Post-harvest quality of beet and cowpea intercropped at different planting densities and under green manuring

ABSTRACT

Properly managed intercropping systems with green manuring and component crop population densities can promote gains in crop productivity and benefits that extend to the post-harvest quality of their products. Thus, the objective of this study was to evaluate the post-harvest quality of the beet-cowpea intercropping at different equitable amounts of hairy woodrose (Merremia aegyptia) and roostertree (Calotropis procera) biomass in the doses of 20, 36, 52, and 68 t·ha^-1, on a dry basis, and at cowpea densities of 80, 120, 160, and 200 thousand plants·ha^-1, in two crops in a semiarid environment. The cultivars planted were beet ‘Early Wonder’ and cowpea ‘BRS Tumucumaque’. The highest values of the beet post-harvest indices for pH (6.41), soluble solids (10.07 ºBrix) and titratable acidity (0.14% citric acid), respectively, were achieved in the combinations of equitable amounts of M. aegyptia and C. procera biomass of 68 t·ha^-1 added to the soil at population densities of 145, 110 and 200 thousand cowpea plants·ha^-1, while the highest values for total soluble solids (7.32%) and betalain (55.90 mg 100 g^-1), respectively, were achieved in the combinations of equitable biomass amounts of the green manures of 68.00 and 47.90 t·ha^-1 added to the soil at the population density of 200,000 cowpea plants per hectare. The highest contents of bioactive compounds in immature cowpea grains were reached at the maximum cowpea population density (200,000 plants·ha^-1) and at the highest biomass amount of the green manures, hairy woodrose and roostertree, of 68 t·ha^-1.

Key words
Merremia aegyptia ; Calotropis procera ; bioactive compounds; organic fertilizer; root quality

INTRODUCTION

Beet and cowpea are crops of great economic and nutritional relevance, especially for the northeast region of Brazil. Beet stands out for its high nutrient and antioxidant content, while cowpea is a valuable source of proteins and vitamins, both with significant potential for human consumption and promotion of food security (ABCSEM 2020). The acceptance of these foods in the consumer market is strongly linked to their sensory attributes, including appearance, flavor, and nutritional quality. These factors, in turn, are directly associated with physicochemical characteristics, including color, chlorophyll and carotenoid content, soluble solids, titratable acidity, and hydrogen potential (Silva et al. 2023).

One way to improve sensory attributes in vegetables and in food crops is through the use of green manuring, as it acts on the soil, improving its structure, fertility, and nutrient availability, which in turn affects the quality of the products (Guerra et al. 2022b). By providing a more favorable environment for plants, green manuring can increase productivity and product quality, including flavor, texture, and aroma. Green manuring with spontaneous species from the Caatinga, such as hairy woodrose (Merremia aegyptia) and roostertree (Calotropis procera), can have a positive impact on the post-harvest attributes of some crops, such as beet and cowpea (Chaves et al. 2020, Lino et al. 2023). These species, when used in adequate doses, maintain the balance between sugar and acidity levels, in addition to improving storage potential. However, excess nutrients can reduce the storage potential of the product produced in the post-harvest period.

Another way to meet product quality requirements in intercropping is through adequately optimized population densities of component crops in an associated system (Lino et al. 2023). It is known that this production factor induces a series of changes in plant architecture, plant growth and development, and the production and partitioning of photoassimilates. Detailed characterization of these changes is essential to assess production efficiency in the system and the quality of the products obtained (Bezerra Neto et al. 2005, Taiz et al. 2017). Studying the post-harvest indices and color parameters of beet roots in intercropped with lettuce under green manure with different equitable amounts of M. aegyptia and C. procera biomass (20, 35, 50, and 65 t·ha^-1 on a dry basis) at diverse lettuce population densities (150, 200, 250, and 300 thousand plants·ha^-1), in two cultivations in a semiarid environment, Guerra et al. (2022b) recorded that the beet crop achieved the best indices when fertilized with amounts of the green manures between 20 and 55 t·ha^-11 and between lettuce population densities of 150 and 300 thousand plants·ha^-1. The color parameters of the purple/red beet roots (well appreciated by consumers) were obtained with amounts of green manures between 25 and 65 t·ha^-1 and between lettuce population densities of 280 and 300 thousand plants·ha^-1.

Therefore, the objective of this study was to evaluate the post-harvest quality of beet roots and cowpea grains produced in intercropping, at different crop densities and equitable amounts of green manures, in two cultivations in the Brazilian semiarid region.

MATERIAL AND METHODS

Two experiments were carried out, the first one conducted from September to December 2022, and the second one from September to November 2023, at ‘Rafael Fernandes’ Experimental Farm of the Universidade Federal Rural do Semi-Árido, at geographic coordinates 5°03’37”S and 37°23’50”W and at altitude of 80 m.

