Assessment of Cassava Pollen Viability and Ovule Fertilizability under Red-Light and Plant-Growth-Regulator Treatments

Abstract

Understanding pollen and ovule fertility as factors influencing fruit and seed set is important in cassava breeding. Red light (RL) and plant growth regulators (PGRs) have been used to induce or enhance flowering and fruit set in cassava without any reference to effects on pollen viability or ovule fertilizability. This study investigated effects of field-applied RL and PGR on pollen viability and ovule fertilizability. Panels of cassava genotypes, with early or moderate flowering responses were used. RL was administered from dusk to dawn. Two PGRs, a cytokinin (6-benzyl adenine, BA) and an anti‑ethylene (silver thiosulphate, STS) were applied. Pollen viability was assessed basing on pollen grain diameter, in vitro stainability, in vivo germinability, ovule fertilizability and ploidy level. Treating flowers with RL led to increased pollen diameter from 143.6 to 148.5µm, stainability from 82.9 to 93.0%, ovule fertilizability from 52.0 to 56.9% in treated female flowers and 47.7 to 64.3% on using pollen from treated male flowers. Fruit set increased from 51.5 to 71.8% in RL-treated female flowers and from 53.7 to 71.1% when pollen from RL-treated male flowers was used. Seed set followed a trend similar to that of fruit set. Ploidy level of pollen from RL-treated flowers increased slightly and positively correlated with pollen diameter (R2 = 0.09*), ovule fertilization (R2 = 0.20*), fruit (R2 = 0.59*) and seed set (R2 = 0.60*); but negatively correlated with pollen stainability. Treating flowers with PGR did not affect pollen diameter and stainability, but increased ovule fertilizability from 42.9 to 64.9% and fruit set from 23.2 to 51.9% in treated female flowers and 31.3 to 54.7% when pollen from treated male flowers was used. Application of combined BA+STS on male flowers caused the highest ovule fertilizability, fruit and seed set efficiency, though this varied among study genotypes. These results show that RL and PGR treatment of flowers increases pollen viability and ovule fertilizability. This is important for planning pollination strategies in cassava breeding programmes.