Dennis, Frank G. and Neilsen, John C. 1999. Physiological Factors Affecting Biennial Bearing in Tree Fruit: The Role of Seeds in Apple. HortTechnology 9(3): 317-322.
This article, rather than a publication of new research, is a review of the existing research to date on the biennial bearing patterns of apples and pears. Research has been aimed at discovering why some varieties exhibit biennial, or every other year, bearing: during “on” years, there is a full fruit crop but floral initiation for the following growing season is inhibited; during “off” years, there is no fruit crop as a result of the previous season’s failure to initiate flowers, but floral initiation proceeds normally during the current year for a full fruit crop the following year, and so on. The idea that flowering inhibition is a result of nutrient partitioning to developing fruit rather than initiating flowers is set aside in favor of two hypotheses that link developing seed-bearing fruit to inhibition of floral initiation.
Chronologically, the referenced research is as follows: in 1918, Kraus and Kraybill’s work with carbohydrates and nitrogen concentrations in Lycopersicon suggested that nutrient aviailability was a factor in flowering, and subsequent studies probed this hypothesis with inconclusive results. Roberts ‘1920 study suggested that “flowering is greatest when shoot length is intermediate,” which was untrue for strongly biennial varieties. Chan and Cain (1967) showed that it was the developing seeded fruit that was inhibitory towards floral initiation unless the flowers/fruit were removed within a few weeks of bloom. In 1996, bourse shoot length was revisited and the Michigan grown, commercial, biennial and seeded ‘Paulared’ was evaluated. The authors reported inconsistent effects of shoot length on flowering, and in this particular variety, floral inhibition was also not consistently correlated with developing fruit. However, it is accepted that in general, parthenocarpic varieties exhibit floral initiation inhibition in the presence of seeded fruit. Two hypotheses roughly fit these existing data and are good candidates for further study: the export of biochemical inhibitors from developing fruit, and priority for florigen, the hypothetical floral promoting hormone.
For the first hypothesis, Luckwill (1969) demonstrated that “seeds contain[ed] relatively high concentrations of gibberellins,” and he proposed diffusion of GAs into the bourse shoot, which was supported by higher concentrations in the bourse shoot tissue adjacent to developing seed. This would explain the apparent relationship advanced by Roberts between floral initiation and shoot length (in some varieties). Though this conclusion is widely accepted, it is not equivocally supported by subsequent research. In fact, there is a disconnect between increased GA concentration in shoots and unexpected lower concentration of GA in developing seeds. In some cases, the GA exudates in the bourse shoot were in another form of GA (GA4) which is sometimes floral inducing. How GA is distributed or diffused between seeds and bourse tissue remains unclear even after studies where GA was radiolabeled for tracking. Green (1987) showed that less than 0.01% of labeled GA was transported, even when accompanied by hypothetical biochemical precursors. Subsequent studies produced similarly small percentages of transport, though there was relatively more GA diffusion from seeded than unseeded varieties, and GA4 levels were found to be lesser during floral initiation.
Additional factors may play significant roles. Callejas and Bangerth (1997) suggested that IAA might be a “secondary messenger in the inhibition of flowering,” but no appreciable differences in IAA levels in tissues were demonstrated between annual and biennial varieties. Additionally, naphthaleneacetic acid (NAA), a thinning agent that also has been shown to stimulate flowering, could possibly produce results that mimic floral initiation at fruit removal. Neither substance’s role is well understood at this time.
The second hypothesis is based on Ryogo’s hypothesis that “seeds have priority for florigen,” referring to the hypothetical hormone thought to promote flower induction (1988). In this scenario, instead of carbohydrate partitioninig or prioritizing, the substance that reproductive tissues might be vying for is florigen, which is thought to be produced by leaves and available relative to leaf area. Adequately long bourse shoot length and adequate leaf area could diffuse enough florigen to make flower initiation possible even with adjacent development of seeded fruit, which fits with Roberts’ findings. The authors also note that this hypothesis “might explain the failure of seeded ‘Bartlett’ pear fruit to prevent flowering in California, where water and sunlight are abundant and growth is vigorous.” This supports the idea that increased leaf area would result in increased concentrations of its assimilate, florigen – enough for seeded fruit development and floral initiation to be concurrent. Substituting “florigen” for “nutrients” in the previous hypothesis of carbohydrate partitioning (Kraus & Kraybill) would corroborate those findings.
At this stage of researching the physiology of biennial bearing, it isn’t clear what the research question is or what mechanism or substance is being sought. In discussing the inhibitor export theory and IAA/NAA effects, the authors acknowledge that inconclusivity may reflect the limits of technology at the time of these referenced studies. Possibly none of the existing scenarios is a good approximation. Luckwill has posited that a balance of phytohormones involving cytokinins may be play a role, as might antagonistic reactions between substances. Practically speaking, complex and as yet understood chemical balances and relationships are certainly germane. Additionally, this article does not discuss other factors outside the scope of completed research, such as why some trees can be induced to biennial bearing after a hard frost. There have been insufficient comparisons among species and cultivars, an aspect of the research that may have to wait until a firmer hypothesis is in place.
In conclusion, what is known with some confidence is that “seeded fruit of biennial cultivars of apple and pear [does] inhibit flowering, whereas seedless [fruit] do not,” and for now, the “effect of the seeds declines as bourse shoot length and/or leaf surface per bourse increases.” This article is a good starting off point for future research, provided inquirers agreed with these hypotheses, but it also shows that previous studies attempting to answer the same question have addressed it in very different ways. As technology and funding are available, it might be possible to answer the questions of inhibitor export and florigen priority.