The climate of the region, according to the Köppen classification, is BSh (hot semiarid), with two distinct seasons: a dry season typically from June to January, and a rainy season from February to May (Beck et al. 2018). The monthly meteorological data collected during the experimental period are presented in Fig. 1.

Figure 1
Average monthly climatic data on maximum and minimum temperatures and relative humidity in each cultivation of the beet and cowpea intercropping.

The soils of the experimental areas were classified as typical dystrophic Red Argisols (Santos et al. 2018). To characterize these soils, composite samples of the surface layer (0–20-cm depth) were collected and subjected to chemical analyses in the laboratory, whose results for 2022 cultivation were as follows: pH (H₂O) = 6.40; electric conductivity (EC) = 0.34 dS·m^-1; organic matter = 7.84 g·kg^-1; P = 6.00 mg·dm^-3; K = 1.35 mmol_c·dm^-3; Ca = 3.28 cmol_c·dm^-3; Mg = 5.90 cmol_c·dm^-3; Na = 1.74 mg·dm^-3; Cu = 0.20 mg·dm^-3; Fe = 15.20 mg·dm^-3; Mn = 10.80 mg·dm^-3; Zn = 1.10 mg·dm^-3, and B = 0.34 mg·dm^-3. In the 2023 cultivation, the results were as follows: pH (H₂O) = 5.80; EC = 0.41 dS·m^-1; organic matter = 13.25 g·kg^-1; P = 7.00 mg·dm^-3; K = 1.77 mmolc·dm^-3; Ca = 8.40 cmolc·dm^-3; Mg = 2.70 cmolc·dm^-3; Na = 1.49 mg·dm^-3; Cu = 0.22 mg·dm^-3; Fe = 18.70 mg·dm^-3; Mn = 4.90 mg·dm^-3; Zn = 0.60 mg·dm^-3, and B = 0.44 mg·dm^-3.

In both experiments, a randomized complete block experimental design was used, organized in a 4×4 factorial scheme with four replications. The first factor comprised different biomass amounts of the mixture of hairy woodrose and roostertree applied to the soil (20, 36, 52, and 68 tons·ha^-1 on a dry basis), and the second factor consisted of cowpea plant densities per hectare (80, 120, 160, and 200 thousand plants). Plots with beet and cowpea monocroppings were included in each experimental block, at optimized biomass amounts of the green manure mixture of 49.87 and 50.48 tons·ha^-1 respectively, as recommended by previous studies (Desravines et al. 2022, Lino et al. 2021).

The intercropping of the beet with cowpea was established in alternating strips, in the proportion of 50% of the area cultivated with beet and 50% of the area cultivated with cowpea. In each experimental plot, the alternating strips were composed of four rows of each crop flanked by two rows of cowpea on one side and two rows of beetroot on the other side, which were used as borders. The total area of each plot was 3.60 m² (3.00 m × 1.20 m), and the harvest area was 2 m² (2 m × 1 m).

The beet monocropping was established in a total area of 1.44 m² (1.20 m × 1.20 m), containing six rows, and a harvest area of 0.80 m² (0.80 m × 1.00 m), comprised of four central rows of the plot in the plant spacing of 0.20 m × 0.10 m. On the other hand, the cowpea monocropping was established in a total area of 3.60 m² (3.00 m × 1.20 m), containing six rows, with a harvest area of 2.00 m² (2.00 m × 1.00 m), comprised of four central rows in the plant spacing 0.50 m × 0.10 m. In both systems, the borders and the plants at the end of the rows were excluded from the data collection. The details of the spacings used in the experimental plots of the sugar beet-cowpea intercrop and in the beet and cowpea monocrops are found in Table 1.

Table 1
Description of the population densities of beet and cowpea used in intercropping systems and monocroppings, in their respective spacings.

Population densities of intercropped crops (thousand plants·ha-1)			Spacings (m)
Beet	Cowpea		Beet	Cowpea
500	80		0.25 × 0.04	0.25 × 0.20
500	120		0.25 × 0.04	0.25 × 0.15
500	160		0.25 × 0.04	0.25 × 0.12
500	200		0.25 × 0.04	0.25 × 0.10
Population densities of the crops in monocroppings (thousand plants·ha-1)			Spacings (m)
Beet	Cowpea		Beet	Cowpea
500	200		0.20 × 0.10	0.50 × 0.10

The experimental areas were prepared for planting by mechanically cleaning the soil using a tractor with a plow. After plowing, the beds were shaped with a rotary bed digger. Then, pre-planting solarization of these beds was carried out with transparent plastic such as Vulca Brilho Bril Flex (30 microns) for 30 days to combat phytopathogenic microorganisms present in the soil that could affect crop productivity (Chaves et al. 2020).

