Diego Pereira Santos · Mauriana da Rocha Sobrinho · Maria de Fátima de Castro Oliveira ·Niedja Bezerra Costa · Tiago Massi Ferraz · Fabrício de Oliveira Reis · Heder Braum · Eliemar Campostrini ·Fábio Afonso Mazzei Moura de Assis Figueiredo

Abstract In scenarios of climatic change when increased global temperatures can be expected, it is essential to search for technologies that favor sapling survival and growth after planting and increase yield in the f ield. Kaolin-based particle f ilms (KBPF) have been applied as barriers against the deleterious ef fects of high levels of solar radiation. The objective of the present study was to assess the ef fects of applying purif ied calcined kaolin-based particle f ilm to young eucalyptus plants. Five treatments were carried out: 0% (just water), 3%, 5%, 7%, and 10% calcined kaolin applied to the adaxial part of the leaves. A complete randomized block design was used with f ive treatments and ten replicates. The plants were assessed for height, diameter at ground level,gas exchanges (net photosynthetic CO 2 assimilation, stomatal conductance and transpiration), instantaneous (WUE)and intrinsic water use ef ficiency (IWUE), chlorophyll a f luorescence (maximum quantum yield of photosystem II(PSII) (F v /F m ), concentration of active reaction centers in relation to the quantity of photons absorbed (RC/ABS) and the maximum ratio of quantum yields of photochemical and concurrent non-photochemical processes in PSII (F v /F 0 ),SPAD reading and leaf ontogeny. The 3% KBPF concentration showed the best responses in biometric assessments 80 days after planting (DAP) and were corroborated by the responses of the leaf, stem and shoot dry matter production as a whole. The leaf ontogeny assessments showed positive responses following KBPF application when considering leaf development, with 7% KBPF concentration resulting in the highest mean values. The mean specif ic leaf mass had negative response to high KBPF concentrations. At 60 DAP,the gas exchange variables during both assessment periods declined with an increase in KBPF concentration. Signif icant dif ferences as a result of KBPF applications were found only at the start of the assessments (34 DAP) for both WUE and for IWUE. Leaf ‘greenness’ (SPAD reading) at 47 days showed a quadratic relationship in both periods. The variables of chlorophyll f luorescence showed a linear response at 34 DAP and a quadratic response 60 DAP. KBPF application increased height and diameter growth in plants treated with 3% KBPF but this response was not associated with photochemical ef ficiency and photosynthetic carbon assimilation values on a single-leaf basis. The best performance among the variables was provided by 3% KBPF application.

Keywords Ecophysiology · Photoprotection · Yield ·Silviculture

Introduction

Eucalyptus plantations have gradually increased in Brazil and some species grow faster than others and have adapted better to the edaphoclimatic conditions of the country, producing high-quality wood with many uses (Pinto et al. 2011;Fernandes et al. 2015). Growth rates depend on genetic qualities, climate and silvicultural practices (Binkley et al.2017). The interactions between environmental and genetic factors can directly af fect growth and plant development(Moraes et al. 2012; Gapare et al. 2015; Pagliarini et al.2016). Understanding the ef fects of ecological factors, both biotic and abiotic, on organisms helps in the search for ways to mitigate their impacts (Otto et al. 2013). Thus, research to understand these factors will help optimize production and ecosystem sustainability.

Although Brazilian clonal silviculture has adopted various technological advances, large and signif icant yield differences are still found in commercial plantations, indicating the need for more studies on the ecophysiological aspects of the plants (Xavier and Silva 2010). In the scenario of changing climates, which will have an impact on agricultural production (Chen et al. 2016), and even though eucalyptus is a high adaptive genus (Brillante et al. 2016), it is essential to develop strategies that mitigate the ef fect of light stress resulting from severe environmental conditions (Xavier et al.2018).

In order to reduce the impact of heat, excessive light, and water stress, calcined kaolin-based particle f ilms (KBPF)have been applied to plant leaves and fruits (Rosati et al.2007; Boari et al. 2015), also contributing to insect pest and pathogen control (D’Aquino et al. 2011; Silva and Silva 2015). The layer created by applying KBPF to the leaves reduces stress caused by high temperatures due to light excess (Glenn et al. 2002; Saavedra Del R et al. 2006, Dinis et al. 2016b).

