vegetative growth and reproductive strategies in ...

VEGETATIVE GROWTH AND REPRODUCTIVE STRATEGIES IN APPLE FRUITING BRANCHES - AN INVESTIGATION INTO VARIOUS CULTIVARS Lauri, P.E. Unité de Formation et de Recherche en Arboriculture Fruitière Institut National de la Recherche Agronomique (I.N.R.A.) Ecole Nationale Supérieure Agronomique de Montpellier (E.N.S.A.M.) 34060 Montpellier cedex 01, France Lespinasse, J.M. Unité de Recherche sur les Espèces Fruitières et la Vigne I.N.R.A. B P 81 33883 Villenave d'Ornon cedex, France Térouanne, E. Département de Mathématiques et Informatique Appliquées Université Paul Valéry. B P 5043 34032 Montpellier cedex, France Keywords: Apple, Mains x domestica Borkh., fruiting branch, extinction, bourse-overbourse, spur pruning Abstract A large proportion of modern French apple growers have adopted management systems based on manipulating "free growing" fruiting branches (Solen, and recently Solaxe). Basically, these systems need only light pruning and use the natural bending of the fruiting branch brought about by the progressive development of fruiting. In the case of erect branches (spur-type cultivars), weights are also used to hasten bending. In this context, one of the main objectives of apple growers is to improve the sustainability of these fruiting branches and the regularity of fruit production from year to year. A comprehensive approach was developed to study the type of each axillary structure (either latent, vegetative, inflorescence without a fruit or with at least one fruit, scar) on unpruned fruiting branches belonging to trees trained as Solen. Since it first developed on 1-yearold wood, the objective was to study its subsequent development in the following years. Each initial node of the fruiting branch was then characterized by a sequence of letters corresponding to the type of terminal bud of laterals observed each successive year. In a previous study we showed that two phenomena may interact in the regulation of the vegetative growth fruiting relationship of the fruiting branch. One phenomenon is the cessation of growth of a certain proportion of sequences ('extinction' phenomenon) and the other is the ability to string bourse over bourse in the remaining sequences. This paper reports further results which show that on regular bearing cultivars, such as 'Granny Smith' and 'Red Winter', this regulation occurred essentially within the first 2 years of sequences. The precocity of this natural regulation mechanism suggests that the effectiveness of artificial spur pruning, which is recommended for spur-type cultivars, may be enhanced i f it is carried out as early as on 1- or 2-year-old wood. 1. Introduction In order to reduce pruning and harvesting costs, many French apple growers have adopted training systems based on the establishment of either one central axis (French Vertical Axis), or two lateral scaffolds (Solen Training System), or a combination of both systems (Solaxe), bearing fruiting branches (Lespinasse, 1996). In the case of a vertical axis, Proc. 6. Int. Symp. on Integrating Canopy, Rootstocks and Environmental Physiology in Orchard Systems. Eds. Barritt, Kappel. Elfving, Flore, Lang, Quamme, Webster. Acta Hon. 451, ISHS 1997.

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branches are vertically distributed all around the trunk, whereas with the Solen tree with 2 scaffolds, branches are arranged horizontally on both sides of the row at 1.8 to 2 m high. In the case of a Solaxe, branches are vertically distributed up to 2 to 3 m high and then horizontally on the bent part of the trunk. A total of 12 to 16 fruiting branches are usually obtained on trees grafted on M.9 (Lespinasse and Delort, 1986; Lespinasse and Lauri, 1996). One of the main objectives of apple growers is to establish a well-structured framework during the first year and then to manage an optimal positioning of fruiting branches during the second year. Basically, both early fruiting and the subsequent control of the vegetative growth fruiting relationships of these fruiting branches are the consequence of the natural bending below the horizontal, brought about by fruit production. In the case of erect branches (spur-type cultivars), weights can be used to hasten bending. These systems only need light pruning. Since the sustainability and regularity of fruiting strongly depend upon the equilibrium between vegetative growth and fruiting, a thorough analysis of the branching and fruiting pattern of fruiting branches is important to optimizing the training method of each cultivar. In a previous study, we showed that two phenomena may interact in the regulation of the vegetative growth fruiting relationship of the fruiting branch (Lauri, et al., 1995). One phenomenon is the cessation of growth of a certain proportion of sequences ('extinction' phenomenon) leading to a drop in the number of laterals; the other is the ability to string bourse over bourse in the remaining sequences. This paper presents some results that show that the number and length of laterals and the ability to fruit terminally on bourse-shoots appear precociously within the first years of fruiting branch development. 2. Material and methods Five cultivars with contrasting patterns of growth and fruiting, 'Golden Delicious' (gd), 'Granny Smith' (gs), 'Oregon Spur Delicious' (os), 'Red Winter' (rw) and 'Reine des Reinettes' (rr) were studied. Trees were grafted on M.9 rootstock, planted in field plots at the National Institute for Agricultural Research (INRA) in Bordeaux, France, in the spring of 1988, and trained to Solen. This system allowed us to select 10 fruiting branches equally distributed on both sides of the rows for each cultivar. These branches were not pruned during the trial. Pollination was provided by bees and was not believed to influence cropping of the different cultivars. Blossoms were first chemically thinned (Rhodofix, (1% N A A ) 120 g hi" and Sevin (85% Carbaryl) 60 g hi" ) and then hand thinned to leave one fruit per inflorescence. Growth and flowering of the axillary bud at each node of the first annual increment of the fruiting branch (1989) was recorded in spring for 5 successive years. Five types were distinguished: latent bud (L), vegetative (V), inflorescence which set fruit (A) and inflorescence which did not (F). V , F and A will be hereafter referred to as growing types (G). If a bud died, the presence of the scar (S) was recorded. The same experiment was carried out on the 1990 and 1991 annual increments of the main axis of the fruiting branch. Hence for each node of this shoot, a sequence of letters designated how the pattern of development of the axillary bud evolved over the years, i.e., with increasing wood age (Fig. la). We then obtained 5-, 4- and 3-year sequences on the 1989, 1990 and 1991 annual increments of the main shoot of the fruiting branch, respectively (Fig. lb). During spring, additional records on the length of the supporting axis (that developed during the preceding year) for all vegetative (V) and reproductive (F and A ) bud types were observed on 1- and 2-year-old wood (Fig. la). For 1-year-old wood, these lengths corresponded to sylleptic shoots (Bell, 1991; Lauri, et al, 1996). 1

