Dr. Alieta Eyles, Dr. Ryan Warren, Prof. Dugald Close, Dr. Stephen Quarrell and Prof. Alistair Gracie
Dr. Alieta Eyles is a Senior Research Fellow, Dr. Ryan Warren is a Junior Research Fellow, Prof. Dugald Close is a Professor in Horticulture, Dr. Stephen Quarrell is a Research Fellow and Prof. Alistair Gracie is a Professor in Horticultural Science. All authors are affiliated with the Tasmanian Institute of Agriculture, University of Tasmania.
Mechanical pollination (MP) is an emerging technique that utilises mechanical devices to transfer previously collected pollen to target flowers. MP is particularly relevant to fruit and nut crops, which are reliant on adequate pollination to maximise both yield and quality.
Sub-optimal crop yields are often due to insufficient pollination caused by a range of factors such as asynchronous flowering, poor pollen compatibility, unfavourable weather and declining pollinator health and diversity. This is especially relevant in the face of increasing global food demand and changing climates. It is therefore critical that modern horticultural systems investigate novel MP approaches to help address these issues.
Current research is focused on developing mechanical systems to supplement and enhance traditional pollination strategies. MP systems offer a number of key advantages to horticultural producers, including a reduced reliance on insect pollinators and the ability to overcome the effects of asynchronous flowering. Pollen quality can easily be assessed to ensure maximum viability, whilst storage and transport of pollen reduces the need for on-site polliniser cultivars, which are typically of less commercial value.
Significant research and development over the past decade have resulted in MP being successfully tested in kiwifruit, date palm, sweet cherry and almond. While the use of MP in kiwifruit is the most advanced, research is still required before it will be broadly adopted in other crops. Furthermore, recent trials have revealed potential for application in cacao, Japanese pear, olive, hazelnut and pistachio crops. However, further testing and optimisation must occur to facilitate widespread adoption.
Overall, MP pollination systems consist of four key steps:
1. Pollen Collection
MP systems rely on the ability to efficiently harvest high-quality pollen. This is typically achieved by collecting pollen/flowers prior to anthesis (bud opening) using a range of different techniques. Mechanised ‘pollen vacuums’ have been used to collect pollen from wind-pollinated species e.g. Douglas fir and cannabis. However, collection is more challenging in fruit and nut trees, which often have ‘sticky’ pollen. As a result, flowers are harvested by hand in these crops, representing increased labour cost. As the technologies advance, there may be crops grown specifically for high-quality, compatible pollen that are designed for efficient mechanical pollen collection. Regardless of collection technique, factors such as flower maturity, weather, genotype and pesticide application must be considered due to the impacts on pollen quality.
2. Pollen Handling
Processing is generally required to separate the anthers/pollen from harvested flowers. This can be achieved via gentle milling and sieving before drying the pollen. Different pollen purity can be achieved depending on the requirements, with a number of commercial mills/extractors available.
3. Pollen Storage
MP systems require the capability to easily store and transport pollen between orchards and seasons. Breeding and conservation efforts have well-established protocols for the long-term storage of pollen, which have since been adapted for MP systems. As such, storage temperatures of -20 °C are recommended to ensure pollen longer viability. Drying of pollen is also essential in maintaining pollen viability, and moisture contents of <11% have been reported to be adequate for bicellular pollen —a type of pollen that is characteristic of 70% of flowering species. However, our studies in sweet cherries have demonstrated that even lower moisture contents of between 6-7% are required to improve long-term pollen viability. Commercial suppliers report successful cold storage of pollen for up to five years, with viability suitable for field application. However, the specific storage techniques must be optimised depending on species/cultivar, with a focus on ideal temperature, humidity, air flow and pollen moisture content.
4. Pollen Application
A number of different techniques have been developed to deliver harvested pollen to the target crop. The majority of studies conducted to date have utilised handheld sprayers to test the feasibility of MP systems on a small scale. Pollen is applied either as a dry powder or in solution. Both techniques rely on a carrier to improve the efficiency and precision of pollen delivery. As MP pollination systems continue to develop commercially, a range of orchard-scale applicators have become available, commonly utilising electrostatically charged boom sprayers.
Although MP is a well-established concept, there is necessity for continued testing and optimisation across a wide range of crops and environments. This requires a strong understanding of the underlying pollination biology of the target species, including flower morphology, stigma receptivity, pollen type, growth habits, agronomic requirements, susceptibility to environment conditions and risk of pollen-vectored viruses. Through consideration of these factors the potential of MP has been identified for many nut and fruit crops such as apple, apricot, date, peach, pear, almond, cashew, chestnut, hazelnut, macadamia and pistachio, recently reviewed by Eyles et al. and Broussard et al.
Mechanical Pollination Research in Tasmania
Researchers at the Tasmanian Institute of Agriculture (TIA) are currently developing MP systems, through investigation of pollen application in the cool-climate orchards of the southern hemisphere. Funded by Hort Innovation Australia, this research focuses on developing MP systems for sweet cherry. This has led to the optimisation of pollen collection, viability assessment, processing techniques and long-term storage. Advancements have been achieved over multiple field seasons, with the development of refined germination tests, prototype milling equipment and novel molecular genetic techniques. Work now continues into optimising pollen application in the field, with an aim to ensure a sustainable future for fruit and nut pollination.
Figure 1. Milling prototype (A) used to collect anthers (B) from fresh sweet cherry flowers. (C) Scanning electron microscopy (SEM) image of sweet cherry pollen that has been dried and stored for mechanical pollination. (D) Pollen tubes growing down the style following in vivo germination of sweet cherry with stored pollen.
Eyles, A., Close, D. C., Quarrell, S. R., Allen, G. R., Spurr, C. J., Barry, K. M., Whiting, M. D., et al. (2022). Feasibility of Mechanical Pollination in Tree Fruit and Nut Crops: A Review. Agronomy, 12(5), 1113. Broussard, M. A., Coates, M., & Martinsen, P. (2023). Artificial Pollination Technologies: A Review. Agronomy, 13(5), 1351.