Use of organic substrates on the quality of watermelon seedlings

Watermelon ( Citrullus lanatus ) is a succulent fruit and vine-like plant that is cultivated in Mexico and it generates employment and currency for the country. However, there is the need to research what local organic substrates can substitute peat moss as a culture medium to produce watermelon seedlings of good quality and at low cost. The objective of this study was to evaluate the physical and chemical properties of five local organic substrates as substitutes of the commercial substrate “Peat Moss”, for the production of seedlings of two watermelon cultivars, Sun Sweet and Jubilee. Five local organic substrates were studied: cacao husk, compost, vermicompost, bovine manure, coconut fiber and the commercial substrate “Peat Moss” as control. The response variables were percentage of germination, indicators of morphological quality and morphological quality indexes, stability of the clod, and relative efficiency of the local substrates. The best morphological indicators and morphological quality index of the seedlings were found with the substrates cacao husk and vermicompost, with a seedling quality similar to those obtained with the commercial substrate. Compost presented the lowest stability of the clod and relative efficiency. The substrates of cacao husk and vermicompost can substitute the commercial substrate “Peat Moss”, in addition to being easy to obtain and of low cost; so they are a viable alternative for rural farmers in the production of watermelon seedlings.


Research
Horticultura Brasileira 40 (3) July -September, 2022 M exico is the second country capturing most currencies from foreign trade of watermelon (Citrullus lanatus). It occupies the tenth place in its production (1,472,459 t) and is the third in exports volume (14% of the total produced) (SIAP, 2020). The state of Tabasco has a surface of 1250,000 ha with high soil-climate potential for the production of watermelon, with potential average yields of 60 t/ ha (Aceves-Navarro et al., 2008). However, actual yields are significantly lower than estimated, usually related to changes in air temperature and rainfall patterns (Rivera-Hernández et al., 2016), which originate non-germinated seeds and delays in watermelon planting directly in the field.
The production of watermelon seedlings in containers, using commercial substrates, is a widely used practice to have uniform and highquality plants, whose characteristics contribute to the survival of plants after transplantation to the field. Peat Moss (most used commercial name) is the standard substrate worldwide to produce seedlings, but it represents 23% of the total production cost. That is why it is necessary to search for other types of substrates of lowest cost (Pascual et al., 2018).
The search for new substrates can be satisfied through the use of local organic substrates, which contribute to the development and healthy seedlings (Schmilewski, 2014). Dalastra et al. (2016) found that watermelon RIVERA, B; QUEJ, VH; GUTIÉRREZ, R; ANDRADE, JL; CARRILLO, E; GONZÁLEZ, V; VILLARREAL, EC. 2022. Use of organic substrates on the quality of watermelon seedlings. Horticultura Brasileira 40: 261-267. DOI: http://dx.doi.org/10.1590/s0102-  Use of organic substrates on the quality of watermelon seedlings Benigno Rivera 1 ; Victor H Quej 2 * ; Roberto Gutiérrez 1 ; José L Andrade 3 ; Eugenio Carrillo 2 ; Vianey González 1 ; Edelia C Villarreal 1 RESUMO Influência de substratos orgânicos na qualidade de mudas de melancia plants that grow from seedlings grown in containers with organic substrate showed a greater vegetative development than plants obtained from direct sowing. In Tabasco, particularly in the region of Chontalpa, studies have not been performed yet to understand the potential of some agricultural and industrial byproducts that could eventually be used as substrates in the production of watermelon seedling, such as coconut fiber, cacao husk, different composts and vermicomposts, and even bovine manure. In addition to being an alternative to decrease the costs of seedling production, the accumulated residue would be used (Pascual et al., 2018). However, the organic substrates destined to substitute peat moss ought to be evaluated by their biological, physical and chemical properties, since these can influence both the germination and the growth of seedlings (Dalastra et al., 2016).
The production of seedlings in greenhouses is a practice that guarantees uniform and well-developed plants, which also ensures a better start in the field and, consequently, good production through proper management. Thus, the quality of the substrate plays an important role in the production of seedlings in terms of its physicalchemical characteristics that will influence their growth and development. Consequently, in the evaluation of substrate quality, those with desirable physical-chemical characteristics are identified, since each species or cultivar will have its own water and mineral nutrient requirements for good root development (Schmilewski, 2014). The present work aims to evaluate the physical-chemical properties of five local substrates of organic origin for seedling production of two watermelon cultivars: Sun Sweet and Jubilee.