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3. Results A first study was designed to calculate the distribution with time of the first occurrence of an S in sequences. Figure 2 shows during the 3- to 5-year period that 90% to 95% of the total number of S occurred within the first two years of sequence life time, except for gd (70%). However, cultivars might be differentiated by the proportion of S appearing before 718

"spring N + l " : 70% to 75% for os and rr, versus 10% to 30% for gd, gs and rw. The 'extinction' phenomenon (Lauri, et al., 1995) accounts for the S which appeared after "spring N + l " . Most occurred between the 1- and 2-year-old wood. They are especially numerous for gd, gs and rw (60% to 80% of all S). Figure 3 represents the percentage of axes, on 1- or 2-year-old wood, cumulated along 4 arbitrary length categories ("< 1 cm", "1 to 3 cm", "3 to 15 cm", and "> 15 cm"). It shows that cultivars present similar axis length characteristics on 1-year-old wood (at least 90% of vegetative or reproductive growth lying within the "< 1 cm" class), whereas greatest differences existed for the 2-year-old wood axis length (about 50% of axes more than 1 cm, for rw, vs. 10% for rr). The study of the bourse-over-bourse phenomenon was carried out by noting the proportion of subsequences ( A -A ) among subsequences ( A -G ,) for each couple of years ("1-2", "2-3", "3-4" and "4-5"). Figure 4 shows that the proportion of bourse-over-bourse for the successive couples of years tended to increase with time (despite a sharp decrease in "4-5" for rr and to a lesser extent for os). Moreover, the curves do not intersect, showing that the bourse-over-bourse observed for the couple of years "1-2" may be a good indicator of the cultivar ability to crop terminally on the bourse-shoot. Y e a r N

Y e a r N + 1

Year N

Y e a r N+

4. Discussion These results were obtained on fruiting branches which were bent below the horizontal (naturally for gs and rw; using weights for os, rr and to a lesser extent for gd). This probably brought about some modifications in the natural growth and fruiting pattern of erect cultivars such as os and rr. Previously (Lauri, et al., 1995) we pointed out that cultivars may be discriminated by two characteristics, first the frequency of 'extinction', defined as the relative proportion of sequences beginning with a G type and ending with an S over the total number of sequences and second the terminal cropping on bourse-shoots. It was also shown that cultivars with high 'extinction' were also characterized by a high bourse-over-bourse tendency. We then hypothesized that these two phenomena could be linked and that the fruiting autonomy of sequences could be maintained only i f a certain number of neighboring growing meristems, i.e., neighboring sequences aborted. The data we analyzed showed that for the 5 cultivars at least 2/3 of all 'extinction' was already observed on "spring N+2". In parallel the bourse-over-bourse tended to increase from its initial value for the couple of years "1-2" over the following years. Cultivars were not differentiated by the 1-year-old wood but rather by the 2-year-old wood axis length. This characteristic lends further support to the idea that a strong relationship exists between three phenomena, the 'extinction' of a certain number of meristems, the general trend toward an increase in length of the remaining ones, and the ability to fruit terminally on bourse-shoots. Our data are consistent with the results of Chan and Cain (1967) which showed that a minimum axis length is necessary for a bourse-overbourse to occur. The physiological mechanism of bourse-shoot initiation may involve hormones or the sink vs. source relationship (Barlow, 1994; Dennis, 1986; Griggs, et al., 1970). The precocity of the bourse-over-bourse mechanism in relation to the 'extinction' phenomenon observed on self-regulating cultivars such as rw and gs would suggest that spur pruning which is recommended on "spur-bound" cultivars (Forshey, et al., 1992; Rom, 1992) may be enhanced i f it is carried out as early as on 1- or 2-year-old wood. The possibility of introducing such methods in young commercial orchards is now being investigated. References Barlow, P.W., 1994. From cell to system: repetitive units of growth in the development of roots and shoots. Pages 19-58 in M . Igbal, ed. Growth patterns in vascular plants. Dioscorides Press, Portland, U S A . 719

