1. Introduction
Over the years, climate change has greatly impacted agricultural production and the food supply, mainly due to harvesting losses and crop yield reduction [
1,
2]. Climate change has stimulated the intensive use of the available water resources and has endangered hydrological balance and sustainable agricultural production [
3,
4]. It is known that the relationship between water, agriculture, and climate is extremely significant. Thus, it is essential to seek techniques that minimize the impacts of water stress in a scenario with major challenges regarding inadequate water availability [
5].
In Brazil, beans (
Phaseolus vulgaris L.) are one of the main crops grown through the entire year and under different edaphoclimatic conditions [
6,
7]. The state of Minas Gerais is the second-largest producer in the country, with 554.039 tons produced per hectare [
8]. The potential yield of beans can be impaired in conditions of water stress due to the lag in the processes of transpiration and absorption as a result of the low water availability [
9,
10,
11,
12].
Studies have shown that strategical water stress can improve crop yield in the field [
13,
14,
15]. Therefore, knowing the right time to expose the crop to a water deficit is fundamental. The deficit irrigation strategy can be seen as a feasible and efficient technique to ensure greater crop yield, without putting in jeopardy their physiological processes and their final yield [
16]. Little has been done to identify the impacts of the water deficit on the growth, yield, and chlorophyll characteristics of beans in the study region and therefore, this work provides important information for producers in the region or even for producers in other regions with the same characteristics as the present study.
Beyond the use of water deficit, another option to increase yield is the planting density technique. The increase in planting density should be carefully chosen so that intraspecific competition does not happen and it results in the best use of available resources for grain’s growth and yield. Some studies have been carried out, in a range of 100,000 to 400,000 plants ha
−1, to verify the best plant density for beans [
17,
18]. However, the combination of water deficit and the planting density of common beans has been little studied and still lacks information, including the study of the physiological characteristics of the plant. In the present study, these questions were studied and the effects were justified by the results.
Studies comparing the use of water deficit associated with different planting densities are still scarce. There is still a need for information that identifies the best condition and the best technical strategy that ensures the highest crop yield under these conditions, or even the use of strategies that warrant savings in water, energy, and seeds, without reducing crop yield. In regions of water scarcity, as is the case in the study area, it is increasingly important to seek management techniques that aim to better use water resources to ensure productivity in a sustainable manner. Therefore, this work aims to verify the impact of irrigation frequency on the bean’s morpho-physiological traits and water productivity under different planting densities and water deficit.
2. Materials and Methods
2.1. Experimental Area
The field experiment was carried out from May to August 2019 in the experimental area of Irrigation and Drainage, from the Department of Agricultural Engineering, Federal University of Viçosa (UFV), Viçosa-MG, Brazil, of which their geographical coordinates are 20°46′9″ S, 42°51′43″ W, with an elevation of 651 m (
Figure 1).
The region’s climate according to Köppen’s climate classification is “Cwa”, tropical of altitude, with a rainy summer and dry winter [
19]. During the experiment, the average air temperature was 18.8 ± 2.4 °C, the average solar radiation was 13.6 ± 3.6 MJ m
−2, and the accumulated rainfall was 82 mm (
Figure 2).
The soil of the experimental area is classified as Red-Yellow Argisol [
20] and its chemical and physical characteristics are shown in
Table 1 and
Table 2, respectively.
2.2. Experimental Design
The randomized block design in a split-plot (3 × 5) scheme with 5 replications was used. In the plot, the irrigation frequencies studied were: 1 (F1), 4 (F2), and 8 (F3) days. In the subplot, the plant densities studied were: 20 (D1), 24 (D2), 28 (D3), and 30 (D4) plants per m2.
Each plot was formed by three rows of plants spaced every 0.5 m, 1.5 m wide, and 2.0 m long, a total of 3.0 m2 for one experimental unit.
2.3. Agronomic Practices
The soil preparation was carried out with plowing and harrowing 15 days before sowing. The sowing of the common bean variety BRS Esteio was done on 7 May 2019, through mechanization, placing the seeds at a depth of 3 cm. To ensure the proper stand and to avoid replanting, 50% more seeds were sown manually, using manual planters, in each treatment. At 18 days after sowing (DAS), the plant stand was adjusted for each treatment and the excess plants were thinned.