Site study and organic substrates
The experiment was carried out in the facilities of the Popular University of Chontalpa, located in the municipality of Cárdenas, Tabasco, Mexico in a greenhouse covered with shade mesh tarp at 50% of the total daily solar radiation. Sowing was performed on November 29, 2019. Five organic substrates were evaluated: cacao husks (CH) that were collected in a farm after extracting the seed; bovine manure (BM), collected in a cattle productive unit after milking; compost (CP) elaborated based on sugarcane cachaza, collected one month after milling; coconut fiber (CF) acquired from the Local Agricultural Association of Coconut Producers (Asociación Agrícola Local de Productores de Coco) in the municipality of Comalcalco, Tabasco, Mexico; and vermicompost (VC) that was obtained with Eisenia foetida fed with sugarcane cachaza plus Canavalia ensiformis in a proportion of 70:30 (v/v). Composting of the residues CH, BM and CP was carried out according to the procedure described by Ceglie et al. (2015). The substrates were dried in the shade, ground and sieved at 2 mm for their analysis. The physicalchemical analysis of the substrates was performed in the laboratory of Colegio de Postgraduados Campus Campeche, Mexico (Table 1). The variables evaluated were: potential of hydrogen (pH) measured with potentiometer in suspension susbstrate solution 1:5; organic matter (OM) by the Walkley and Black procedure; nitrogen (N) extracted with potassium chloride 2N and determined by vapor distillation; assimilable phosphorus (P) by the Bray P1 method; sodium potassium (Na, K) extracted in ammonium acetate 1.0 N, pH 7.0, 1:20 rate and determined by flame emission spectroscopy; calcium and magnesium (Ca, Mg) extracted in ammonium acetate 1.0, pH 7.0, in a 1:20 rate and determined by atomic absorption spectroscopy; iron and magnesium (Fe, Mg) extracted with a DTPA (Diethylenetriamine pentaacetic acid) 1:4 rate and determined by atomic spectroscopy; bulk density test by the tube method (BD). The methodology for the analysis of these variables was carried out according to the Mexican Official Norm NOM-021-SEMARNAT-2000.

Vegetative material
The vegetative materials used were two watermelon cultivars (cv), Sun Sweet and Jubilee, because these are the ones most sown by producers in the state of Tabasco. Polyethylene cups with a volume of 100 mL were used as containers, considering as ideal to produce the watermelon seedling (Araújo et al., 2010).

E x p e r i m e n t a l d e s i g n a n d treatments
A completely random experimental design was used, considering the different substrates as treatments, separately analyzing their effect on watermelon cultivars Sun Sweet and Jubilee. Each treatment with four repetitions of 150 seedlings per experimental units and each container with a seedling was the experimental unit.

Morphological analysis and quality indexes
To record the response variables, 25 plants per experimental unit were selected at 35 days after sowing (DAS). The evaluated indicators of morphological quality were percentage of emergence (% E), which was obtained from counting the emerged plants at 6 and 12 DAS; seedling height (SH, cm), root length (RL, cm), measured with a ruler; number of roots per seedling (NR), visually quantified after washing the roots with tap water; final stem diameter (SD, mm), measured with a digital stainless steel vernier; number of true leaves (NL); foliar area (FA, cm 2 /seedling) estimated with the procedure described by Sauceda-Acosta et al. (2017); specific leaf area (SLA, cm 2 /g of dry weight), estimated as a relation between FA and dry leaf biomass (DLB, g); dry aerial biomass (DAB, g); dry root biomass (DRB, g) and total dry biomass (TDB, g), obtained after drying the seedlings in a Shel Lab FX28 model stove (Sheldon Manufacturing, Cornelius, Oregon, EUA) at 70°C until obtaining constant weight. The morphological quality indexes evaluated were the DRB/ DAB rate; the slenderness index (SI) (Duryea, 1985); and Dickson's quality index (DQI) (Dickson et al., 1960). The stability of the clod according to the pictorial scale by Gruszynski (2002) where: 1 when more than 50% of the clod is retained in the container; 2 when the clod is separated from the container, but does not remain cohesive; and 3 when the whole root is separated from the container and more than 90% of the clod remains cohesive; the ease of extraction was evaluated according to the procedure described by Acevedo-Alcalá et al. (2020), where the release of the seedling from the tray was classified as easy (1), medium (2) and difficult to extract (3). In the last two variables 80 seedling extractions were evaluated per organic substrate. Lastly, the percentage of relative efficiency (RE) of organic substrates was quantified, according to the procedure described by Silva et al. (2017a), with the following formula: (1) where CS is the commercial substrate, and LOS are the local organic substrates.
Statistical analysis A one-way variance analysis was carried out for each of the response variables evaluated, and the means, when significant, were compared with Tukey's test (p≤0.05).

