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by Dr Linda Falzari and Professor Robert Menary
April 2007
RIRDC Publication No 06/134 RIRDC Project No UT-38A
Who is the report targeted
at?
Identification of productive
genotypes and increases in crop oil yields through improved propagation
techniques will be invaluable to the extension and field officers associated
with the traders and wholesalers in persuading growers to undertake the
production of peppermint and to manage their crops well. Highly productive
peppermint crops will also benefit growers by adding an attractive alternative
to the suite of crops available for inclusion in crop rotations.
Background
Peppermint (Mentha piperita
L.) is a key species to the Australian essential oil industry, which itself
is a significant sector of Tasmania’s agricultural economy. Current Australian
demand for peppermint oil exceeds the current production of approximately
ten tonnes per annum. Increases in production could supply some of this
demand but also be exported for trade in international markets.
The decision by primary producers to grow particular agricultural crops is determined largely by the gross margins they can expect to achieve. Crops with high gross margins produce the greatest farmgate returns and therefore are likely, not only to be grown by primary producers but also to receive the greatest level of attention their management. Crops perceived by growers to produce low returns may either fail to be planted or fail to receive the level of care required for optimum production. This lack of care, in turn leads to low productivity and/or failed crops, contributing to further dissatisfaction in the producers and further decline in the likelihood of future production.
Methods and Findings
Genetic variability
All the commercial peppermint
in Tasmania originated from two parent plants imported from the USA. Because
the clone “Black Mitcham” is a sterile hybrid, this material should be
of a single genotype and the same as the original parent material. However,
anecdotal evidence suggests that genetic mutations have produced a series
of somaclonal variants that differ with respect to the economically important
traits of oil yield and oil composition. It is also possible that there
are differences within the Tasmanian mint population with respect to disease
resistance e.g. to the fungal pathogen rust, Puccinia menthae.
The trials to establish the level of genetic variability in the Tasmanian peppermint population were conducted on 200 mother plants collected from two local producers. These were tested under controlled conditions, in replicated glasshouse trials. Significant genetic differences were found between these clones. Several potential peppermint genotypes were selected and steps taken to test these outside the glasshouse. Problems with strong response to environmental conditions led to inconclusive results as to the optimum genotypes for commercial production. These clones are available for testing under commercial growing conditions and it is recommended that the testing continue on a population rather than single clone basis.
Propagation of nursery
stock
Commercial peppermint crops
in Tasmania have traditionally been established through division and replanting
of an existing crop but this can cause problems with pest and disease transfer
from the old site to the new. The current trend is to make use of an intermediate
purpose-grown nursery.
Peppermint is a sterile hybrid and as such is vegetatively propagated. Three planting materials available for propagation are stolons or turions, tip cuttings and micropropagated plantlets. These vary in the level of crop hygiene that can be achieved and in the intensity of management required.
Propagation by stolons is the cheapest method in terms of both variable and capital costs but results in the greatest potential for transfer of pests, diseases and weeds to the new nursery area. Propagation by tip cuttings is a more labour intensive system and has greater production costs, especially if the tip cuttings are rooted in the glasshouse prior to planting in the field nursery. The advantage is the reduced disease and pest load because soil-borne problems can be drastically reduced and only systemic infections are carried to the new site.
The third method of propagation is micropropagation using meristem culture. This is by far the most expensive technique, in terms of time, variable costs of production (labour and consumables) and infrastructure, but it has the potential to eliminate all pests and diseases, including systemic diseases, such as viruses and mycoplasmas. It also has the potential to produce plants that are physiologically more vigorous than the parent material from which the cultures are established.
A field trial was established to compare these methods of propagation. Data were collected over two growing seasons. The trial supported the hypothesis that both micropropagated plants and tip cuttings were more vigorous than the stolon propagated material. Oil yield from the tip cuttings was high, especially in the second season, when a mean yield of approximately 90 kg.ha-1 was achieved from the tip cuttings, compared with 64 kg.ha-1 for the stolon propagated material.
Oil composition was also affected by the propagation method. Tip cuttings, with their faster, more vigorous growth and lower ratio of immature to mature leaves, produced oil higher in menthol and lower in menthofuran than the stolon propagated material. The tip cuttings also matured more quickly than either the micropropagated or stolon propagated material.
A cost analysis of the three propagation methods is presented along with details of the many assumptions made in its preparation. Because micropropagation is labour intensive and as such extremely costly, the budget for this method was prepared based on weaning a number of plantlets from tissue culture and then multiplying these through tip cuttings. The cost of producing a 50 m by 50 m nursery from either tip cuttings or micropropagation was calculated to be approximately $25k. This is much more expensive than the traditional method of propagation but the tip cuttings particularly give rise to a more productive crop.
This additional cost should be recovered through the improved productivity of the crop.
Implication and Recommendations
It is recommended that the
industry use the tip cutting method of propagation for the establishment
of nursery sites, which can then be expended into future commercial crops.
Each new crop should be expanded from a newly established nursery so as
to take advantage of the rejuvenating effect of tip cutting propagation.
These nurseries should be established from the most productive peppermint genotypes. To this end urgent testing of the selected genotypes is necessary, in order to determine their relative performance under standard commercial production conditions. It is strongly recommended that this research not pause at this critical stage but that the industry immediately commences testing the selections to avoid further propagation of unproductive clones in this perennial crop.
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