Fertilization was carried out during sowing, using the formulated fertilizer 4-14-8 (N-P-K) at a dose of 250 kg ha−1. After sowing, the fertilized ammonium sulfate (20% N) was applied at a dose of 10 kg ha−1. The weeds were controlled by manual weeding. Pests and diseases were controlled through chemical methods. The harvest was carried out on 31 August 2019, 21 days after the last irrigation and 116 days after sowing.
2.4. Irrigation System and Management
A surface drip irrigation system was used. This comprised a 16 mm drip tape (Toro, model Aqua-Traxx, Plentirain, China) with emitters spaced every 30.5 cm and 1.50 L h−1 of flow, with an operating pressure of 1 bar. A drip tape was installed in each plant row. After sowing, 40 mm of water was applied by the irrigation system to ensure germination.
The irrigation depth applied in each treatment was determined based on the current soil moisture. For this, in each treatment, two tensiometers were installed in soil layers 0–20 and 20–40 cm. The readings of the tensiometers were taken daily, between 8:00 and 10:00 a.m. The frequency of the irrigations depended on each treatment (1, 4, or 8 days). The irrigation management period was 77 days, starting 18 days (25 May 2019) after sowing and ending on 10 August 2019, 15 days before harvest.
The tensiometers readings were converted into soil moisture through the soil water retention curve, obtained by the Richards chamber method. The Van Genuchten model [
21] was adopted, considering the hydraulic characteristics of the soil shown in
Table 2.
The irrigation depth was calculated for each treatment according to Equation (1).
where, LI is the irrigation depth, mm; θ
CC is the volumetric soil moisture at field capacity, m
3 m
−3; θ
a is the volumetric current soil moisture before irrigation, m
3 m
−3; Z is the crop’s root system depth, mm; and Ef is the efficiency of the irrigation system by field test, 0.98.
2.5. Water Consumption
The actual water consumption during the bean cycle was calculated based on the soil water balance [
22] through Equation (2).
where WC is the water consumption, mm; I is the applied irrigation, mm; P is the rainfall, mm; Cr is the capillary rise, mm; Dp is the percolation, mm; Rf is the runoff, mm; ΔS is the change in soil-water storage between sowing and harvesting, mm.
In Equation (2), Cr was considered null because the water table was more than 15 m below the surface, Rf was also assumed to be insignificant because the experimental area is flat, and Dp was significant only when there was precipitation.
2.6. Evaluated Parameters
The plant height was measured weekly in three random plants from each plot. The evaluation of the chlorophyll a (Cfa) and b (Cfb) levels in the leaves were obtained with the aid of a chlorophyll meter (model chlorofilog, Falker, BRA) in plants 67 DAS. After the grain filling phase (95 DAS), three plants were collected in each plot. Their stem, leaves, and pods were separated for the total and stratified quantification of fresh matter and percentage of aboveground dry matter, after drying at 65 °C for 72 h in the heating chamber. Also, at 95 DAS, the height of the insertion of the first pod of the three plants collected was measured. At harvest, the weight of 1000 grains and yield (ton ha
−1) were quantified. Finally, irrigation water productivity (ton m
−3) was determined using the method of [
23].
2.7. Statistical Analysis
The effects of the treatments were evaluated by variance analysis and when there was a significant difference, the means were separated by the Tukey test (
p < 0.05) using the R software [
24].
4. Discussion
The irrigation frequency changed the temporal distribution of soil moisture. Increasing the irrigation frequency decreased the fluctuations in soil moisture and it remained closer to the field capacity moisture. However, decreasing the irrigation frequency causes greater fluctuations in soil moisture, which can reach higher levels of matrix tension, as verified in the present study. Soil’s water depletion associated with a higher planting density can cause water stress and negatively impact biomass production and yield as observed in this study.
A suitable irrigation frequency for different planting densities can increase yield and reduce water consumption. In a corn crop, reducing the irrigation interval to 6 days increased productivity [
22]. Corn yield is severely impaired by the increase in planting density in conditions of low water availability [
25]. For tomato crops, increasing the irrigation intervals from 1 to 7 days increased the efficiency of carboxylation and the photosynthesis rate, which increased yield by 35% [
25]. The irrigation frequency as well as the amount of water must be adequate for each planting density. Furthermore, the right irrigation frequency depends on edaphoclimatic parameters and the hydraulic characteristics of the irrigation system.
