2.1. Field experiments
The experiments were conducted for two cropping seasons in Casa Nova, BA state, northeastern Brazil (Figure 1). During the study, the temperature in the region ranged between 21.5 and 33.0 ºC, the average humidity was 51 %, precipitation was 309 mm, and evapotranspiration was 260.8 mm (Figure 2). Two areas of commercial table grape (Vitis vinifera) were evaluated, respectively, with the seedless cultivars Arra 15® (9° 20.593' S, 40° 48.950' W) and BRS Vitoria (9° 25.754' S, 40° 46.483' W), during two seasons: September to December 2020 (humid season) and January to April 2021 (dry season).
(Figure 1)
(Figure 2)
BRS Vitoria is the most cultivated table grape in the study area, occupying nearly 1,500 ha. It is a vigorous black cultivar adapted to wide climate settings and has excellent horticultural performance in several regions. The bunches are compact, requiring careful management practices, including growth regulators for elongation and berry thinning with scissors [24, 25]. In its turn, Arra 15® is second to BRS Vitoria regarding the cultivated area in NE Brazil. It is a white seedless grape with large, elongated, particularly crunchy berries planted worldwide due to its adaptability to differing climates, outstanding shipping performance, and long shelf life.
The soils of the experimental areas were classified as Ultisol in the cultivation of Arra 15 and Entisol in the cultivation of the Vitoria grape [26]. The physical and chemical characterization of soils at a depth of 0-20 cm was performed according to standard methods described by the Brazilian Agricultural Research Corporation [27] (Table 1). Soil available Si was extracted with 0.01 mol L-1 calcium chloride followed by colorimetric determination [28].
(Table 1)
2.2 Experimental design
An experiment was set up in randomized blocks with three replicates for each cultivar. The cultivars BRS Vitoria and Arra 15 were five and four years old, respectively, and cultivated in a trellis-type system with 3.5 x 3.0 m spacing with drip irrigation. Silicon was supplied as a granular fertilizer (2-5 mm) derived from amorphous silica (Agrisilica), containing 26 % Si, 2 % Ca, 1 % Mg, and 1 % Fe [9]. The treatments consisted of three rates of ASF (0, 175, and 350 kg ha-1) banded along the row with no incorporation five days after the production pruning in each evaluated crop season. The treatment plots were sized at 10.5 x 100 m, comprising three cultivation lines; the central line was considered the usable plot area for sample collection (Figure 1).
2.3 Chlorophyll fluorescence
Four chlorophyll fluorescence measurements were performed in each treatment plot at the time of fruit harvest. The determinations took place in the middle third of the leaf opposite the first bunch counted from the apex of the branch. First, the leaves were kept in the dark through clipping for 30 min to reach the maximum oxidation state of the photosynthetic electron transport system. After this period, the leaves were exposed to pulses of high-intensity saturated light (2250 mmil m-2 s-1) and the fluorescence was measured using a Fluorpen fluorometer, FP 100 model (Photon Systems Instruments). By determining the fast kinetic fluorescence, the initial fluorescence (Fo), maximum fluorescence (Fm), variable fluorescence (Fv = Fm – Fo), and quantum yield of photosystem II (Fv/Fm) were obtained.
2.4 Harvest and Chemical Analyses
Leaf samples were taken at the first and second cropping seasons for Arra 15 (November 2020 and April 2021) and BRS Vitoria (November 2020 and March 2021). Ten leaves were collected in each plot to form a composite sample. The leaves were washed in running water, subjected to a triple wash with distilled water, dried in an oven (Solab SL 102/42) at 65 °C for 72 h, and subsequently crushed in a Wiley mill (Tecnal TE-648).
To Si analysis, the leaves were digested by hydrogen peroxide and sodium hydroxide solution in an autoclave. Silicon was measured by photocolorimetry (NI 2000UV, Nova Instruments, Brazil) at a wavelength of 410 nm using ammonium molybdate as a complexing agent [28]. The contents of P, K, Ca, Fe, Cu, Mn, and Zn were determined in extracts from the digestion of leaf samples with HNO3 + H2O2 solution (3:1) in a microwave oven (Milestone – Ethos Easy) at 180 °C for 10 min according to modified 3050B methodology [29]. Phosphorus, Ca, Fe, Cu, Mn, and Zn were determined by inductively coupled plasma optical emission spectroscopy (ICP – OES Perkin Elmer Optima 7000 DV). Potassium was measured by flame photometry. The N content was obtained by digesting 0.2 g of the samples in sulfuric acid at 350 ºC, using the Kjeldahl method [30].
The analytical quality control used blank samples and SRM 1570a (Spinach Leaves) certified reference material from the National Institute of Standards and Technology (NIST). The recoveries of elements in the reference material ranged from 76 to 94 %.
2.5 Yield and postharvest quality of fruits
The yield was determined by weighing all bunches of four plants in each experimental plot. In addition, six bunches were randomly collected per plot to evaluate the postharvest quality; they were immediately taken to the laboratory for refrigeration and physical and chemical analyses. The physical variables bunch weight (BW), berry diameter (BD), berry length (LC), the number of berries per bunch (BB), berry crunchiness (BC), and berry firmness (BF) were determined. The chemical composition analyzed were soluble solids (SS), titratable acidity (TA), and SS/TA ratio.
Soluble solids (SS) were determined in juice samples using a digital refractometer PAL-1 (Atago, Brazil) with automatic temperature compensation. The results were expressed in percentage. Titratable acidity (TA) was evaluated by titration of 1 mL of juice diluted in 50 mL of distilled water with a solution of 0.1 N NaOH until pH 8.1. Results were expressed as percentage of tartaric acid in the juice. Berry firmness was determined as the maximum force required to press 20 % of the fruit diameter with a P/75 (75 mm) pressure plate, whereas berry crunchiness was determined by the penetration of a P/2 (2 mm) probe for 6 mm into the fruit, using a TA.XT.Plus Texture Analyzer (Extralab®, Brazil). Flesh firmness and crunchiness results were expressed in kilogram (kg).
2.6 Statistical analysis
Data were submitted to analysis of variance (one-way ANOVA) using the F test (p ≤ 0.05), and, when significant effects were found, a Tukey test for comparing means was performed (p < 0.05). All statistical procedures were performed using the SISVAR software (v 5.6).