Introduction

The Brazilian forest sector has considerable relevance in the national economy, accounting for 6.9% of the industrial gross domestic product of this country. Of this sector, pine (Pinus spp.) and eucalyptus (Eucalyptus spp.) are the most important plants, totaling 7.83 million hectares of planted area (Ibá 2019). Specifically for pine, the planting areas are concentrated in the southern region. The state of Santa Catarina is responsible for 34% of the area planted with pine in Brazil. In 2018, the national productivity was 30.1 m3 ha−1 year−1, making Brazil the world's largest producer of pine (Ibá 2019). Thus, the development of genetic improvement programs, as well as cultural treatments (such as weed control), are fundamental factors to ensure the development of this sector (Stape et al. 2004; Pereira et al. 2012).

Glyphosate, N-(phosphonomethyl) glycine, is the most widely used herbicide for weed control in reforestation areas due to its low cost and broad spectrum of activity (Kogan and Alister 2010; Rodrigues and Almeida 2011). However, because it is a non-selective herbicide, it is often used for direct spray application in eucalyptus and pine cultivation areas, or sprayed between the planting rows using protected bars (Tuffi Santos et al. 2006). Although, due to the inadequate environmental conditions and the preparation of spraying equipment, there might be displacement of the application spray to cultivated plants (Carvalho et al. 2013). This phenomenon is named as drift, and it can cause damage to crop plants, resulting in productivity losses (Tuffi Santos et al. 2007; Carvalho et al. 2015; Monquero et al. 2016).

Previous studies have reported that phosphate transporters are involved in plant glyphosate absorption due to the phosphonate group present in the formation of the herbicide molecule. Thus, phosphate competes with the herbicide for transmembrane transporters, causing its pumping into the cell to be reduced, thereby not reaching its site of action (Denis and Delrot 1993; Morin et al. 1997; Pereira et al. 2019). However, it is important to point out that in the presence of phosphate, glyphosate absorption also occurs, but more slowly and in smaller amounts, via a transporter that recognizes the amino group in the glyphosate molecule (Morin et al. 1997; Xu et al. 2016). Since lower amounts of available phosphorus may stimulate increased expression of high affinity phosphate carrier protein encoding genes (Raghothama 1999; Pereira et al. 2019), it is important to study the effect of phosphorus fertilizer as an attenuator of glyphosate effects on a possible drift. Thus, it would be possible to apply this herbicide between rows without protecting the spray bar, reducing both the phytotoxic effect of this herbicide and the costs of weed control.

Phosphorus is an essential component of important plant cell compounds and participates in a variety of biological processes such as nucleic acid synthesis, enzyme activation and inactivation, carbohydrate metabolism and nitrogen fixation (Raghothama 1999; Vogel et al. 2005; Vitousek et al. 2010). The availability of this nutrient in most soils is rarely adequate for optimal plant growth and development (Abel et al. 2002). Thus, several studies have reported adverse effects on multiple classes of plant metabolites under phosphorus deficiency conditions such as phenylpropanoids, tricarboxylic acids, amino acids and carbohydrates (Hernández et al. 2007; Huang et al. 2008; Warren 2011). In Brazil, phosphorus is one of the most limiting nutrients in forest production due to its natural low levels in soils and its high ability to interact with colloids abundantly present in some soil types (Barros and Novais 1996).

In view of the increasing number of Pinus spp. genotypes used by the forest sector, the understanding of how these genetic materials respond to variations in soil nutritional availability are necessary for the appropriate targeting of genotypes available in the market, aiming at maximizing crop management and, consequently, productivity (Fox 2000; Codesido and Fernández-López 2009). Therefore, research in this direction can provide useful knowledge to producers, breeding programs and the scientific community in general.

Thus, with the hypothesis that phosphorus fertilization mitigates the toxic effect of glyphosate, this study aimed to evaluate the influence of phosphorus fertilization on the initial growth of Pinus taeda L. genotypes submitted to application of glyphosate subdoses during warm and cold seasons.

