Biomass and Rootstock Quality of Guava ( Psidium guajava L . ) Saline Water Irrigated under Nitrogen Fertilization

The increase in N concentration in the root zone of plants under saline conditions can inhibit the absorption of chloride and reduce the osmotic, toxic and nutritional effects caused by this ion. Thus, this study aimed to evaluate biomass and rootstock quality of guava. cv. Paluma irrigated with saline water under four N rates. The experiment was carried out in a greenhouse at the Center of Science and Agri-food Technology of the Federal University of Campina Grande (UFCG) at Pombal-PB, Brazil. The experimental design was randomized block with a 5 × 4 factorial arrangement. The treatments were levels of electrical conductivity of water ECw (0.3. 1.1. 1.9. 2.7 and 3.5 dS m), with 70, 100, 130 and 160% of the N rate recommended for guava seedlings (541, 773, 1004.9 and 1236.8 mg N dm of soil) and four replicates totaling 80 plots, each one with five plants. Salt stress caused by electrical conductivity of irrigation water of 1.4 dS m onwards affected negatively the formation of phytomass of guava cv. Paluma rootstock, and this effect was mitigated on root dry matter and Dickson quality index by the increase in nitrogen rate up to 819.38 mg of N dm of soil.


Introduction
The common guava (Psidium guajava L.) is a species over all the subtropical and tropical regions worldwide, due to its easy adaptation to different edaphoclimatic conditions.In the Brazilian Northeast region, it is among the fruit crops of high economic value cultivated under irrigation (Gurgel et al., 2007), since its fruit has great acceptance in domestic and foreign markets, because of its pleasant taste, strong aroma and protein-mineral quality (Cavalcante et al., 2005).
In the semiarid region of the Northeast, a large portion of the water used for irrigation comes from small and medium-sized reservoirs (superficial water) and wells (ground water), which have high salt content with an ECw between 1.97 and 2.98 dS m -1 (Medeiros et al., 2003).The use of this water for irrigating crops, including guava, may compromise formation of seedlings and productive capacity (Cavalcante et al., 2007).
The excess of salts in the irrigation water may increase soil pH, electrical conductivity in the saturation extract of soil and compromise Na/Ca, Na/Mg and Na/Ca+Mg ratios in plants, causing nutritional imbalance, toxic and osmotic effects (Pereira et al., 2006).According to Nobre et al. (2010), Na + and Cl -are the most frequently accumulated ions in salinized soils and the main ones to harm plant metabolism.Additionally, under saline conditions high Cl -concentration may interfere with the absorption of NO 3 -through ionic competition, causing N deficiency (White & Broadley, 2001).In studies using saline water in irrigation, salinity negatively affected the production of guava seedlings of cultivars 'Paluma', 'Ogawa', 'Rica', 'Pentecoste', 'Surubim' and 'IPA-B38', reducing growth and phytomass accumulation in plant parts (Ferreira et al., 2001;Cavalcante et al., 2005;Gurgel et al., 2007;Cavalcante et al., 2010).
For the expansion of guava plantations in regions with low quality irrigation water, such as saline water, besides evaluating genotypes tolerant to salinity (Cavalcante et al., 2005;Gurgel et al., 2007), technologies for reducing effects of salts on plants during seedling formation should be evaluated.In this regard, Del Amor et al. (2000) report evidence of competition in absorption of nitrogen (nitrate) and chloride, so that an increase in N concentration in the root zone under saline conditions can inhibit absorption of chloride and minimize osmotic toxic effects caused by chloride.Therefore, N fertilization is an alternative because it enhances plant growth and reduces the effect of salinity on plants (Flores et al., 2001).The explanation may be related to the structural function of N, because it participates in organic compounds for plants, such as amino acids, proteins, proline, which increases the capacity of plants for osmotic adjustment, increasing the resistance to the water stress caused by salinity (Parida & Das, 2005).
Guava is highly demanding in N, and this nutrient is the second most required during the initial growth stage.Nitrogen accumulations were 552 and 585 mg plant -1 120 days after transplantation in cultivars 'Paluma' and 'Século XXI', respectively (Franco et al., 2007).This result was confirmed by Dias et al. (2012) who observed positive effects of 773 mg N dm -3 of substrate on growth, phytomass production and quality of guava seedlings cv.'Paluma' 120 days after transplantation.
There are studies about the importance of N fertilization on the initial growth of guava crop, but only a few of them evaluated the interaction between salinity and N fertilization, especially with the cv.'Paluma', one of the most used cultivars in the Brazilian northeast region.Thus, this study aimed to evaluate the effect of saline water on phytomass accumulation and rootstock quality of guava cv.'Paluma', under different nitrogen rates.