Irrigation frequency and planting density significantly influence the biometric and physiological traits of plants [
26] and the grain’s quality [
25]. Usually, water deficit greatly affects the biometric traits of beans [
12]. With mechanized harvesting, the traits related to the architecture of the plants are essential to reduce losses. In this study, the plant height and the insertion height of the first pod were not affected by the treatments. Normally, the bean’s biometric traits are more affected by row spacing than different planting densities [
27]. Larger stem diameter is important as it supports the plant and prevents lodging, which is important for mechanized harvesting.
The use of planting density may have induced different interceptions of photosynthetically active radiation by the common bean, which had an influence on the grain yield of the crop. The use of density up to D4 generated a good use of light, water, and nutrients, reflected in grain yield. Therefore, by controlling the density of plants in the area, it is possible to obtain higher quality in the use of photosynthetically active radiation and return of the commercial product [
28].
The chlorophyll index indicates water stress in plants [
29] as this pigment is responsible for generating energy from water [
30]. In this study, the lowest irrigation frequencies increased the chlorophyll index “a” but did not affect the chlorophyll index “b”. This result may indicate that the bean increased the synthesis of chlorophyll “a” in the leaves to overcome the greatest fluctuations in soil moisture in treatments F2 and F3.
Limited water availability negatively affects the number of pods and consequently, the beans yield [
29,
31]. In general, treatments with higher yields had higher numbers of pods per plant and seeds per pod. F3 had higher numbers of pods per plant and seeds per pod than F2. Possibly, increasing the irrigation interval may have stimulated greater root growth followed by greater photosynthetic efficiency and carboxylation, which leads to a higher yield [
26]. In legumes, the most sensitive stage to water stress is the reproductive stage, in which a lack of water during the flowering phase, pod formation, and grain filling can cause abscission of flowers and young pods [
32]. However, in situations where there is a high density of plants and water stress, ethylene, a phytohormone present in the plant, can be influenced for greater production for a survival strategy, through the perpetuation of the species through the seed.
It was observed that as the planting density increased, the productivity also increased, however there was a reduction in productivity when the frequency of irrigation increased. It was observed that at the frequency of 8 days (F3), there was a 33.3% reduction in productivity in the treatment with a planting density of 24 plants, in relation to the same planting density with an irrigation frequency of 1 day (F1). This can be justified because the low water levels in field conditions promote an increase in evapotranspiration and hinder the process of transforming light energy into chemical energy, which leads to reduced yield since the plant cannot successfully perform its photosynthesis cycle [
33,
34].
Water productivity (WP) indicates harvest yield per unit of water consumed by the crop [
35]. WP was the highest for F1 irrigation frequency and higher planting densities. The decrease in water consumption was possibly due to the reduction of the soil evaporation component. Soil evaporation can, on average, account for 30% of total water consumption [
36]. Probably, the higher production of dry biomass may have favored the canopy’s bigger soil coverage, confirming those results obtained by [
37]. A bigger development of the plant canopy can increase aerodynamic resistance and intercept more solar radiation, which increases photosynthetic efficiency, yield [
33], and reduces soil evaporation [
38].
5. Conclusions
Plant height, number of pods, and insertion height of the first pod were not impacted by the treatments applied, but a significant difference was observed for the variables stem diameter, chlorophyll index in the leaves, weight of 1000 seeds, and number of grains per pod.
The biomass was significantly influenced by the treatments applied, with the highest biomass being observed in the treatment with a density of 30 plants, reaching 55% more biomass in the frequency of 8 days when the density of 20 plants in the same frequency was purchased.
In the frequency of 8 days, there was a reduction in productivity of 33.3% in the treatment with a planting density of 24 plants, in relation to the same planting density with an irrigation frequency of 1 day (F1).
The highest yields were found in the treatments F1D3 (2968 kg ha−1), F1D4 (2997 kg ha−1), and F3D3 (2946 kg ha−1). For water productivity, treatments with F1 irrigation frequency and higher planting densities were the most efficient in the use of water.