Avocados in cool climates
Avocados, while essentially of semi-tropical origin, are adaptable to a range of climatic conditions. The variety Hass has demonstrated this capacity by being successfully grown from subtropical conditions in North Queensland to cool Mediterranean conditions in the South-West of Western Australia.
Growing avocados commercially outside of their optimum growing conditions, such as under cool Mediterranean conditions, presents a range of issues for commercial growers. Some include the temperature impact on flowering, pollination, fruit set and carbohydrate partitioning. Under cooler conditions, the avocado will also progressively tend towards more biennial bearing with corresponding fluctuating levels of spring leaf drop, root flushes and shoot flushes.
The overall yields of Hass avocados in the South-West of Western Australia are not as consistently high as growers would like. In 2006/07, the overall WA state avocado yield was just over 10 tonnes per hectare (Cutting 2007). However, growers consistently comment about achieving much higher yields for at least part of their orchards. The challenge is to identify the causes of low yields and to develop strategies to improve yields on a consistent yearly basis.
There is a range of theories as to why growers are struggling to consistently produce high yields from Hass avocados in cool Mediterranean conditions:
- poor pollination
- insufficient flowers
- insufficient carbohydrate production
- shoot growth competition with flowers.
Poor pollination and fruit set
Availability of viable pollen
Poor pollination can be the result of several issues, including lack of pollen, pollen transfer or pollen viability. Weather conditions during flowering have a significant effect.
Flowering in avocados is well documented, and the nature of the flowering pattern of type A and B flowering varieties generally well understood. Under ideal flowering temperature conditions of 25/20°C maximum/minimum, (Sedgley and Annells 1981) the A/B flowering patterns are quite typical which result in excellent cross-over of functionally male and female flowers between the A and B types.
Under these ideal conditions, the variety Hass (type A) can also have a short cross-over of functionally male and female flowers during the middle of the day within a single stand of Hass plantings (Figure 1). In this case, the functionally female stage remains open for a short period during the early stage of the functionally male stage, allowing for close-pollination (pollen from flowers of the same or nearby trees of the same variety) and satisfactory fruit set. As a result, many orchards have been planted as a monoculture of Hass without inter-planting a complementary type B variety.
Under cooler conditions, the flowering story becomes more complicated. Extensive investigations into the effect of temperature on flowering of avocados have been carried out (Ish-Am and Eisikowitch 1991, Sedgley and Annells 1981, Sedgley and Grant 1933, Sedgley and Alexander 1983). Cold temperatures alter the flowering cycle by delaying the normal opening and closing routine of the avocado flower, extending the overall period of flowering, delaying the release of pollen, slowing pollen tube growth and reducing the number of flowers open on a given day.
The impact of this for a single variety Hass orchard is that the expected period with both functionally male and female flowers open may be severely reduced, or eliminated altogether, thus reducing the possible period of close-pollination and reducing fruit set.
Investigations by Sedgley and Annells (1981) on Hass were under controlled and sustained temperature regimes. Under natural conditions, there are daily fluctuations in night and day temperatures during flowering resulting in regular changes in the floral sequence. As a result, there will be periods when pollination is much more likely than others.
The temperature effects and low average yields of Hass have led growers to reconsider the use of a suitable cross-polliniser variety to ensure sufficient pollen is available for pollination. However, the temperature story does not stop with Hass. Research by Sedgley and Grant (1983) into other varieties, both type A and B, has shown similar effects to different temperature regimes, though with some differences between varieties.
The delaying effect to type B varieties is so pronounced at low temperatures that the functionally female stage was often not recorded. This has a dual effect:
- If there are few functionally female flowers then the cropping potential of the type B varieties will be severely affected.
- Delaying of flower opening has been recorded to result in the peak pollen release period occurring during the night (Sedgley and Annells 1981).
Therefore, as the temperature regime is affecting the flowering cycle of flowers of both type A and B varieties, then the choice of a suitable cross-polliniser is not a simple case of choosing a known complementary flowering type. You will need to investigate the effect of the local temperature conditions on both varieties to see if they are indeed complementary. A single year of observations in 2009 in the South-West of Western Australia demonstrated similar results to those reported in the literature — see ‘Cross-pollinisers for Hass avocado’ for more details.