Material and methods

Experimental site and growing conditions

Four experiments were conducted in an open and semi-controlled area located at the Center of Agriveterinarian Sciences of the Santa Catarina State University, in Lages, SC, Brazil (27° 47′ 33″ S, 50° 18′ 10″ W and 915-m altitude). According to the Köppen (1948) classification, the experimental area is characterized as Cfb, with a temperate climate, humid and evenly distributed rainfall throughout the year.

Two simultaneous experiments were carried out starting in a warm season from November 2014 to June 2015 (Table 1) while the other two simultaneous experiments were conducted in a cold season from March to June 2016 (Table 2). Plants were grown in 10-L pots filled with a substrate derived from a mixture of silt-textured soil and commercial organic substrate (Tecnomax®, Brazil) in a proportion of 2:1 (v/v). The substrate was previously fertilized with 200 kg ha−1 NPK (formulation 5-2-10) added by 30 kg ha−1 urea (45% N), hereafter named as planting base fertilization (PBF). The experiments were conducted without water restriction.

Table 1 Meteorological data from the region of Lages, SC, Brazil, during warm season; year 2014/2015
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Table 2 Meteorological data for the region of Lages, SC, Brazil, during cold season; year 2016
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Plant material

Four genotypes of Pinus taeda were used in this study, hereafter named as Genotypes 1, 2, 3 and 4. Seedlings of each genotype were obtained and produced under trade privacy in a private nursery of the company Klabin® in Lages, SC, Brazil, and further supplied for our research.

Each experiment was conducted by using only one P. taeda genotype. Genotypes 1 and 2 were used in the warm season experiments and Genotypes 3 and 4 were used in the cold season experiments, according to company’s availability.

Treatments and experimental design

Treatments consisted of a 2 × 4 factorial scheme, being the presence or absence of a supplementary phosphorus fertilization (SPF) (25.2 kg P2O5 ha−1, equivalent to 2 g per pot), as the factor 1 and the application of four subdoses of glyphosate (0, 72, 144 and 288 g a.e. ha−1) as the factor 2. The same treatments were used for all experiments.

Each pot, with one plant placed in the center of the pot, was considered as an experimental plot. All experiments were set up in a completely randomized block design with five repetitions.

Phosphorus fertilizer, herbicide application and equipement used

Adding to PBF, a triple superphosphate (TSP) fertilizer (Delta Adubos®, Brazil) at dose equivalent to 60 kg ha−1 was used in this study for the SPF treatments. The TSP is originated from the phosphoric acid attack on the phosphorus rock. It is obtained in a granular form with 42% P2O5, supplying ~ 25.2 kg P2O5 ha−1. The TSP fertilizer was placed over the substrate and incorporated at 10-cm depth. The SPF occurred just before placing the pine seedlings to the pots, correspondent to 30 days before herbicide application.

For herbicide treatments, a commercial formulation (Roundup Original®, Monsanto, Brazil) containing an isopropylamine-salt glyphosate-based herbicide, with concentration of 480 g active ingredient (360 g acid equivalent—a.e.), was used in this study. Herbicide was applied using a CO2-pressurized backpack sprayer (Herbicat®, Brazil) equipped with a bar containing four 80.02 VS fan nozzles (TeeJet®, USA), spaced 0.5-m apart, and calibrated to deliver 200 L ha−1 spray volume.

The glyphosate subdoses were selected from a drift simulation, equivalent to 0, 5, 10 and 20% of the recommended commercial dose of Roundup® Original (4 L ha−1). These doses are equivalent to 0, 72, 144 and 288 g a.e. ha−1 (hereafter named doses 0, 1, 2 and 3, respectively) and were applied to all the four experiments. The herbicide was applied directly over the pine plants after an acclimatation period of 30 days after planting (DAP). In the warm season, glyphosate was applied to pine plants showing 16.5-cm average plant height and 2.3-mm average stem diameter, for both Genotypes 1 and 2. Herbicide application was performed at 8h30 am, under 67 °F temperature, 1.36 m s−1 wind speed, 84.8% relative humidity and with no rainfall. In the cold season, glyphosate was applied to pine plants showing 16.9-cm and 26.3-cm average plant height, and 4.1-mm and 4.1-mm average stem diameter, for Genotypes 3 and 4, respectively. Herbicide application was performed at 8:30 am, under 53 °F temperature, 0.69 m s−1 wind speed, 87.0% relative humidity and with no rainfall.

