resistance when supplied with adequate amounts of this element. Thus, plants deficient in silicon are more susceptible to tipping and fungal infection. Freitas et al. 26 determined the role of silicon in reducing the severity of yellow Sigatoka (Mycosphaerella musicola) in banana trees grown in nutrient solution. The treatments had five concentrations of silicic acid (H4SiO4): 0. 0.5, 1.0, 1.8 and 3.6 mmol/L. The AUDPC for plants in the 3.05 mmol/L H4SiO4 treatment was 49.27% lower than that for plants without supplementary H4SiO4. In contrast, plants grown in a 3.6 mmol/L solution of H4SiO4 showed 23.53% more Si content in the leaves than plants grown without H4SiO4 supplementation. Thus, Si could be used in disease management.
Approximately 65% of oil palm trees had excess K and Mg deficiency. Potassium is the second most required nutrient in plants, and in the soil solution, it appears as the ion K+, which is absorbed in the roots of plants. In addition, K+ permeates plasma membranes, making it easily absorbed and transported in xylem and phloem. However, at high concentrations of this nutrient, the absorption of Ca2+ and Mg2+ is reduced by competitive inhibition 25. Mg deficiency induced by excess K in fertilization is common in crops such as banana and coffee, as they require large amounts of K 26,. Mg is present in the chlorophyll molecule. In addition, it participates in enzymatic activation, acting as a cofactor for phosphorylative enzymes by forming a bridge between ATP or ADP pyrophosphate and the enzyme molecule. Therefore, deficiency in this element is characterized by yellowing of the older leaves 23.
In the soil classes PACal md/arg. PACd md/arg and PACal + PACd md/arg, with the highest amount of clay, there was a higher frequency of plots with FY incidence. The textural class is determined by the particle size distribution and affects other physical properties, such as drainage and water retention, aeration and soil consistency. The texture and structure of the soil define the surface area and porosity, which are the main factors associated with the storage and availability of water in the soil and the drainage of water from the surface to deep layers of the soil profile (Costa et al. 2014). Silveira et al. 42 analyzed the physical and chemical properties of 19 pedological profiles distributed in different plots of crop plantations. The authors classified the soil as a medium-textured yellow Latosol and showed the presence of compaction (1.365 kg/m³ to 1619 kg/m³) at depths of 30 and 60 cm. Thus, saturation of the soil in the superficial layer occurred during the period of greatest rainfall in the year, causing oxygen deficiency in the superficial layer of the soil. These conditions can be harmful to oil palm, causing root rot, due to its root system being dominantly distributed horizontally close to the soil surface. Regarding water in the soil and drainage, Bernardes 45 measured the water potential in the soil with a tensiometer with low soil aeration with values between 0 bar and 0.075 bar, while 0.075 bar and 0.30 bar are more suitable for the plant. Thus, areas at risk of flooding, which is related to the soil class, can cause anoxia of the oil palm's superficial roots. triggering nutritional deficiency and weakening the plant's resistance to diseases of biotic origin.
In addition, epidemiological analyses identified stress-related proteins in FY and Asy plants with little difference in abundance between the two conditions. These prevent us from stating that the initial cause of FY is pathogenic infection.
Reactive oxygen species (ROS) are related to the regulation of signaling pathways and initial responses that occur in response to environmental stresses of biotic or abiotic origin 46, 47. Previous studies carried out with proteomic analyses have shown that ROS scavenging is an important mechanism for plant resistance and oxidative stress in flooded soils 48, 49. Antioxidant system-related proteins were identified in FY and Asy plants with little difference in their abundance. In this case, oxidative stress can be an indication of stress from soil flooding in the area of plant cultivation.
In the data, primary metabolism-related proteins involved in energy production were also identified as having little difference in abundance between conditions. Despite this, they were overall more accumulated in Asy plants.
