Insights into secondary growth in perennial plants: its unequal spatial and temporal dynamics in the apple (Malus domestica) is driven by architectural position and fruit load

Abstract

Background and aims: Secondary growth is a main physiological sink. However, the hierarchy between the processes which compete with secondary growth is still a matter of debate, especially on fruit trees where fruit weight dramatically increases with time. It was hypothesized that tree architecture, here mediated by branch age, is likely to have a major effect on the dynamics of secondary growth within a growing season.

Methods: Three variables were monitored on 6-year-old 'Golden Delicious' apple trees from flowering time to harvest: primary shoot growth, fruit volume, and cross-section area of branch portions of consecutive ages. Analyses were done through an ANOVA-type analysis in a linear mixed model framework.

Key results: Secondary growth exhibited three consecutive phases characterized by unequal relative area increment over the season. The age of the branch had the strongest effect, with the highest and lowest relative area increment for the current-year shoots and the trunk, respectively. The growth phase had a lower effect, with a shift of secondary growth through the season from leafy shoots towards older branch portions. Eventually, fruit load had an effect on secondary growth mainly after primary growth had ceased.

Conclusions: The results support the idea that relationships between production of photosynthates and allocation depend on both primary growth and branch architectural position. Fruit load mainly interacted with secondary growth later in the season, especially on old branch portions.

Fig. 1.

(A) Solaxe tree with branch portion age (from ‘6’, at trunk bottom, to ‘1’, current year shoot). (B) Detail of a current-year bourse-shoot with an adjacent fruit.

Fig. 2.

Shoot primary growth. Evolution of (A) the proportion of growing shoots, and (B) length and (C) total shoot leaf area of these growing shoots when more than four shoots, depending on tree fruit load (H, high; M, medium; L, low fruit load, respectively) and the presence (F, fruit) vs. absence (NF, no fruit) of an adjacent fruit.

Fig. 3.

Fruit growth depending on tree fruit load from 30 April until harvest (15 September). Fruit load adjustment was made on 15 May. Fruit volume is estimated from the equatorial diameter (Volume = 0·0018 × Diameter^2·65). Values are means (for clarity, the standard error is not shown). Significant differences between fruit loads were observed from the 27 May (black arrow) onwards. The last values (grey symbols, 15 September) were measured on a sub-sample of fruits. On 27 May and 15 September, different letters indicate significant differences in fruit volume according to the HSD Tukey test at P < 0·05.

Fig. 4.

Secondary growth from full bloom to harvest (15 September) as a function of branch portion age: (A) cumulated secondary growth, (B) proportion of shoots with active secondary growth, for the three merged fruit load values, and (C) ln(RDAI) as a function of branch portion age for L- (low fruit load) trees [similar patterns of variation throughout the growing season and order between branch portion ages were observed for H (high) and M (medium) fruit loads]. Branch portion age varies from 1 (current-year shoot) to 6 (trunk). The growth phases (phases I, II and III) are noted in (A). Data are means when more than four growing branch portions are included. Negative values of ln(RDAI) result from mean RDAI values <1. Statistics of the effects on RDAI of age, phase and fruit load and their interactions are given in Table 2.