Aeration Stress in Plant Tissue Cultures

The requirement for sterility in plant tissue cultures avoidance of dehydration can impose sealing requirements that severely limit the rate of gas exchange in and out of the culture vessel. Conditions within the culture vessel such as the depth of any water cover, the presence of gelling agents, the bulk and porosity of the tissue and the temperature also strongly influence in vitro rates of gas exchange, primarily driven by diffusion. This article uses elements of Fick’s Law of Diffusion to identify key factors limiting gas exchange between culture and its immediate surroundings. In particular, it identifies static liquid media, gelling agents, large tissue mass and warm temperatures as imposing severe limits on diffusive flux for gases such as O2, CO2 and ethylene. The principle barrier to diffusive exchange of gases between the in vitro and ex vitro atmospheres is the wall of the enclosing vessel. This is invariably made of glass or plastic that is gas-impermeable and well-sealed against evaporative drying or entry of micro-organisms. Cultures enclosed in this way will, inevitably, asphyxiate unless a compensating pathway for diffusive gas exchange is contrived or replaced by some system of convective flow that carries gases to and from the tissue. Supplementing diffusive aeration with convective flow is the basis of most successful hydroponics systems for whole plants and may be a prerequisite for securing levels of aeration suitable for autotrophic cultures. The paramount consideration is the extent to which the total rate of consumption or production of a particular gas by the cultured tissues is matched by the maximum rates of gas transport imposed by the culture itself, its immediate surroundings and the ventilation and sealing system of the culture enclosure.