In Vitro Slow Growth Storage of Fruit Rootstocks inside Gas-Tight or Gas-Permeable Containers

The maintenance of stock cultures in darkness and at low temperatures is by far the most common slow growth storage technique used in commercial laboratories. This study was carried out with in vitro shoot cultures of three fruit rootstocks: cherry rootstock ('Gisela 5®'), apple rootstock ('M26'), and pear rootstock ('A74'). The shoot cultures were stored at 4°C in darkness inside 500-ml glass jars or "StarPac" plastic bags. Moreover, with 'Gisela 5®' rootstock, three different sucrose concentrations (10, 20, and 30 g L-1) were tested in the storage medium. Very high levels of CO2 were detected inside the gas-tight containers, particularly at the highest sucrose concentration which determined over 130,000 ppm accumulation after just 8 weeks of conservation. Differently, the gas-permeable bags proved to be effective in maintaining low levels of CO2 accumulation. In both containers, ethylene accumulation never exceeded 10 ppm. After 9 months of 'Gisela 5®' conservation in glass jars, a lower growth rate was recorded from shoot cultures stored in the lowest sucrose concentration. However, with both containers, shoots stored this way were highly affected by hyperidricity and decay, and had to be discarded in great quantity. Best conditions of conservation and regrowth were obtained when shoot cultures were stored in a medium containing 30 g L-1 sucrose. As regard 'M26' apple and 'A74' pear rootstocks, the shoot cultures maintained a good standard of quality for the whole period of storage at 4°C in darkness, regrowing easily when moved again to proliferative conditions. Differently, the use of mannitol as an osmotically active compound allowed a satisfactory conservation only for the pear rootstock.