After this solarization, the materials used as green fertilizers were incorporated into the soil in two distinct stages, using manual tools such as a hoe. In the first stage, 50% of the total amount of the green manures biomass equitable mixture from each treatment was incorporated into the soil, before planting the crops. The second stage of incorporation occurred 20 days after sowing, with the remaining 50% of the green manures equitable amounts, being added in open furrows between the planting lines (Guerra et al. 2022a).

The hairy woodrose and roostertree, used as green manures, were collected in several areas of the rural area of Mossoró (RN), Brazil, in the pre-flowering phase, in order to ensure a higher concentration of nutrients. After collection, the plant material was crushed in a forage mill until fragments of 2–3 cm in length were obtained. The materials were then dehydrated at room conditions until they reached approximately 10% moisture content. Moisture content of the green manures was determined by collecting some samples of the material (with known weight). These samples were placed in an oven with forced air flow at 60°C until a constant weight was obtained, and then the moisture content was determined. In addition to this, a qualitative assessment was made by touching and visually observing the material.

The chemical attributes of green manures in the 2022 cultivation for hairy woodrose were: N = 18.98 g·kg^-1, P = 2.65 g·kg^-1, K = 34.6 g·kg^-1, Mg = 5.18 g·kg^-1, Ca = 18.43 g·kg^-1 and C:N = 25:1; and for roostertree were: N = 15.35 g·kg^-1, P = 1.25 g·kg^-1, K = 33.0 g·kg^-1, Mg = 5.29 g·kg^-1; Ca = 10.28 g·kg^-1 and C:N = 27:1. In the 2023 cultivation, the chemical attributes for hairy woodrose were: N = 18.59 g·kg^-1, P = 2.58 g·kg^-1, K = 32.80 g·kg^-1, Mg = 4.48 g·kg^-1; Ca = 17.88 g·kg^-1 and C:N = 24:1, and for roostertree were: N = 14.66 g·kg^-1, P = 1.27 g·kg^-1, K = 31.00 g·kg^-1, Mg = 5.40 g·kg^-1; Ca = 11.60 g·kg^-1 and C:N = 28:1.

The cultivars planted were ‘Early Wonder’ for beet and ‘BRS Tumucumaque’ for cowpea. The seeds were planted in holes with a depth of approximately 3 cm, with three or four seeds per hole and covered with commercial substrate to promote germination. The crops were thinned 14 and seven days after sowing, respectively, with only one plant per hole being maintained in both crops, in both the intercropping and monocropping systems. During the crop cycles, manual weeding and hilling operations were carried out in the beet crop whenever necessary. The bio-insecticide Azamax was used in the two cultivations at the beginning of the appearance of caterpillars, generally at 45 days after planting, and three sequential applications were made, spaced seven days apart, to control the action of the caterpillars.

Crop irrigation was carried out daily using a micro-sprinkler system, divided into two shifts: morning and afternoon. The amount of water applied was determined based on the cultivation coefficient (Kc) of the main crop of the intercropping (beet), with an average value of 0.83 for the region (Oliveira Neto et al. 2011), with a water depth of approximately 8 mm being provided per day, sufficient to meet the water needs of the crops in the two cultivations.

The beet was harvested 65 days after planting (DAP) in the first cultivation and 66 DAP in the second cultivation. The cowpea was harvested at different times: in the first cultivation, harvests were carried out at 54, 57, and 60 DAP, while in the second cultivation, harvests occurred at 53, 56, and 59 DAP. After harvest, both the beet and the cowpea were sent for laboratory analysis.

The beet roots classified as commercial were subjected to a preparation process for analysis. This classification was carried out using the diameter of the roots (RD) in extra (4 < RD < 5 cm); extra A (5 ≤ RD < 6 cm); extra AA (6 ≤ RD < 7 cm); and larger (RD > 7 cm), being all damaged and cracked roots measuring less than 4 cm in diameter considered scraps (Batista et al. 2016). Initially, they were washed in running water and dried at room temperature. Then, they were crushed in a Juiceman 3 in 1 JM3000 juice extractor processor until approximately 70 mL of juice was obtained. The juice obtained was fractionated into aliquots for each specific analysis, and all determinations were performed in duplicate.

