Yam (Dioscorea spp.) of the Dioscoreaceae family, is a tuber crop with high nutritional, economic and socio-cultural importance in the tropical world and especially in West Africa where more than 90% of world production is recorded [1] [2] [3]. In this region of Africa, yam trade accounts for about 32% of farmers’ income [4]. It is a financial source for a significant portion of small producers, especially women, who are mainly involved in production, processing and marketing [5].

In Côte d’Ivoire, yam is the leading food crop with an estimated production of more than 7.148 million tons in 2017 [4]. It is the staple food for more than 56% of the Ivorian rural population [6]. It is consumed in several forms: boiled, crushed, stewed, roasted and fried. Two species contribute mainly to production: Dioscorea rotundata, a species native to West Africa and Dioscorea alata native to Southeast Asia and introduced into Africa during the 16th century [7] [8] [9].

Yam tubers are intended for two types of use. They can be directly consumed or used as planting material [10]. Yam is mainly vegetatively propagated using whole tubers or different portions of tubers [11] [12]. This vegetative propagation technique has the disadvantage that a significant portion of the harvest is saved as seed for new cultivation [10] [13]. This reduces the proportion of production available for food. On average, it is estimated that 25% to 50% of the yam harvest is converted back into seed [12] [14] [15] [16].

The availability and cost of yam seeds have thus been identified as a major limiting factor for the expansion of yam cultivation. Indeed, yam seeds are not only expensive, but also scarce [17]. They represent 40% to 60% of the total variable cost of yam production and are also bulky [18] [19] [20]. These constraints are attributed to the relatively low multiplication rate of yams, which varies from 4 to 8, as opposed to 300 for cereals [21] [22].

In addition, the increase in demographic and land tenure pressure in Côte d’Ivoire is leading to a very high demand for yam seeds [23]. It is, therefore, necessary to reduce the pressure due to the demand for seed on ware yams and to solve the seed shortage caused by the consumption of tubers. Thus, other methods of yam propagation that do not require the use of tubers or a large quantity of tubers are necessary.

Yam vine cuttings to obtain seeds are a promising alternative [12]. This technique has been tested with some success with D. alata and D. rotundata. Survival rates of 30% to 80% and average numbers of minitubers of 0.5 to 2 have been recorded [12] [22] [24]. However, the survival rate, rooting and production of minitubers remain generally low and unstable for D. rotundata, the most popular and commercialized species in Africa. These rates have varied from 40% to 80% on low soil or carbonized rice husk environments and are not under control [25] [26]. The aeroponics method that gives better results [11] requires the use of advanced equipment that everyone cannot afford. However, a simple nursery phase in a liquid nutrient medium could significantly increase the survival rates of cuttings [27]. To improve the rooting of yam vine cuttings, some works with varying results have been carried out using auxins or synthetic hormones [28] [29], natural hormones [30] [31] and inorganic fertilizer [32].

The objective of this study is to develop a simple and effective technique for D. rotundata yams vine cuttings. Specifically, the effect of two types of phytohormones (Indole-3-Acetic Acid and Gibberellic Acid) and a liquid fertilizer (NPK 8-8-8) on the survival rate, rooting and tuberization of vine cuttings are tested.

The results recorded concern the survival rate and the number of roots and minitubers initiated by vine cuttings in the different growing media (Figure 1).

Among all the growing media tested, the best survival rates of Dioscorea rotundata vine cuttings were obtained in media containing 0.25% NPK 8-8-8 (100%), 25 ppm AIA₃ (93.3%), 25 ppm AIA₃ + 5 ppm GA₃ + 0.5% NPK (66.6%) and 2.5 ppm GA₃ (60%). It appears that the survival rates in media containing 0.25% NPK 8-8-8 and 25 ppm AIA₃ respectively are higher than those obtained by Igwilo (2003) [35] and Dibi et al. (2014) [25] on solid media consisting of coarse construction sand and carbonized rice husks. Indeed, these authors reported survival rates of 71% and 68% respectively. In addition, AIA₃ favoured root development in D. rotundata vine cuttings. Indole-3 acetic acid (AIA₃) is an auxin which is a plant growth hormone essential for plant development. Also known as cuttings hormone, auxin acts at a high concentration (between 10-8 and 10-3 g·L⁻¹) on rhizogenesis, promoting the appearance of roots on cuttings [36]. The liquid growing medium containing mineral elements such as nitrogen (N), phosphorus (P) and potassium (K) at a rate of 0.25% favored the survival of vine cuttings of Dioscorea rotundata. In fact, the positive action of mineral elements on the development of yam vine cuttings has been reported by several authors. According to Otoo et al. (2016) [22], in general, improving the nutritional status of mother plants through nutrient supplementation (NPK 15-25-8) before cutting vines has improved survival rates by more than 16% and ensures good subsequent development of minitubers. It has also been found that spraying urea on vine cuttings of Dioscorea alata facilitates rooting, increases the size of the minitubers and increases the number of roots per cut; but the survival rate decreases with increasing urea concentration [32]. The average survival rate of vine cuttings in the growing medium containing 2.5 ppm GA₃ (60%) is similar to that obtained by Behera et al. (2009) [32]. These authors obtained survival rates of 60.02% for Dioscorea alata vine cuttings when dipped in a solution containing 10 ppm GA₃ followed by NPK (100-100-100) on a solid medium consisting of a mixture of sand and cow dung.

