Exploiting Polyploidy in Napier Grass (Cenchrus purpureus Schumach) for Increased Forage Yield

Napier grass (Cenchrus purpureus Schumach) is an important forage crop and livestock feed. However, its yield and quality in Kenya are often limited by Napier grass headsmut and stunt disease. Napier grass genetic improvements through mutation breeding and selection could avail cultivars with increased forage. This study investigated the response of embryogenic calli to different levels of colchicine in inducing polyploidy in the two germplasms of Napier grass; South africa and Bana grass. The experiments were carried out as a factorial experiment in a completely randomized design (CRD). The colchicine concentrations used were 0, 0.05, 0.1, and 0.2%, and the exposure durations were 24, 48, and 72 h. During the shoot regeneration stage, culturing explants on an MS medium (Murashige and Skoog) supplemented with 0.2 mg L−1 Benzyl Adenine (BAP), 0.1 mg L−1 dichlorophenoxyacetic acid (2, 4-D), and 0.1 mg L−1 indole-3-butyric acid (IBA) was more suitable for shoot regeneration. Chromosome doubling was confirmed by genomic DNA and the stomata size and number. Culturing explants on an MS medium supplemented with 1 mg L−1 IBA, 1 mg L−1 2, 4-D, and 0.5 mg L−1 BAP was more suitable in inducing embryogenic calli in both genotypes. Polyploidy results revealed that a 0.1% concentration of colchicine with two days of treatment established the maximum number of octoploid plantlets induced in vitro, while a 0.2% concentration was very toxic. The stomata size and number of derived octoploid plantlets were bigger with a lower density, a shorter plant height, and a smaller stem diameter, and despite being the first to produce tillers, they were significantly higher than their progenitors. Induced mutants also had a significantly higher number of chromosomes and showed different band patterns and distances during gel electrophoresis. However, we recommend the use of flow cytometry to confirm the ploidy level. The superior mutant plantlets can be selected and recommended for characterization across representative agro-ecologies for large-scale production and used in Cenchrus purpureus breeding programs in Kenya and its environments.