The hydrogen potential (pH) was measured using a benchtop pH meter, model 016A. A portable digital refractometer, model 104-D, with automatic temperature correction, was used to determine the total soluble solids (SS). Two drops of the juice were placed directly on the refractometer prism, and the reading was expressed in °Brix. The titratable acidity (TA) was determined by acid-base titration using a 1-mL aliquot of the juice diluted in 49 mL of distilled water. After adding two drops of phenolphthalein as an indicator, the sample was titrated with a standard 0.1 N NaOH solution until the turning point (light pink). The titration results were used to calculate the TA, expressed as % citric acid (Eq. 1). The ratio between total SS and TA was calculated according to the methodologies described by the Association Official Analytical Chemists (2012) and Instituto Adolfo Lutz (2008).

$Citric acid \% = \frac{10 x acid factor x NaOH factor x NaOH spent \left(ml\right)}{sample weight \left(\mathrm{g}\right)}$ (1)

The quantification of total soluble sugars (TSS) was performed by the anthrone method, proposed by Yemm and Willis (1954). A 1-mL aliquot of the juice was diluted in 99 mL of distilled water, and then a 50-μL aliquot of this diluted solution was added to a test tube containing 950 μL of distilled water. The tubes remained immersed in ice water while 2 mL of anthrone solution was added to them, then they were placed in a boiling water bath for 8 minutes, and then cooled in cold water. To construct the standard curve, glucose solutions at different concentrations (0, 5, 10, 15, 20, 25, 30, 35, and 40 μg·L^-1) were used. The readings were performed in a spectrophotometer at 620 nm, and the results were expressed as a percentage of total sugars, calculated by Eqs. 2, 3, and 4.

$Concentration=\frac{(Absorbance\pm b)}{a}$ (2)

where: a and b: the glucose standard curve.

$Material mass=\frac{(Sample weight (g) x aliquot (\mu g\left)\right)}{diluition \left(ml\right)}$ (3)

$\mathrm{TSS}\mathrm{\%}=\frac{(Concentration x 100)}{material mass}$ (4)

To quantify the betalain content of beet roots, 1 mL of beet extract diluted in 99 mL of distilled water was used. The reading was then performed on a spectrophotometer at 536 nm. Quantification was performed according to the Beer-Lambert-Bouguer Law, modified by Tang and Norziah (2007) (Eq. 5):

$B(\mathrm{mg}/100\mathrm{g})=\left(\frac{Abs\times 550\times 1000}{60000\times 1}\right)\times 10$ (5)

The determination of the internal color of the beet roots was performed using a Minolta CR-410 colorimeter calibrated on a white porcelain surface, under lighting conditions and expressed in the coordinate modules L*, a* and b*, which describe the color uniformity in three-dimensional space. For this purpose, the roots were cut transversely, and the color reading was performed in two different positions. The L* coordinate represents the luminosity, ranging from 0 (black) to 100 (white). The a* coordinate indicates the color position between green (-a*) and red (+a*), while the b* coordinate represents the position between blue (-b*) and yellow (+b*). In order to obtain more precise information about the hue and saturation of the color, the values of a* and b* were converted to the hue indices h° (Hue angle) and saturation C* (Chroma) (Eqs. 6 and 7), using the equations proposed by McGuire (1992).

$h^{\circ }=arctan\left(\frac{b\ast }{a\ast }\right)$ (6)

$C^{\mathit{\star }}\mathit{=}\sqrt{{\mathit{a}}^{\mathit{\star }\mathit{2}}\mathit{+}{\mathit{b}}^{\mathit{\star }\mathit{2}}}$ (7)

In the cowpea crop, the following bioactive compounds were evaluated: chlorophyll a, b, total, and carotenoid. For this, two grains of green cowpea were macerated in a mortar, then the macerated sample was weighed and placed inside screwed test tubes with 15 mL of 80% acetone. After a period of 24 hours, the reading was taken on the spectrophotometer (SP2000 UV model) at wavelengths of 470, 645, 652, and 663 nm. With the values of the weight, volume of the extract, and the absorbance at the indicated wavelength, the contents of chlorophyll a (Chlo a), chlorophyll b (Chlo b), total chlorophyll (Total Chlo) and carotenoids (Car) were calculated, according to Eqs. 8, 9, 10, and 11 (Barbosa et al. 2008):

$Chlo\mathit{ }a\mathit{=}\frac{\mathit{(}\mathit{12}\mathit{,}\mathit{7}\mathit{ }\mathit{\ast }\mathit{ }{\mathit{A}}_{\mathit{663}}\mathit{ }\mathit{-}\mathit{ }\mathit{2}\mathit{,}\mathit{69}\mathit{ }\mathit{\ast }\mathit{ }{\mathit{A}}_{\mathit{645}}\mathit{)}\mathit{ }\mathit{\ast }\mathit{ }\mathit{V}}{\mathit{1000}\mathit{ }\mathit{\ast }\mathit{ }\mathit{P}}$ (8)