Additionally, with regard to energy metabolism-related proteins, alcohol dehydrogenases were identified in the roots of FY and Asy plants. Alcohol dehydrogenases are involved in alcohol fermentation during hypoxia or anoxia. There is a decrease in energy production through oxidative phosphorylation. Fermentative metabolism promotes energy compensation through recycling of NAD+ to the glycolytic pathway28. Increases in the glycolytic pathway and alcohol fermentation-related proteins has been identified as a key response of plants to flooded soils 48, 50. Adaptive mechanisms to handle oxygen deprivation include synthesis induction of a set of approximately 20 proteins known as anaerobic stress proteins (ANPs), which in addition to alcohol dehydrogenase, also includes carbohydrate metabolism-related proteins, such as those identified in this study 28. In this context, the results indicated that the analyzed plants were likely subject to hypoxic conditions, which were probably caused by soil flooding in the crop area before or during the sampling period of the roots analyzed in this study.
It is important to highlight the identification of flooding stress-related proteins, such as polygalacturonase. This protein acts on the degradation of the cell wall, which leads to the formation of lysigenous aerenchyma. In response to different stimuli, enzymes of the antioxidant system are expressed in response to hypoxia under flood conditions49. These aerenchymal formations enhance aeration in the roots. Increases in both transcript and polygalacturonase protein levels have been identified in plants subjected to this stress 30, 51. In maize submitted to flooding stress, the increase in polygalacturonase expression and formation of aerenchyma tissue during waterlogged stress is related to an adaptive response and allows greater tolerance of the plants to flooding or waterlogged conditions 51, 52. Here, polygalacturonase was downregulated in FY plants. These plants were submitted to a hypoxic environment. This may be one of the factors why Asy plants are less debilitated.
Recently analyzing the protein profile of oil palm roots of FY and Asy plants, Nascimento et al. 31 observed high levels of energy metabolism- and fermentation-related proteins, such as alcohol dehydrogenase, in both plant conditions. In FY plants, this protein showed high levels from the onset to more advanced stages of FY. However, proteins related to biotic stresses presented greater abundance in only the more advanced stages. In this study, the alcohol dehydrogenase protein was also present in high levels in both samples.
Protein-protein interactions play an important role in achieving proper cellular function. In hypoxic conditions, plants seek to develop adaptive mechanisms that include changes in the levels of proteins such as those involved in carbohydrate metabolism (glycolytic pathways and fermentative metabolism) and signaling- and stress-related proteins, including antioxidant system and programmed cell death proteins. The presence of these proteins and the interactions between them are important for the survival of plants under adverse conditions but also provide information about changes in their physiological state due to exposure to stress. Interactions between most proteins acting on different metabolic pathways were observed, confirming the importance of the protein pool identified in this work to oil palm roots and the importance of its roles in responses to the environmental conditions that influence the development of FY in oil palm and the greater resistance of Asy plants to both the development and worsening of the FY symptoms.
Therefore, from all the analyses carried out, the edaphoclimatic conditions related to the accumulation of water in certain soil classes were related to the FY of the oil palm, which was related to the fertility and nutrition of the plant. This hypothesis was also confirmed by proteome analysis, which identified the environmental stress conditions of the roots in plants with symptoms. Under these conditions, several pathogens, both on the ground and in the aerial parts, may be opportunistic and associated with the symptoms observed, but they are not the primary cause of the disease. Therefore, a better understanding of the abiotic factors associated with this disease is essential to understand its etiology and in choosing locations and soils appropriate for the crop and which management strategies should be used. Thus, considering the abiotic origin of the disease caused by temporary flooding in oil palm cultivation sites, it is recommended that before planting that the soil pedology be surveyed, that soils less subject to flooding and good drainage are chosen, and that the soil is unpacked. Planting in ridges is also recommended to prevent root anoxia. In addition, periodic analyses of plant nutrition should be performed to maintain good soil fertility to avoid nutritional deficiencies and imbalances. In this way, planting can help plants adapt to the biome in the environment, achieving financial and social sustainability for forest populations.