RESULTS AND DISCUSSION
The treatment with the sugarcane c a c h a z a c o m p o s t p r e s e n t e d a significantly lower seed emergence percentage at 6 and 12 DAS in the two watermelon cultivars (Figure 1), which is probably due to a greater compacting in the container (Abad et al., 2001;Silva et al., 2017b). In addition, crust formation was observed on the surface of the container due to a higher substrate bulk density (Table 1), and as has been reported in previous experiments (Silva et al., 2014(Silva et al., , 2017b. Pascual et al. (2018) point out that high values of bulk density affect root aeration and therefore their development. Except sugarcane cachaza compost, all other substrates are within the ranges of total porosity (70-85%) considered ideal (Pascual et al., 2018).
The growth of seedlings in the substrates cacao husks and vermicompost promoted significantly greater seedling height and final stem diameter (Table 2) in both watermelon cultivars. The values of SH and SD obtained by cacao husk and vermicompost were statistically similar to the values obtained with the use of the commercial substrate peat moss in cv. Sun Sweet; the same trend was observed in both variables in cv. Jubilee, with identical statistical results, although with different values. This favorable effect of the cacao husk and vermicompost is attributed to the higher content of nitrogen in both (Table 1). Nitrogen is one of the most essential elements for plants and originates the production of additional proteins, allows the leaves of the plants to grow more, and for these to have greater surface available for photosynthesis. In addition, nitrogen also influences the increase of root biomass, which results in a higher absorption of nutrients and soil water, leading to higher biomass area (Cavalcante et al., 2019). The stem diameter is a variable that is associated with seedling vigor and represents a higher probability of seedlings not bending during transplant (Luna et al., 2014).
A significantly lower number of leaves per seedling in cv. Sun Sweet was present in the treatments with compost and coconut fiber, while in cv. Jubilee this was seen only in the compost treatment. However, all the substrates presented seedlings with more than four leaves, one of the quality criteria that define the ideal moment to transplant watermelon seedlings (Andrade et al., 2017). The root length of the seedlings grown in the commercial substrate was similar (p≤0.05) to the one obtained in cacao husk and vermicompost in the two watermelon cultivars. Again, in the treatments with compost and coconut fiber, a lower number of roots per seedling was observed in cv. Sun Sweet, while in cv. Jubilee seedlings a significantly lower value was seen in root length in the compost treatment. It seems that watermelon seedlings growth is assigned toward a higher number of roots than root length, which is most marked in cv. Jubilee (Table 2). Larger root formation allows seedlings to further explore the volume of available substrate, allowing for greater water and nutrient absorption. (Souza et al., 2013).
The production of DAB and DRB resulted equal (p>0.05) in the substrates cacao husk, vermicompost and peat moss in the two watermelon cultivars (Table 2). This higher production of DAB and DRB can be attributed to the higher number of leaves (Table  2) and the higher foliar area (Table  3) in these substrates (Wright & Westoby, 2001;Wei et al., 2020), which promoted higher light absorption; this indicates that the seedlings grown in these substrates presented higher photosynthetic capacity (Melo et al., 2019). The higher production of DAB and DRB has been attributed primarily to the nitrogen content, but it is also possibly due to the interaction Use of organic substrates on the quality of watermelon seedlings between nitrogen and phosphorus that interacted positively to increase the dry matter of the plants (Cavalcante et al., 2019). The cachaza compost presented slightly lower nitrogen content than that obtained in the cacao husk and vermicompost, but it presents much higher values in phosphorus, which indicates that the development of the seedling in the compost is limited by physical properties (Table 1). A statistically higher accumulation of total dry aerial biomass (1.024 g) was obtained in vermicompost, which exceeded in 15.44% the value obtained in the commercial substrate (0.887 g) in cv. Sun Sweet, while in cv. Jubilee no significant differences were found (p>0.05) between vermicompost, cacao husk and peat moss in this variable (Table 3). Seedlings with higher root biomass and aerial biomass are more likely to overcome, after transplant, the negative effects of drought and soil compacting (Castillo et al., 2014;Yanyan et al., 2018).
