Effect of Salinity and Potassium on Phytomass and Quality of Guava Rootstocks

Potassium fertilization is one of the main techniques that has been studied to mitigateeffects of salt stress in plants, probably because potassium reduces the toxic effect of sodium by competitive inhibition and provides greater tolerance to genotypes of plants to salinity. Hence, this study aimed to evaluate the effect of different salinities of irrigation water in the formation of phytomass and quality of rootstocks of guava cv. Paluma, fertilized with increasing doses of potassium, in an experiment conducted using eutrophic Fluvic Neosol with sandy loam texture under greenhouse conditions, in the municipality of Pombal-PB, Brazil. The experimental design was randomized blocks in 5 × 4 factorial scheme, and the treatments resulted from the combination of five levels of irrigation water electrical conductivity (ECw = 0.3; 1.1; 1.9; 2.7 and 3.5 dS m) and four K doses (70, 100, 130 and 160% of K), in which the dose of 100% K corresponded to 726 mg of K dm of substrate, with four replicates and two plants evaluated in each plot. Irrigation with water salinity from 0.3 dS m compromises the total dry matter accumulation and the Dickson quality index of guava rootstocks cv. Paluma at 225 days after emergence (DAE), independent of potassium fertilization. Fertilization with different potassium doses did not promote differences in phytomasses and quality of rootstocks. There was no significant effect of interaction (salt × doses of K) on the studied variables.


Introduction
Guava (Psidium guajava L.) is among the fruit species of highest expression in the Brazilian agribusiness, with great potential for expansion, notably in the Northeast region, due to the favorable edaphoclimatic conditions (IBGE, 2016), and the cultivar 'Paluma' stands out as the most widespread in Brazil and preferred by the most diverse consumer markets (Ramos et al., 2010).
Despite the good adaptation of this fruit crop to Northeast Brazil, this region poses limitations involving both quantitative and qualitative aspects of water resources, especially regarding the presence of salts in the irrigation water (Souto et al., 2013).However, due to the increasing demand for food, the use lower-quality water, such as saline water, becomes necessary.Nevertheless, using this waterfor irrigation increases the contents of salts in the soil solution, causing negative effects on plants through the inhibition of germination, emergence, growth and biomass accumulation, as a consequence of the osmotic and toxic effects of ions Na + and Cl -and nutritional imbalance in plant metabolism (Cavalcante et al., 2010).
It should be pointed out that the effect of water salinity on crops varies among species, genotypes, saline levels, edaphoclimatic conditions, irrigation management and fertilization (Brito et al., 2014); thereby, studies have been carried out using saline water in the Northeast region, especially on the formation of guava seedlings (Cavalcante et al., 2010;Souza, et al., 2016).However, these studies are very incipient regarding the interaction between saline levels and potassium fertilization, which highlights the importance of further research on this fruit crop in this growth stage.drought resistance and salt stress (Marschner, 1995).However, under conditions of salinity, the presence of excess ions from the salts, can prevent through chemical competition the absorption of essential elements for the growth of the plant, such as potassium (Tester & Davenport, 2003), which may possibly compromise the accumulation of phytomass and quality of rootstock, as in the case of guava.Cuartero and Muñoz (1998) report that the most direct method to reestablish normal levels of K in the plant in saline conditions would be to increase the concentration of this nutrient in the root zone up to a certain level by increasing the dose of potassium, which would possibly, cause a higher absorption of K in relation to Na, presenting lower Na/K ratios in the leaves and, consequently, a nutritional balance more adequate to the plants.Schachtman and Schroeder (1994) assumed the existence of a common K and Na uptake mechanism in higher plants, which would be regulated by the concentrations of these elements in the substrate; thus suggesting that high levels of K in the substrate could modify the uptake and transport of Na and limit the toxic damages attributed to this element under salinity conditions (Blanco et al., 2008).
Therefore, it is necessary to adopt strategies of water and soil management, such as mineral fertilizationwith potassium, to reduce the deleterious effects of high saline concentrations on plants (Sá et al., 2016), since potassium is one of the most required nutrients by guava seedlings (Franco et al., 2007), besides having an osmoregulatory function (Epstein & Bloom, 2006).
In this context, it is proposed with this work, to evaluate the effect of different levels of salinityof irrigation water in the formation of phytomass and quality of rootstocks of guava cv.Paluma, fertilized with increasing doses of potassium.