Measurements

Plant height, stem diameter and shoot dry matter was measured in each experiment. In the warm season, measurements of plant height and stem diameter were performed at 30, 60, 90, 120, 150 and 180 days after herbicide application (DAA), and shoot dry matter was determined at 180 DAA, the end of the experimental period. In the cold season, plant height and stem diameter were measured at 7, 14, 21, 35, 49, 63 and 81 DAA, and the determination of shoot dry matter was performed at 81 DAA.

The plant height was measured from the base to the top of the main stem using a graduated yardstick (cm). The stem diameter was measured at 2-cm from the soil using a digital caliper (mm). For dry matter determination, the plants were firstly cut close to the soil and then the plant material was dried in a forced air convection oven at 60 °C until constant weight. In sequence, dried plant material was weighed using a semi-analytical balance (0.01 g).

Statistical analysis

Data were subjected to analysis of variance (ANOVA) by F test and means were compared by Tukey test, at 5% probability level. The experiments were compared to each other within each season separately. Thus, genotype 1 was compared to genotype 2 (both conducted in the warm season), while genotype 3 was compared with genotype 4 (both conducted in the cold season).

In a first moment, the fertilization conditions were compared for each genotype separately, and in a second moment, the genotypes were compared in each fertilization condition. The AgroEstat® software (version 1.1.0.626, Barbosa and Maldonado Júnior 2011) was used for statistical analysis and graphics were made with Origin® 8.0 software.

Results

Warm season

When comparing the effect of glyphosate subdoses for Genotype 1, regardless of SPF, glyphosate application at dose 2 showed a positive effect on pine growth (p < 0.01, Table 3) compared to the control (dose 0) (Table 4). For the interaction between the factors (p-values on Table 3) for the plant height variable, in the condition without SPF, glyphosate at dose 3 provided an increase of 17% (p < 0.01), not differing from treatments with SPF (Table 4A).

Table 3 Two-way ANOVA summary table for plant height (PH), stem diameter (SD) and shoot dry matter (SDM) of two Pinus taeda genotypes cultivated during the warm season
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Table 4 Effect of supplementary phosphorus fertilization and glyphosate subdoses on plant height (PH), stem diameter (SD) and shoot dry matter (SDM) of two Pinus taeda genotypes cultivated during the warm season
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Regarding Genotype 2, plants treated with SPF had higher height growth than untreated plants (p < 0.01, Table 3), independently of glyphosate subdoses (Table 4). For stem diameter, the plant growth behavior was different considering the fertilization conditions (Table 4B). In the absence of SPF, glyphosate applied at dose 2 provided greater increment in stem diameter, differing from control and also from the condition with SPF (p < 0.05). When SPF was used, glyphosate at dose 1 stimulated pine growth, differing from the others and also from the condition without SPF (p < 0.01) (Table 4B). However, although this genotype did not respond positively to any of the glyphosate subdoses tested for shoot dry matter (Table 4C), the SPF caused higher plant growth compared to the treatment without SPF (p < 0.05), except for glyphosate application at dose 2 (Table 4C). It is noteworthy that there was no negative effect resulting from the application of glyphosate subdoses for any of the characteristics assessed in both genotypes.

Comparing the growth increment of genetic materials throughout the experimental period, regardless of glyphosate subdoses, Genotype 2 was more responsive to SPF, since the addition of this nutrient made its growth equal to that of Genotype 1 (Fig. 1A). In condition without SPF, the height increment of Genotype 2 was lower than Genotype 1 (p < 0.05) (Fig. 1B). Also, the plants of Genotype 1 showed no difference in height growth when compared the absence and presence of SPF. In contrast, plants of Genotype 2 in the presence of SPF obtained greater growth than those carried out in the absence of SPF (p < 0.01, data not shown).