$Chlo b=\frac{{\displaystyle (22,9\ast A_{645}-4,68\ast A_{663})\ast V}}{{\displaystyle 1000\ast P}}$ (9)

$Total Chlo=\frac{(A_{652}\ast 1000)\ast \left(\frac{V}{1000\ast P}\right)}{34,5}$ (10)

$Car=\frac{{\displaystyle \left[\frac{\left(1000\ast A_{470} - 3,27\ast Cl a-104\ast Cl b\right)}{229}\right]}}{1000*P}$ (11),

where: A: absorbance at the indicated wavelength; V: volume of acetone and cowpea grains; P: weight of cowpea grains. The contents were expressed in mg·g^-1.

Univariate analysis of variance was used to assess the homogeneity of variances between cultivations in each variable analyzed. Given the homogeneity of variances (observed by fulfilling the assumption that the ratio of the mean squares of the errors of the two cultivations should not be greater than 7), an average of the values obtained in each treatment in the two cultivations was calculated (Pimentel-Gomes 2022). Then, a regression analysis was performed for each variable, using a procedure to fit a response surface as a function of the amounts of green manure biomass and population densities of cowpea, through the Table Curve 3D software (Systat Software 2021). The F test was used to verify whether there was a significant difference between the intercropping and monocropping systems.

RESULTS AND DISCUSSION

Beet crop

The results of the analysis of variance and regression of the quality attributes—pH, SS, TA, SS/TA ratio, TSS, and betalain (B) content in beet roots are presented in Table 2. No significant interaction was observed between the factors population densities of cowpea and amounts of green manures tested in these quality attributes, except for B content. However, significant differences were observed between the cropping systems (monocropping and intercropping) in the variables pH, SS, TA, SS/TA ratio, and TSS, with the intercropping system standing out from the monoculture system in SS and TA, and the monocropping standing out from the intercropping system in the pH, SS/TA ratio, and TSS. No significant difference was observed between the cropping systems in B content (Table 2).

Table 2
F values for hydrogen potential (pH), soluble solids (SS), titratable acidity (TA), SS and TA ratio, total soluble sugars (TSS), and content of betalain (B) in beet roots intercropped with cowpea in different equitable amounts of Merremia aegyptia and Calotropis procera biomass incorporated into the soil and cowpea population densities.

Sources of variation	DF	pH	SS	TA	SS/TA	TSS	B
Blocks	3	1.02ns	0.24ns	0.74ns	0.08ns	0.38ns	0.54ns
Amounts of hairy woodrose and roostertree biomass (A)	3	10.43**	3.31*	1.08ns	1.00ns	3.36*	263.39**
Cowpea population densities (D)	3	7.87**	1.69ns	0.17ns	1.28ns	5.95**	332.68**
A × D	9	2.13ns	0.99ns	0.001ns	0.73ns	0.24ns	33.70**
Monocropping × Intercropping	1	6.01*	8.85*	7.09*	101.73ns	4.25*	2.91ns
Regression (Response surface)		11.39**	16.34**	567.55**	10.28**	38.94**	10.07**
Error		0.00594	0.03499	9.5454e-08	2.44524	0.02001	12.24383
Cropping systems			SS (°Brix)	TA (% citric acid)	SS/TA	TSS (%)	B (mg/100g)
Intercropping		6.08b	9.50a	0.15a	36.78b	5.98b	44.22a
Monocropping		6.23a	8.37b	0.13b	70.76a	6.60a	47.82a
CV (%)		2.03	7.78	11.03	9.51	8.90	8.60

• ^*
  Significant at p ≤ 0.05 by F test;
• ^**
  significant at p ≤ 0.01 by F test;
• ^ns
  non-significant at p > 0.05 by F test;
• CV: coefficient of variation; DF: degree of freedom;
• ^†
  means followed by the same lowercase letters in the column do not differ statistically from each other by the F test at 5% probability level.

A response surface was fitted to the attributes pH, SS, TA, and SS/TA ratio evaluated in the beet roots as a function of the treatment-factors, amounts of green manures and population densities of cowpea plants (Fig. 2). The highest values of pH (6.41), SS (10.07 ºBrix), and TA (0.14% citric acid), respectively, were achieved in the combinations of equitable amounts of hairy woodrose and roostertree biomass of 68 t·ha^-1 added to the soil and population densities of 145, 110 and 200 thousand cowpea plants·ha^-1 (Figs. 2a, 2b and 2c). The SS/TA ratio showed its highest value (74.41) in the amount of 47 t·ha^-1 of the green manures in the population of 130 thousand cowpea plants·ha^-1 (Fig. 2d).