Transfer of pollen
Viable pollen being available for transfer to a receptive stigma is just the first step in achieving effective pollination. The pollen must actually be transferred to the stigma, while it is still receptive, by some means and in sufficient quantity to result in the desired level of fruit set.
The more common view is that pollen transfer requires intervention of a small pollen vector, or pollinator. Most commonly discussed is the honey bee (Ish Am and Eisikowitch 1993), but a range of other vectors has also been monitored (Ish-am et al. 1999). An alternate view has been discussed by Davenport (1989, 2003 and 2007) whereby pollen is transferred by wind, both in the form of close-pollination and cross-pollination (pollen from flowers of nearby trees of a different variety).
Davenport (2007) has questioned the belief that pollination can only occur during the first stage of flower opening — that is, during the functionally female stage. That pollination can also happen during the second opening period, or functionally male stage. This again raises the question about the need for cross-polliniser varieties. Equally, there has been research showing proximity effects on yield of Hass with various cross-polliniser varieties when planted within an orchard (Fetcher et al. 2001).
Another complication with pollen transfer is rain or moisture, often associated with cooler weather during the flowering period in the South-West of Western Australia (Figure 2).
If we are to promote the benefits of honey bees to be our main pollinator, then we also need to consider the impact of cold or wet weather on bee activity.
A cold and wet winter will reduce the number and health of native bee populations. Growers don’t have to rely on native populations and can introduce healthy hives into their orchards at flowering time. But even healthy hives will be less active in cold or wet conditions. Bees prefer day temperatures above17°C (Dixon et al. 2007). Wet conditions are also not favoured. It is also possible rain may wash pollen from flowers, thus reducing the amount of pollen available.
Bees are attracted to the best and most accessible source of pollen and nectar. Cold weather will reduce the number of flowers open on an avocado tree (Sedgley and Annells 1981) and can delay the release of pollen. Plus the closing nature of avocado flowers between the independent functional stages may make avocado flowers less attractive to bees (Figure 3). The independent functional stages also add complication as bees often set out to collect either pollen or nectar. To do this they may actively seek out either functionally male or female flowers (Ish Am and Eisikowitch 1993). This reduces the chance of depositing pollen onto the stigma of a functionally female flower.
Focus has been on the European honey bee as the main pollinator of avocados in the South-West of Western Australia, but a range of other insects visit flowers including hover flies (Figure 4), native bees and common flies. There is little information as to the benefit, if any, these visitors provide in pollen transfer.
Temperature and pollen growth
Temperature not only affects the actual flowering cycle, but also affects pollen germination, the speed of the pollen tube growth as well the embryo growth rate (Sedgley and Annells 1981). However, while a lower temperature regime of 17/12°C (maximum/minimum) did slow the speed of all this, it did not prevent fertilisation or ovule growth, which still occurred within 6–24 hours.
Zamet (1990) suggested that 10°C was a critical temperature for avocados in Israel. That is, a period of five to six days with a minimum above 10°C increased the chance of a fruitlet not abscising. He found that the lower the number of favourable fruit set temperature periods during the flowering period, the lower the average yield per hectare.
In New Zealand, they tested an effective pollination period of two consecutive days of maximum temperature above 17°C and a minimum temperature above 11°C, but only demonstrated a very weak correlation between this and fruit set (Dixon et al. 2007).
Summary
It appears logical to accept that the cool and wet conditions experienced during avocado flowering are having an impact on the production capacity of Hass avocados. However, further investigations are warranted to accurately quantify these effects to assist in providing the best options for improving pollination.
The use of cross-pollination to take advantage of potentially limited good pollination events is one method of improving pollination and fruit set. However, it is not just a simple case of inter-planting with a known type B variety. You must select a variety that will consistently flower at the same time and in a truly complementary manner.