Several studies have reported the benef its of using kaolin in various crops such as tomato (Cantore et al. 2009),apple (Glenn 2009 ), papaya (Campostrini et al. 2010), grape(Glenn et al. 2010; Dinis et al. 2016a), and cof fee (Steiman et al. 2007). The positive ef fects include leaf and fruit heat regulation, increased photosynthetic carbon assimilation,water use ef ficiency and reduced sun scalding of the fruit(Glenn and Puterka 2005; Boari et al. 2015; Sharma et al 2015). However, Brilhante et al. ( 2016) reported controversies regarding its impact on gas exchanges and raised doubts about the mechanism of action.

Limiting environmental factors frequently impose stress conditions on plants, af fecting their growth and survival(Xavier et al. 2018). Strategies to improve physiological responses during the initial growth of eucalyptus plants may be one way of increasing yields. Competition for resources among species may cause reductions in plantation development and yield. The development and survival of many tree species in commercial plantations have reduced yields due to the negative ef fects of weeds (Caron et al.2012).

Furthermore, high light intensity can result in photoinhibition (Hartmann et al. 2011), af fecting photosynthetic carbon assimilation (Long et al. 1994). Therefore, protection from excess temperature stress using KBPF can improve gas exchanges and plant yields (Glenn et al. 2003; Glenn and Puterka 2005). Thus, the object of this study was to assess the ef fects of applying calcined KBPF on the photosynthetic capacity and growth of young eucalyptus plants.

Material and methods

Experimental area

The experiment was carried out on the faculty farm of the State University of Maranhão (UEMA), located in the municipality of São Luís, Maranhão state, Brazil(02º35′04.0′′ S and 44º12′33.3′′ W). The climate according to the Köppen and Geiger classif ication is Aw (hot and humid) (Trinta 2007).

Temperature, relative air humidity (RH %), air vapor pressure def icit (VPDair), rainfall and photosynthetically active radiation (PAR) were determined daily throughout the experimental period using a mini meteorological station(Fig. 1 a–e). For daily PAR averages, only values recorded between 6 a.m. and 6 p.m. were considered and values of 0 were discarded.

Establishing the experiment

Ninety day-old clonal saplings ofEucalyptus grandisW.Hill. and aEucalyptus urophyllahybrid in 53 cm3polyethylene tubes were planted in 15 L pots exposed to the open and irrigated to keep the substrate at f ield capacity. The substrate used to f ill the pots consisted of soil collected on the farm and fertilized with 18-18-18 NPK formulation at 6.67 g per liter. Half of a dose of 20 g per pot of potassium oxide (60%K2 O) and urea (40% N) used on [cover fertilization] was applied at 30 days after planting (DAP) and half was applied at 60 DAP.

Applying the treatments

Used kaolin based-particle f ilm (KBPF) is produced from calcinate purif ied kaolin (Surround WP®; TK Inc., Phoenix, AZ., USA), a white and with a low abrasive compound of aluminum silicate (Al4 Si4 O10(OH)8) which is chemically inert and highly soluble in water (Glenn et al. 2010). The KBPF was applied by spraying the product at concentrations of 3%, 5%, 7%, and 10% mixed with water, with the treatments placed in rows. Controls were plants that were sprayed only with water. Treatments started August 11th 2018, with three applications every 15 days throughout the experiment.

Fig. 1 Mean daily values of a temperature (°C), b relative air humidity (RH%), c air vapor pressure def icit [VPD air (kPa)], d photosynthetically active radiation [PAR (μmol m −2 s −1 )], e and rainfall (mm)during the period of cultivation in pots (July 24 to October 23, 2018);values within the boxes refer to the daily means for the variables during the f irst, second and third physiological assessments, respectively

The calcined kaolin suspension was applied in the mornings using a manual 2-L capacity atomizer sprayer. For this,the pump was pressurized continually until resistance was met (internal air maximum pressure). The suspension was sprayed evenly from a distance of 50 cm on the adaxial part of the leaves to cover the crown completely. An estimate of the quantity of KBPF actually applied to the leaves was 4.1, 7.4, 8.5, and 11.8 g m−2, for 3%, 5%, 7%, and 10%treatments, respectively, which were obtained applying the product for 40 s in 1 m2area with f ive petri dishes that were weighed and randomly distributed. After the product dried,the petri dishes with the KBPF were weighed once again and their area calculated to obtain the amount of product.

Biometric and photosynthetic capacity measurements

Height and diameter

Height and diameter at ground level were assessed when the saplings were planted in the 15-L pots (time zero) and at 15,30, 60 and 80 DAP using a graded ruler and digital Vernier calipers, respectively.