Experiment Localization and Treatments
The experiment was conducted from March 2015 to December 2015, in a greenhouse at the Center of Sciences and Agri-food Technology of the Federal University of Campina Grande, Campus of Brazil (6º47′03″ S;37º49′15″ W;193 m).
The experimental design was in randomized blocks with treatments arranged in a 5 × 4 factorial scheme, relative to five levels of irrigation water salinity (ECw = 0.3; 1.1; 1.9; 2.7 and 3.5 dS m -1 ) associated with four doses of K fertilization [(70, 100, 130 and 160% of recommended dose of K, corresponding to 508.2, 726,943.8 and 1,161.6 mg of K dm -3 of substrate)].Each plot consisted of two plantswith four replicates, totaling 80 plots (5 salinity treatments × 4 fertilization treatments × 4 replicates).Potassium dose corresponding to 100% was based on itsabsorption rate in the guava seedling formation stage determined in a hydroponic system by Franco et al. (2007).
Irrigation solutions were prepared through the addition of NaCl, CaCl 2 •2H 2 O and MgCl 2 •6H 2 O in water from the local supply system (Pombal-PB), which had electrical conductivity of 0.3 dS m -1 , maintaining equivalent proportions of 7:2:1 for Na:Ca:Mg, whose quantity (Q) was determined based on Richards (1954), according to Equation 1, calibrated using a portable conductivity meter.Subsequently, the solutions were stored in 200-L plastic containers to avoid evaporation and/or contamination by external agents.
Where, Q = quantity of salts to be applied (mg L -1 ) and ECw = desired level of water electrical conductivity (dS m -1 ).
The cultivar 'Paluma' was used in the experiment because it is a genetic material adapted to the edaphoclimatic conditions of the Brazilian Northeast region and one of the most cultivated in Brazil (Dias et al., 2012), due to the easy access to it, high yield, vigor, suitability for fresh and industrial consumption, tolerance to pests and diseases, especially rust (Puccinia psidii Wint.) (Oliveira et al., 2012).
'Paluma' guava seeds used for rootstock formation in the experiment were obtained from a commercial plantation in the municipality of Aparecida/PB.Plants were standardized according to the criterion of vigor, lack of pests, and sanitary health.Healthy, physiologically mature fruits with homogeneous size were harvested and washed in running water after removal of pulp and dried in the shade on paper towel for 3 days.Four seeds were equidistantly planted in polyethylene bag at 1.0 cm depth and, when plants showed on average two pairs of true leaves, thinning was performed to leave onlyonemost vigorous plant per bag.
Soil moisture was maintained close to field capacity through water balance in the substrate, irrigated using low-ECw water (0.3 dS m -1 ), until the beginning of treatment application (40 days after seedling emergence -DAE).Irrigation wasmanually performed in the early morning (8 h) and late afternoon (17 h), and the applied water volume was determined by the drainage lysimetry method, obtained by the difference between the applied volume and the volume drained in the previous irrigation, plus a leaching fraction of 0.15 (Bernardo et al., 2006), to reduce the salinity level of the substrate saturation extract.ECw readings were taken after each irrigation event, according to the pre-established treatments, using a portable conductivity meter.
The bags had two holes at the bottom to allow drainage and plastic bottles were placed below to monitor the drained water volume and estimate water consumption by plants.
Potassium fertilization started at 40 DAE and was divided into 24 equal applications, weekly performed.Potassium nitrate -KNO 3 (14% N and 48% K) was used as K source, manually applied using a Beaker, to simulate a fertigation with ECw of 0.3 dS m -1 , individually in each plot, according to the treatments.
In addition, 24 nitrogen (N) fertilizations were weekly applied using urea (45% N) as source, as recommended by Dias et al. (2012) for guava rootstocks propagated by herbaceous cuttings, at dose of 773 mg of N dm -3 of substrate considering the N percentage of 14% supplied by the potassium nitrate.
Cultivation practices consisted of manual weeding, superficial scarification of the substrate to remove compacted layers and pruning of lateral branches, since there was no incidence of pests and/or diseases.

Variables Measured
To analyze treatment effects on the phytomass production of guava rootstocks at 225 DAE, when the plants presented a diameter suitable for grafting (above 4 mm) (Bastos & Ribeiro, 2011), plants were collected, roots were washed to remove the adhered soil and each plant was separated into leaves, stem and roots.This material was placed in previously identified paper bags and taken to the laboratory for the determination of leaf area according to Lima et al. (2012), using Equation 2. Subsequently, the material was dried in a forced-air oven at 65 ºC for 72 hours and weighed on analytical scale to determine stem dry phytomass (StDP), leaf dry phytomass (LDP), root dry phytomass (RDP) and shoot dry phytomass (ShDP) (stem + leaves), which were summed to provide the value of total dry phytomass (TDP: stem + leaves+root).
Rootstock quality was measured using the Dickson quality index (DQI) for seedlings, through the formula of Dickson et al. (1960), described in Equation 4.

Statistical Analysis
The variables were subjected to analysis of variance by F test (0.01 and 0.05 probability levels) and, in cases of significant effect, linear and quadratic regression analyses were applied using the statistical software SISVAR-Version 5.3 (Ferreira, 2011).The regression model was selected through the best fit based on coefficient of determination (R 2 ) and considering a probable biological explanation for the treatments.Due to the data heterogeneity evidenced by the coefficients of variation (Tables 2 and 3), an exploratory analysis became necessary and the data were transformed to √x for the variables leaf dry phytomass, root dry phytomass, shoot dry phytomass, total dry phytomass, root/shoot ratio, leaf area ratio and Dickson quality index.

Results and Discussion
According to the analysis of variance summary (Table 2), the levels of irrigation water salinity had significant effect (p ≤ 0.01) on the dry phytomass of stem (StDP), leaves (LDP), shoots (ShDP) and roots (RDP) at 225 DAE.For K doses, significant effect (p ≤ 0.05) occurred only on stem dry phytomass.There was no significant interaction between the factors salinity levels and K doses (S × KD) for any of the studied variables.
Various authors report that K + is the main nutrient related to osmotic functions of plants and have observed a better performance of some genotypes under conditions of salt stress, associated with adequate potassium nutrition (Blanco et al., 2008;Gurgel et al., 2010;Parveen et al., 2016).However, Gurgel et al. (2010) argue that increasing the proportion of K + in saline medium does not always result in beneficial effects for plants; and may offer a non-significant effect with increased K doses, as verified in the present study.Such phenomena may be related to the use of low doses that do not contribute to the decrease of the Na + /K + ratio in the leaves.Note.ns = non-significant; ** and * = significant at 0.01 and 0.05 probability levels (p ≤ 0.01 and p ≤ 0.05); FD = Freedom degree; CV = coefficient of variation.

Table 1 .
Physical and chemical characteristics of the substrate used in the experiment