Effect of a Pseudomonas Fluorescens-Based Biofertilizer on Sweet Potato Yield Components

Modern agricultural systems promote the cultivation of high-input and high-yielding crop species and it implies to use a limited number of species, and it has caused a decline in crop diversity in agricultural systems across the world. Therefore, there are no doubts that humanity needs an alternative agricultural development paradigm, one that encourages more ecologically sound, biodiverse, resilient, sustainable and socially just forms of agriculture where genetic diversity is vital [1]. At the same time, the agricultural productivity and hence, food and nutrition security are being affected by climate changes while new thousands of people require food every day. Consequently, it is necessary to move towards a more sustainable and resilient production, in which the agroecological approach is the way forward to produce the food needed by the growing world population.

The ecological resilience of agroecosystems is closely linked to social resilience [2] and this in turn, requires the implementation of environmentally friendly crops and strategies, as well as timely training of producers for its use. Facing the great challenges that climate change implies for countries’ food sovereignty implies combining traditional crops and techniques with advances in science and technology. Sweet potato (Ipomoea batatas (L.) Lam.) (batata, boniato, camote) is a species of American origin [3] and constitutes the sixth most important food crop in the world [4]. Globally, 112 835 316 t of this tuberous root are produced in 9 202 777 ha with a yield of 12.26 t ha-1; in Cuba, sweet potato yields on smallholder farms stand at an average of 10.87 tha-1 [5]. It is a typical food for food safety since it can be harvested in just 4-6 months, an easily propagated crop able to provide carbohydrates, minerals and β-carotenes (pro-vitamin A). Its ability to face adverse climatic conditions, effective response to meteorological phenomena, its versatility and vegetative reproduction, places it above other crops of higher production [6].

As in other crops, fertilization is very important to get high yields. Sweet potato productivity is constrained by poor fertility, especially low potassium (K), phosphorus (P), nitrogen (N), sulfur (S), and some micronutrients [7,8]. Phosphorus requirements are quantitatively lower than K and N doses, but it affects increasing the average weight and the number of roots [9]. Therefore, P is one of the nutrients that, when deficient, most intensely affects crop yields, although plants need a smaller amount than other macronutrients for growth and development. Due to its slow diffusion and high degree of fixation, phosphorus is generally less available in the soil solution but, its uptake and utilization are essential on the final yield of agricultural crops [10].

Correct and timely fertilization are one of the requirements to take into account, however, the availability of chemical fertilizers, which are the most used in cultivation, is deficient many times. On the other hand, the application of chemicals in agriculture is often the cause of soil erosion and environmental deterioration, mainly when used indiscriminately. Fortunately, the use of natural sources, as biofertilizers, have been promoted in the nutrition of crops during the last decades; the relationships between plant and microorganism improve the assimilation of nutrients and, therefore, that allows to obtain better yields. Among the most commonly used biofertilizers in agricultural crops are mycorrhizae, azotobacter or phosphorin [11,12]. Other bioproducts, such as: Fitomas [13], VIUSID agro® [14] that stimulate vegetative development, flowering and fruiting, are also used successfully. Many studies have been carried out on sweet potato growth and productivity, including the effect of organic and inorganic fertilizers applications [15,16].

Among the most important beneficial microorganisms, different bacterial species of the genus Pseudomonas have been described; they act in a double way on crops: they promote plant growth and suppress pathogenic microorganisms. It has also been suggested that they stimulate the establishment of other beneficial microorganisms associated with roots, such as mycorrhizae [17,18]. Pseudomonas produces an increase in the availability of phosphorus and nitrogen in an assimilable way for the plant, due to the production of phytohormones that stimulate a vegetative activity, as well as the degradation of ethylene precursors [19]. Pseudomonas fluorescens Migula is a Gram-negative, aerobic bacilliform bacterium that has several polar flagella. They are known for their ability to stimulate the growth of plants that live in contact with them [20].