Leaf ontogeny measurements

After applying the treatments to each plant, a young leaf was selected to assess leaf ontogeny. At two- and three-day intervals, the length of the central vein (LCV) was measured over a two week period. The LCV was measured using a ruler.

Gas exchange and leaf temperature

Gas exchanges [net photosynthetic CO2assimilation (A),stomatal conductance (gs), transpiration (E) and leaf temperature (Tleaf)] were measured from a single leaf per plant located in the mid third of the crown that was healthy, completely open and exposed to the sun. Data were obtained by an infrared gas analyzer (IRGA, LI-6400XT, LI-COR, Lincoln, NE, USA) and assessments carried out in the morning between 8 a.m. and 10 a.m. and in the afternoon between 12 noon and 2 p.m.

The equipment was adjusted to 1500 μmol m−2s−1PAR,a 500 μmol air f low inside the chamber and CO2concentration adjusted to (400 ± 20) ppm. The mean values inside the IRGA chamber for the variables CO2concentration, VPDairand RH are shown in Table 1.

Table 1 Means of the CO 2 concentration, air vapor pressure def icit(VPD), air temperature and relative humidity of the air inside the chamber on assessment days for gas exchanges at dif ferent times

Water use ef ficiency (WUE) and intrinsic water use ef ficiency (IWUE)

WUE was estimated from the ratio between the photosynthetic CO2assimilation values (A) and leaf transpiration (E)(A/E), while IWUE was estimated from the ratio between the photosynthetic CO2assimilation and stomatal conductance (A/gs) (Hatf ield and Dold 2019).

Chlorophyll a f luorescence analysis

Chlorophyll f luorescence measurements were carried out on the same leaves used for gas exchange assessments and the following were assessed: maximum quantum yield of photosystem II (Fv/Fm), the energy absorbed per active reaction center (RC/ABS), and the energy f low captured by dissipated energy, i.e., maximum ratio of quantum yields of photochemical and concurrent non-photochemical processes in PSII (Fv/F0).The data were obtained using a portable non-modulated f luorimeter, model Pocket–PEA (Hansatech, Norfolk, UK),that was previously adapted to the leaf in the dark for 30 min using specif ic leaf clips for analysis to completely open the reaction centers with minimal heat loss (Bolhar-Nordenkampf et al. 1989; Strasser et al. 2000) and complete QA oxidation.

SPAD reading: leaf ‘greenness’

The Soil Plant Analysis Development (SPAD) reading was assessed in the morning between 7 a.m. and 9 a.m. and in the afternoon between 12 noon and 2 p.m. using a portable chlorophyll meter (SPAD-502 Plus, Konica, Japan). For the assessment, f ive points were sampled on dif ferent leaves of the plant.

Shoot dry matterAt the end of the experiment, five plants per treatment were assessed for shoot dry matter content (SDMC). The selected plants were cut at the base, separated into stem and leaves, washed with water and the parts placed in a chamber at 72 °C until attaining constant weight. After drying, the material was weighed using a precision analytical balance to obtain the dry matter weight per plant (g plant−1). Shoot dry matter was estimated from the sum of the values for stem dry matter (SDM) and leaf dry matter (LDM).

Specif ic leaf mass (SLM)

This was obtained by taking 10 fresh leaf discs per plant using a 0.58 cm diameter cutting perforator. The discs were washed with water and placed in a chamber at 72 °C until constant weight. After drying, the discs were weighed on precision analytical balance. SLM was calculated from the ratio of dry leaf matter and leaf area (Deus 2014).

Statistical analysis

For each KBPF concentration, a one-way ANOVA was used to analyze the ef fect of treatments on plant growth. At different periods and times of the day, one-way ANOVAs were used and regression analysis performed to analyze the ef fect on variables af fected by KBPF concentration using linear and quadratic models. The models were chosen based on the biological occurrence of the response and signif icance of the coef ficients of regression using at-test with up to 10% probability and evaluation of the coef ficient of determination. All analyses were performed in R version 3.6.3 (R Core Team 2019) using the ExpDes.pt package for ANOVAs (Ferreira et al. 2018).

Results

KBPF application af fected height and diameter of the eucalyptus plants (p< 0.001). The 3% KBPF concentration had the best impact in the biometric assessments 80 days after planting, with a mean height of 95.0 cm (± 4.8) (Fig. 2 a),superior to the controls by 21.6%, and with a mean diameter of 16.8 mm (± 0.7) (Fig. 2 b), and 17.3% greater than the controls. However, concentrations over 3% did not stimulate growth and the means were statistically the same as the controls.

