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.