Embryo formation

To this point, we have examined diffusion growth in terms of nucleatlon and embryo formation. Let us now explore the actual species which diffuse in the lattice. 

Let us now reconsider our nucleation models of 4.4.1., specifically Models B, D and E. These are examples of phase-boundary controlled growth involving random nucleation. We now assume an exponential embryo formation law (see 4.4.7), with isotopic growth of nuclei in three dimensions and k2 as the rate constant. By suitable manipulation of 4.4.6.,... 

However, here, too much of a good thing is not so hot. If retinoic acid is administered systemically during embryo formation (in experimental animals to be sure), severe malformations result in the offspring. Thus, retinoic acid is a teratogen. 

Figure 4-3 Contributions to Gibbs free energy for homogeneous embryo formation.

Note the increase in complexity of embryo formation, the interface structure and the number of steps, 6, needed to do so. In the simple case, B grows at the expense of A. However, in the second case involving parallel reactions, two embryos form from A. In the third case, at least two steps are involved in the two consecutive reactions where "A" transforms to B", which transforms to "C". 

We have already shown that embryo formation leads to nucleation, and that this nucleation precedes any solid state reaction or change of state. 

Further improvements in embryo quality are achieved by using lower levels of 2,4-D for induction of embryo formation. By treating alfalfa cultures with 10 uM 2,4-D (instead of 50 uM) a 50% increase in embryo conversion was achieved (4). Embryos treated in this manner were morphologically more similar to zygotic embryos than were those produced after a high 2,4-D treatment. Furthermore, the biochemistry of the improved-quality embryos was altered. When the level of embryo-specific IIS storage protein was compared in 10 and 50 uM... 

Kevers, C Gaspar, T. Dommes, J. (2002). The beneficial role of different auxins and polyamines at successive stages of somatic embryo formation and development of Panax ginseng in vitro. Plant cell. Tissue and Organ Culture, Vol.70, No.2, pp. 181-188, (August 2002), ISSN 0167-6857... 

To achieve in vitro mass propagation of ES, cell suspension cultures using hypocotyls-derived callus have been firstly conducted by Choi et al. (1999a). However, the somatic embryo formation capacity of suspension cultured cells was significantly lower compared to that from callus cultures. Later, improved cell suspension cultures were observed that 35 g dotyledonaiy embryos (about 12,000) were converted to 567 g fresh mass of plantlets with initially culture in 500-ml flask, followed by culture in 10-1 plastic tank, and then low-... 

Explants used in the study The most optimal explant somatic embryo formation (%) References... 

Table 8. Explants of callus initiation and somatic embryo formation. — means undefined in the relevant reference...

From studies on the explants of different species it appears that the direction of cell division can be a marker of cells undergoing changes in cell fate. In Arabidopsis explants, during DSE, the protodermal cell is involved in somatic embryo formation and divides... 

However, not all of the results described so far are in agreement with those presented above. In the case of Pineapple guava symplasmic, isolation was not detected during the formation of somatic embryos (Canhoto et al., 1996). Plasmodesmata were present between the cells of the embryo, but also between the embryo and the surrounding cells. This suggests that symplasmic isolation is not a prerequisite for somatic embryo formation (Canhoto et al., 1996). In other tissue culture systems, the same conclusion was drawn (Jasik et al., 1995 Thorpe, 1980 Williams Meheswaran, 1986). 

Choi, Y.-E. Soh, W.-Y. (1997). Enhanced somatic single embryo formation by plasmolyzing pretreatment from cultured ginseng cotyledons. Plant Science, Vol. 130, No. 2, (December 1997), pp. 197-206, ISSN 0168-9452... 

Kong, L. and Yeung, E. (1995) Effects of silver nitrate and polyethylene glycol on white spruce Picea glauca) somatic embryo development enhancing cotyledonary embryo formation and endogenous ABA content. Physiol. Plant., 93 298-304. 

Salaj, T. Salaj, J. (2003/4). Somatic embryo formation on mature Abies alba x Abies cephalonica zygotic embryo explants. Biologia Plantarum, Vol. 47, pp. 7- 11. 

Alternatively, the multi-cellular mass that is liberated from the rupture of the microspwre wall may proliferate to form a callus, from which organogenesis occurs by the regeneration of shoots and roots following transfer to culture media with ap>prop)riate growth regulators. It may sometimes be possible to obtain androgenic haploids either via embryo formation or organogenesis from callus in the same species by the manipulation of the chemical components of the culture medium... 

Several explants have been tested with diverse results, being the most responsive immature infloresencenes and plumules in increasing order (Blake and Hornung, 1995 Chan et al., 1998 Perez-Nunez et al., 2006). For this reason plumules have been more extensively used to improve on the different developmental changes in the process callogenesis, embryo formation, germination and conversion. 

The effect of the brassinosteroid 22(S), 23(S)-homobrassinolide on initial callus, embryogenic callus and somatic embryo formation in coconut plumule explants was tested. The explants were exposed (during a 3 or 7 d pre-culture) to different concentrations (0.01, 0.1,1, 2 and 4 pM) of the brassinosteroid. The explants responded favorably to the brassinosteroid increasing their capacity to form initial callus, embryogenic callus and somatic embryos. The largest amount of somatic embryos formed, 10.8 somatic embryos / explant, was obtained exposing the explants for 3 d to the brassinosteroid at 0.01 or 0.1 pM, whereas 3.8 somatic embryos / explant were obtained from untreated explants. Efficiency-wise the overall effect of HBr increases the total amoxmt of somatic embryos formed p>er explant 2.8 times (Azpeitia effll. 2003). 

Thus, because of lack of knowledge about a, the kinetic problem of calculating the nucleation rate has been transformed to the thermodynamic problem of evaluating the equilibrium droplet distribution (the discussion of purely kinetic theories that avoid a thermodynamic formulation is beyond the scope of this article the interested reader should consult [5], and the interesting papers by Ruckenstein and coworkers [19-24]). Returning now to Equation (7), the calculation of the nucleation rate requires knowledge of the energetics of embryo formation, which we now address. 

The microscopic theory of the physically consistent cluster due to Reiss and co-workers [25, 63-68] addresses the rigorous calculation of the energetics of embryo formation from statistical mechanics. This approach is only applicable to nucleation in supercooled vapors. The key result of the theory is an expression for the free energy of embryo formation. 

The work of embryo formation is a function of two variables (embryo radius and pressure). At equilibrium, we must have [5]... 

The above treatment can be applied to the calculation of the energetics of nucleation [100]. To this end, one considers the formation of an embryo within a bulk phase of initial density p. At constant temperature and volume, the reversible work of embryo formation is simply the change in Helmholtz energy associated with the above process. 

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