Leaf ontogeny assessments (Fig. 2 c) showed positive ef fects of KBPF applications considering leaf development(p< 0.001). At 13 days, 7% KBPF concentration resulted in the highest means, with growth 148.8% superior to the controls.

Fig. 2 Ef fects of the KBPF concentrations (0, 3%, 5%, 7% and 10%) on a height growth, b diameters, and c leaf ontogeny of eucalyptus plants;vertical bars indicate standard error of the mean (n = 10). ***: signif icant at 0.01% probability by the t-test

Fig. 3 a, b and c Net photosynthetic CO 2 assimilation (A), d, e and f stomatal conductance (g s ), g, h and i leaf transpiration (E), and j, k,and l leaf temperature (T leaf ) in eucalyptus plants after applying calcined kaolin-based particle f ilm (KBPF) at dif ferent periods and times of the day; vertical bars are standard error of the mean (n = 10); ***signif icant at 0.1% probability, ● : signif icant at 10% probability, ns:not signif icant

Sixty days after planting, photosynthetic CO2assimilation(A) (Fig. 3 c) showed that, during both assessment periods,(A) was reduced with increases in KBPF concentrations(p< 0.01). However, in the morning, 3% KBPF treatment had a mean value of 15.3 μmol m−2s−1(± 2.0), 8.1% higher than that of the controls. In the afternoon, there was a reduction of 17.4% for the same concentration when compared to the controls.

Signif icant dif ferences in stomatal conductance were found 34 days (p< 0.01) (Fig. 3 d) and 60 days (p< 0.001)after planting (Fig. 3 f). At 34 DAP, the model was quadratic with a maximum point estimated to be 0.66 mol m−2s−1for the morning measurements and 0.56 mol m−2s−1for the afternoon, relative to an estimated FP concentration of 3.2%and 2.8%, respectively. At 60 DAP, there was a decreasing response with increasing KBPF levels, with the lowest means for the 10% treatment at both measurement times.

Regarding transpiration assessment (Fig. 3 g–i), a negative ef fect was observed only at 60 DAP (p< 0.01) with increasing KBPF concentrations. Ten percent KBPF gave the lowest means, with values 30.4% below the controls in the morning and 64.9% lower in the afternoon assessment.

There were no ef fects of KBPF on leaf temperatures at 34 and 47 days. At 60 days, an increase in KBPF concentration resulted in an increase in leaf temperature (Fig. 3 j–l).

Signif icant dif ferences as a result of KBPF applications were found only at the start of the assessments (34 DAP)for both water use ef ficiency (WUE) (p< 0.001) (Fig. 4 a–c)and for intrinsic water use ef ficiency (IWUE) (p< 0.01)(Fig. 4 d–F). For these two variables, 3% KBPF resulted in the lowest means for both parameters. For WUE, the averages for the groups were 3.04 μmol mmol−1(± 0.2) for the morning measurements and 2.47 μmol mmol−1(± 0.08) for the afternoon measurements, values 15.4% and 8.0% lower than the controls, respectively. Whereas for IWUE in the morning, a mean 29.5% lower than the controls without KBPF application was observed. In the afternoon, the IWUE results were 17.1% lower than those in the controls.

With regards to the SPAD reading, there was a signif icant ef fect at 47 days (p< 0.05) (Fig. 5 a–c). The data showed a quadratic relationship, with a reduction in the means of the variables with a minimum point estimated of 40.7 leaf‘greenness’ in a estimated concentration of 2.8% in the morning, and a minimum of 37.0 leaf ‘greenness’ in the afternoon group with the estimated concentration of 2.1%,followed by an increase in the averages with higher KBPF concentrations.

With regards to the assessment of maximum quantum yield of photosystem II (PSII) (Fig. 5 d–f), there were no signif icant dif ferences found for KBPF applications 47 DAP.In the assessment at 34 DAP (p< 0.05), a higher Fv/Fmratio occurred with KBPF applications, the 10% treatment had in the morning the highest means among all treatments(0.84 ± 0.002). At 60 DAP (p< 0.01), the results showed a quadratic response with increases in the means giving an estimated concentration of 2.7% KBPF in the morning and 3.1% in the afternoon.

