Exploring plant growth regulator use in new pearsResearch & Extension
Researchers at the Horticulture Centre of Excellence in Victoria are exploring the effects of using plant growth regulators (PGRs) in new pear cultivars on bud development, fruit set and lateral shoot growth.
While there are a number of PGRs used in Australia, most are not registered on pears. We wanted to further understand the effects of PGRs to see if there was potential for any of them in pears, and specifically on the new red-blushed pear cultivar ANP-0131, which will be marketed as Deliza®. This work was part of the PIPS Orchard Productivity Program, a strategic levy investment under the Hort Innovation Apple and Pear Fund.
About plant growth regulators
Plant growth regulators (PGRs) are widely used to control vegetative and reproductive growth in commercial fruit production. Chemical and hormonal PGRs can advance the transition of buds from vegetative to reproductive growth and alter the proportion of bud types to maximise fruit quality and productivity.
PGRs can also promote lateral shoot development in young trees, slow shoot growth in more vigorous cultivars and improve fruit set to overcome issues with low chill or poor pollination. Conversely, different PGRs can act as flower thinners to reduce crop loads and improve fruit size.
PGRs use on pears
Most recent PGR research in pear production has focused on breaking dormancy, reducing shoot growth and improving fruit set. Research has shown that timing, rate and cultivar are important factors in pear responses to PGRs.
As new pear cultivars get more widely planted and markets for them develop, growers will want advice and help on how to best manage them and PGRs could be part of their ‘toolbox’.
We were able to test a number of PGRs in pears under the APVMA’s Permit PER7250, which allows suitably qualified persons to undertake small scale research trials on unregistered products.
From a grower perspective it must be remembered that the on-label use of agricultural chemicals is mandatory in most states across Australia. However, in Victoria, it is legal to use chemicals, other than ‘restricted or prescribed use’ chemicals, off-label providing that:
- The maximum label rate is not exceeded.
- The label frequency of application is not exceeded.
- The maximum residue limit is not exceeded.
- Any specific label statements prohibiting the use are complied with.
As a result we want to emphasize that the findings in this report should not be considered as recommendations as method and rate of application may affect PGR efficacy.
Below is a brief summary of the effects of six common PGRs used in fruit production.
Maxcel is a cytokinin. Cytokinins applied to dormant buds can induce growth. Maxcel® has been shown to reduce fruit set and increase fruit size in apple.
Cytolin is a combination of gibberellins and cytokinin. Research has shown that combination of PGRs can be more effective in modifying plant growth than individual growth regulators. Cytokinin is used to stimulate cell division whilst gibberellin stimulates cell expansion. Cytolin® is also used to manipulate fruit shape in apple.
Dormex® can be used to advance foliation and flowering and reduce the period of flower bloom. It is useful in seasons or locations where there is insufficient chill to promote good bud break and flowering.
ProGibb® is a gibberellin (GA). The ability of GAs to promote fruit set in pears by stimulation of parthenocarpic fruit development (i.e. seedless fruit) has been known since the early 1960s. However, dependent upon the cultivar and rate and timing of GA applied, GA can affect the amount of return bloom, increase fruit drop and cause fruit malformation.
Regalis® is an inhibitor of gibberellin biosynthesis, and is effective at slowing shoot growth. It also reduces the production of ethylene, which can be used to reduce fruitlet abscission and thus increase the amount of fruit set.
ReTain® inhibits the production of ethylene in plant tissues. Ethylene affects plant processes such as fruit maturation, ripening, and pre-harvest fruit drop. Inhibiting ethylene production with ReTain® can help manage fruit maturation in the orchard, reduce pre-harvest fruit drop, delay harvest (which can enhance fruit size), maintain or increase fruit firmness and enhance storage potential.
Results of PGR experiments in pears
PGRs to promote shoots on young trees
We conducted an experiment to investigate the effects of several PGRs on the promotion of side lateral development on two-year old ANP-0131 pear trees on D6 rootstock. The PGRs were applied at three different times during the growing season in the lower half of the tree where there was minimal shoot development.
