Local and growing conditions.
Two experiments were conducted in a hydroponic growing system in the greenhouse at the School of Agricultural and Veterinarian Sciences (UNESP), Jaboticabal, Brazil.
Seeds of common beans (cv. BRS Estilo) were obtained from the Brazilian Agricultural Research Corporation of the Ministry of Agriculture, Livestock and Food Supply, Brazil.
The use of plant parts in the present study complies with international, national, and/or institutional guidelines. This research was not conducted with endangered species and was conducted in accordance with the is in accordance with the Declaration of IUCN Policy on Research Involving Endangered Species.
The first experiment was carried out to obtain the best Si concentration and source for Si leaf spraying, which occurred between August and the end of the crop cycle, which lasted 115 DAE (days after emergence). Based on results of the first experiment, a second experiment was conducted to evaluate the effect of Si on the physiology and dry matter yield of common beans plants under K deficiency, starting in December and maintained until the emergence of K deficiency symptoms, corresponding to the phenological stage R5 (28 DAE).
The relative air humidity and maximum and minimum temperature were recorded throughout the experimental period. There was a high variation in the average relative humidity (34.39 ± 9% | 32.79 ± 8%), minimum temperature (17.9 ± 7°C | 19.47 ± 5°C) and maximum temperature (44.8 ± 8°C | 38.56 ± 7°C) to the first and second experiment respectively. High temperatures may have induced plants to possible stresses, considering that the average temperature for optimum beans crop growth is between 18 and 24°C [18].
Growing conditions.
For the first experiment, the seeds were sown in a trays. Then the seedlings at five DAE were transplanted to 7 dm³ polypropylene pots (upper diameter: 16 cm; lower diameter: 11cm; height: 33 cm), filled with 6 dm³ medium texture sand, previously washed with water, 1% HCl solution and deionized water, maintaining two plants per pots. These were irrigated daily with nutritive solution applied in order to maintain 70% water-holding capacity in the substrate.
For the second experiment, the seeds were also sown in trays, and the seedlings at five DAE were transplanted to polypropylene pots (length: 44 cm; width: 19 cm and height: 14 cm, with capacity for 10 liters), also filled with the nutritive solution.
The nutrient solution used in both experiments was proposed by Hoagland and Arnon [19]. The solution concentration during the first and second week of the growing season was maintained at 10 and 25%, respectively. From the third week until the end of the experiments, the concentration was raised to 50%. The pH value of nutritive solution was maintained between 5.5 and 6.5, adjusted using NaOH (1 mmol L− 1) and HCl (1 mmol L− 1) solution. In the second experiment, the hydroponic solution was modified with different levels of K, as per the treatment (Table 1), and renewed every week to replace the water, Si and nutrients absorbed by the plants.
Table 1
Amount of K provided and adjustment between control (–Si), Si via roots (SiRO) and Si via leaf spraying (SiLE) treatments in the second experiment.
K supply route
|
-K
|
+K
|
|
-Si
|
SiRO
|
SiLE
|
-Si
|
SiRO
|
SiLE
|
------------------------------mmol L− 1-----------------------------------
|
Via root
|
0.2
|
*
|
0.2
|
6
|
5.8*
|
6
|
Via leaf
|
0.5
|
0.5
|
**
|
0.5
|
0.5
|
**
|
Total K
|
0.7
|
0.7
|
0.7
|
6.5
|
6.5
|
6.5
|
* received 0.2 mmol K via SiNaK; ** received 0.5 mmol via SiNaK |
Experimental design.
The first experiment was carried out in a randomized complete block design in a 2 x 4 factorial scheme, with two Si sources: sodium and potassium silicate stabilized with sorbitol (SiNaK) (113.4 g L− 1 of Si and 18.9 g L− 1 of K2O, pH 11.8) and potassium silicate without stabilizer (SiK) (128 g L− 1 of Si and 126 g L− 1 of K2O, pH 12.0). Four concentrations: 0.0; 5.4; 10.8 and 16.2 mmol L− 1 of Si. All treatments were conducted with four replicates.
The second experiment was arranged in completely randomized blocks in a 2 x 3 factorial scheme, with two concentrations of K in the nutrient solution: deficient (-K) (0.2 mmol L− 1 of K) and sufficient (+ K) (6 mmol L− 1 of K), and two modes of Si supply: roots via nutrient solution (SiRO) (2 mmol L− 1 of Si), leaves (SiLE) (5.4 mmol L− 1 of Si per application) and control (–Si) (0 mmol L− 1 of Si), in four repetitions.
Si application and K adjustment.
For the first experiment, foliar Si applications (SiNaK and SiK) were performed in three stages of development: V4 (emergence of the 3rd trifoliate leaf), R6 (flowering – opening of the first flower) and R7 (pod formation). The volume of the solution applied varied according to plant size and 8, 16 and 24 ml of the solution were sprayed in stages V4, R6 and R7 respectively.