Bell, A . D . , 1991. Plant form : A n illustrated guide to flowering plant morphology. Oxford University Press, Oxford. Chan, B.G. and J.C. Cain, 1967. The effect of seed formation on subsequent flowering in apple. Proc. Amer. Soc. Hort. Sci. 91: 63-67. Dennis, F.G., 1986. The physiology of flowering and fruit-set in tree fruits. Annual ReportOregon Horticultural Society (USA) 77: 33-43. Forshey, C.G., D.C. Elfving and R.L. Stebbins, 1992. Training and pruning of apple and pear trees. Amer. Soc. Hort. Sci., Alexandria, V A , 166 pp. Griggs, W.H., G.C. Martin and B.T. Iwakiri, 1970. The effect of seedless versus seeded fruit development on flower bud formation in Pear. J. Amer. Soc. Hort. Sci. 95(2): 243-248. Lauri, P.E., E. Térouanne, J.M. Lespinasse, J.L. Regnard and J.J. Kelner, 1995. Genotypic differences in the axillary bud growth and fruiting pattern of apple fruiting branches over several years: A n approach to regulation of fruit bearing. Scientia Hort. 64(4): 265-281. Lauri, P.E., E . Térouanne and J.M. Lespinasse, 1996. Quantitative analysis of relationships between inflorescence size, bearing-axis size and fruit-set: A n apple tree case study. Annals of Botany 77: 277-286. Lespinasse, J.M., 1996. Apple orchard management practices in France. From the vertical axis to the solaxe. Symposium of the International Dwarf Fruit Tree Association, British Columbia, April 1996 (in press). Lespinasse, J.M. and F. Delort, 1986. Apple tree management in vertical axis: appraisal after ten years of experiments. Acta Hort. 160: 120-155. Lespinasse, J . M . and P.E. Lauri, 1996. Influence of fruiting habit on the pruning and training of fruit trees. Symposium of the International Dwarf Fruit Tree Association, British Columbia, April 1996 (in press). Rom, C.R., 1992. Spur pruning 'Delicious' apple for improved spur quality and yield. Acta Hort. 322: 55-67.

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5-YEAR-LONG SEQUENCE

4-YEAR-LONG SEQUENCE

3-YEAR-LONG SEQUENCE

Fig. 1 - (a) Drawing of a 5-year-long sequence as it appears during spring of year 5, with the corresponding sequence of letters ( A V V A A ) , and (b) diagram of 5- to 3year-long sequences branched on successive annual increments of the main axis of the fruiting branch. " L I " , "L2": lengths of the supporting axis on 1and 2-year-old wood, respectively.

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Fig. 2 - Cumulative percent of the first occurrence of an S in sequences over 5 years ("spring N + l " to "spring N+5"), for 5 cultivars. The number of the first S being low in the fourth year (for 4- and 5-year-long sequences), as well as in the fifth year (for 5-year-long sequences), 3-, 4- and 5-year-long sequences are gathered, "spring N " : beginning of growth of the bearing shoot, gd: 'Golden Delicious', gs: 'Granny Smith', os: 'Oregon Spur Delicious', rw: 'Red Winter'; rr: 'Reine des Reinettes'. Note the major increase in the number of S between "spring N + l " and "spring N+2", corresponding to the strongest period of 'extinction'.

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a

One-year-old wood

< 1 cm

1 to 3 cm 3 to 15 cm

> 15 cm

length categories Two-year-old wood

< 1 cm

1 to 3 cm 3 to 15 cm

> 15 cm

length categories Fig. 3 - Percentage of axes, on 1- (a) and 2- (b) year-old wood, cumulated along 4 arbitrary length categories ("< 1 cm", "1 to 3 cm", "3 to 15 cm", and "> 15 cm"), for 5 cultivars. Cultivar abbreviations, see Fig. 2.

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1-2

2-3

3-4

4-5

couple of years Fig. 4 - Variation of the percentage of A A subsequences (bourse-over-bourse) over all A G subsequences, for the successive couple of years "1-2", "2-3", "3-4" and "4¬ 5", and for 5 cultivars. Cultivar abbreviations, see Fig. 2.

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