- ORCHID BIOTECHNOLOGY 179
B.In VitroSeed Germination
Orchid seeds are quite small and unique. They are produced in the
ovary, which develops into a capsule that dehisces by a number of lon-
gitudinal splits. Orchid seeds are produced in large amounts inside
the capsules, varying from a few thousand to estimated millions in
some species, such asCymbidiumsp. (1.5 million seeds),Maxillaria
sp. (1.7 million seeds),Anguloa ruckeri(3.9 million seeds), andCyc-
noches chlorochilon(4.0 million seeds), among others (Arditti 1992;
Arditti and Ghani 2000). One particular characteristic of orchid seeds is
that in general they do not contain endosperm, the tissue that provides
nutrients for embryo germination. Therefore, germination depends on
a symbiotic association with mycorrhizal fungi for most orchids. Under
natural conditions, the chances of disseminated orchid seeds to meet
suitable mycorrhizal fungi are very small and in general less than 5%
of seeds germinate (Rao 1977; Nikishina et al. 2001). Furthermore, both
seed maturation and seed coat characteristics vary largely among orchid
species and consequently affect germination (Lee 2011).
Manz et al. (2005) defines seed germination by the emergence of the
radicle through the seed coat. Kauth et al. (2006) specified six specific
stages for orchid seed development upon germination: (1) seed imbi-
bition, (2) seed enlargement, (3) protocorm formation with shoot apex
and rhizoids, (4) protocorm with developing leaves and rhizoids, (5)
seedling formation, and (6) seedling development with evident roots
and two or more leaves. Upon seed germination, the orchid embryo
becomes rounder than normal to form a small spherical corm-like organ
defined as a protocorm, possessing an apical meristem and a leaf pri-
mordium. Protocorm is a term coined to designate a stage of develop-
ment in the orchid embryo. Once developed, protocorms become green,
capable of photosynthesis and accumulate carbohydrates. Only when
sufficient organic matter has been stored in the protocorm, shoot and
root can appear followed by normal seedling growth.
Most orchids can be germinatedin vitrowithout the need for spe-
cific mycorrhizal fungi and therefore high rates of germination can be
achieved. This is referred to as asymbiotic seed germination. Asymbi-
oticin vitroorchid seed germination has come a long way since the
introduction of the Knudson C medium (Knudson 1922) and the addi-
tion of tomato juice to the germination medium by Vacin and Went
(1949). Orchid commercial production via propagation as well as breed-
ing became much easier and accessible, and it also facilitated research
of the different aspects related to orchid seed germination (Yam et al.
2002).In vitroseed germination of orchids has been studied for species