There is a possibility that even with significant cross-over of male and female flowers, from the use of a polliniser, that the climatic conditions are still not favourable for effective pollination due to effect of the climate on pollen growth or bee activity. Also, are European honey bees the only significant pollinators of avocados in the South-West region of Western Australia? All these issues require further investigation.
Producing sufficient flowers
All the research into flowering and pollination accepts the basic belief that avocado trees are prolific flowerers, but are they always? A wander through most avocado orchards during flowering, particularly ones displaying alternate bearing problems, will display a great array of flowering intensity – from massive flowering to virtually none on different trees and in some cases different major limbs within a single tree.
The first aspect is to consider what triggers flower initiation. The general consensus is that a period of low temperature (below 20°C) and short day length (less than 10 hours) is required to initiate the transition from vegetative bud to floral bud (Buttrose and Alexander 1978, Nevin and Lovatt 1990, Salazar-Garcia et al. 2006).
The term ‘irreversible commitment to flowering’ is used to describe the time when the apical bud becomes committed to reproductive growth. Generally, this is achieved after the accumulation of about 28 days of conditions suitable for flower initiation (Salazar-Garcia and Lovatt 2002, Salazar-Garcia et al. 2006).
The statement of ‘irreversible commitment to flowering’ can be a little misleading. It implies that once achieved, the bud will continue to develop as a floral bud regardless of conditions. However, certain events can arrest further development of the floral bud. For example, a moderate frost event on 17 June 2006 resulted in what appeared to be significant damage to buds (Figure 5), even on shoots with only minor leaf burn.
Based on the requirement of 28 days below 20°C and the temperature conditions normally experienced in the South-West, it could be anticipated that irreversible commitment to flowering had occurred prior to the frost. However, after the frost event the majority of buds that would have been expected to flower actually developed into vegetative growth in the following spring, with generally only a few weak late flowers. Therefore it would seem that the period of extreme cold temperature had either damaged the developing flowers or almost totally inhibited their further development while promoting vegetative growth.
The application of gibberellic acid (GA3) to avocados has been shown to prevent further initiation of floral buds (Salazar-Garcia and Lovatt 1996). When applied during the floral initiation period it results in those flowers already initiated continuing to develop, but stops further initiation of new flowers.
Given an equal set of conditions, a heavy flowering tree has a greater chance of a heavier fruit set than a lighter flowering tree. This was demonstrated over several years in New Zealand with an observable relationship shown between the number of open flowers recorded and fruit set (Dixon et al. 2008).
Avocado flowers are borne on new season growth, that is, shoots produced during the previous season’s vegetative flush. Therefore, growth of shoots is required to produce buds that can develop into flowers. In the South-West of Western Australia, three vegetative flushes are normally observed – a spring flush, summer flush and autumn flush, similar to New Zealand (Dixon et al. 2008).
Flowers can develop on any of the flushes, but the spring flush reportedly provides the greatest contribution in Mexico (Salazar-Garcia et al. 2006) and New Zealand (Cutting 2003). These were both in minimally irrigated orchards that resulted in a strong spring flush — that is, a greater number of shoots produced per branch, compared to later flushes.
Salazar-Garcia et al. (1998, 2006) observed that under Californian and Mexican conditions, crop load did not have a significant impact on the number of shoots produced. The percentage of floral to vegetative shoots produced from these shoots the following flowering period was affected in California but not in Mexico. In California, the ratio of inflorescences to vegetative shoots was significantly higher after a light crop as compared to a heavy crop. What was not reported was the length of the shoots produced or the total number of flowers produced as a result of differing crop load. However, Salazar-Garcia et al. (1998) reported that in California the return flowering after a heavy crop was less intensive than after a light crop.
Dixon et al. (2008b) estimated that a shoot producing at least six panicles gave the best initial fruit set. This was estimated to be a shoot of about 150 to 200mm long. Dixon also noted that in a heavy flowering year, there was a higher percentage of initial fruit set per inflorescence than in a light flowering year. Unfortunately, what was not reported was the total number of shoots produced in each year, to determine the impact of crop load on the total number of flowers, rather than just the impact on the individual shoots.