Recently, because of the need to promote the production of bioproducts with agricultural interest on a larger scale, researchers from the Cuban Business Group LABIOFAM developed a biofertilizer based on a P. fluorescens strain phosphate solubilizer [21]; the new product is in the process of technical validation for its future use in agriculture.

In the case of sweet potato, the effect of this biofertilizer or the most effective way to apply it to achieve sustainable productions is unknown. It is not also known if with the combined use of this bioproduct and doses of mineral fertilizer, the current doses of the chemical, expensive and environmental pollutants, could be reduced. Therefore, the objective of this work was to determine the effect of a P. fluorescens-based biofertilizer on the development and yield of sweet potato (Ipomoea batatas (L.) Lam) cv. INIVIT B 240- 2006.

The experiments were conducted on a neighboring experimental field site located at the Farm “La Dora” (Cienfuegos city, Cuba), between December/2018-April/2019 (low rainy period). The farm “La Dora” is on a slightly wavy relief (2.1-4.0%) to Wavy (4.1-8.0%); the field is situated 14m above sea level. This farm occupies a total area of 3.42 hectares of which 2.75ha represents the agricultural area. The last 20-years average annual rain fall is 1304.8mm (only 240.3mm during the growing season) with average temperatures from 20.9 °C (minimum) to 30.9 °C (maxim). The soil is brown without carbonates [22] with 3.2% of organic matter and a pH=6.8.

The commercial sweet potato cultivar ʹINIVIT B 240-2006ʹ was used. It is an early cultivar that has many appreciated characteristics by the producers: a good culinary quality of its elongated tuberous roots, the flesh or pulp is white, sweet and without fibers. The skin is light red and smooth with a very good presence for commercialization. It has a short production cycle (four months) with potential yields greater than 55t.ha-1.

The effect of a P. fluorescens-based biofertilizer was evaluated at the dose recommended by the manufacturer (Labiofam) of 20Lha-1. The application method was by immersion of cuttings before sowing for 5, 10 and 15 minutes and the studies included the combination of the bioproduct with doses of 0, 50 and 100 % of the NPK mineral fertilizer (complete formula: 9-13-17). In field, plants from all tested groups were exposed to the same agricultural conditions according to the sweet potato Technical Instructive [23]. On-farm field experiments were conducted with 12 treatments:

T1. Absolute Control (a control group without the application of any mineral fertilizer or Pseudomonas)

T2. Cuttings with immersion in P. fluorescens for 5min

T3. Cuttings with immersion in P. fluorescens for 10min

T4. Cuttings with immersion in P. fluorescens for 15min

T5. Cuttings with 100% of complete fertilization (NPK) (a control group technically recommended by [23])

T6. Cuttings with 100% NPK fertilization plus immersion in P. fluorescens for 5min

T7. Cuttings with 100% NPK fertilization plus immersion in P. fluorescens for 10min

T8. Cuttings with 100% NPK fertilization plus immersion in P. fluorescens for 15min.

T9. Cuttings with 50% NPK fertilization

T10. Cuttings with 50% NPK fertilization plus immersion in P. fluorescens for 5min

T11. Cuttings with 50% NPK fertilization plus immersion in P. fluorescens for 10min

T12. Cuttings with 50% NPK fertilization plus immersion in P. fluorescens for 15min.

100% NPK fertilizer was applied as 90kg N ha-1, 75kg P2O5 ha-1 and 150kg K2O ha-1, according to the technical recommendations for this edaphic condition [23].

The vine cutting, 20-30cm in length, was used as planting material. A unique planting distance of 0.90m X 0.30m was used according to [23]. Plants were grown in field conditions and the experiments were conducted in completely randomized blocks with 12 treatments in three replicates (plots of 10.80m2). The plantation was manually, on the ridge and with the soil properly humid, and in independent plots. Routine agronomic package of practices and plant protection measures recommended for this crop were applied to raise a good crop [23]. Each treatment was represented in three replicates with 50 plants. The measurements were recorded on ten randomly selected plants from the central ridges of each plot, and they consisted in different qualitative and quantitative characters, especially those related with the yield: Number of Total Tuberous Roots per plant (NTTR), Number of Commercial (NCTR) and Noncommercial (NNCTR) tuberous roots per plant, Fresh Weight of Commercial (FWCTR, g) and Noncommercial (FWNCTR, g) tuberous roots per plant, and commercial yield per hectare (CY, tha- 1). The collected data were averaged to get mean values of these characters that have been affected by the studied treatments.

