Prospects for a rapid in vitro regeneration system for propagation of the pesticidal tree Melia volkensii, Gurke.

Abstract

An in vitro system for regeneration of Melia volkensii Gurke shoots is described. M. volkensii, a drought-tolerant tree native to East Africa, has under-utilized potential as a source of botanical pesticides. Extracts of its fruits and seeds have larvicidal, growth retarding and anti-feedant effects on mosquito larvae and locusts. The species has been over-exploited for its timber. Production of adequate planting material for replenishment of declining stock is constrained by difficulties in propagation via seed and cuttings. The objective of this study was to develop an in vitro culture protocol for rapid and efficient shoot regeneration in two ecotypes of M. volkensii. Mature zygotic embryos were aseptically cultured on Gamborg et al’s B5 medium containing the plant growth regulator Thidiazuron. High frequency callus induction and regeneration were achieved. Callus subcultured to hormone-free ½ MS or B5 medium formed multiple somatic embryos which grew into micro shoots. ½ MS was superior to B5 medium for induction of somatic embryos. Thidiazuron concentration had no significant effect on callus induction but significant effects were observed on fresh mass of callus and somatic embryo induction (Ftest, p < 0.001). Microshoots elongated well on B5 medium containing 0.1 mg/l Benzylaminopurine plus 5 or 10% (v/v) coconut water. Frequent multiple shoot induction, with 4 to 12 shoots per initial shoot, was also observed on the elongation medium. This protocol produced phenotypically normal shoots of height ≥ 5.0 cm in 3.5 months. Further work is in progress to attain rooting of the shoots. This study offers a simple protocol that could be optimized for exploitation in large scale in vitro cloning of M. volkensii mother trees with elite genotypes and phenotypes.