Influence of Weeds on Yacon Initial Growth and Development

Abstract

Experiments with tuberous roots and tubers, such as cassava, carrot and potato, suggests that weed interference and inadequate management, especially in the initial development stage, contribute significantly to reduced yield (Silva and Silva, 2007; Blanco, 2008), demonstrating the demand of weed interference studies in yacon, to support decision making when managing this crop. RESULTS AND DISCUSSION Yacon (Smallanthus sonchifolius) initial growth (90 days) was affected by weed interference, with Commelina benghalensis and Amaranthus viridis as the most competitive species reducing yacon development, decreasing plant height, leaf area, number of stems and leaves. Please see PDF.] These results demonstrate the yacon sensibility to competition with some weeds in the early crop cycle stages, indicating the requirement of management techniques for these plants, since this crop has a long cycle plant (8-12 months) and slow initial growth (Silva et al., 2018a), providing a favorable growth window for the weeds and allowing them to compete for resources, mainly water and nutrients. Please see PDF.] The higher competitive ability observed for A. viridis, C. benghalensis may be explained for the more efficient growth of these species, since both accumulated more shoot biomass, and A. viridis also presented higher biomass for the root system, whereas C. rotundus was the weed with lower biomass accumulation (Table 2).

INTRODUCTION

Yacon (Smallanthus sonchifolius), which is an Andean species belonging to the Asteraceae family, is mainly produced in some South American countries, such as Colombia, Ecuador, and Peru. This plant has reserve roots, where high concentrations of fructooligosaccharides (FOS) are present, including from the inulin type, charactering yacon as a functional food and increasing the interest of food and pharmaceutical industry to use this plant (Gusso et al., 2015).

With regard to health benefits, expectations were opened about its cultivation as a new product to be explored and applied at social, agricultural, technological and scientific levels, in Brazil and worldwide (Santana and Cardoso, 2008). Yacon has been entering the market gradually, but there are no commercial studies or exact sales figures due to its relative novelty in the consumer market, which in turn reflects the low level of knowledge of the product. In addition, yacon production is still predominant in family agriculture, and is cultivated as an innovative non-conventional vegetable, mainly from a nutraceutical point of view. It is based on the concept of healthy food, setting the agronomic importance of its study (Oliveira et al., 2013).

One important research topic to be explored is the effect of weeds on yacon cultivation, since this crop has a relatively long cycle, up to 11-12 months, and slow initial growth (Silva et al., 2018b), the crop performance may be affected by weed interference (Seminario et al., 2003).

Weeds represent one of the biotic factors of horticultural cropping systems, affecting the growth, development, and productivity of these crops. When not properly managed, weeds can interfere in the plant development, competing for resources such as water, light, nutrients, and also releasing allelopathic substances, affecting crops and reducing yield and quality, interfering in crop management and harvest (Soares et al., 2003; Medeiros et al., 2016) .

Experiments with tuberous roots and tubers, such as cassava, carrot and potato, suggests that weed interference and inadequate management, especially in the initial development stage, contribute significantly to reduced yield (Silva and Silva, 2007; Blanco, 2008), demonstrating the demand of weed interference studies in yacon, to support decision making when managing this crop. The objective of this study was to evaluate the yacon initial growth and development under weed interference.

MATERIALS AND METHODS

The experiment was conducted in a greenhouse, located in the city of Alegre, Espírito Santo state, Brazil, with geographic coordinates of -20.761833o S and 41.537026o W, during the months of April, May and June 2017. The climate of Southern Espírito Santo is classified, according to Köeppen, as type “Aw”, with two well defined seasons during the year, hot and rainy between the months of October and March, and cold and dry between April and September, with average annual temperature of 23 oC and rainfall of 1,200 mm (Pezzopane et al., 2012).

The experiment was conducted in a completely randomized design, with five treatments and seven replicates. The treatments were yacon growing under the interference of four weed species: Cyperus rotundus L. (nutgrass), Commelina benghalensis L. (tropical spiderwort), Amaranthus viridis L. (slender amaranth) and Bidens pilosa L. (hairy beggarticks); and control (weed-free yacon). Rhizomes were used as propagules for nutgrass (C. rotundus L.) and stolons for tropical spiderwort (C. benghalensis L.). Seeds were used for slender amaranth (A. viridis L.) and hairy beggarticks (B. pilosa L.).