Figure 2
Chemical’s parameters of beet roots: (a) pH, (b) soluble solids, (c) titratable acidity, and (d) soluble solids/titratable acidity intercropped with cowpea in different equitable amounts of biomass of Merremia aegyptia and Calotropis procera and population densities of cowpea.

It is known that factors such as nutritional management of plants, population densities of component crops in intercrops, among others, interfere in the production of sugars and acids in agricultural products produced in the cultivation system (Batista et al. 2016, Portela et al. 2012). Therefore, it was observed in this study that the pH and the SS/TA ratio were most affected by both the amount of green manures incorporated into the soil and the densities of cowpea, secondary crop (Figs. 2a and 2d), while the TA and total SS were affected only by the population density of cowpea (Figs. 2b and 2c).

The results of this research showed that both the population density of the secondary crop and the amounts of green manures based on hairy woodrose and roostertree played an important role in determining the post-harvest parameters of beet roots. These results disagree with those obtained by Guerra et al. (2022b), intercropping beet with the secondary crop of lettuce, in which they observed that the pH, total SS, and the SS/TA ratio were more affected by the densities of the secondary crop of lettuce. These differences in the behavior of the post-harvest indices of beet in the intercropping are due to the type of competition exerted by the secondary crop. Lettuce is a very competitive crop for environmental resources, differentiating its behavior from cowpea in the intercropping development.

It is known that parameters such as pH, SS, TA, and the SS/TA ratio directly influence the flavor of foods. Decreasing acidity increases sweetness and intensifies the flavor of fruits and vegetable crops. SS are used to evaluate the degree of sweetness, while the SS/TA ratio reflects the balance between sugars and acids, determining the final flavor of the product (Chitarra and Chitarra 2005, Pacheco et al. 2021). A higher SS/TA ratio results in a sweeter flavor (Silva et al. 2023). Therefore, the evaluation of these parameters is crucial, since high productivity does not guarantee consumer acceptance if the flavor of the beet is not pleasant. The results obtained with beet in the intercropping with cowpea reflected the behavior of these quality attributes.

Regarding TSS and B contents, a response surface was fitted as a function of the factors-treatments, amounts of green manures, and population densities of cowpea plants intercropped with beet (Fig. 3). The highest values of TSS (7.32%) and B (55.90 mg 100·g^-1), respectively, were achieved in the combinations of equitable amounts of hairy woodrose and roostertree biomass of 68.00 and 47.90 t·ha^-1 added to the soil with the population density of 200 thousand cowpea plants·ha^-1 (Figs. 3a and 3b).

Figure 3
Chemical’s parameters of beet roots: (a) total soluble solids and (b) content of betalain intercropped with cowpea in different equitable amounts of biomass of Merremia aegyptia and Calotropis procera and population densities of cowpea.

The TSS were affected by both the amount of green manures and the population density of cowpea, while the B content was more affected by the population density of cowpea. These results partially agree with those obtained by Lino et al. (2023), when they intercropped beet with arugula and observed that the dosage of hairy woodrose and roostertree and the population of the secondary crop affected only the TSS of beet roots.

The TSS content may be related to the fixation of atmospheric N by cowpea and the mineralization of K and N by hairy woodrose and roostertree throughout the beet cycle. These nutrients participate in the process of formation and translocation of sugars and increase the concentration of sucrose, acting on the composition of amino acids (Taiz et al. 2017).

B content is directly related to the color of beet roots, with the reddish-purplish hue resulting from the presence of these pigments. According to Sokolova et al. (2024), B content in brown beet cultivars can be up to 77% higher than in yellow beet cultivars. The nitrogen content in the soil during beet development can directly influence the B content in its roots, since B is a nitrogenous pigment derived from betalamic acid (Pereira et al. 2022).

Beet color parameters

Table 3 presents the results of the analysis of variance for the beet color parameters, luminosity (L*), Hue angle (hº), and Chroma (C*). There was no significant interaction between the population densities of cowpea and the amounts of green manures based on hairy woodrose and roostertree incorporated into the soil for any of the color parameters studied in the beet crop. There were also no statistically significant differences in the color mean parameters between the monocropping system and the intercropping system. This indicates that intercropping did not influence the color of the beet roots.

Table 3
F values for the beet roots color parameters, luminosity (L*), Hue angle (h°) and Chroma (C*) in intercropping with cowpea at different equitable amounts of Merremia aegyptia and Calotropis procera biomass and cowpea population densities.

