Micropropagation of Tropical Tree Species
Tree species are important in tropical regions as a source of timber, nutrition, medicine, fuel, income and livestock food. Many vegetable species are not adapted to production in tropical climates, further highlighting the importance of tropical fruit species in these regions. Although considerable research has been applied to temperate species, research into tropical tree species has been limited despite an increase in recent years. Also, many tropical tree species have represented the greatest challenge to in vitro culture and some are still regarded as recalcitrant in vitro. The list of difficult tropical species includes both dicotyledons, e.g. cacao, and monocotyledons, e.g. coconut. The level of recalcitrance varies between species within a genus, e.g. Eucalyptus; and between genotypes within a species, e.g. mango. A number of factors affect micropropagation of tropical tree species. Media components and concentrations affect growth of shoots, roots and callus in vitro. Root initiation can be optimized by short exposure to auxins and low concentrations of minerals before transfer to PGR free medium containing higher concentrations of minerals to optimize root growth and minimize callus growth. The source of nitrogen supplied to the medium and the concentration of potassium have been shown to have a marked effect on root initiation and growth. Media interactions, such as photo-oxidation of auxins by riboflavin, can rapidly affect the stability and thus affect various medium components. The choice of explant has been shown to be very important in micropropagation of recalcitrant species. Establishment of in vitro cultures requires the use of apomictic seeds of mangosteen, immature ovules of mango, immature embryos of coconut, and very young leaves of litchi and longan. High levels of microorganisms are present in tropical plants, thus pretreatment of mother stock is important to establish cultures free of contamination such as endogenous anaerobic bacteria in papaya explants. Cultural conditions of daylength, irradiance levels, temperature, aeration of the root zone and gaseous components in the flask headspace can all affect plantlet survival and growth at all five stages of micropropagation. Many problems still need to be solved, e.g. excess callus production that severely reduces acclimatization percentages of plantlets in some species, e.g. Moringa olifera and Azadiracta indica. A comparison of the research effort that has been applied to temperate tree species such as apple, to that of tropical tree species such as rose apple or star apple, shows there is need for a much larger research effort to elucidate protocols for micropropagation of many tropical tree species.
Plant Biology not elsewhere classified
Biological Sciences not elsewhere classified