Researchers have for more than 30 years sought to improve the fruit set and retention of apples, using sprays of various plant growth regulating chemicals. The principal ones tested have been
- Auxins
- Auxin transport inhibitors
- Gibberellins
- Gibberellin biosynthesis inhibitors
- Cytokinins
- Mixtures of auxins, gibberellins and cytokinins
- Ethylene biosynthesis inhibitors
- Polyamines
Auxins
Attempts to improve apple pollination and fruit set with sprays of auxins, such as IAA and NAA have proved ineffective.
- However, when combined with gibberellins (GA3 or GA4+7), auxins such as NAA, NAAm and 2,4,5-TP have proved effective in increasing fruit set (see below).
- Although auxins such as IAA are strongly implicated in the processes associated with fruit set and retention, sprays of IAA are largely ineffective in improving fruit set.
- One of the contributory reasons for this may be the high instability of IAA, when dissolved in aqueous solutions and sprayed onto trees.
Auxin transport inhibitors
Attempts by researchers at Long Ashton Research Station to increase fruit set on several tip or spur bearing apple varieties, using sprays of 2,3,5-tri-iodobenzoic acid (TIBA) at 150 ppm, were unsuccessful.
- The only effect of the sprays was to reduce fruit size at harvest.
Gibberellins
Gibberellins have been shown to be an essential hormone if parthenocarpic fruit set is to be induced in apple (Schwabe and Mills, 1981).
- However, they are largely ineffective used alone on varieties such as Golden Delicious, Jonagold, Cox’s Orange Pippin and Boskoop (Wertheim, 1993; Varga, 1966).
- Research in Germany over a period of 10 years showed that sprays of either GA3 or GA4+7 to frost-affected flowers had no significant effect on fruit set (Bangerth and Schröder, 1994).
- Studies conducted in Canada (Looney et al., 1985) suggested that whilst GA3 and GA7 had inhibitory effects on apple flower initiation, GA4 had a stimulatory effect.
- These results could not be repeated, however, in German experiments (Bangerth and Schröder, 1994).
Gibberellin biosynthesis inhibitors
Trials with daminozide (SADH or Alar) in the early 1970s showed that, not only did the sprays increase flowering on trees of Discovery in the year after spraying, they also stimulated pollen tube growth in the styles of the flowers produced (Stott et al., 1973; Stott, et al., 1974).
- Alar is no longer available for use on fruit trees.
Cytokinins
- Treatments of apple flowers with cytokinins on their own do not induce fruit set.
- However, in combination with gibberellins stable cytokinin-type compounds such as diphenylurea or CPPU can have beneficial effects on fruit set and yields.
Mixtures of auxins, gibberellins and cytokinins
Research conducted in the 1960s in the USA (Williams and Letham, 1969) first showed that combinations of gibberellins with cytokinins might have promise as setting agents for apples.
- The ‘Wye Mixture’, developed by researchers based at Wye College in Kent, enjoyed some popularity in the 1970s and early 1980s as a fruit setting spray for use with apple varieties in the UK (Kotob and Schwabe, 1971; Goldwin, 1978; Goldwin, 1981).
- Goldwin and colleagues demonstrated the feasibility of inducing parthenocarpic (seedless) fruit set in the apple variety Cox’s Orange Pippin. It had been known for many years that low concentration, high volume sprays of gibberellic acid GA3 could have a very beneficial effect on pear varieties such as Conference.
- Applied following severe frost-damage to flowers, the sprays stimulated the production of a crop of parthenocarpic fruits.
- Trials on apples using GA3 sprays with no other hormones added were not successful, however, when applied after frost damage.
The work conducted at Wye College demonstrated that, if the GA3 (200 ppm) was supplemented by an auxin and a ‘cytokinin’, the spray mixture was capable of inducing good fruit set.
- Various auxins were tested including 1-naphthaleneacetic acid (NAA), 2‑naphthoxyacetic acid (NOXA at 50 ppm) and 2,4,5-trichlorophenoxy propionic acid (2,4,5‑TP at 10 ppm) and although there were small variations in response all seemed to work well with the other two components of the mixtures.
- The ‘cytokinin’ used was not a true cytokinin, but a product known to have cytokinin type activity in plants, namely NN’‑diphenylurea (DPU at 300 ppm).
Trials in Germany, however, (Bangerth and Schröder, 1994) showed no consistent benefits when using this Wye Mixture on varieties such as Golden Delicious, Jonagold and Boskoop.
Trials in the USA (Greene, 1980) have also shown that applications of gibberellins plus 6‑benzylaminopurine (a cytokinin) caused reduced rather than improved fruit set and this was attributed to increased ethylene levels induced in the flowers by the sprays.
Blasco et al., (1982) showed that whilst sprays of the Wye Mixture increased fruit set of Cox trees on M.9 rootstock, no similar effects were recorded on trees on M.26, M.7 and MM.106 rootstocks.
- However, initial fruit set was increased on trees on all rootstocks, irrespective of their vigour.