Treatments included a control, Cytolin, Maxcel, notching, Cytolin plus notching, and Maxcel plus notching. The application rate of PGRs was equivalent to 475 ppm (i.e. 25 ml/L). The treatments were applied using a one litre spray bottle with the addition of a non-ionic wetter (ai. 1000 g/L polysorbate 20) on three occasions:
- Date 1: 25 September at first flower (slight leaf in upper half of tree).
- Date 2: 10 October at slight leaf in the upper and lower half of the tree.
- Date 3: 31 October at full leaf emergence.
There were six replicates (one tree per replicate) of two-leader upright trees on D6 rootstock planted to an Open Tatura Trellis at 0.5 m spacing. The treatments were assessed eight weeks after each application for the number of extinct (or latent) buds, active buds, spurs (1 – 2.5 cm), sprigs (2.5 – 5 cm), short shoots (5 – 20 cm) and long shoots (> 20 cm).
The most effective treatment for the promotion of long shoot development was the combination of Maxcel plus notching and Cytolin plus notching, with Date 2 being the most effective timing of an application.
For the first two application dates, the PGR and notch treatments were more effective at promoting shoot development than PGR alone, whilst the effect of PGR alone and notching alone were similar. This suggests that notching aides the effectiveness of PGR application.
The effectiveness of PGRs appears to be related to the stage of leaf emergence from the buds with slight leaf emergence along the entire trunk or leader providing the optimum time for PGRs in combination with notching to stimulate spur and lateral shoot growth.
PGRs and spur development
Whilst the PGR treatments were less effective at stimulating shoots on Date 3, they still stimulated more spurs than earlier applications.
On the first two spray dates, there were more terminal flower clusters on the shoots due to notching and PGR or both. There were less spur flower clusters on the PGR and PGR plus notch treatments on date 1 and 2 than the control, which was likely due to fewer spurs and more shoots. There was no difference in total flower cluster between treatments, partly attributed to a large variation in total number of flower clusters in each treatment/date combination, which is not unusual for young trees in their first cropping year.
ProGibb on mature trees
We also conducted another experiment to investigate the effect of ProGibb on the promotion of fruit set on five-year old ANP-0131 trees on BP1 rootstock.
Treatments included a control and 37.5 mg/L (37.5 ppm) of ProGibb applied at 90 per cent bloom on five replicates (one tree per replicate). Trees were planted on Open Tatura Trellis at 3,333 trees/ha. The PGR spray was applied to run-off (1.4 L/tree) on 25 September.
We recorded smaller fruit on the ProGibb treatment compared to the control, higher yield, and higher fruit number per tree. However, the next season’s crop was dramatically reduced with significantly lower numbers of flower clusters on the ProGibb treatment compared to the control.
Similarly, in Conference pears, phytohormone treatments that induce parthenocarpic fruit (i.e. seedless fruit) have up to 12 per cent smaller fruit than fruit produced from cross-pollination with compatible pollen. In Packham’s Triumph pears, both 100 and 200 mg/L of AVG (ReTain®) applied two weeks after full bloom increased fruit set and yield but fruit size was reduced.
ProGibb applied at 90 per cent full bloom promoted fruit set with a higher number of fruit and greater yield at harvest on treated trees than on the control.
ProGibb did not affect fruit shape or number of seed, but appeared to adversely affect the next season’s crop with a large reduction in the number of flower clusters.
PGRs will continue to be used in pear orchards to improve tree architecture, fruit bud formation and fruit set. Research at Tatura is continuing with respect to improving fruit set in pears without impacting on fruit quality and the following season’s crop.
This project was funded through the Productivity Irrigation Pests and Soils (PIPS) program by Hort Innovation using the apple and pear levy and matching funds from the Australian Government with co-investment from the Victoria’s Department of Economic Development, Jobs, Transport and Resources. The technical assistance of David Cornwall and Wendy Sessions was greatly appreciated.
About the authors
Susanna Turpin, Lexie McClymont and Ian Goodwin are all from the Horticulture Centre of Excellence, Tatura. For more information contact Lexie McClymont on 03 5833 5222 or firstname.lastname@example.org.