Introduction

Annona squamosa L., popularly known as sugar-apple, is a short tree belonging to the Annonaceae family. Its yield in Brazil is still low, below 6 t ha^-1, and the Northeast is the main producing region (^{Lemos, 2014}). In general, the crop is produced in small areas using low technological level (^{Oliveira et al., 2016}). In the planting of the orchards, several phytotechnical aspects must be considered and one of the first and fundamental ones is the acquisition or production of seedlings, because their vigor may affect crop yield (^{Santos et al., 2017}).

One of the main problems for production in Northeast Brazil is the availability of water, as well as its quantity and/or quality. Presence of high quantity of salts, which would correspond to electrical conductivity values above 4 dS m^-1, affects water absorption and cause toxic effects (^{Munns & Tester, 2008}; ^{Taiz et al., 2017}), directly hampering physiological and biochemical processes (^{Prisco et al., 2016}), resulting in lower growth of seedlings (^{Cavalcante et al., 2010}; ^{Sousa et al., 2011}; ^{Nunes et al., 2012}; Sá et al., ²⁰¹³, ²⁰¹⁵; ^{Oliveira et al., 2017}).

The use of water-absorbing polymer has become a low-cost measure aimed at more efficient use of water and reduction of problems related to salinity (^{Kant & Turan, 2011}; ^{Ljubojevi et al., 2017}), as it is possible to reduce irrigation depth and consequently the addition of salts. Furthermore, management in irrigation frequency can be determinant in water availability (^{Navroski et al., 2015}), as well as the adjustment of container volume, which is directly linked to the availability of space and nutrients for root growth, possibly leading to greater growth and vigor of seedlings (Costa et al., 2009^a,^b; ^{Vallone et al., 2010}; ^{Mesquita et al., 2012}; ^{Santos et al., 2012}). However, larger containers are more expensive and, therefore, it is essential to reduce container volume to minimize costs, but without compromising seedling quality.

Thus, this work was conducted to evaluate substrate salinity and growth of sugar-apple seedlings as affected by irrigation frequency, irrigation water electrical conductivity, water-absorbing polymer and container volume.

Material and Methods

The study was conducted in the screened greenhouse (6° 58’ 9.4” S, 35° 42’ 59.3” W and 522 m of altitude) of the Department of Soils and Rural Engineering of the Center of Agrarian Sciences, at the Federal University of Paraíba, situated in the municipality of Areia, Paraíba State, Brazil.

Treatments were arranged in a [(2² + 2 x 2 + 1) x 2] + 2 scheme, where (2² + 2 x 2 + 1) was obtained by combinations between five doses of the polymer Hydroplan-EB/HyA (0; 0.2; 0.6; 1.0 and 1.2 g dm^-3) and five levels of irrigation water electrical conductivity (0.3; 1.1; 2.7; 4.3 and 5.0 dS m^-1) using the Box’s Central Composite Design, associated with two irrigation frequencies (daily and alternating), and two additional treatments to evaluate the effect of container volume (0.75 and 1.30 dm³), as can be observed in Table 1. The statistical design was randomized complete blocks, with four replicates, and the experimental unit consisted of four containers with one plant each.

The substrate consisted of a mixture of soil, sand and bovine manure at 3:1:1 proportion, respectively. The soil was collected in the 0-20 cm layer of a profile of Red Yellow Latosol (^{Santos et al., 2013}), at the Chã do Jardim experimental station, located in the municipality of Areia, Paraíba State, Brazil. Fertility attributes were: 5.9 of pH, 0.46 dS m^-1 of electrical conductivity (1 soil – 2.5 water suspension); 0.50; 9.47; 1.74; 1.28; 0.23 and 0.88 cmol_c dm^-3 of Al³⁺, H⁺+Al³⁺, Ca²⁺, Mg²⁺, Na⁺, K⁺, respectively, 45 mg dm^-3 of phosphorus and 24.3 g dm^-3 of organic matter. The saturation extract, which indicates soil salinity, had pH of 5.9 and electrical conductivity of 1.97 dS m^-1. Physical characteristics were: 68.67% of sand, 18.17% of silt and 13.16% of clay, 1.35 and 2.64 g cm^-3 of bulk and particle densities, respectively, and porosity of 48.86%. Analyses of substrate were carried out according the methodologies compiled by ^{Teixeira et al. (2017)}.