Experiment Localization and Treatments
The experiment was carried out in a greenhouse from March to October 2014, at the Center of Sciences and Agri-food Technology (CCTA) of the Federal University of Campina Grande (UFCG), in Pombal-PB, Brazil (6º47′20″ S; 37º48′01″ W; 184 m).
Saline waters were prepared using supply water, from the Piancó River, Pombal-PB.with ECw of 0.3 dS m -1 , mixed with requisite amounts of salts of NaCl, CaCl 2 •2H 2 O and MgCl 2 .6H 2 O in equivalent proportion of 7:2:1, which is the predominant ratio in the main water sources available for irrigation in the Brazilian northeast region.The calculations were based on the following relation between ECw and salt concentration: mmol c L -1 = EC × 10 ( Rhoades et al., 1992).Nitrogen rates were determined based on the mean rate of 773 mg dm -3 , recommended by Dias et al. (2012) for seedling production of guava.cv.'Paluma' propagated by herbaceous cutting method, which corresponded to a 100% N rate.

Plant Material and Management of the Experiment
The guava cultivar 'Paluma' was sown on March 18, 2014, at a depth of 1.0 cm, using two seeds per polytube (capacity = 288 cm 3 ), supported on metal benches, at a height of 0.8 m from the ground.
The polytubes were filled with a substrate composed of Fulvic Neosol + sand + weathered cattle manure, in 82%, 15% and 3% (volume basis) proportion, respectively.The substrate was analyzed for physical and chemical characteristics (Table 1) in the laboratory, according to the methodologies of Claessen (1997).
For phosphate fertilization, the rate of 100 mg of P dm -3 was applied in the form of single superphosphate, which was ground and mixed with the substrate at planting (Corrêa et al., 2003).For potassium fertilization (potassium chloride), the rate of 726 mg of K dm -3 of substrate, recommended by Franco et al. (2007), was divided into four equal applications at 60, 90, 120 and 150 days after emergence (DAE), via fertigation with water of EC of 0.3 dS m -1 for all treatments.
Thinning was performed when plants showed two pairs of fully expanded true leaves (15 DAE), leaving only the most vigorous seedling per tube.
Saline water application started at 25 DAE.Irrigations were applied according to the treatments, based on plant water demand, corresponding to the difference between the applied volume and the volume drained in the previous irrigation.A leaching fraction of 0.15 was applied in intervals of fifteen days, based on the volume applied during this period.Note.Ca 2+ and Mg 2+ extracted with KCl 1 M at pH 7.0; Na + and K + extracted with NH 4 OAc 1 M at pH 7.0; Organic matter: determined by wet digestion Walkley-Black method; pH PS : pH of saturated paste of the substrate; ECse: Electrical conductivity of the saturation extract of the substrate at 25 °C.
Fertilization began at 25 DAE and was divided into 14 equal applications, performed every 10 days, using urea (45% of N) as the N source, and application was similar to that used for K fertilization.

Variables Measured
Fresh and dry phytomass accumulation and rootstock quality of guava were evaluated at 190 DAE.Variables were: stem fresh matter (StFM), leaf fresh matter (LFM), shoot fresh matter (ShFM), as well as stem dry matter (StDM), leaf dry matter (LDM), shoot dry matter (ShDM), root dry matter (RDM) and total dry matter (TDM).
The stalk of each plant was cut close to the soil, separated into stem and leaves and immediately weighed on a precision scale (0.001 g), for evaluating StFM and LFM; the sum of these two variables accounted for the ShFM.
For DM evaluation, roots were removed from the substrate with a 3-mm-mesh sieve, and leaves, stems and roots were separately placed in identified paper bags and dried in a forced-air oven at 65 ºC until constant mass, for determination of LDM, StDM and RDM; ShDM was determined as the sum of StDM and LDM, and TDM as the sum of ShDM and RDM.
Rootstock quality was determined through the Dickson quality index, according to Equation 1 (Dickson et al. 1960).

Statistical Analysis
The data were evaluated through analysis of variance by F test (p ≤ 0.05).If significant, linear and quadratic polynomial regressions were performed using SISVAR/UFLA.The selection of the regression was based on the best fit, considering R 2 and a plausible biological explanation.

Results and Discussion
Irrigation water salinity and N rates had a significant effect (p ≤ 0.05) on StFM, StDM, LFM, LDM, ShFM, ShDM and TDM (Table 2).In addition, according to the ANOVA, there was significant interactive effect (p ≤ 0.05) between the factors irrigation water salinity and N fertilization on RDM and IQD.
There was quadratic response of StFM and StDM with the increase in salinity of the irrigation water at 190 DAE and the highest values (2.98 and 1.25 g), were obtained at the ECw levels of 1.4 dS m -1 and 1.3 dS m -1 , respectively (Figure 1A).This behavior may be related to the adaptation of the rootstock to the saline stress because, according to Flowers (2004), plants adapt or acquire tolerance to salinity when they are able to adjust the osmotic potential of the cells with that in the soil, or even maintaining high contents of K, Ca and NO 3 and low contents of Na and Cl inside the tissues, especially in the leaves (Dias & Blanco, 2010).Cavalcante et al. (2005) and Cavalcante et al. (2007) also observed adaptation of guava seedlings, cv.'Surubim', to irrigation water salinity at 180 DAE, with greater accumulation of StDM in plants irrigated with water of 1.5 dS m -1 .

Figure
Figure 2. L 30 of N dm -3 ) (Fig of1.1 and 1.9 ngs under hyd he data best fi N rate of 106% d

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