In each pot, 50 seeds of slender amaranth and hairy beggarticks were randomly planted. Five nutgrass rhizomes, with a diameter of 1.0 cm, were planted arranged in a circle, and four stolons with four axillary buds were planted in “X”, for tropical spiderwort. Plants were thinned 30 days after planting, reducing the weed population to 4 plants per pot, following a uniform and quantitative distribution of the plants. Weeds were then pruned (single time) at 10 cm from the soil, to simulate a mowing. Spontaneous plants not related to the study were removed weekly in all the pots throughout the study period.

Yacon rhizophores were transplanted right after the weed pruning. Rhizophores with 4 to 5 buds were previously sanitized using a sodium hypochlorite solution the 2% for 10 minutes to prevent contamination and planted at 5cm depth in plastic boxes (70 x 50 x 20 cm) filled with 20 kg of washed sand, where they remained for 15 days for conservation. Black plastic pots with 10 dm3 of volume were filled with 20.0 kg of a mix of soil and composted bovine manure, in a ratio of 3:1 (v/v). Subsequently, two rhizophores were transplanted in each pot at 5 cm of depth.

The soil was classified as red-yellow latosol, medium texture (Santos et al., 2013), collected at depth 0-20, air dried and sieved in a 5 mm mesh. The soil sample (0-20 cm) was submitted to chemical analysis, resulting in the following: pH 6.64 in water, 17.79 mg dm-3 of P, 229 mg dm-3 of K, 7.0 Na dm-3, 2.70 cmolc dm-3 Ca, 1.12 cmolc dm-3 Mg, 0.00 cmolc dm-3 Al, base saturation of 75%. The composted manure was also analyzed, presenting the following results: 13.019 g kg-1 N; 2.756 g kg-1 of P; 9.129 g kg-1 K; 6.193 g kg-1 Ca and 4.563 g kg-1 Mg.

The evaluations started at 30 days after transplanting and included, for agromorphological parameters: plant height, number of leaves, number of stems, leaf area, chlorophyll A, chlorophyll B and total chlorophyll (clorofiLOG; Falker, Porto Alegre, RS, Brazil). To estimate the leaf area, the leaf length was measured along the leaf midrib, from the base to apex without the petiole; and leaf width was measured perpendicular to the midrib from one end of the leaf to the other. These readings were then used to estimate the leaf area by applying the equation developed by Erlacher et al., 2016.

ƒLW=−27.7418+(3.9812LW/lnLW) ƒLW=-27.7418+3.9812LW/ln⁡LW ; where L is the leaf length and W is the width.

After the end of the greenhouse experiment, yacon plants were collected, fractionated and weighed to obtain a fresh weight for thin and tuberous roots, rhizophores, stems and leaves. Roots and rhizophores were washed previously in running water. The plant fractions were oven dried until reaching a constant weight, to determine the dry biomass of rhizophores, tuberous roots, thin roots, stem, and leaves. Roots with a diameter larger than 0.5 cm were classified as tuberous and the remaining were considered thin roots (Machado et al., 2004).

The data were submitted to analysis of variance using the F test (p>0.05), and when significant, means were separated using Tukey test (p>0.05).

RESULTS AND DISCUSSION

Yacon (Smallanthus sonchifolius) initial growth (90 days) was affected by weed interference, with Commelina benghalensis and Amaranthus viridis as the most competitive species reducing yacon development, decreasing plant height, leaf area, number of stems and leaves. Bidens pilosa and Cyperus rotundus less affected the crop comparing to the other weeds (Table 1).

[Table Omitted. Please see PDF.]

These results demonstrate the yacon sensibility to competition with some weeds in the early crop cycle stages, indicating the requirement of management techniques for these plants, since this crop has a long cycle plant (8-12 months) and slow initial growth (Silva et al., 2018a), providing a favorable growth window for the weeds and allowing them to compete for resources, mainly water and nutrients.