Fig. 1
figure 1

Plant height of two Pinus taeda genotypes in the presence (a) and absence (b) of supplementary phosphorus fertilization with 60 kg ha−1 triple superphosphate—SFP (45% P2O5) added to the planting base fertilization (PBF) with 200 kg ha−1 NPK (formulation 5-2-10) plus 30 kg ha−1 urea (45% N), regardless of glyphosate subdoses, in experiments conducted during the warm season. Lages, SC, Brazil, 2014/2015. Vertical lines indicate the standard error of means (N = 5) and * indicates difference between treatments (p < 0.05)

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Regarding herbicide subdoses in the presence of SPF, glyphosate application at dose 3 provided a positive effect on pine plants (average of both genotypes) with an increase of plant height by 13.2% (p < 0.05, Tables 3, 5). Herbicide application also had a stimulatory effect on stem diameter and shoot dry matter, so that the glyphosate application at dose 2 was the best for Genotype 1, and the subdose of dose 1 was the best for Genotype 2 (for stem diameter only) (Table 5A, B, respectively). In addition, Genotype 2 showed the best tendency of response to SPF, since both stem diameter and shoot dry matter presented higher values for doses 0 and 1, comparing to Genotype 1 (Table 5A, B).

Table 5 Effect of glyphosate subdoses on plant height (PH), stem diameter (SD) and shoot dry matter (SDM) of two Pinus taeda genotypes grown in the absence or presence of supplementary phosphorus fertilization during warm season
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Considering the condition without SPF, the same response pattern was observed for stem diameter and shoot dry matter, in which the glyphosate application at dose 2 was the only one providing higher growth than the other treatments, regardless of the genotypes evaluated (p < 0.01, Tables 3, 5). For plant height, Genotype 1 showed greater growth than Genotype 2 in the two highest subdoses tested (p < 0.01) (Table 5C). Considering only Genotype 1, the dose 3 also caused higher growth than both the control and dose 1, but not differing from dose 2 (p < 0.01) (Table 5C).

Despite some specific characteristics in which doses 1 and 3 provided higher values to the evaluated variables (Tables 4A, B, 5A, C), in general, dose 2 was the one that provided the highest growth to pine plants, especially in relation to shoot dry matter (Fig. 2). Considering the average of both fertilization conditions, shoot dry matter increased by 34% (p < 0.01) in Genotype 1 plants, whereas regarding only the PBF + SPF condition, the increase in shoot dry matter was by 51.1% (p < 0.05) compared to control without SPF (Fig. 2).

Fig. 2
figure 2

Shoot dry matter increment (compared to controls) of two genotypes of Pinus taeda submitted to different fertilization conditions in relation to glyphosate subdoses, in experiments conducted during the warm season. Lages-SC, Brazil, 2014/2015. PBF = planting base fertilization with 200 kg ha−1 NPK (formulation 5-2-10) added by 30 kg ha−1 urea (45% N); SPF = supplementary phosphorus fertilization with 60 kg ha−1 triple superphosphate (45% P2O5). Dose 0 = control without glyphosate application; Dose 1 = glyphosate at 72 g a.e. ha−1; Dose 2 = glyphosate at 144 g a.e. ha−1; Dose 3 = glyphosate at 288 g a.e. ha−1

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Similarly, observing the means of the genotypes in the condition without SPF, glyphosate application at dose 2 was the only subdose that provided a stimulatory effect to pine growth, with the shoot dry matter increasing by 19.8% (p < 0.01) (Fig. 2).

Cold season

During the cold season, the SPF provided a positive effect on pine plant growth of Genotype 3 (p < 0.05, Table 6), regardless of herbicide application (Table 7). Regarding the interaction between the factors (p-values on Table 6) for shoot dry matter, none of the glyphosate subdoses differed from the control (Table 7A), however, there was a different behavior in relation to the fertilization treatments. Plants supplied with SPF and exposed to glyphosate at dose 2 showed higher shoot dry matter (p < 0.01), whereas the effect was the opposite for dose 3 (p < 0.05) (Table 7A).

Table 6 Two-way ANOVA summary table for plant height (PH), stem diameter (SD) and shoot dry matter (SDM) of two Pinus taeda genotypes cultivated during the cold season
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Table 7 Effect of supplementary phosphorus fertilization and glyphosate subdoses on plant height (PH), stem diameter (SD) and shoot dry matter (SDM) of two Pinus taeda genotypes cultivated during the cold season
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For Genotype 4, the SPF provided larger stem diameter and shoot dry matter (p < 0.01, Table 6), regardless of glyphosate subdose (Table 7). As found in the Genotype 3, no positive effect occurred due to herbicide application, since none of the subdoses differed from the control (Table 7).