After 130 days of growth, the plots were harvested and data for the mentioned characters, especially the number and weight of marketable tubers per plant were recorded. Root and vine characteristics were described previously after 90 days of growth, according to sweet potato classification system defined by [24,25].

The collected data were subjected to the analysis of variance (ANOVA) appropriate to the design of completely randomized blocks with factorial arrangement (3x4), where the factors were: A-four immersion times in a P. fluorescens-based biofertilizer (0, 5, 10 y 15min), and B-three doses of NPK mineral fertilizer (0, 50y 100%). All statistical procedure was according to the experimental design and using the tools from the SPSS/PC+ statistical package version 15.0 for Windows®. Whenever differences existed among means values, the comparison of them was carried out with the Tukey’s honest significant difference (HSD) for P≤ 0.05.

Observations of the sprouting percentage 20 days after planting always behaved above 95 % (absolute control treatment without any fertilizer) for a general average of 98.06%. In all cases, the result coincides with the one expected by this commercial cultivar and it was higher to the 90%, technically demanded as a permissible value on the sweet potato crop production [23]. The vigor and the positive response observed in all treatments is firstly related with the good quality of cuttings (tips) that came from areas of categorized seed (original seed) of the cultivar at the Research Institute of Tropical Roots and Tuber Crops (INIVIT). In addition, cuttings with 30 cm are the best option to get a better development of this crop; a recent study made by Dumbuya et al. [26] confirmed that 30cm-long sweet potato vine cuttings produced the greatest growth and yield. Therefore, the planting material used had enough moisture and nutrients to achieve an optimum sprouting, after the agrotechnical work to the soil was carried out properly before, during and after sowing, especially the irrigation.

The effect of different treatments on the growth parameters of sweet potato was observed. However, the morphological characteristics did not change, especially those related with the color of immature (lightly purple) and mature leaf (green), the tuberous root form (lengthened), and the predominant light red skin color and the white tuberous roots pulp. All plants, regardless of the treatment received, maintained the characteristics of the cultivar INIVIT B-240-2006, among them: vigorous foliage with green stems and green nodes. The leaves were heart-shaped, slightly dentate of medium size and green color, with green ribs slightly pigmented on the underside with the purple limbo-petiole insertion point.

On the other hand, a positive response was measured of the evaluated agronomic variables in all treatments where the immersion of cuttings in the solution of P. fluorescens (20 tha-1) was carried out (Table 1); when the biofertilizer was combined with 100 % of NPK fertilization, the results were significantly higher over the rest of treatments. No statistical differences were shown among the treatments for the Number of Total Tuberous Roots (NTTR), the Number of Noncommercial Tuberous Roots (NNCTR) and its Fresh Weight (FWNCTR). However, significant differences appeared for other variables, especially for those related with the yield. The worst treatment for all variables was the absolute control (without any fertilizer) with significant statistical differences with the rest of combinations. The control treatment with 100% of the mineral fertilizer (NPK) and its combinations with 5,10 and15min immersion in the biofertilizer expressed statistical differences regarding to the rest of treatments. The mixture of 50% NPK and the immersion during 15min in the biofertilizer don’t differ statistically with the control with 100% NPK. That’s a good finding because it shows the possibility to save mineral fertilizer to increase new areas when immersions for 15min in the P. fluorescens-based biofertilizer could be made to improve sweet potato production significantly.

Table 1:Effect of a P. fluorescens-based biofertilizer on the yield components on the sweet potato cultivar INIVIT B 240- 2006 at La Dora farm (Abreus, Cienfuegos).