The type of inflorescence is also reported to impact on the level of fruit set, with determinate inflorescences (Figure 6) setting a higher number of fruit per inflorescence than indeterminate, Figure 7 (Salazar-Garcia and Lovatt 1998, Dixon et al. 2008). Dixon et al. (2008) also found that in New Zealand in a heavy flowering year (on tree carrying a light crop), the proportion of determinate inflorescences to indeterminate was higher. However, Salazar-Garcia and Lovatt (1998) found the opposite with a higher percentage of indeterminate inflorescences produced.
As the size of the crop load can influence the intensity of flowering which can influence fruit set, it has been considered that manipulating the intensity of flowering can also manipulate the size of the crop. Manipulation of flower intensity can be achieved by removal of flowers. This can be achieved by mechanical removal of flowers (flower pruning) or by chemical thinning.
One method of chemical flower thinning is using GA3. The timing of application has been investigated by Salazar-Garcia and Lovatt (1996, 1998) as a tool to manipulate the intensity of flowering. Dependent on the timing, it could result in no flowering, reduced flower intensity, a change in the ratio of determinate and indeterminate inflorescences, or advanced vegetative flush.
Shoot growth and carbohydrates
As discussed, there is a relationship between flowering intensity and fruit set, and an implied relationship between shoot growth and flowering potential. It therefore suggests that by increasing shoot growth you should be able to increase potential for fruit set. But what is controlling the level of shoot growth from year to year and can this be manipulated?
Stored carbohydrate levels in the major limbs of the avocado tree just prior to flowering are considered to be an important indicator of flowering potential. Cyclic patterns of accumulation and depletion of carbohydrates have been recorded by Scholefield et al. (1985), Whiley et al. (1996) and Dixon et al. (2005). Low starch levels are recorded at the end of the autumn flush and highest levels just before flowering and shoot flush in spring.
Whiley et al. demonstrated a reasonable relationship between increasing trunk starch concentration in July and increasing yield for the next season. Therefore it is possible the starch concentration had an effect on the developing flowers and fruit set during winter and spring.
Scholefield et al. (1985) demonstrated an apparent relationship between crop load and the depletion and accumulation of starch, with a deeper depletion of starch following a light crop year and a lower accumulation of starch after a heavy crop year. This was for Fuerte grown in the Murray Valley Irrigation Area.
Whiley et al. (1996) did not find any significant relationship between heavy and light cropping years and changes in the cyclic pattern of starch accumulation on Hass in subtropical Queensland. Scholefield et al. (1985) recorded very high levels of starch accumulation of up to about 18%, while Whiley et al. (1996) and Dixon et al. (2005) recorded much lower maximum levels of 6 to 7%. These high differences could be due to differing extraction methods.
Dixon et al. (2008b) found limited effect of light or heavy flowering on the timing or duration of the subsequent vegetative spring flush. However, they did report a reduced overall level of growth of shoots and roots after heavy flowering as compared to that after a light flowering. Interestingly, they also noted an even more extreme drop in level of shoot and root growth in one year following a light flowering accompanied by a period of moisture stress. Dixon et al. (2008b) also demonstrated the link, albeit not consistently, between a heavy crop and following light flowering and vice versa.
While a relationship has been drawn between carbohydrate levels in the trunk and subsequent yield, equally it is likely that a relationship can be drawn between carbohydrate levels and the strength of the vegetative flush. Other conditions equal, a strong vegetative flush, with a higher leaf area, could be expected to result in higher accumulation of starch than a weaker vegetative flush. As a result, a strong vegetative flush should lead to a higher accumulation of starch.
Is it the heavy flowering and/or subsequent heavy crop that are potentially drawing too heavily on the plant's starch reserves to allow adequate shoot growth? The shoot growth is required for the development of sufficient inflorescences to allow for a follow up good flowering. Or is it that low carbohydrate reserves following a heavy cropping year limit the final development of the inflorescences?
