In Vitro Acclimatization and Rooting20

The term acclimatization is defined in this chapter as the environmental adaptation of a tissue-cultured or micropropagated plant that has been moved to a new environment, i.e., a greenhouse or a field environment. During the acclimatization, the plant's environment is changed gradually with time,

Table 28.3. Experimental description of Coffea arabusta

Treatment code*

Sucrose concentration

(g l-1)

Supporting material

No. of air exchanges (h-1)

Ventilation rate (ml h-1 / platelet)

SAL

20

Agar

0.2

27

SAH

20

Agar

2.3

310

SFL

20

Florialite

0.2

27

SFH

20

Florialite

2.3

310

FAL

0

Agar

0.2

27

FAH

0

Agar

2.3

310

FFL

0

Florialite

0.2

27

FFH

0

Florialite

2.3

310

* S and F at the left represent sugar-containing and sugar-free medium; A and F at the middle represent agar and Florialite; L and H at the right represent low and high number of air exchanges per hour of the vessel, N, or vessel ventilation rate = N. vessel air volume/ No. of plantlets in the vessel).

starting with the 'near' in vitro environment and finishing with the 'near' greenhouse or field environment. Acclimatization conducted in a greenhouse or in a field under shade is called 'ex vitro acclimatization'. In photoau-totrophic micropropagation, acclimatization can be almost completed in the tissue culture vessel, which is called 'in vitro acclimatization'.

Rooting in vitro of many woody plant species is known to be difficult, and application of plant growth regulators or other chemicals to the medium has been tried for promoting rooting, without success in many cases. Thus, rooting is often conducted during the ex vitro acclimatization, which is often called 'direct (ex vitro) rooting'.

Under photoautotrophic micropropagation, root formation, development and growth in vitro of physiologically and morphologically normal plants are common. The plants are able to control the transpiration and thus control water loss when exposed to the ex vitro environment; thus, wilting is not obvious even without the necessity of any specialized ex vitro acclimatization. This is probably due to:

1. A higher net photosynthetic rate and thus a greater accumulation of carbohydrates;

2. A higher production rate of phytohor-mone by the in vitro plant itself;

3. A higher transpiration rate and thus a higher uptake rate of minerals in the medium during the in vitro culture; and

4. A higher air porosity of the supporting material, which gives a higher dissolved oxygen concentration around the shoot base.

In photoautotrophic micropropagation, plants with supporting material can be transplanted to the soil in the greenhouse or field, with little bacterial and fungal contamination,

Fig 28.3. Coffea arabusta plantelets in vitro on day 40 as affected by presence/absence of sugar in the culture media, types of supporting materials (agar and Florialite) and high/low number of air exchanges of the vessel.12-14 For treatment codes, see Table 28.1.

provided that the sugar-free supporting material consists of inorganic components and its surface is dry. Transplanting of plants with supporting material reduces root damage, saves labor costs, and gives a possibility of automatic transplanting.

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