Size relationships between contiguous organs in the same year
The apple inflorescence – leaves and flowers – and the 2–5 proximal leaves of the bourse-shoot(s) are preformed in the over-wintering bud (Bijhouwer, 1924; Pratt, 1988). Therefore, the bourse-shoot is almost totally neo-formed during the growing season. Since the bourse-shoot is borne by the inflorescence and develops after inflorescence expands (Abbott, 1960; Lauri & Térouanne, 1995), our results strongly suggest a causal and positive relationship, both spatial and temporal, between the size of the preformed inflorescence and that of the bourse-shoot that develops from it. This finding is in agreement with previous results from other species, about the positive relationships between the size of the preformed bud, or the number of preformed appendages, and the size of the neoformed shoot which develops from it (Kozlowski, 1973; Powell, 1991; Greene & Autio, 1994).
Fruit size is a characteristic of the cultivar and is affected by the year of development of the fruitful shoot in relation to the age of the parent branch, with fruit in a lateral position in Y1, that is the year following the growth of the parent branch, being smaller than fruit that developed in Y2 and Y3 (Lespinasse, 1970; Volz et al., 1994; Lauri & Lespinasse, 2001). Our results, which included fruit in terminal position in Y1 but in small proportion (0.20 and 0.11 for ‘Chantecler’ and ‘Pitchounette’, respectively; data not shown), confirmed these effects, and showed that year of development of the shoot also affected the size relationships between R shoot and fruit. In Y1, fruit developed preferentially on R shoots with < 12 leaves (Fig. 3) representing 65% and 84% of all fruitful R shoots in Y1 for ‘Chantecler’ and ‘Pitchounette’, respectively (data not shown), whereas larger R shoots existed in Y1 (up to 36 leaves; Fig. 4a,b). In Y2 however, the same scheme was not observed, and large R shoots with up to 35 leaves were fruitful (Fig. 3).
Considering the R shoot as a subunit with its own morphological and physiological characteristics, these contrasting patterns might be interpreted in terms of source–sink relationships (Watson & Casper, 1984; Minchin & Thorpe, 1996). At the branch level, although an acrotonic gradient exists along the annual shoot (Simons & Chu, 1967; Costes, 2003), lateral inflorescences in Y1 usually have fewer leaves and flowers of smaller size, smaller flower receptacles, and weaker vascular connections (May, 1970; Dennis, 1986; Volz et al., 1994) compared with inflorescences that developed in Y2 and Y3. As a consequence, an inflorescence in Y1, especially in lateral position, has a lower ‘sink strength’ (Dennis, 1986; Marcelis, 1996) than an inflorescence in Y2. It is then more dependent on other sources of assimilates than an inflorescence in Y2 or Y3 with respect to fruit-set and early fruit growth (Lauri et al., 1996; Lauri & Térouanne, 1999). The relationships between fruit-set and fruit growth, and the bearing R shoot, are usually split into two main phases: early in the season, inflorescence leaves are primary sources of photosynthates for flowers and fruitlets while the growing bourse-shoot may be concurrent at that time; later in the season bourse-shoot leaves strongly contribute to fruit growth, final fruit size and calcium content (Abbott, 1960; Quinlan & Preston, 1971; Ferree & Palmer, 1982; Volz et al., 1994). Reciprocally, fruit sink stimulates photosynthesis of adjacent leaves (Hansen, 1977). These interactions would lead to a positive relationship between the size of the whole R shoot and fruit size. Our results, notably on ‘Chantecler’, would follow this scheme when combining fruitful R shoots in Y1 and Y2 but not if we consider each year independently (Fig. 3). However, the tendency to lower fruit-set (Table 3), although not statistically significant in this study but well documented in literature (Dennis, 1986; Lauri et al., 1996), as well as the lower fruit size and number of leaves on fruitful R shoots in Y1, (Fig. 3) suggested that, compared with R shoots on Y2, stronger competitions exist within the R shoot in Y1, that is between the bourse-shoot and the inflorescence (flowers and fruitlets), and also between adjacent shoots on the parent branch. In Y2, inflorescences have a higher number of leaves, with possibly less competition with the adjacent bourse-shoot, and also between shoots on the parent branch. In Y2, R shoots are then more autonomous for fruit-set (Table 3) and fruit growth.
Shoot size relationships between two consecutive years and fate of the terminal bud
As previously stated, there is a positive relationship between the size of a shoot in a given year and the size of the shoot it gives rise to in the following year (Powell, 1991; Barlow, 1994). According to Alaoui-Sosséet al. (1994) on Quercus robur, and Goldschmidt & Koch (1996) on Citrus, the previous growth cycle provides assimilates – from either reserves and/or current photosynthates according to the species – to the new developing growth increment. The present findings on the size relationships between these two consecutive years of shoot growth support these previous results and it has been shown here that each cultivar had a specific pattern of size relationships, that is higher slope and lower intercept for ‘Chantecler’ compared with ‘Pitchounette’. On ‘Chantecler’ the parabolic adjustment is better for the two transitions leading to a decrease in Y2 of the size of the longer shoots in Y1. On ‘Pitchounette’ the trend was linear for three out of four transitions leading to shorter shoots in Y2 compared with Y1 for long shoots. Both cultivars thus exhibited a tendency towards a decrease in shoot size in Y2 compared with Y1, illustrating two expressions of the phenomenon of ageing, that is reduction in the annual growth increment (Wareing, 1970; Poethig, 1990; Costes et al., 2003).
