GENETIC AND PHYSIOLOGICAL ANALYSIS OF FLOWERING IN CASSAVA (Manihot esculenta Crantz)
ABSTRACT
Due to increased utilisation and discovery of novel end products, the demand for cassava has increased significantly. Cassava breeders have the responsibility to release varieties with traits that best suit the end users. However, cassava breeders are faced with the challenge of a lengthy breeding cycle. A major component of the breeding cycle is the flowering habit. Cassava pollination activities are hampered due to uneven synchrony, lateness in flowering and sparse flower production. A participatory rural appraisal (PRA) was conducted using questionnaires and focus group discussions (FGD) to learn about the importance farmers attribute to cassava flowers and branching habits in farmer preferred genotypes in selected communities in Edo and Niger states of Nigeria. Furthermore, genetic studies of cassava flowering were evaluated using a population (genetic gain population) of 750 genotypes. The trial was established in two locations (Ubiaja and Ibadan) for three years. Broad sense heritabilities were estimated for the flowering and branching traits. DNA samples were genotyped using the Genotype by Sequencing (GBS) and subjected to genome wide association studies (GWAS) for identification of marker trait associations. Three approaches were used to induce flowering in cassava, they include grafting, temperature control and plant growth regulator (PGR) application. Twenty-one rootstocks with good flowering ability and 5 poor flowering scion genotypes were used for the grafting experiment. Temperature treatments were carried out in growth chambers where temperatures imposed as treatments were 35ºC/20ºC and 22.5ºC/22.5ºC. PGR experiments involved the use of several PGRs including Silver thiosulfate (STS), Benzyladenine (BA), Salicylic acid (SA) and Amino-ethoxyvinylglycine (AVG). Further experiments were conducted using only STS at different concentrations. Grafting and PGR experiments were carried out in IITA cassava fields at Ibadan while the temperature experiment was performed at Guterman Bioclimatic laboratory at Cornell University.
The PRA study revealed that 58% and 70% female and male respondents respectively knew almost nothing about cassava flowers. Despite the low knowledge of cassava flowers, a large percentage of farmers preferred branching varieties. Most farmers allowed volunteer cassava genotypes in their fields. The farmers who allowed volunteer genotypes to grow in their farms were 83% and 81% in Edo and Niger states respectively. In summary, farmers do not pay particular attention to flowering in cassava and are ready to adopt varieties that have their preferred traits regardless of the branching and flowering habits. The genetic and phenotypic study of the genetic gain population revealed location, year, location x year interactions effects on flowering and branching traits. All traits had moderate to high broad-sense heritabilities ranging from 33% to 75%. The phenotypic coefficient of variation (PCV) for all traits was higher than their corresponding genotypic coefficient of variation (GCV); this indicated that the traits are more sensitive to environmental influence. Days to branching was found to be highly correlated with days to anthesis. By GWAS, novel 18 genomic regions for flowering and branching traits were discovered using the MLM model and 327 using GLM. Neither model could detect significant associations for the number of branch levels. Results from induction experiments showed that grafting did not improve flowering but promising root stocks and scion genotypes in terms of ability to survive the grafting procedure were identified. PGR experiments revealed STS and BA and their combinations as the most effective PGRs to influence cassava flowering as they gave best results as regard to flowering time, flower production and longevity. Cooler temperature treatments (22.5ºC/22.5º) triggered flowering time between 65-112 days while the plants exposed to the warm temperature (35ºC/20ºC) did not flower at all. Induction experiments conducted revealed the use of PGRs as the most effective and feasible approach of all three approaches tested because it produced significant results and it is amenable to field conditions.