In the substrate cacao husk, leaf area values were observed on seedlings that were statistically equal to the Table 2. Seedling height (SH, cm), stem diameter (SD, mm), number of leaves per plant (NL), root length (RL, cm), number of roots per seedling (NR), weight of the dry aerial biomass (DAB, g), weight of the dry root biomass (DRB, g)  ones obtained in peat moss, in both watermelon cultivars (Table 3). Low values of specific leaf area indicate higher ability of the seedling to resist transplant due to greater thickness and lignification of the leaves (Castillo et al., 2014). According to Cavalcante et al. (2019), the leaf growth rate is directly influenced by the contribution of nitrogen. The higher values of leaf area measured in cacao husk and vermicompost are due to a higher number and growth of the leaf lamina of the seedling. The relation between dry root biomass and dry aerial biomass (DRB/ DAB) did not present significant differences between the different organic substrates in any of the two watermelon cultivars (Table 3). The values found in this study represent a balanced relation between DRB and DAB (Bantis et al., 2019a). However, several responses interact among each other when many factors are coincidentally superposed, and therefore, the volume of roots can be a crucial element to evaluate seedling quality (Nakano, 2007;Bantis et al., 2019a). This study did not evaluate the root volume, but it is logical to think that the volume would be higher in cacao husk and vermicompost, since greater root length and higher number of roots were found in these substrates.
The highest SI was found in the substrates bovine manure, peat moss and coconut fiber in cv. Sun Sweet, while in cv. Jubilee it was seen in the same substrates plus the compost. This occurs because the SD in the seedling was smaller, which represents an imbalance in its growth. The most balanced growth of the seedling was found in the treatment of vermicompost and cacao husk.
In the substrates vermicompost, cacao husk and peat moss, statistically higher values were found in DQI in both watermelon cultivars. The DQI seen in this study are higher than those reported by Oliveira et al. (2015) for cv. Crimson Sweet. Normally, with higher value of DQI the quality of the seedling will be greater (Wei et al., 2020), indicating that the seedlings were more vigorous in these substrates. The DQI is considered one of the best quality indexes of the seedling because it considers the robustness and the equilibrium of the seedling when calculating biomass distribution of seedlings, considering the results of several important attributes in a single index (Melo et al., 2019;Bantis et al., 2019b).
In the variables of stability and extraction of the seedling from the containers (Table 4), it was observed that, when extracting the seedlings from the substrates cacao husk and peat moss, more than 80% was extracted, which is considered high stability of the substrate and easy extraction (code 3) to medium extraction (code 2). The ease of extraction and the stability of both substrates could be because they present better physical characteristics, since they do not compact and present sufficient cohesion to not detach the root from the root ball and therefore not causing physical damage to the root at the moment of the transplant (Acevedo-Alcalá et al., 2020).
Vermicompost and cacao husk presented the highest RE of the five  Means with different letters in columns are statistically different (p≤0.05).
Use of organic substrates on the quality of watermelon seedlings organic substrates evaluated compared to the commercial substrate peat moss ( Figure 2). Thus, the RE values of SH and TDB variables are close to the origin for cacao husk and vermicompost substrates for both cv. Jubile and cv Sun Sweet. Organic substrates of compost, bovine manure and coconut fiber presented the most negative values, indicating that they are less efficient compared to peat moss. The morphological variables of SD, LA, SH, TDB and FA presented values above the origin in the cacau husk and vermicompost substrates, indicating that these substrates exceed the efficiency of the commercial substrate peat moss.
The response of the relative efficiency is different between cv. Sun Sweet and cv. Jubilee, both in the trend and in the values of the growth variables. I n c o n c l u s i o n , c o n s i d e r i n g morphological quality indicators for both watermelon cultivars, the cacau husk and the vermicompost substrates showed the best results for the growing watermelon seedlings; and the compost and coconut fiber substrates obtained the lowest results for seedlings. Likewise, the cacau husk and the vermicompost substrates were evaluated statistically equal to those obtained with the commercial substrate Peat Moos, so they represent a viable alternative for rural producers in the production of watermelon seedlings, whose advantage lies on their low cost and on how easy to get are.