Thirty-four days after planting (p< 0.05) and 60 DAP(p< 0.01), there were signif icant dif ferences in the quantity of active reaction centers per photon unit absorbed by photosystem II (Fig. 5 g–i) with positive responses to increasing KBPF. At 34 DAP from the morning measurements,the 5% KBPF treatment had the highest mean (3.7 ± 0.3),29.3% higher than the controls, and in the afternoon, the highest average was with the 7% KBPF treatment, with a means of 1.5 (± 0.1), showing a 31.7% increase relative to the controls. At 60 DAP, the ef fect observed was quadratic,with increases up to 3.1% of KBPF for the morning measurement and 3.5% in the afternoon.

Fig. 4 a, b and c Water use ef ficiency (WUE) and d, e and f intrinsic water use ef ficiency (IWUE) in eucalyptus plants after applying KBPF at dif ferent periods and times of the day; vertical bars are standard errors of the mean (n = 10); ***:signif icant at 0.1% probability, ● signif icant at 10% probability, ns: not signif icant

Fig. 5 a, b and c SPAD (leaf ‘greenness’); d, e and f Maximum quantum yield of photosystem II (F v /F m ); g, h and i density of active reaction centers (RC/ABS) and j, k and l ef fective quantum energy yield conversion maximum ratio of quantum yields of photochemical and concurrent non-photochemical processes in PSII (F v /F o ) in eucalyptus plants, after applying KBPF at 3%, 5%, 7% and 10% at dif ferent periods and times of day. Vertical bars show standard error of the mean (n = 10); ***: signif icant at 0.1% probability; ●: signif icant at 10% probability; ns: not signif icant

With regards to the Fv/F0parameter, significant differences were found at 34 DAP (p< 0.05) and 60 DAP(p< 0.001) (Fig. 5 j–l). At 34 DAP during the morning measurements, the best response was for 10% KBPF with a mean 7.8% higher than the controls. While in the afternoon, the highest mean was for the 7.0% treatment, with a mean of 3.8% (± 0.11), a value 16.8% higher than the controls. In the assessment at 60 DAP, a quadratic model explained the data, with an estimated maximum point of 4.8, associated with 2.7% KBPF for the morning measurement and 4.0 related to 3.2% KBPF for the afternoon measurement.

The highest averages in height (Fig. 2 a) and diameter (Fig. 2 b) were for plants treated with the 3% solution and corroborated with leaf responses, and stem and shoot dry matter production (Fig. 6 a); biomass production decreased in treatments over 3%.

Although the results show a linear but negative response to KBPF applications (p< 0.001), the 3% KBPF treatment produced the highest means for leaf, stem and shoot dry matter (Fig. 6 a), with values 54.9%, 60.9% and 57.7% higher than the controls, respectively.

The mean specific leaf mass (SLM) (Fig. 6 b) was negative in response to high KBPF concentrations (p< 0.05).The highest mean SLM value was for 5% KBPF, with a mean of 87.4 g m−2(± 0.002), a result 20.9% superior to the controls.

Discussion

Applying calcined kaolin-based particle film increased the leaf surface albedo since the presence of the product increased the ref lection of incident radiation (Glenn 2012).In fact, the white coating of KBPF ref lects part of the photosynthetically active radiation that reaches the leaves (Jifon and Syvertsen 2003; Nanos 2015).

Although assimilate production is directly related to photosynthetically active energy conversion (Caron et al. 2012),the biomass gains associated by applying 3% KBPF were not due to gas exchange optimization (Fig. 5). Lombardini et al. ( 2005) reported that applying KBPF to pecan (Carya illinoinensis(Wangenb) K. Koch) leaves did not increase gas exchanges. The demand for assimilates by other parts of the plant, for example the roots, can prevent excess assimilates reaching the shoot. In this context, the biomass gain by 3%KBPF may be associated with other factors, such as lower root respiration and/or better scatter light within the canopy(Wunsche et al. 2004), rather than optimization of the photochemical machinery and photosynthetic CO2assimilation.

With regards to leaf ontogeny (Fig. 2 c), 5%, 7% and 10%KBPF resulted in the highest mean values throughout the assessment. This may have been due to increased shading of the leaves caused by the higher levels of KBPF deposited on the leaves. According to Alabadí et al. ( 2004), a reduction in light radiation to leaves can increase cell lengthening due to higher levels of gibberellin synthesis.

