Protocols for Micropropagation of Woody Trees and Fruits

APRICOT MICROPROPAGATION
2.2.2. Hyperhydricity
Hyperhydricity is a physiological disorder frequently related to the in vitro environment
during micropropagation of woody plants. Apricot shoot tips at the prolife-
ration stage are very sensitive to hyperhydration (Figure 5), although the degree of
symptoms is genotype-dependent.
Treatments that decrease hyperhydricity, without affecting micropropagation
rates in apricot, are the application of a bottom cooling system for 1 or 2 weeks in
each culture cycle; and an increase in the agar concentration of the culture medium
(0.6–0.8%) or the use of Agargel TM (0.5%) as gelling agent (Pérez-Tornero et al.,
2001). However, the effectiveness of each method depends on the genotype and only
bottom cooling have consistently prevented hyperhydricity in all apricot cultivars
tested.
Figure 5. Hyperhydric apricot explants of two different cultivars.
Bottom cooling system. High relative humidity (RH) is probably the most important
environmental factor that induces hyperhydricity. The bottom cooling system
(Figure 6) decreases the RH inside the in vitro culture jar by condensing water on
the cooled culture medium (Vanderschaeghe & Debergh, 1987). Vessels are placed
on an iron plate cooled by a system of copper tubes. The water, cooled by a cryostat
(Selecta, Frigiterm-10), circulates constantly through the tubes. The temperature of
the water is adjusted to 13° C in the cryostat to obtain a gradient of approximately
4° C between the bottom and the top of the jar. RH within the jar is then decreased to
approximately 75% (Vanderschaeghe & Debergh, 1987). Hyperhydricity can be
avoided by placing proliferating shoots in the bottom cooling system in the first
week of the culturing cycle. More than 64% of hyperhydric shoots could be returned
to normal by keeping them for 3 weeks in the bottom cooling system.
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