Multiple shoot induction and leaf and flower bud abscission of Annona cultures as affected by types of ventilation

Nodal explants of Annona squamosa L. and Annona muricata L. were cultured in vitro under various types of ventilation: airtight vessel (sealed condition; number of air exchange 0.1 h-1), natural ventilation (via a polypropylene membrane; number of air exchange 1.5 h-1), and forced ventilation (5.0 cm3 min-1 in a 60 cm3 vessel; number of air exchange 5.0 h-1). In both species, numbers of leaves, leaf areas and numbers of nodes per shoot increased with improving standards of ventilation, while leaf abscissions were substantially reduced; all the leaves had abscised in the airtight vessels after 12-15 days, but none had done so with forced ventilation. Flower-bud abscission in A. muricata showed a similar trend after 21 days. These effects were associated with reductions in the accumulation of ethylene within the culture vessels, produced by increasing the efficiency of ventilation; ethylene was not detected in those fitted with a forced ventilation system. CO2 concentrations in culture headspaces and the net photosynthetic rates of the plantlets were also evaluated. CO2 concentrations decreased well below the ambient in the natural and airtight vessels; however, under forced ventilation, CO2 concentrations were significantly higher during the photoperiod, compared to those of the natural ventilation and airtight vessel treatments. In general, net photosynthetic rates per unit leaf area increased with increasing photosynthetic photon flux (PPF) and rates were highest in plantlets grown under forced ventilation, intermediate under natural ventilation and lowest in the airtight vessels. Eighteen different media were investigated for their effects on multiple shoot induction in both species. The best medium for multiple shoot induction and growth in A. squamosa was Murashige and Skoog medium (MS) + 6-benzylaminopurine (BA; 1.5 mg l-1) + casein hydrolysate (1.0 g l-1) and for A. muricata MS + BA (1.0 mg l-1) + naphthaleneacetic acid (NAA; 0.1 mg l-1).