Fig. 6 Ef fect of KBPF concentrations on a: leaf dry matter(LDM), stem (SDM) and shoot dry matter (SDMC); b: specif ic leaf mass (SLM) in eucalyptus plants; vertical bars show standard error of the mean (n = 5);***: signif icant at 0.01% probability; ●: signif icant at 10%probability

In the assessment of leaf ‘greenness’ 47 days after potting(Fig. 5 b), the highest values were observed with increases in KBPF. Increase in leaf ontogeny (Fig. 2 c) and the higher chlorophyll concentrations following application of 7% and 10% KBPF may be an adaptive response to the reduction light available within the mesophyll (Espindola-Júnior et al.2009), similar to that which occurs with umbraphiles, plants growing in the shady sub-canopy (Almeida et al. 2005).These responses may be due to the benef icial ef fect of KBPF on leaf surface albedo and consequently on the ref lectance of ultraviolet and infrared wavelengths (Glenn 2012). A similar ef fect in leaf surface albedo with KBPF application was reported in research on walnut and almond trees (Rosati et al. 2006), orange, lemon and rubber trees and the common bean (Abou-Khaled et al. 1970).

SLM (Fig. 6 b) is directly related to the incidence of photon intensity on the leaf surface due to the quantity and organization of the parenchyma cells that comprise the leaf tissues, thus correlating with leaf thickness (Aranda et al. 2004; Sanches et al. 2009; Taiz et al. 2017). However, although 7% and 10% KBPF concentrations resulted in higher growth rates of the central leaf veins (Fig. 2), the increase in KBPF concentrations did not af fect SLM (Fig. 6).

In general, the gas exchange variables (Fig. 3 a–i) have a correlation between themselves (Marenco et al. 2014), and thus a smaller stomatal opening, as observed here, corresponded to a reduction of CO2entry and assimilation, and water loss through the stomata during the transpiration process (Costa et al. 2015).

The WUE (Fig. 4 a–c) and IWUE (Fig. 4 d–f) may be related to the control of stomatal opening that, in order to prevent drastic reductions in water potential, limits gas exchange in the stomata (O’Grady et al. 2008). This may interfere in net photosynthetic CO2assimilation (Steiman et al. 2007) that added to the ef fect of KBPF on light energy absorption and may have intensif ied at this time, reducing water use ef ficiency in the process. However, there was no association between plant growth and KBPF concentrations as shown in the present study. In fact, Maletsika and Nanos( 2015) showed that, forOlea europaeaL. plants, KBPF applications did not alter instantaneous or intrinsic water use ef ficiency.

In all the assessments, the mean Fv/Fmratios (Fig. 5 d–f)were within the 0.75–0.85 range, an indication that there was no photoinhibition (Bolhàr-Nordenkampf et al. 1989)or non-associated damage by light stress on photosystem II(PSII), suggesting that the plants were in adequate photosynthetically active radiation intensity conditions (Silva et al.2011). The high mean Fv/Fmvalues for leaves sprayed with KBPF may be associated with protecting the leaves from over excitation of photosystem II (Jifon and Syvertsen 2003).

Regarding Fv/F0(Fig. 5 g–i) which is, according to Lichtenthaler and Buschmann ( 1984), a more sensitive variable than the Fv/Fmratio, it was observed that during the f irst assessment period, 10% KBPF gave the best response for morning measurements compared to other treatments. This indicates that lower KBPF concentrations contributed to photon absorption and capture, resulting in greater activation of the reaction centers and thus favoring the photochemical process (Mengarda et al. 2012).

In line with observations for RC/ABS (Fig. 5 j–l), these responses demonstrate that absorbed energy was used more ef ficiently in the photochemical processes at points evaluated (Bispo 2010), thus expressing, as reported in literature,that the capacity of the plants in maintaining Quinonaain an oxidative state was not af fected (Zhao et al. 2017). This is generally associated with the absence of damage to the photosynthetic apparatus, corroborating with that observed for Fv/Fm. However, these responses did not positively af fect net photosynthetic CO2assimilation.

Conclusions

Plant growth as measured by height and diameter responded positively to KBPF applications and the highest increments for both were observed for 3% KBPF treatment. Photochemical ef ficiency did not ref lect gains in net photosynthetic CO2assimilation on a single-leaf basis after the application of KBPF. Biomass gains were not due to the optimization of single-leaf gas exchange measurements and there was also no association between the instantaneous or intrinsic water use ef ficiency and the KBPF concentrations. Of the KBPF applications, the best performance by the plants was with the 3% treatment.

AcknowledgementsThe authors thank CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior), FAPEMA (Fundação de Amparo à Pesquisa e ao Desenvolvimento Científ ico e Tecnológico do Maranhão) and FAPERJ (Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro) for f inancial support, and CNPq fellowships awarded 312959/2019, to Campostrini E.