Sources of variation	DF	L*	h°	C*
Blocks	3	0.26ns	0.03ns	0.02ns
Amounts of hairy woodrose and roostertree biomass (A)	3	6.27**	4.53**	6.49**
Cowpea population densities (D)	3	6.06**	4.13*	2.79ns
A × D	9	0.48ns	0.97ns	0.92ns
Monocropping × intercropping	1	0.29ns	0.00ns	0.13ns
Regression (Response surface)		20.23**	18.37**	9.02**
Error		0.78778	0.00017	1.25046
Cropping systems		L*	h°	C*
Intercropping		36.46 a	0.31 a	42.62 a
Monocropping		35.73 a	0.31 a	42.16 a
CV (%)		7.40	10.16	6.05

• ^*
  Significant at p ≤ 0.05 by F test;
• ^**
  significant at p ≤ 0.01 by F test;
• ^ns
  non-significant at p > 0.05 by F test;
• CV: coefficient of variation; DF: degree of freedom; †means followed by the same lowercase letters in the column do not differ statistically from each other by the F test at 5% probability level.

A response surface was fitted for the color parameters of beet roots as a function of the treatment factors studied (amounts of green manures and cowpea population densities). The maximum values of luminosity (L*), hue angle (h°), and color saturation (C*) of beet roots of 39.14, 0.34°, and 45.00, respectively, were achieved in the combinations of equitable amounts of green manures biomass and cowpea population densities of 47.27 and 136, 68 and 80, and 42.97 t·ha^-1 and 146 thousand cowpea plants, respectively (Figs. 4a, 4b and 4c).

Figure 4
Color parameters of beet roots, (a) luminosity, (b) saturation expressed by the Hue Angle h°, and (c) chroma intercropped with cowpea in different equitable amounts of biomass of Merremia aegyptia and Calotropis procera and population densities of cowpea.

The colorimetric representation of a crop product is based on three color attributes that can be arranged together to create a three-dimensional solid (Fig. 5): luminosity, which determines the intensity of light or dark colors, ranges from 0 to 100, with values closer to 100 representing lighter colors, and values closer to 0 darker colors.

Figure 5
Representation of the color space section of beet roots, under fertilization with hairy woodrose and roostertree biomass from the averages of the coordinates luminosity (L*), a*, b* and the configurations chroma (C*), and Hue angle (h°).

Color saturation is related to the concentration of the coloring element. Neutral colors, such as gray, have low saturation, while pure colors, such as red or blue, have high saturation, being brighter in human perception. Hue, represented by the Hue angle (h°), is a qualitative attribute of colors defined as reddish, greenish, among others (Ferreira and Spricigo 2017). The 0° angle is defined as red, the 90° angle, yellow, the 180° angle, green, and the 270° angle, blue. The hues form the outer rim of the solid, with lightness as the central axis and saturation advancing horizontally in the rays.

The results obtained from beet roots fertilized with hairy woodrose and roostertree under different cowpea densities when positioned on the color wheel suggest a predominantly dark red color, which is the most attractive color and most accepted by consumers, due to the presence of B (Fig. 5). These results came from the following average values: lightness (35.73), color saturation (42.16), Hue angle (0.31º), red color component (a = 36.94) and yellow color component (b = 11.49).

Cowpea crop

The F values for Chlo a, Chlo b, total Chlo, and Car contents of green cowpea grains are presented in Table 4. A significant interaction was recorded between the factors cowpea density and quantities of hairy woodrose and roostertree for the variables Chlo a and Car. For Chlo b and total Chlo, no significant interaction was observed between the treatment factors.

Table 4
F values for chlorophyll a (Chlo a), chlorophyll b (Chlo b), total chlorophyll (Total Chlo), and carotenoid (Car) contents in green cowpea grains intercropped with beet in different equitable amounts of Merremia aegyptia and Calotropis procera biomass and cowpea population densities.

Sources of variation	DF	Chlo a	Chlo b	Total Chlo	Car
Blocks	3	0.18ns	2.31 ns	0.73ns	0.56ns
Amounts of hairy woodrose and roostertree biomass (A)	3	7.87**	4.77**	7.52**	8.47**
Cowpea population densities (D)	3	74.38**	26.72**	52.22**	41.99**
A × D	9	3.06*	1.05ns	1.51ns	4.44 **
Monocropping × intercropping	1	3.23nns	0.05ns	3.50ns	1.36ns
Regression		19.06**	31.91**	31.92**	14.88**
Error		0.01185	0.00292	0.01627	0.00440
Cropping systems		Chlo a	Chlo b	Total Chlo	Car
		---------------------- mg·g-1----------------------
Intercropping		0.82ª	0.60a	1.53a	0.46a
Monocropping		0.72ª	0.59a	1.34a	0.42a
CV (%)		15.00	15.57	14.35	17.16

• ^*
  Significant at p ≤ 0.05 by F test;
• ^**
  significant at p ≤ 0.01 by F test;
• ^ns
  non-significant at p > 0.05 by F test;
• CV: coefficient of variation; DF: degree of freedom; †means followed by the same lowercase letters in the column do not differ statistically from each other by the F test at 5% probability level.