The rooting is an essential parameter in the management of cuttings. It is observed 15 days to 3 weeks on average after transplanting vine cuttings of the species D. rotundata and D. cayenensis [37]. This is confirmed by Ayankanmi et al. (2010) [38] who found the formation of roots of vine cuttings from two clones of D. rotundata three weeks after transplanting onto a medium containing sand. In addition, Igwilo (2003) [35] reported that after 14 days, cuttings of the Obiaoturugo variety (D. rotundata) showed a high rooting rate. In our study, the highest mean root number was recorded with growing media containing 0.25% NPK 8-8-8 (20.3), 25 ppm AIA₃ (17.7) and mixed media 25 ppm AIA₃ + 5 ppm GA₃ + 0.5% NPK (16.5). These environments favored the increase in the number of roots from the 21st day to the 35th day. These average numbers of roots of cuttings are similar to those obtained by other authors. Work by Acha et al. (2004) [28] showed that auxin (AIA₃) induced an increase in the number of roots of vine cuttings of 10 cultivars of D. rotundata from 15 to 30 days after transplanting. It induces the formation of new roots by overcoming root apical dominance resulting from the cytokinin effect [39]. Behera et al. (2009) [32] showed that treatment with 2% urea increased the number of roots per cut (38.16%) on the 20th day after transplanting. The results obtained in our study corroborate those of Igwilo (2003) [35]. According to these authors, the use of a hormonal solution composed by 50 ppm AIA₃, 5 ppm GA₃ and 10 ppm Benzyladenin increases the number of roots per vine cuttings of D. rotundata. However, the average number of roots of cuttings in GA₃-based growing media would be lower than that of the control because the concentrations of GA₃ used would appear low to induce an increase in the number or growth of roots. However, the root numbers obtained with 2.5 ppm gibberellic acid are higher than those obtained by Behera et al. (2009) [32] who found that with 1 ppm GA₃, the root number per cut can be improved.

The average numbers of minitubers initiated by vine cuttings in the different growing media at 25 ppm AIA₃, 0.25% NPK 8-8-8 and the medium containing 25 ppm AIA₃ + 5 ppm GA₃ + 0.5% NPK 8-8-8 are 4.6 ± 0.3 ; 5.0 ± 0.0 and 3.3 ± 0.0 respectively. These numbers of minitubers obtained are higher than those reported by Dibi et al. (2014) [25] which were 1.03 ± 0.3 and 1.73 ± 0.86. Ayankanmi et al. (2010) [38] also obtained poorer results on a rice husk-based growing medium with 0.6 and 0.7 per cut. These differences could be due to the types of media used. The nutrient richness of the medium, its texture and structure influence the formation and development of minitubers [22]. In this study, liquid media were fortified with growth hormones and nutrients, while most authors used solid media based on soil and carbonized rice husks. GA₃ growing media produced very little or no minitubers. No significant differences were observed between the numbers of minitubers initiated by cuttings in GA₃. Our results are in line with those of Kefi et al. (1995) [40] who reported that gibberellic acid inhibits tuberization in yams. This is confirmed by Behera et al. (2009) [32] who found no significant difference between the numbers of minitubers initiated by vine cuttings treated with different concentrations of gibberellic acid.

The nature and age of the mother plants of the cuttings used could also explain the differences in results observed from one experimenter to another [24]. In this study, vine cuttings were collected from seedlings of 5-month-old grown in a screen house. Other authors have used vitro-plants as mother plants or yam plants grown in the field, whose age varied from 2 to 5 months [22] [35] [38].

The growing media used in this work produced average survival rates of Dioscorea rotundata vine cuttings between 60% and 100% and average root numbers between 5.3 and 17.7. Growing medium containing 0.25% NPK 8-8-8 has been identified as the one that allows better survival rate, root emission and minituber production by yam vine cuttings. As this nutrient compound is cheaper and more accessible, it is possible to improve the survival rate and rooting of vine cuttings that will allow the production of yam seeds without the use of tubers. However, for the standardization of this technic for yam vine cuttings, further studies will have to be carried out, including the definition of the stage and conditions for transplanting vine cuttings from the nursery room to the field.

The authors thank the National Centre for Agronomic Research (CNRA-Côte d’Ivoire) for facilities to operate the research. They are also thankful to Bill and Melinda Gates Foundation who funded this study through “AfricaYam Project”. CNRA research technicians, involved in the project are acknowledged. They thank International Institute of Tropical Agriculture (IITA) for the partnership.

The authors declare no conflicts of interest regarding the publication of this paper.

Brice, D.K.E., Michel, K.A., Elisée, A.L.-O.D.G., Bernard, B.L.J., Innocent, D.-A., Sidoine, E.B., Boni, N. and Emmanuel, D.A. (2019) Effect of Plant Growth Hormones and Liquid Fertilizer on Rooting and Tuberization of Yam (Dioscorea rotundata Poir.) Vine Cuttings. American Journal of Plant Sciences, 10, 1903-1920. https://doi.org/10.4236/ajps.2019.1010134