Liquid Culture Systems for in vitro Plant Propagation

Development of photoautotrophy in Coffea 325
endosperm as a coating to the somatic embryos (Redenbaugh and Walker,
1990; Redenbaugh et al., 1988). This approach has achieved little success,
perhaps because of the poor uptake of the added nutrients by the embryo
axis, leaching of the nutrients during conversion, or toxicity of the coating.
Coffee plays a major role in the economy of many African, American
and Asian countries. Coffea arabusta has been clonally propagated to obtain
genetically uniform transplants using microcuttings. However, the growth of
microcuttings in vitro is slow (Dublin, 1980) therefore somatic
embryogenesis is considered to be an effective method for the mass clonal
multiplication of C. arabusta (Dublin et al., 1991).
This review describes the recent research on photoautotrophic culture of
coffee somatic embryos by Afreen et al. (2002a, b) to overcome the
problems in conventional propagation system of somatic embryos. We also
discussed the photosynthetic ability of different stage coffee somatic
embryos, the development of photoautotrophy in somatic embryos,
optimization of the photoautotrophic growth and advantages of
photoautotrophic culture of somatic embryos for mass production of clonal
transplants.
2. Photosynthetic ability in coffee somatic embryos
Naturally, somatic embryos developed in sucrose-containing medium are
heterotrophic or mixotrophic and use sugar in the medium as carbon and
chemical energy source for their dry mass accumulation. Afreen et al.
(2002a) investigated some of the physiological variables in relation to the
photosynthetic ability of coffee somatic embryos.
1. The CO2 concentration in the culture headspace during the photoperiod,
was normally less than ambient (370 µmol mol -1 ) in converted embryos
and little above ambient in cotyledonary stage embryos. This indicates
that these embryos have photosynthetic ability and that the CO2 uptake
rate, or carbon assimilation rate, of converted embryos was positive
(Figure 2b). In the case of torpedo and precotyledonary stage embryos,
the occurrence of comparatively higher CO2 concentrations than the
ambient air indicated that plant respiratory activity masked any
photosynthetic assimilation. This was probably because of there being
few chlorophyll-containing tissues and a significant amount of
chlorophyll-free tissues without stomatal development.
2. The development of stomata is essential for the physiological processes
of plants because stomata act as portals for entry of CO2 into the leaf for
photosynthesis (Willmer, 1983). Anatomical studies revealed that,
generally, stomata did not form in torpedo (Figure 1a, b) and