The Factors involved in the efficient operation of a Mist Propagation unit

 

The advent of the intermittent mist propagation unit, in the early 1960s, as a commercial piece of equipment, was a major advance in the then technology for environmental control in plant propagation. It permitted the regeneration of soft wood cuttings (with their high inherent regenerative capacity) to be achieved relatively easily and successfully by maintaining their survival during the root initiation and development process. It allowed a much greater range of plants to be propagated from soft wood cuttings and this substituted many of the traditional techniques which had been necessary and costly in order to achieve their propagation.

 

The principle aim of the system is to maintain the normal water status of a soft wood cutting – by reducing water loss from transpiration to a minimum and increasing atmospheric humidity. This reduces the chances of wilting and will maintain the turgor of the cells at optimal status. This condition enhances root initiation and at the same time the system provides suitable and relevant levels of light, carbon dioxide and temperature for the cutting to develop without stress.

 

The operation of the system is achieved by spraying fine water (50 to 100 µm) droplets, from above, onto the surface of the cuttings to create a complete water film. As a result of evaporation this film is reduced until it is no longer a complete cover and a sufficient amount is then applied, automatically, to replace the film.

 

The required effect is achieved by the action of evaporation – this cools the exposed leaf surfaces and the surfaces of the internal cells so that the intercellular spaces have a high vapour pressure and thus does not encourage water loss from the cell surfaces. In addition there is a cooling of the immediately surrounding atmosphere.

 

If however the cutting does become stressed the stomata (usually on the lower side of the leaf) will close to reduce water loss but this will also mean that the movement of carbon dioxide into the leaf for photosynthetic activity is curtailed. 

 

The success of operating the system is to develop the efficacy of the control mechanism so that it applies only sufficient water to maintain the evaporating water film and to ensure that on reduction it is replaced without delay.

 

Excess water application leaches nutrients and minerals from the leaves and so reduces the efficiency of root initiation, development and subsequent establishment. This factor becomes more significant the longer the cutting takes to root and therefore is more important for those species which are difficult (slow) to root and occupy the unit for long periods.

 

The excessive application of water will also necessarily cool the rooting medium unnecessarily and, in order to maintain the required temperature in the medium for rooting, will inevitably increase energy costs. It may also reach, if not properly monitored and controlled, a level at which the medium becomes waterlogged and thus affects the level of aeration of the medium. Hence the constituents used in the composition of the rooting medium must permit satisfactory drainage.

 

The system also allows the access of light – which enhances the potential for photosynthesis. However maximum summer light is not necessarily required as the amount of carbohydrate production required for the physiological activity in the production of adventitious roots is not particularly high – apparently there is some evidence to suggest that high carbohydrate levels can even be counterproductive.

 

Light is a source of energy and hence high light intensity will raise the ambient temperature to the detriment of maintaining the balance of humidity in the atmosphere surrounding the cutting and inside the leaf itself. Thus it is probable that shading in summer may be necessary to mitigate this effect and, as indicated above, this may not be detrimental.

 

Heating of the rooting medium will be required to maintain the temperature level of ‘bottom heat’ needed to optimize root initiation – this is generally set at 20̊C for the great majority of subjects.

 

Thus the efficient operation of a mist unit is to achieve a balance of all the environmental factors such that the survival of the cutting and its normal metabolism and activity are not compromised. This involves attention to the relationship and balance between the following environmental features:- water, temperature, light, carbon dioxide, oxygen and nutrition.

 

For the most efficient operation of an intermittent mist unit the significant feature is to create the most effective and evenly spaced application of the water droplets so that the entire system is operating uniformly and that this is achieved with minimum water usage. As the normal system of control for water application is to provide an artificial leaf surface that will produce an evaporative response equivalent to that of a leaf, the reaction will therefore very much be a response to local weather conditions. The trick therefore is to site the artificial leaf so that it successfully achieves the required responses.

 

Because of the high humidity and temperature levels maintained in such a unit the situation is ideal for the development of disease especially and it therefore paramount that intensive attention is paid to the maintenance of hygiene in the entire unit.

 

Attention will also need to be paid to the quality of the water and the regular maintenance and servicing of the water delivery system so that it remains efficient and uniformly effective. Problems can arise with hard water, the development of algae and the occurrence of liverworts and mosses on the benches and rooting medium.

 

Although the operation of an intermittent mist system can be seen as a simple, elegant and useful adjunct for the propagation of soft wood cuttings - and it has indeed revolutionized the potential for the successful propagation of such cuttings – its operation is in fact a complex of many factors which need to be monitored regularly and individually for an efficient outcome.

 

In conclusion although it is reasonably clear as to what can be done to maintain the efficiency of the unit – in practice the unit is subject, in the context of the British climate, to the vagaries of the local situation and will require individual attention to local conditions. Latitude also has an input insofar as daylength – varying through the seasons will also have its influence.