*Means followed by the same letter in a column are not significantly different according to Tukey’s HSD test for P≤0.05. Legend: NPK-mineral fertilizer (complete formula 9-13- 17); Pf: P. fluorescens; NTTR: Number of Total Tuberous Roots Per Plant; NCTR: Number of Commercial Tuberous Roots Per Plant; FWCTR(g): Fresh Weight of Commercial Tuberous Roots Per Plant; NNCTR: Number of Non- Commercial Tuberous Roots Per Plant; FWNCTR: Fresh Weight of Non-Commercial Tuberous Roots Per Plant(g); CY: Commercial Yield(tha-1).

When analyzing the effect of the bioproduct on the yield, the combination with 100% NPK and the immersion in P. fluorescens for 15min showed the highest yield (56.09 tha-1), followed by the other treatments with 100% NPK and without statistical differences among them. It was interesting that the treatment with 50% NPK and the immersion in P. fluorescens for 15min (49.58 tha-1) had no statistical differences with the control variant (100 % NPK, 51.60 tha-1). In general, the effect of this biofertilizer and its ability to improve plant growth and productivity confirms an option to minimize the agricultural chemical footprint on sweet potato.

Beneficial Pseudomonas strains are frequently found associated with plants where they act as Plant Growth Promoting Bacteria (PGPB) by suppressing growth of pathogens or by producing plant growth hormones [27]. Bio-fertilizer liberates growth promoting substances and vitamins and helps in maintaining the soil fertility. They acts as antagonists and suppress the incidence of soil borne plant pathogens [16]. They are one of the most important organic sources, containing beneficial viable organisms which have ability to mobilize nutritionally important elements from nonusable to usable form through biological processes [28]. On the other hand, there are many examples where the combination of inorganic fertilizer with biofertilizers produce better productive results than when they were applied individually; Abdel-Razzak et al. [29] reported an improved root quality and productivity in sweet potato when combining arbuscular mycorrhizal fungi (G. mosseae) inoculum with the recommended P level (100% P2O5) (superphosphate fertilizer).

Usually, chemical fertilizers make the difference necessary to guarantee the increase of yields [30], but the excessive mineral fertilization to one side have adverse financial effects and also represents an environmental burden [31]. The vision to reduce the dependency on synthetic fertilizers requires effective biologicalbased alternatives. In this sense, P. fluorescens is a new input for sweet potato crop that can reduce the application rates of chemical fertilizers, offering an alternative to traditional agricultural practices like in other crops [32,33].

Due to the worldwide current multifactorial crisis (ecological, economic and social), the increase of the world population and the necessity of foods for millions of people, where climate change represents a threat to agriculture and food security, agroecological methods offer comprehensive solutions for food systems. Farmers have to fight with all sorts of problems, including the high prices of fertilizers, the soil degradation, the reduction of crops productivity, the increment of plagues and droughts, among others. In this context, the application of environment-friendly farming practices is not an alternative, is the urgent response to guarantee the food sovereignty. In consequence, although it would be necessary to know the productive response during the rainy period (in execution), the results of this research are the first scientific report about the use of this new P. fluorescens-based biofertilizer to improve the sweet potato yield significantly. This is a very important alternative for more sustainable agricultural practices without affecting the growth and productivity of this important crop. The study concludes that the P. fluorescens-based biofertilizer stimulated the commercial yield of sweet potato (Ipomoea batatas (L.) Lam) cv. INIVIT B 240-2006; the combination of 50% NPK fertilization and the immersion of vine cuttings in P. fluorescens during 15 min after planting, can equal the productive results reached with 100% of the mineral fertilizer.

The authors would like to thank Ms. Geisy Díaz-Roche for her contribution to the revision and comments that greatly improved the original manuscript. Thanks to the National Fund of Science and Innovation from the Cuban Ministry of Science, Technology and Innovation for the financial support for the research. The authors would also wish to thank the Bioali-Cyted Network for its support in the development of this research.

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