A signifficant difference in the plant height of genotypes was found throughout the experimental period (Fig. 3). Genotype 4 showed higher plant height than Genotype 3, regardless fertilization condition. However, that difference is simply due to a non-homogeneous seedling size between the Genotypes 3 and 4 since experiment starded. In addition, a low plant height increment was found for both genotypes during the 81 days after treatment application.

Fig. 3
figure 3

Plant height of two Pinus taeda genotypes in the presence (a) and absence (b) of supplementary phosphorus fertilization with 60 kg ha−1 triple superphosphate—SFP (45% P2O5) added to the planting base fertilization (PBF) with 200 kg ha−1 NPK (formulation 5-2-10) plus 30 kg ha−1 urea (45% N), regardless of glyphosate subdoses, in experiments conducted during the cold season. Lages, SC, Brazil, 2016. Vertical lines indicate the standard error of means (N = 5)

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Genotype 4 had higher plant height and stem diameter than Genotype 3 (p < 0.01, Table 6), reaching 47.7% more plant growth in the presence of SPF, regardless glyphosate subdoses (Table 8). No positive effect of glyphosate on shoot dry matter was found in SPF condition for both pine clones (Table 8A). However, there was a difference between doses 0 and 3, where Genotype 4 had higher plant height and stem diameter than Genotype 3 (Table 8A).

Table 8 Effect of glyphosate subdoses on plant height (PH), stem diameter (SD) and shoot dry matter (SDM) of two Pinus taeda genotypes grown in the absence or presence of supplementary phosphorus fertilization during cold season
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In the condition without SPF, Genotype 4 had grown by 55.7% higher than Genotype 3 (p < 0.01, Table 6), but not differing for stem diameter (Table 8). For shoot dry matter, glyphosate-treated plants did not differ from untreated plants (Table 8B). However, Genotype 3 accumulated more shoot dry matter than Genotype 4 only for the dose 1, having an opposite effect for the dose 2 (Table 8B).

Discussion

The stimulatory effect caused by glyphosate subdoses on pine growth, observed under both fertilization conditions in warm season, can be qualified as part of the response of a phenomenon characterized as hormesis (Tables 4 and 5, Fig. 2). Hormesis is conceptualized as the positive effect resulting from the low dose action of a chemical that would be toxic in high amounts (Calabrese et al. 2007). This stimulatory effect has already been observed for several plant species and chemicals, such as coffee (Carvalho et al. 2013), soybean (Velini et al. 2008; Silva et al. 2015), chickpea (Abbas et al. 2015), sugarcane (Silva et al. 2009; Carbonari et al. 2014), eucalyptus (Pereira et al. 2013; Bacha et al. 2018; Pires et al. 2019), including pine in response to glyphosate (Velini et al. 2008).

For Pinus caribaea, Velini et al. (2008) observed that doses of glyphosate above 72 g a.e. ha−1 caused irreversible damage to the plants, reaching reductions of more than 60% compared to the control, for the highest dose tested (2,280 g a.e. ha−1). On the other hand, doses of up to 14.4 g a.e. ha−1 caused the plants to obtain greater growth when compared to the control, with increments of up to 20% in the total dry matter (Velini et al. 2008). In the present study, increments of up to 51.1% were observed in the growth of P. taeda Genotype 1 at 180 DAA (Fig. 2). The difference in increments between the previously mentioned studies is due to the fact that the occurrence of the hormesis phenomenon is related to several factors, such as the genotype/cultivar used (McDonald et al. 2001; Bacha et al. 2017); the conditions under which plants were exposed after application of the products (Belz and Cedergreen 2010; Cedergreen and Olesen 2010); and the final assessment point (Cedergreen et al. 2009; Belz et al. 2011).

Thus, considering the importance of the initial growth of forest crops to ensure good yield (Nambiar and Sands 1993; Esen et al. 2003; Garau et al. 2009), the pine increments observed in this study are significant. However, it is important to note that more studies must be conducted at the field level until the harvesting process. In this way, it will be possible to assess the real increment in productivity and in the characteristics of the wood produced.