In substrate preparation, phosphorus contents were increased to 300 and 100 mg kg^-1 of nitrogen were applied (^{Novais et al., 1991}), using single superphosphate and urea, respectively. The water-absorbing polymer was hydrated in distilled water before being mixed with the substrate. Irrigation was performed with saline waters, prepared by the addition of sodium (Na⁺), calcium (Ca²⁺) and magnesium (Mg²⁺) ions in the form of chloride, following the proportion of 5:2:1, respectively, on mass basis (^{Silva Júnior et al., 1999}), in the public-supply water, with electrical conductivity of 0.3 dS m^-1. In the irrigation with alternating frequency, every two days, a volume equivalent to 70% of that applied in the daily frequency was applied. The daily water depth was applied to maintain the substrate close to field capacity, until drainage began.

At 120 days after sowing, the seedlings were evaluated for the following parameters: stem diameter at substrate level (SD), using a digital caliper; plant height (PH), measured with millimetric ruler from collar to apical bud; number of leaves (NL), determined by counting. After removal of sugar-apple seedlings, the substrate was sieved and electrical conductivity was measured in the substrate-distilled water suspension at 1:2.5 proportion, respectively. Dickson quality index (DQI) was calculated by the formula (^{Dickson et al., 1960}):

where:

TDM - total dry matter, g;

PH - plant height, cm;

SD - stem diameter, mm;

DMS - dry matter of shoots, stem + leaves, g; and,

RDM - root dry matter, g.

The data were subjected to analysis of variance. Quantitative effects of polymer doses and levels of irrigation water electrical conductivity were subjected to regression analysis, whereas irrigation frequency and container volume were evaluated by contrasts, based on F test (p ≤ 0.05). Statistical analysis was carried out using the software SAS^® University Edition.

Results and Discussion

The effects of irrigation water electrical conductivity, water-absorbing polymer, irrigation frequency and container volume on the electrical conductivity of the substrate, growth and quality of sugar-apple seedlings are presented in Table 2.

Substrate electrical conductivity was affected by irrigation water salinity and polymer doses (Table 2). Irrigation increased substrate salinity, which changed from 0.5 to 2.3 dS m^-1 (360% increase) from the initial condition to the condition of irrigation with 0.3 dS m^-1 water, being intensified with the increment in irrigation water electrical conductivity (Figure 1A). It was estimated that unit increase in irrigation water electrical conductivity led to increment of 0.22 dS m^-1 or 9% in substrate salinity, which changed from 2.3 to 3.3 dS m^-1, respectively, at salinity levels of 0.3 and 5.0 dS m^-1. ^{Bezerra et al. (2014)} observed that the non-saline character of the substrate used in the production of passion fruit seedlings changed to saline when irrigated using water from 0.57 to 1.1 dS m^-1 and to salic with water above 1.1 dS m^-1, demonstrating that even water with no limitations with respect to salinity can alter the saline character of the substrate.

* and **: Significant at 0.05 and 0.01 probability levels by F test, respectively

Figure 1 Substrate electrical conductivity after production of sugar-apple seedlings, as a function of irrigation water electrical conductivity (A) and water-absorbing polymer (B) 

Increasing polymer doses had effect on substrate electrical conductivity (Figure 1B), which showed maximum value of 2.8 dS m^-1 at the dose of 0.6 g of hydrogel kg^-1 of substrate, whereas doses higher than 0.6 g kg^-1 caused reduction in substrate electrical conductivity. ^{Navroski et al. (2015)}, mixing commercial substrate with hydrogel, also found increased salinity with the increment in polymer doses.

