ReviewQuantifying key parameters as elicitors for alternate fruit bearing in cv. ‘Elstar’ apple trees
Graphical abstract
Orchard plot of yields of consecutive 210 cv. ‘Elstar’ trees (x axis) in three rows (y axis) in 2010 (top) and in 2009 (bottom) showing alternate bearing in fruit trees and random distribution of biotic bearing within the same row; colours indicate yield ranges from 0 to 10 (red-orange), 10 to 15 (yellow), 15 to 20 (green) and >20 kg/tree (blue).
Introduction
Alternate bearing, the change between large and small yields, is a research topic of growing interest in plant physiology [1], [2], [3]. Alternate bearing affects most fruit, nut and forest trees worldwide, particularly as hail and frosts increase due to recent climate change [4]. Many new cultivars exhibit this phenomenon such as apple cv. ‘Honeycrisp’, ‘Pacific Rose’ etc. The current theory is that alternate bearing is either instigated by (a) resource allocation (Satake and Iwasa [5]) or (b) abiotic stress caused by microclimatic events, such as late frost, which destroys the flowers or fruitlets in year x, followed by excessive flowering and a bumper harvest in year x + 1; all trees within an orchard are affected in the same way [6], [7].
This study draws attention to another form of alternate bearing – a phenomenon that affects trees or groups of trees within the same tree row in the same year. Since this inherent alternate bearing seems to be of biotic origin, which may well be accelerated by loss of light available to the crop, the current increase in hail nets over fruit crops may enhance this phenomenon [8] possibly by adversely affecting flower bud initiation [2].
The apple (Malus domestica Borkh.) cv. ‘Elstar’ is a prime example that exhibits this biotic alternate bearing phenomenon. This major variety is cultivated on 12,000 ha with 1.6 M tonnes in Europe and high regional consumer affinity [9] and is one of the most difficult apple varieties to grow and presents a challenge to both growers and plant physiologists alike.
The objectives of this study were (a) to analyze the possible biotic form of alternate bearing using three approaches. The first, traditional approach is an evaluation of the three different versions of the alternate or biennial bearing index (BBI) using individual trees rather than an orchard average. Yields of year x and x + 1 were correlated with the number of flowers in year x, using a large number of trees grown under different light conditions to obtain a wide variability. The next two approaches, using the intersect of yield curves of individual trees over the years and the summation method are two new ideas based on individual tree data; both new ideas have the advantage over the BBI of providing a target number of flower clusters for thinning strategies. The other objectives will be to (b) assess this number of flowers or clusters required for consistent yields of trees affected by biotic alternate bearing for sustainable cultivation and (c) to explain its physiological background and to identify physiological key elicitors and the underlying regulatory mechanisms as (d) a base for a range of possible countermeasures.
Section snippets
Materials and methods
The experiment comprised 2086 fourteen-year-old apple (M. domestica Borkh.) cv. ‘Elstar’ trees (2009–2011) grown at Campus Klein-Altendorf, University of Bonn, Germany. All the apple trees were on M9 rootstock and came from the same nursery. The trees were trained to slender spindles with long branches and spaced 3.5 × 1.5 m, representing the common growing system for ‘Elstar’. The excellent properties of the luvisol (on 8 m alluvial loess) with 92 out of a 100 soil score scale and high nutrient
Results
Three approaches were employed to assess this biotic tree-to-tree alternate bearing in cv. ‘Elstar’. While tree-to-tree yields in apple commonly vary by ±2–4 kg/tree, the wide variation of cv. ‘Elstar’ yield (between 0 and 22 kg/tree) in one year x in an orchard (Fig. 1) indicates its biotic alternating character, as also shown as a colour plot for 210 consequtive trees in three tree rows in supplement S1.
Underlying physiology and regulatory mechanisms of biennial bearing
The present work (Fig. 1, Fig. 2, Fig. 3, Fig. 4) highlights the biotically-induced alternate bearing feature. Apple cv. ‘Elstar’ represented a prime example of this peculiar physiological phenomenon, where almost all trees alternated all the time, but not synchronously. A possible cause for this physiological disorder or phenomenon, the tree-to-tree biotic alternate bearing e.g., in cv. ‘Elstar’ may be competition for resources such as photo-assimilates (sources) between flowers and growing
Conclusions
To our knowledge, this is one of the most comprehensive experiments and data sets evaluated for biotic alternate bearing. The results identified the fruit cultivar as well as possibly excessive early cropping as possible elicitors, but excluded four factors as elicitors for alternate bearing:
Genetic variation, since all trees came from the same nursery and were on the same rootstock, which is vegetatively propagated, and thus exogenic.
Cultivation and farming practices, since all 2086 trees were
Acknowledgments
We are grateful to H.J. Wiesel and H.J. Weber for instigating this interesting project and the team at Campus Klein-Altendorf for counting the numerous flowers over the three years and Dr. Dereck P. Hucklesby, Bristol, UK for revising the English, (emer.) Prof. Dr. Fritz Bangerth for stimulating discussion and Prof. Jonathan Gressel for editorial guidance.
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