It is important to highlight that the significant effects of weed competition on yacon were observed along the crop cycle, resulting in plant leaf area reduction, with C. benghalensis and A. viridis L. (slender amaranth) as the weeds causing more interference in the development of yacon. Additionally, one important result was C. rotundus always being the least prejudicial to yacon (Figure 1).

[Figure omitted. Please see PDF.]

The higher competitive ability observed for A. viridis, C. benghalensis may be explained for the more efficient growth of these species, since both accumulated more shoot biomass, and A. viridis also presented higher biomass for the root system, whereas C. rotundus was the weed with lower biomass accumulation (Table 2).

[Table Omitted. Please see PDF.]

In addition to the fact of competition, certain weed species have the allopathic potential, with the release of substances by means of exudates from roots, and or leachate. These substances impair growth and hamper the normal development of established plants (Trezzi and Vidal, 2004). For nutgrass, Cyperus rotundus, for example, the allelopathic effect from extracts obtained from its rhizomes is known (Andrade et al., 2009).

The effects of weed interference may also be demonstrated by the leaf chlorophyll content of yacon plants. The weeds, with exception to C. rotundus, led to a decrease in the total chlorophyll content in relation to control, with emphasis again for C. benghalensis and A. viridis, since these species resulted in more substantial decreases. For chlorophyll A in yacon leaves, only C. benghalensis and A. viridis affected it negatively. Relatively to chlorophyll B, once again the C. benghalensis and A. viridis most affected this parameter, but B. pilosa also reduced the yacon chlorophyll concentration (Table 3).

[Table Omitted. Please see PDF.]

The biomass accumulation in the initial phase of the yacon cycle (90 days after transplanting) was reduced by weed competition, and from the studied weeds, A. viridis caused more reduction in plant growth, since less fresh biomass weight was observed for all plant sections, including stem, leaves, rhizophores, fine roots and tuberous roots (Table 4).

[Table Omitted. Please see PDF.]

When the results for fresh biomass of tuberous roots are analyzed, becomes evident how weeds are capable of interfering in plant development and commercial production of yacon, since the tuberous roots are main plant part to be consumed. The effect of competition on yacon yield varies for each weed, but in some situations, the yield reduction may reach 67%, as observed for A. viridis (Table 4). Other weed species were less competitive to yacon, resulting in less reduction, such as C. rotundus which resulted in 6% of yield loss (Table 4).

The more efficient competition of A. viridis is related to its faster plant development, especially for the root system (Table 2), providing to the plant more ability to compete more aggressively for water and nutrients, reducing the formation of tuberous roots in yacon. Similar results were observed for the potato crop, where the production of tubers was negatively affected by weed interference, leading to losses varying between 6.0 and 60%, and this variation was related to the presence of weeds with different competitive capacity (Costa et al., 2008).

The results for dry biomass production in the different parts of the yacon plants were similar to the trends observed before and already discussed, where the weed competition affected the yacon crop, with A. viridis leading to more intense reduction in the accumulation of dry biomass for all parts of the yacon plant, and the C. rotundus being the species that least affected, with results for rhizophores and thin roots not differing from the control (weed-free yacon) for dry biomass of rhizophores and fine roots (Table 5).

[Table Omitted. Please see PDF.]

Plant dry biomass production responds to efficient vegetative growth and adequate formation of shoots and leaves since these parts have an essential function for optimal photosynthesis. Thus, it is observed that yacon was negatively affected by weed competition in different levels, according to the aggressiveness of each specie, since weeds may limit crop production when not correct managed (Pitelli, 1985).

The development and initial growth of yacon (Smallanthus sonchifolius) were affected by weed interference, with A. viridis as the most competitive and C. rotundus less affecting the yacon crop.

ACKNOWLEDGEMENTS

The authors are grateful to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and the Fundação de Amparo à Pesquisa e Inovação do Espírito Santo (FAPES), for financial support, and FAPES, for the second author’s research grant.

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