Whiley et al. (1996) demonstrated that delaying the harvest of mature fruit until flowering exacerbated alternate bearing. Garner and Lovatt (2008) found that there was no relationship between crop load of the previous season and subsequent abscission of the reproductive structures during flowering. This tends to suggest that crop load is likely affecting the development of the shoots and accompanying inflorescences during the growth season rather than impacting on the actual flowering and fruit set stage. If it is the energy draw of the flowering, can this be overcome with different management strategies?
Irrigation and nutrition
Dixon et al. (2008b) noted the potential of increased water stress on reducing the vegetative flush. Therefore, ensuring adequate moisture levels are maintained during spring and summer would be one step to improving vegetative flush.
Lovatt (2001) demonstrated that selective timing of nitrogen fertiliser application can modify yields and the severity of alternate bearing. Applications during autumn, after irreversible commitment to flowering resulted in increased fruit size at harvest. While applications in spring after anthesis assisted in reducing the severity of alternate bearing.
Spring flush timing
The spring flush is also in competition for carbohydrates at flowering. The spring flush normally will start around the same time as flowering and fruit set. This competition is often considered to be one of the triggers of low fruit set. Salazar-Garcia and Lovatt (1998) have suggested that modifying the timing of the vegetative growth may offer an opportunity to reduce the level of competition at critical times.
Observations in the South-West of Western Australia have hinted that earlier flushing of vegetative growth in spring results in a stronger spring flush after a heavy flowering than if the shoot-flush is later. This could assist in a better return flowering. Salazar-Garcia and Lovatt (1998) demonstrated that application of GA3 could be used to advance the development of the vegetative flush of indeterminate inflorescences.
Small fruitlet abscission
Abscission of immature fruit immediately after flowering continues to concern growers. Growers believe this to be an even greater problem for fruit set from late-season flowers. Garner and Lovatt (2008) found that peak abscission of immature fruit occurred about one month after completion of flower abscission. But there was no indication as to what flowers set this fruit. What causes this immature fruit to abscise is still to be determined, but does not appear related to the size of the previous crop.
Genetics
It may be that the variety Hass does not have the capacity to consistently set large crops and in cooler climatic zones this leads to significant alternate bearing and that future efforts may need to consider variety improvements rather than management improvements.
Where to from here?
- Encourage careful monitoring of soil moisture management and recommend growers trial strategic timing of nitrogen application.
- The value of cross-pollinisers needs to be further investigated in the South-West of Western Australia. This should include investigation of flowering timing for pollination potential and visitation habits of pollinators to different flower stages and different trees with different flower intensity.
- Investigations into the use of GA3, to determine capacity to modify flowering intensity, timing of flowering and shoot flush. This will require better knowledge of the timing of the various stages of floral development.
- Investigate further the timing of spring vegetative flush, what may cause seasonal variations in this timing and the impact of this on flowering and fruit set. Further investigate potential options to modify the timing of the vegetative flush if shown as useful.
- Investigate the setting of fruit, what climatic conditions coincide with fruit set and determine if late setting fruit is subject to increased immature fruit drop.
Other resources
Further reviews on the issue of alternate and irregular bearing have been commissioned by Avocados Australia. The reports are available via the Avocados Australia website (member services login required).
Presentations have been delivered on pollination and fruit set and alternate and irregular bearing as part of Avocado Study Groups and Qualicado workshops. Copies presentations are available via Avocados Australia's Best Practice Resource (password login required).
Acknowledgements
Special mention must be made of Dr Cengiz Erol who greatly assisted in the collation and review of the research literature on this subject.
This publication was previously published as Appendix G in ‘Improving technology uptake in the WA avocado industry’, a final report for Horticulture Australia Limited project AV06002. This was funded by the Agricultural Produce Commission — Avocado Growers Committee, Horticulture Australia Limited and the Department of Agriculture and Food, Western Australia. The complete final report is available electronically via Avocados Australia website (member services login) or Horticulture Innovation Australia.
References
Most of the following references and many more are available electronically via avocadosource.com, an extensive internet database of avocado information from around the world — hosted by the Hofshi Foundation.
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