- It is hypothesised that the sprays facilitate the retention of fruitlets, which have initially set with very few seeds.
- Later in the season, when competition for assimilates with extension and bourse shoots increases, these fruits are shed from the trees on the more invigorating rootstocks.
- On M.9, where shoot competition is less, they are retained.
- Intuitively, if this hypothesis is correct, one would expect treatment with chemicals which reduce excessive shoot growth to aid the retention of hormone-stimulated fruitlets on trees grafted on invigorating rootstocks.
- However, as most of the growth retardant chemicals currently available reduce natural gibberellins in the tree (e.g. Cultar and Regalis, they are likely to partially negate the value of the gibberellins in the setting mixtures.
According to Bangerth and Schröder (1994), the best effects are achieved with cytokinins which are biologically stable such as the phenyl urea type and natural cytokinins such as zeatin-riboside give poorer results.
- In these trials, the best cytokinin type compound was N‑(2-chloro-4-pyridyl)-N-phenylurea (CPPU) at 20 ppm.
- The best combinations were of GA4 or GA7 (100 ppm) together with CPPU; GA3 and GA5 gave poorer results.
- It is hypothesised that the effects are brought about by the hormones’ direct action on the fruit ovary and not by any changes in fruit ‘sink strength’.
- It is possible that a short hormone pulse, provided by the treatments, establishes hormone autonomy in the unfertilised ovary (Browning, 1989).
- The addition of the cytokinin may enable this hormone autonomy to be sustained through until harvest.
- It is interesting that fruits set using this mixture are not susceptible to abscission induced by seeded fruits or strong shoot growth close by.
- In other crops such as kiwi fruits, CPPU causes a reduction in the natural levels of the inhibitor ABA.
- Unfortunately, the GA + CPPU mixtures used in the trials described above reduced flowering significantly in the subsequent season.
- Also, the CPPU in these mixtures causes russeting and flattening of the fruit shape. Because of this and because registration may prove difficult, it is unlikely that these gibberellin CPPU mixtures will become available to growers in the near future.
One recent suggestion is that neither the gibberellins nor the cytokinins have a direct inhibitory effect on flower initiation, but it is the increased fruit set induced by them that inhibits subsequent flowering (Bangerth and Schröder, 1994).
Ethylene biosynthesis inhibitors
Ethylene is involved in the abscission of flowers and young fruitlets and application of chemicals that inhibit ethylene production by the apple tree can sometimes reduce flower and fruitlet abscission.
- Aminoethoxyvinylglycine (AVG) has been reported to inhibit both ethylene production (Baker et al., 1978) and its action (Beyer, 1976); AVG inhibits the enzyme system that converts S-adenosylmethionine to 1-aminocyclopropane-1-carboxylic acid.
- This same enzyme system is also influenced by the endogenous levels of auxin (IAA) in the fruits and spurs.
Sprays of 200 ppm AVG applied at bloom time to Red Delicious apple trees increased fruit set but reduced fruit size and increased the length diameter ratio of fruits at harvest time (Greene, 1980).
- In further studies Greene (1983a) showed that sprays at 500 ppm pre harvest increased the initial but not the final set in the subsequent season of McIntosh, Spartan and Spencer apple trees grown in Massachusetts.
- Two clones of the variety Red Delicious also responded similarly, but here the treatments increased final as well as initial set.
- However, final fruit sizes were reduced by the treatments (10% to 20%) and the length diameter ratios of the fruits were increased.
- Applications of fruit thinners were shown to be ineffective in reversing the negative influence of AVG on fruit sizes (Greene, 1983b).
- It has been argued that the increase in fruit set achieved with the autumn AVG treatments is due to reduced post petal fall abscission of very young fruitlets (Williams, 1981).
- Williams also showed that the AVG treatments suppressed ethylene production in the leaf buds and induced increases in bud break vegetative growth.
Trials conducted in the early 1980s at Long Ashton Research Station (Child et al., 1986) showed that sprays of AVG at 250 ppm applied at full bloom increased the initial and final fruit set of Cox’s Orange Pippin apple trees.
- The AVG inhibited the ethylene produced following treatment with gibberellins but had no effect on the ethylene induced by treatments with the thinner NAA.
Other known inhibitors of ethylene production by the tree, such as aminooxyacetic acid and silver nitrate have generally proved ineffective in increasing apple fruit set (Dennis et al., 1983; Rahemi, et al., 1997).
Recent studies by Rahemi, et al., (1997) led the authors to suggest that differences in ethylene evolution are not responsible for differences in initial fruit set.
- They argued that the effects of AVG in increasing set are independent of its effects on ethylene evolution.
- In earlier work, UK scientists had suggested that the AVG sprays delayed the senescence of ovules and increased the setting capacity of older flowers (Child et al., 1982).
Polyamines
Sprays of the aliphatic polyamines, spermine, spermidine and putrescine, have been shown to increase the retention of very young fruits on apple trees (Costa and Bagni, 1983).
- However trials conducted at HRI-East Malling gave diappointing results with these spray treatments (Knight, personal communication).