Low yields in avocados (Persea americana) : is poor pollination a contributing factor and can it be enhanced?

Abstract

The avocado (Persea americana Mill.) is a widely demanded fruit worldwide. While its palatable taste is one of the reasons it is favoured so, the fleshy interior contains a wide range of vitamins, minerals and other health boosting components. Not only this, but the pulp and seed are also valued in the oil-processing, pharmaceutical and cosmetic industries. The demand for avocados is thus extensive, and production of the fruit has increased five-fold over the last 35 years to meet this demand. Despite centuries of selection and hybridisation, avocado trees are still characterised by a low-yielding nature. The oil-rich flesh, alternate bearing, competition between vegetative and reproductive flushes, and disease proliferation all contribute to low yields in commercial orchards. There is another factor, widely investigated and yet poorly understood, that further reduces yields in avocados: sub-optimal pollination. When cultivated in its native region of central Mexico, many endemic insect species facilitate pollination between the synchronous, dichogamous avocado flowers. As avocado cultivation expands outside its native region, honey bee hives are installed in orchards to improve pollination. However, more attractive blooms of other angiosperms (citrus, for example) can cause a mass honey bee exodus out of the avocado orchards. This results in reduced pollination and consequently, fruit set and yield are seriously compromised. This dissertation aimed to explore the notion of low pollination rates in avocados. Factors inherent to the avocado’s physiology, such as floral morphology and scent were considered in conjunction with external influences, including cold stress and nutrient availability. Furthermore, practical management strategies to combat low pollination in avocado orchards were surveyed. Simple methods were used to manually count honey bee activity amongst the canopies of a range of cultivars. Honey bees appeared to favour ‘B-type’ cultivars, such as P. americana ‘Fuerte’ and ‘Zutano’. Visitations to flowers open in the female and male phases showed honey bees are more partial to functionally male flowers. This may be due to the dual reward of pollen in addition to nectar, as well as the grander floral display when the flowers are open in the male phase. Floral volatile exudation also plays an important role in pollinator attraction. Solid-phase microextraction (SPME) and thermal desorption with comprehensive gas chromatography with time-of-flight mass spectrometry (GC x GC-TOFMS) showed that depending on the avocado flowers’ cultivar and sexual phase, distinct floral volatiles are released. Flowers sampled in the actively male phase generally released a higher concentration of volatiles, which may contribute to them being preferred by honey bees. In addition to this, linalool (a volatile known for its honey bee attracting properties) was detected in very little amounts in the avocado flowers. This may explain why flowers that exude higher concentrations of linalool (like citrus, for example) are more attractive to honey bees. With these findings in mind, practical methods to encourage honey bee foraging amongst avocado flowers were investigated. Flowering lavender (Lavandula stoechas) and lemongrass (Cymbopogon citratus) essential oils were used to attract honey bees to an avocado orchard in bloom. Honey bee activity was then monitored over a period of time during the flowering season. The lemongrass lures proved effective in increasing honey bee activity, while the lavender plants increased fruit set in trees experiencing an ‘off’ production year. This highlights the importance of promoting pollination in orchards suffering from reduced flowering sites during ‘off’ years. Therefore, this study pioneered possible solutions that can be introduced into commercial avocado orchards to mitigate the problem of low pollination by honey bee vectors. However, these methods will not be effective if the pollen itself is compromised. As avocado production expands into sub-tropical and temperate regions the trees experience more cold stress. A novel pollen counting methodology was used in conjunction with light microscopy to evaluate the effects of cold temperature on pollen development at flower anthesis. There were many problems observed with the anthers and pollen, including absent and empty compartments, and malformed pollen grains. P. americana ‘Fuerte’ anthers appeared to be more adept at maintaining pollen health during cold periods. The number of pollen counted in flowers sampled during the actively female and male phases was also different, suggesting the overnight closure between the phases is sensitive to cold temperature. Considering the avocado trees used in this study are regularly exposed to temperatures below the minimum required for successful meiosis in tropical plants, it is reasonable to suggest these trees will suffer from inadequate pollen production and development. These findings highlight the need for further investigation into how pollen development in avocado trees is affected by cold stress, especially when the trees are cultivated in cooler climates. Lastly, nutrient analysis of floral material (a factor poorly understood in its relation to avocado production) was analysed using nitric acid (HNO₃) digestions and inductively coupled plasma (ICP) analytical methods. The results showed how flowers accumulate varying amounts of potassium, boron, phosphorous, and calcium based on their cultivar, sexual phase, and sampling location. Furthermore, there was a differential accumulation of nutrients between the reproductive and vegetative growth points. These findings show how the critical reproductive sites have a greater ‘pull’ for nutrients. This was the first known attempt to investigate avocado flower nutrition and consider the impact thereof on pollination. While the direct role of many of these nutrients on pollen development in avocados is not well known, other reports claim these nutrients can protect against the effects of cold stress, improve overall pollen health, and even repel honey bees when present in high concentrations. Ultimately, this dissertation highlighted the importance of applying holistic investigative methods to explore the notion of low pollination in avocados. This study found that many factors, including honey bee preferences for certain flowers based on sexual phase and scent, in addition to external factors such as cold stress and nutrient availability, may play interconnected roles that limit pollination and fruit set in avocados. Furthermore, the possibility of improving pollination in avocado orchards by using attractive measures was highlighted. Therefore, this study was successful in evaluating the causes for sub-optimal pollination in avocados and potential solutions thereof. However, as is the case with scientific investigation, this study also uncovered additional facets that require deeper understanding. Future research should thus focus on long-term studies in commercial orchards to evaluate the roles played by alternate bearing, drought, and insecticide sprays on pollinator activity. In addition, the influence of cold stress and nutrient deficiencies on specific physiological processes such as pollen production must be investigated. This will also require the development of floral nutrient statuses, which can be achieved during long-term studies.