The levels of bioactive compounds evaluated in green cowpea grains were affected by both treatment factors, but mainly by the cowpea population density. The highest contents of Chlo a, Chlo b, total Chlo, and Car of 1.08, 0.80, 1.82, and 0.62 mg·g^-1, respectively, were reached at the maximum cowpea population density (200,000 plants·ha^-1) and at the highest biomass amount of the green manures, hairy woodrose, and roostertree of 68 t·ha^-1 (Figs. 6a, 6b, 6c, and 6d). The relationship between high cowpea planting densities and higher grain chloroplast content is complex and can be explained mainly by greater competition for resources, such as light and nutrients, at high densities. This leads to an increase in the photosynthetic efficiency of plants, which in turn can result in increased chloroplast production to optimize light capture and the photosynthetic process.

Figure 6
Photosynthetic pigments of green cowpea grains intercropped with sugar beet at different equitable amounts of Merremia aegyptia and Calotropis procera biomass and cowpea population densities. (a) Chlorophyll a, (b) chlorophyll b, (c) total chlorophyll, (d) carotenoids.

This increase in the contents of bioactive compounds in green cowpea grains may be related to the greater biological fixation of N in cowpea and the greater availability of N from the decomposition of hairy woodrose and roostertree. According to Lacerda et al. (2020), approximately 50 to 70% of the total N present in the leaves is part of enzymes associated with chloroplasts. These same authors observed that the chlorophyll content in cowpea leaves increased with the increase in N added to the soil.

The total Chlo and Car contents observed in the present study differed from the values found by Silva et al. (2017), being significantly lower. The green color of cowpea grains depends on their degree of maturation, since chlorophylls are responsible for the green color in cowpea grains throughout grain development. These pigments are synthesized and degraded, but during the maturation phase, there is only the degradation of chlorophylls and pigments such as carotenoids and anthocyanins, as they were hidden by the dominant green color (Oliveira et al. 2021, Vieira et al. 2021).

Factors such as pH, enzymatic activity, temperature, light, and oxygen influence the degradation speed of photosynthetic pigments, such as chlorophylls and Car (Vieira et al. 2021).

CONCLUSION

The highest values of the beet post-harvest indices for pH (6.41), SS (10.07 °Brix) and TA (0.14% citric acid), respectively, were achieved in the combinations of equitable amounts of M. aegyptia and C. procera biomass of 68 t·ha^-1 added to the soil at population densities of 145, 110 and 200 thousand cowpea plants·ha^-1, while the highest values for TSS (7.32%) and B (55.90 mg 100 g^-1), respectively, were achieved in the combinations of equitable biomass amounts of the green manures of 68.00 and 47.90 t·ha^-1 added to the soil at the population density of 200 thousand cowpea plants·ha^-1. The maximum values of luminosity, hue angle, and color saturation of beet roots of 39.14, 0.34°, and 45.00, respectively, were achieved in the combinations of equitable amounts of green manure biomass and cowpea population densities of 47.27 and 136, 68 and 80, and 42.97 t·ha^-1 and 146 thousand cowpea plants, respectively. The highest contents of Chlo a, Chlo b, total Chlo, and Car of 1.08, 0.80, 1.82, and 0.62 mg·g^-1, respectively, were reached at the maximum cowpea population density (200,000 plants·ha^-1) and at the highest biomass amount of the green manures, hairy woodrose and roostertree of 68 t·ha^-1. The beet and cowpea can be grown in intercropping without loss of post-harvest quality of their products.

ACKNOWLEDGMENTS

Not applicable.

• How to cite:
  Azevedo, M. C., Bezerra Neto, F., Lima, J. S. S., Santos, E. C., Justino, G. F., Braga, S. A., Silva, F. B. and Assis Filho, J. S. (2025). Postharvest quality of beet and cowpea intercropped at different planting densities and under green manuring. Bragantia, 84, e20250015. https://doi.org/10.1590/1678-4499.20250015
• FUNDING
  Conselho Nacional de Desenvolvimento Científico e Tecnológico
  Grant No.: 305222/2019-8
  Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
  Finance Code 001

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available on request from the corresponding author.

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Edited by

• Section Editor:
  Osvaldo Guedes Filho https://orcid.org/0000-0001-8550-8505

Publication Dates

• Publication in this collection
  17 Oct 2025
• Date of issue
  2025

History

• Received
  23 Jan 2025
• Accepted
  28 July 2025