Optimizing in-vitro conservation and rapid multiplication regimes for exotic and local taro genotypes

Abstract

Taro (Colocasia esculenta (L) Schott) is an important staple food crop grown throughout many part of the world for it fleshy corms and nutritious leaves. Suckers are the main planting materials used in West Africa; therefore, it is traditionally conserved in field collections. This is not an effective conservation strategy for medium to long term preservation of taro genetic resources due to extreme climatic conditions, pests and diseases. Tissue culture is one of the in vitro techniques which have in recent years become major agricultural importance in the area of plant propagation, disease elimination and production of secondary metabolites in-vitro. However, with tissue or micropropagation conservation, challenges include: High growth rate of both the local and exotic genotypes of taro which causes sub-culturing in every eight weeks and also high shoot proliferation and all axillary buds produce shoots, leading to a dense population competing for limited nutrients in the glass vessel. Taro is a crop that is amenable to tissue culture. A study was conducted to optimize rapid multiplication and establish slow growth systems for Colocasia genotypes in-vitro. Thus, taro genotypes used for the experiment were obtained from the Taro Breeding Programme at Crop Research Institute which includes SA0 002, CE/MAL/32, SA0 006 and CE/MAL/ 14, BL/SM/80, KA 019 as corms. The study was carried out at the Tissue culture labouratory-Crops Research Institute (CRI), Fumesua. The experiment was categorized into 5 stages thus: In-vitro Rapid Multiplication, In-vitro Medium-Term Conservation; In-vitro Recovery, Fourier Transform Infra-red analysis for concentration of Mannitol absorption content in Colocasia genotype and Biochemical analyses. The genotypes were evaluated in a complete random design. Means were separated using LSD at 5%. Rapid multiplication experiment cultures were maintained at 25±10C at 16hr photoperiod. Two modes of explant excision (Transverse Section (TS) and Longitudinal Section (LS)) were used. For the rapid multiplication set up results, the highest number of shoots (26.00) and leaves (40.00) were produced on the multiplication medium of transverse section (TS) shoot excision for BL/SM/80 genotype, and lowest number of shoots (24.00) and leaves (39.67) occurred on medium culture of longitudinal section (LS) shoot excision. However, for KA 019 genotype, the minimum number of shoots (30.60) and maximum leaves (38.13) were observed in the culture of LS excision, while the maximum number of shoots (32.00) and minimum leaves (37.40) were observed on culture of TS of shoot. The in-vitro cultures were evaluated for their growth, re-growth, mannitol absorption rate and enzyme activity (Catalase and peroxidase activities) as a response to the conservation conditions of the in-vitro collection. The four genotypes studied showed a good in-vitro response, when the mannitol was used as a growth retardant. The medium term conservation cultures were incubated at 20±10C and 12 hours photoperiod (3000lux) conditions. Explants were subjected to different concentrations of mannitol (0, 20, 25 and 30g/l) and White medium (which is a supposed low nutrient as compare to MS basal media). No growth regulators were added to the various media. Survival rate was high and explants remained almost green and healthy on all mannitol treated media when conserved for 6 months on conservation medium. However, with White medium, the growth decreased as the conservation period increased. High number of shoots (6.33) and leaves (22.67) occurred on medium containing 20 and 25g/l mannitol in genotype SAO 006, whereas 30g/l mannitol was the best to restrict growth for the entire 6 months period in terms of shoot height (22.50cm) among all the genotypes conserved. Results regarding recovery and regrowth showed that regeneration percentage, number of shoots per ex-plant and their length (cm) decreased with increasing concentration of mannitol from control to 30g/l. Determination of mannitol absorption rate by the plantlet, enzyme activities were performed by FTIR spectroscopy and UV/VIS spectrophotometer respectively. The analysis of FTIR and UV spectroscopy both confirm the rate of absorption of mannitol and enzyme activities after 6 months period of conservation. This showed that Transverse (TS) section of explants has more axillary and accessory buds occurring at the base than Longitudinal (LS) section, therefore rapid multiplication can be achieved, whereas mannitol supplemented culture media could reduce the growth of Colocasia plantlets especially in stem height. Results showed that conserved explants can be regenerated after transferring and culturing on recovery medium and incubating under regular usage conditions. The osmolite mannitol, at all concentrations tested, did not affect the viability of the whole explants, probably influenced ROS scavenger system, affecting the antioxidant enzymes activity. The time of FTIR analysis is considerably reduced compared to the classical methods; this demonstrates that FTIR analysis of mannitol offers a quick and efficient means of sugar analysis.