These results suggest that growth correlations, once established in Y1, may lead to a self-organization process (Nozeran et al., 1971; Champagnat, 1974), which tends to maintain the growth potential of the underlying shoot developed in the previous year. This 3-yr study showed that the development of a shoot on the parent branch is a highly integrated process where the number of primordia in the preformed bud in Y1 strongly determined shoot (Fig. 2), and to a lesser extent, fruit (Fig. 3) size in the same year and in the following year (Fig. 4). This chain of consecutive and possibly causally related events complement previous results on apple, on the positive relationship between the size of a shoot and the size of the inflorescence in terminal position (Lauri et al., 1996). According to recent results (H. Cochard et al., unpublished) on Fagus sylvatica, hydraulic conductance in an annual growth has a strong influence on the number of preformed organs in the terminal bud, and the length of the shoot in the following year. These findings would suggest that timing and intensity of vascular connections of the bud in Y1 can have a dramatic effect on shoot development in the following years. Since hydraulic properties of a shoot depend on thickness and more generally on the allometric relationships between length and diameter (Kervella et al., 1994), which is related to leaf area (Barcellos de Souza et al., 1986; Bond & Midgley, 1988; Brouat et al., 1998), it is likely that the structural proportion of the 1-yr-old parent branch would give additional information on the initial developmental phases of the terminal or the lateral shoot.
In our results, the relationship between shoot size and fate of the terminal bud was clearly cultivar dependent. The ‘in-built tendency’ for spurs to alternate (Browning, 1985; Davenport, 2000) was not verified here, even though the two cultivars exhibited contrasting patterns of flowering. On ‘Chantecler’ return-bloom, that is the transition from R shoot in one year to R shoot in the following year, was high with no substantial relation to shoot size. On ‘Pitchounette’ return-bloom was clearly dependent on shoot size, and in Y2 was even higher than transition from V (Fig. 5b). This is in agreement with the findings of some authors (Feucht, 1976; Weinbaum et al., 2001) that the relationships between shoot size and terminal flowering may be influenced by genotypic characteristics. Here, shoot size was assessed by counting foliage leaves and thus did not include scales and transition leaves. From the results obtained for R and V shoots in Y1, on ‘Chantecler’ and ‘Pitchounette’, respectively, we hypothesized that the typical parabolic, that is an increase followed by a decrease, relationship observed for three out of four transitions for ‘Pitchounette’, might be a common scheme of the relationships between shoot size in the first year and terminal flowering in the following year (Fig. 5) or 2 yr later (Fig. 6). According to this hypothesis, the left portion of a theoretical curve, which would include scales and transitional leaves, may be hidden for certain cultivars if only foliage leaves are considered (Fig. 7). On ‘Chantecler’, with high flowering in Y1, Y2 and Y3, an increase in flowering frequency is likely to occur on shoots with only transition leaves (V shoots in Y1) or with a few foliage leaves (R shoots in Y1), and in all cases reaches maximum values with the very first foliage leaves (Fig. 5a). On ‘Pitchounette’, transition to flowering is more dependent on the previous year shoot type and size, and year of development in relation to parent branch age (see difference between V shoots in Y1 and Y2; Fig. 5b). For the latter cultivar, the relationship between transition to flowering and number of leaves is in accordance with the statement of Feucht (1961) that flowering frequency is the highest in terminal position on shoots 1–15 cm long. The possibility to forecast the fate of the terminal bud up to 3 yr would suggest that, without extrinsic manipulation, the shoot enters as soon as shoot inception in Y1 on the parent branch, a predetermined behavior.
Figure 7. Theoretical curves of the relationships between the number of appendages of an apple shoot and flowering frequency in terminal position.
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Finally, the two contrasted habits of ‘Chantecler’ and ‘Pitchounette’ suggested that, on a between-cultivar basis, shoot size and terminal flowering were partly autonomous phenomena. Indeed, the ability to flower in terminal position on long shoots, including strong erect water-sprouts, is variable depending on cultivar and has been proposed as an easy-to-use discriminating feature between apple cultivars (Lauri, 2002). From our results, it can be concluded that the analysis of the flowering pattern of a cultivar should not only focus on the relationships between shoot length, or leaf number and area as is usually done, but should integrate other variables at both shoot and parent branch levels. Lespinasse & Delort (1993) showed on a range of cultivars that bourse volume is positively related to return-bloom, suggesting that the location of stored assimilates close to the terminal bud of the bourse-shoot is of primordial importance for inflorescence and fruit development in the following year. At the branch level, (Lauri et al., 1995; Lauri et al., 1997) showed that the extinction phenomenon that reduces branching density between Y1 and Y2 of regular bearing apple cultivars may be causally related to the high return-bloom ability of the remaining shoots. Hence, the hypothesis of the autonomy of the shoot with regard to the relationship between shoot size and fate of the terminal bud is not an absolute rule (Lauri & Térouanne, 1999; Sprugel, 2002) and varies with cultivar and position in tree architecture. On ‘Pitchounette’, shoot size in Y1 may determine fate in both Y2 and Y3. On ‘Chantecler’, with high extinction between Y1 and Y2, and high flowering in all years (Lauri & Lespinasse, 2001), whole parent branch features are likely to strongly determine the flowering pattern.