Almond trees develop fruit laterally on extension shoots and short shoots called spurs on 2-year-old or older wood (Kester et al., 1996). The pattern of shoots and spurs along branches is of major importance because they determine tree architecture and cropping potential (Bernad and Socias i Company, 1998). In almond, diverse tree architectures among cultivars are mainly the result of their genetically determined branching patterns rather than pruning practices because trees are usually minimally pruned to maximize yield (Gradziel, 2012). Previous characterization of almond branching patterns has been based on the degree and type of lateral branching in the previous and current growing season in different cultivars (Gradziel et al., 2002). However, those patterns have not been well quantified. Branching patterns of shoots range from total suppression of lateral sylleptic and proleptic shoot production, resulting in whip-like shoots, to little lateral suppression of sylleptic and proleptic shoots, resulting in highly branched shoots (Gradziel et al., 2002).
Three types of shoots developed on a 2-year-old shoot according to their dormant time and length: proleptic, or “delayed,” shoots originating from axillary meristems after a dormant period; sylleptic shoots, or “immediate laterals,” elongating from axillary meristems immediately after their initiation without any evident period of rest (Barthélémy and Caraglio, 2007); and spurs that are proleptic shoots with minimal elongation of the internodes and constitute the main fruit-bearing shoot type in mature almond trees (Kester et al., 1996). Spurs produce up to six lateral floral buds (Polito et al., 2002) and typically remain viable for 3 to 5 years (Weinbaum and Spiegel-Roy, 1985). Sylleptic branching of most almond cultivars in California is moderate and this is considered commercially desirable (Gradziel et al., 2002). This moderate branching allows for an open tree architecture and permits opportunities for fruit wood renewal without excessive shading of the interior canopy (Kester et al., 1991).
Hallé et al. (1978) developed a scheme to describe branch and tree architecture of tropical trees that has been applied to many tropical and temperate tree species (Costes et al., 2006; Fisher, 1984). These analyses have helped to identify genetically determined branching patterns involved in plant construction (Barthélémy and Caraglio, 2007). This led to studies of branching patterns using stochastic models, especially in apple [Malus ×domestica (Costes and Guédon, 1996, 2002; Renton et al., 2006)]. In particular, hidden semi-Markov models have been used to describe the organization of lateral meristem fates in a succession of zones along shoots, where each zone has a characteristic composition of observation types such as shoots, spurs, blind nodes, or others (Guédon et al., 2001). These branching studies usually are conducted after the second growing season of shoot development, after sylleptic and proleptic shoots have developed (Costes and Guédon, 2002; Renton et al., 2006).
In the present study, three almond cultivars, which have been described as having different tree architectures, were selected for analysis of branching patterns using the hidden semi-Markov models. ‘Nonpareil’ is the most important cultivar in California (U.S. Department of Agriculture, 2011). This cultivar has consistent and high productivity, bearing on both spurs and long shoots (Asai et al., 1996). The tree is large and upright to spreading with limited lateral shoot development on both 1- and 2-year-old shoots (Gradziel et al., 2002). ‘Aldrich’ is a large and upright tree (Gradziel, 1997), with sparse sylleptic branching but copious production of proleptic spurs and subsequent blooms (Aldrich, 1984). In contrast, ‘Winters’ has an upright and highly branching architecture. Production occurs on spurs, on terminal 1-year-old shoots, and on characteristic, medium-short sylleptic shoots ranging in length from 5 to 20 cm. Gradziel et al. (2007) suggested that this type of shoot increases fruit wood renewal and may promote high and consistent productivity as well as producing an open tree architecture that favors light penetration to the canopy interior.
This study aimed at characterizing the branching patterns of 2-year-old proleptic shoots of these three almond cultivars (Nonpareil, Aldrich, and Winters) with contrasting tree architectures and analyzing the effect of these patterns on flowering. All observation types (i.e., blind nodes, spurs, proleptic and sylleptic shoots) and the number of flower buds on 2-year-old proleptic shoots of different length categories were assessed to determine whether cultivars have shoots with different branching patterns and flowering. Thus, in this study specifically 1) branching patterns were modeled by a single hidden semi-Markov model of different shoot lengths for each cultivar; 2) the models of each cultivar were used to evaluate gradients in branching patterns among shoots of different lengths; and 3) the numbers of flower buds were determined on each shoot length category in each cultivar.
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