The process of glyphosate-induced hormesis is not fully understood. However, it seems to be related to the level of shikimic acid concentration in the plant, and the inhibition of the enzyme 5-enolpyruvylshikimate-3-phosphorus synthase (EPSPs), since glyphosate-resistant soybean did not show changes in growth and concentration of shikimic acid, unlike soybean susceptible to the herbicide (Velini et al. 2008; Maeda and Dudareva 2012). Approximately 20% of the carbon fixed by the plant is destined for the shikimic acid pathway (Haslam 1993). Thus, the inhibition of the EPSPs enzyme will result in changes in the synthesis of proteins, vitamins, alkaloids and several important phenolic compounds, such as flavonoids and phytohormone auxin (Velini et al. 2009). Low doses of this herbicide are also related to the inhibition of lignin synthesis, making cell walls more elastic for a longer period, resulting in greater longitudinal growth (Duke et al. 2006). In another study, low doses of glyphosate have also been shown to cause an increase in the efficiency of carbon assimilation, probably related to the increase in the rubisco activity, the triose phosphate usage and/or to the RuBP regeneration (Cedergreen and Olesen 2010).

Phosphorus fertilization is an important activity in the cultivation of several crops, since phosphorus is the central component of fundamental metabolic processes for plants, such as photosynthesis and respiration (Raghothama 1999), being extremely important for the early development in P. taeda (Vogel et al. 2005). And as such, the effects of this nutrient on plant growth were observed on height (Tables 4A and 7), stem diameter (Table 7) and shoot dry matter (Tables 4C). When phosphorus is available in adequate amounts and in an easily absorbable biological form, the result is usually increased plant growth rates (Vitousek et al. 2010).

The influence of this nutrient on plant height increment and dry matter production was also noted in Pinus maestrensis, in which phosphorus was the most important compound (Molina et al. 1987). As observed in the shoot dry matter production, the plants of Genotype 2 showed higher values for this characteristic (Table 5B). One of the possible causes may be a high production of needles, which reflect the plant's ability to intercept photosynthetically active radiation, and consequently produce more photoassimilates, resulting in dry matter accumulation of the plant (Xavier and Vettorazzi 2003). In addition, this nutrient also participates in several biological processes, such as nucleic acid synthesis, enzyme activation and inactivation, carbohydrate metabolism and nitrogen fixation (Raghothama 1999).

The better use of phosphorus fertilization by Genotype 2 (Fig. 1) is probably related to the genetic characteristics of these plants, since lower amounts of available phosphorus may stimulate increased expression of genes of high affinity phosphorus carrier proteins (Pereira et al. 2019). Plants can reduce phosphorus deficiency by coordinating the expression of the genes involved in their transport. It is believed that there are two phosphorus absorption systems in inorganic form, one of high affinity, activated under conditions of low phosphorus availability, and another of low affinity, constitutively expressed (Raghothama 1999). There is evidence that these transporters are involved in glyphosate uptake by plants due to the phosphonate group present in the formation of the herbicide molecule. Thus, phosphorus competes with glyphosate for transmembrane transporters, reducing its pumping into the cell and thereby not reaching its site of action (Denis and Delrot 1993; Morin et al. 1997; Pereira et al. 2019). However, it is noteworthy emphasize that the herbicide absorption also occurs in the presence of phosphorus, but in smaller amounts, through carrier that recognizes the amino group in the glyphosate molecule (Morin et al. 1997; Xu et al. 2016).

Although glyphosate subdoses had a positive effect on pine growth in both fertilization conditions (Table 5 and B), the greatest increase in shoot dry matter observed in this study was obtained with the addition of SPF in the environment, in which the plants of Genotype 1 reached values ​​51% higher than the control (Table 5B). Thus, since the glyphosate-induced hormesis process causes high cellular activity (Cedergreen and Olesen 2010), we hypothesize that the addition of SPF in the system provided greater availability of nutrients to suppress the demand for this greater metabolic activity in the plants, justifying the results found. It is important to note that this interaction also varies according to the genetic material, since the addition of SPF added to the effect of glyphosate subdoses did not cause an increase in the shoot dry matter of Genotype 2 (Table 5B). We also assume that the plant's response may be dependent on growing seasons. Despite this, the comparison of plant responses between the warm and cold seasons is not robust, since the pine genotypes used in this study were different in each growing season.