Growth and quality of sugar-apple seedlings were affected by irrigation frequency, water salinity and container volume (Table 2). Daily frequency of irrigation led to the highest values of stem diameter (4.8 mm), plant height (23.9 cm), number of leaves (12 units) and Dickson quality index (0.25), and reductions of 13, 20, 20 and 12% were caused when the alternating frequency was adopted (Figures 2A, D, G and J, respectively). This trend was also observed by ^{Zonta et al. (2009)}, in coffee seedlings, when the increase in irrigation interval reduced the growth in diameter and height. ^{Carvalho et al. (2013)}, in yellow passion fruit seedlings, and ^{Tsukamoto Filho et al. (2013)}, in seedlings of Myracrodruon urundeuva Fr. All of them observed that the growth in stem diameter, height and number of leaves decreased when irrigation frequency changed from daily to alternating, and these results can be associated with the water stress that may occur as the intervals between irrigations increase.

Means followed by the same letter do not differ by F test (p ≤ 0.05). ^ns, * and **: Not significant and significant at 0.05 and 0.01 probability by F test, respectively. Actual effect of the contrast: large container (1.30 dm³) – small container (0.75 dm³)

Figure 2 Stem diameter at soil level (A, B, C), plant height (D, E, F), number of leaves (G, H, I) and Dickson quality index (J, K, L) in sugar-apple seedlings as a function of irrigation frequency, irrigation water electrical conductivity and container volume at daily and alternating irrigation frequency, respectively 

Increase in water salinity, at both irrigation frequencies, reduced growth and quality of sugar-apple seedlings. The reduction rates per unit increase in irrigation water electrical conductivity, at daily and alternating frequencies of irrigation, were, respectively, 5.9 and 5.4% (0.34 and 0.26 mm) for stem diameter (Figure 2B), 9.5 and on average 6% (3.0 and on average 1.4 cm) for plant height (Figure 2E), 7 and 6% (1.0 and 0.7 leaf) for number of leaves (Figure 2H) and 14.2 and 11% (0.06 and 0.04) for Dickson quality index (Figure 2K). Among these parameters, Dickson quality index is the most expressive to indicate the deleterious effect of salt stress of the irrigation water.

Lower growth and reduction in the quality of sugar-apple seedlings due to water salinity are related to the accumulation of salts in the substrate, which leads to a decrease in osmotic potential and consequently in water absorption (^{Taiz et al., 2017}). According to ^{Prisco et al. (2016)}, salinity directly hampers physiological and biochemical processes of plants. Reduction in the growth of seedlings under saline water irrigation has been reported in various crops, such as papaya (^{Cavalcante et al., 2010}; ^{Sá et al., 2013}), cashew (^{Sousa et al., 2011}), sugar-apple (^{Nunes et al., 2012}; ^{Sá et al., 2015}) and jackfruit (^{Oliveira et al., 2017}).

In regard to container volume, the effects were associated with the irrigation interval and, when significant, containers with larger volume led to higher means. Growth in diameter (Figure 2C) and number of leaves (Figure 2I) in sugar-apple seedlings, under daily irrigation, increased on average by 0.64 mm (15%) and 2.6 leaves (27%). Under alternating frequency, the increments were 0.82 mm (23%) and 3.8 leaves (60%), when the container volume changed from 0.75 to 1.30 dm³.

For plant height, the gain with the increase in container volume was observed only under alternating frequency of irrigation, from 14.5 to 20.6 cm in the containers of 0.75 to 1.30 dm³, respectively (Figure 2F). ^{Lemos et al. (2010)}, evaluating container volumes in the production of sugar-apple rootstocks, also found higher values of stem diameter, height and number of leaves when the seedlings were produced in containers with larger volume.

Dickson quality index was also affected by container volume, but only under daily frequency of irrigation, increasing by 100% when the seedlings were produced in the larger container (Figure 2L). Negative effect of the reduction in container volume has also been observed in seedlings of coffee (^{Vallone et al., 2010}), papaya (^{Mesquita et al., 2012}) and passion fruit (Costa et al., 2009^a, ^b). These results can be caused by a greater development and vigor of the root system of plants grown in larger containers, which led to greater availability of nutrients and porous space for root growth (^{Costa et al., 2009b}).

Conclusions