For the second experiment, SiNaK was applied as a Si source. In the SiRO treatment, the Si supply via root was performed in a nutrient solution throughout the experiment.
To perform the foliar application in the second experiment (SiLE treatment), a solution with a concentration of 5.4 mmol L− 1 of Si (SiNaK) was made and, then, the application to the leaves was carried out manually. The volume of Si solution applied increased according to plant size, with 0.56; 0.84; 1.12 and 1.40 ml of the silicate solution per plant for the first, second, third, and fourth spraying, respectively, at 8, 13, 18 and 23 DAE.
The solutions used for leaf spraying in both experiments were adjusted with a solution of NaOH and HCl to maintain a pH of 6.0 ± 0.2. Silicon was applied to the leaves immediately after solution preparation.
The SiNaK and SiK sources contains K in its composition, after Si sprayings, foliar applications were performed with potassium chloride (KCl) to balance the K of the treatments. In the second experiment, the K provided by the SiNaK source was also adjusted for the root supply (Table 1).
It is important to highlight that 0.7 mmol L− 1 of K from SiNaK does not meet the demand of 6 mmol L− 1 suggested by Hoagland and Arnon [19] to supply K to plants, and nutrient deficiency of this nutrient is expected.
Temperature (oC) and relative humidity (%) in both experiments were measured during the foliar applications, obtaining values between 9 and 22 oC and 60 and 80% respectively.
Plant analysis.
In the first experiment, assessments were conducted in stage R7, and at twenty five DAE for the second experiment, both in the upper third of the trifoliate leaf.
Quantum efficiency of photosystem II and Gas exchange parameters. In the first experiment, the quantum efficiency of photosystem II (QEPII) was measured with a fluorimeter (Opti-Science®-Os30P+).
In the second experiment, Gas exchange parameters were determined between 9:00–11:00 a.m, using four replicates for each treatment. Photosynthetic rate and transpiration rate were measured using an open infrared gas analyser (IRGA LcPro-SD, ADC BioScientific Ltd., Hoddesdon, Reino Unido). The IRGA chamber was irradiated with a photosynthetic photon flux density of 1200 µmol m− 2 s − 1 and under ambient CO2 concentration (400 ± 10 µmol m− 2 s− 1). Water use efficiency (WUE) was calculated as net photosynthetic rate (A) per transpiration rate (E): WUE = A/E.
Total chlorophyll and carotenoid content. Total chlorophyll (a + b) and carotenoid content in the first and second experiment were measured by an absorbance spectrophotometer at 663 nm for chlorophyll a, 647 nm for chlorophyll b, and 470 nm for carotenoids. Pigments concentrations were determined following the methodology of Lichtenthaler and Wellburn [20].
Electrolyte leakage and relative water content. In the second experiment, the electrolyte leakage index and relative water content (RWC) were measured according to the methodology proposed by Dionisio-Sese and Tobita [21] and González and González-Vilar [22], respectively.
Plant growth analysis and dry matter. Leaf area of the plant was measured with a LI – 3100 Area Meter®. Moreover, the root system was analyzed using the Delta-TScan system and the length measured using the method developed by Harris and Campbell [23]. Root density was calculated by the ratio between root length and solution volume in the pot.
The plants were cut and separated into shoots and roots. Next, the samples were washed with deionized water, 0.1% detergent solution, 0.3% HCl solution and again with deionized water, and dried in a forced air oven at a temperature of 65°C ± 5, until reaching constant weight. After drying, root and shoot dry matter were obtained, followed by grinding in a Wiley mill.
Si accumulation and K use efficiency. To determine the Si content, shoot dry matter (first experiment) and shoot and root dry matter (second experiment) were used. For Si analysis, the samples were extracted following the methodology proposed by Kraska and Breitenbeck [24], and measured in a spectrophotometer at 410 nn to obtain Si content, following the methodology described by Korndörfer et al. [25].
In the second experiment, K content was analyzed by digestion in nitric perchloric acid solution, followed by atomic absorption spectrophotometer reading according to the methodology described by Zasoski and Burau [26]. K use efficiency was estimated considering the dry matter production and K content, according to the methodology described by Siddiqi and Glass [27]: (total dry matter production)2 / (total nutrient content in the plant).
Based on Si, K and dry matter values, the accumulation of these elements in the entire plant (shoots and roots) was calculated following the formula: Element accumulation = ((element content g kg− 1) * (plant mass g per plant))/1000.
Statistical analysis.
Experimental data were submitted to analysis of variance applying the F-test, and when significant for qualitative variables, to Tukey’s test (p < 0.05) to compare the means, using SAS statistical software 9.2 [28].