Nevertheless, it is noteworthy that in warm season, plants without SPF also showed increase in some evaluated characteristics, such as the plant height of Genotype 1 for dose 3 (Table 4A); in the stem diameter of Genotype 2, for dose 2 (Table 4B); and shoot dry matter (mean of the two genotypes) also for the dose 2 (Table 5). This may be due to the fact that the subdoses used in this study were not sufficiently toxic to the plants, not requiring the presence of phosphorus to mitigate the effects of the product, as observed in previous works (Velini et al. 2008; Carvalho et al. 2013; Pereira et al. 2013; Abbas et al. 2015).

In both warm and cold seasons, it was possible to observe the different growth of the genotypes studied with the applied treatments. Specifically for the warm season, in the condition without SPF, Genotype 1 showed higher plant height (Fig. 1B), but without providing significant increase in shoot dry matter (Table 5). When SPF was supplied, Genotype 2 was able to match up the plant height of Genotype 1 (Fig. 1A). The addition of SPF also caused the plants of Genotype 2 to obtain higher values for stem diameter (Table 4B) and shoot dry mass (Table 4C), compared to those that did not receive SPF. The same did not happen for Genotype 1 (Table 4). This shows the greater responsiveness of Genotype 2 to phosphorus fertilization. Tyree et al. (2009) found that the “narrow crown” genotype of P. taeda showed better utilization of phosphorus and nitrogen insertion in the system compared to the “broad crown” genotype, with a 21% increase in stem volume. Similarly, other authors have also elucidated differences in nutrient utilization from distinct genetic materials (Li et al. 1991; Retzlaff et al. 2001; King et al. 2008).

With the advancement of the forest sector to several areas, the understanding of how genotypes respond to variations in soil nutritional availability is necessary for the appropriate targeting of genetic materials available in the market, aiming to maximize crop management, and consequently productivity (Fox 2000; Codesido and Fernández-López 2009). Thus, the best response of Genotype 2 to SPF, in addition to the stimulatory effect provided by subdoses of glyphosate, is an important information for the choice of genotype to be used in areas where this herbicide will be applied between the planting lines, without the need of spray bar protection, thereby reducing the cost of weed management.

In cold season, although the significant difference between the genotypes, plants of both genotypes showed neither pronounced growth during the experimental period (Fig. 3). Regarding glyphosate treatment, none of the tested doses provided plant's growth greater than the control (Tables 7, 8), although they varied in response to each other, for example, in doses 1 and 2 of genotype 3 for shoot dry matter (Table 8B). Such response is probably due to the period of vegetative dormancy. In the annual conifer cycle (e.g. pine plants), some species alternate warm growth periods with winter dormancy and in cold seasons there is a marked decrease in plant growth due to low temperatures (Havranek and Tranquillini 1995; Machado et al. 20102014). During this period, changes in cellular metabolic activities occur along with changes in cytoplasmic structures (Öquist and Strand 1986), which allow the survival of some conifers to winter desiccation (Havranek and Tranquillini 1995). Low temperatures also inhibit photosynthesis due to the inactivation of enzymes in the carbon reduction cycle (Strand and Öquist 1985), in addition to the photosystem II photoinhibition resulting from high light levels (Strand and Öquist 1988).

Conclusions

Subdoses up to 288 g a.e. ha−1 of an isopropylamine-salt glyphosate-based herbicide cause non-toxic effect on plant height, stem diameter and shoot dry matter of Pinus taeda genotypes, during both warm and cold seasons, regardless of a supplementary phosphorus fertilization with ~ 25.2 kg P2O5 ha−1 added to the recommended planting base fertilization.

The exposure of P.taeda to glyphosate subdoses may provide a stimulatory effect to plant growth during the warm season, regardless of the phosphorus fertilization. However, the intensity of this effect is dependent on the genotype.

The supplementary phosphorus fertilization may increase some plant growth characteristic of P. taeda (e.g. plant height, stem diameter and/or shoot dry matter) in both warm and cold season, regardless of the exposure to glyphosate subdoses. However, the plant response is also dependent on the genotype.