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- Materials and Methods
Tree architecture is the expression at a given time of the balance between endogenous factors and exogenous constraints (Halléet al., 1978). Endogenous factors determine characters that are not influenced by the environment. In the apple tree, all cultivars belong to the architectural model of Scarrone: all axes are orthotropic with rhythmic growth and sexuality in the terminal position (Halléet al., 1978), with a similar location of branching zones, each being characterized by a homogeneous composition of lateral types (vegetative, floral, latent) along the 1-yr-old shoot (Costes & Guédon, 2002). However, several features differ depending on the cultivar, including branching density (Lespinasse & Delort, 1986; Forshey et al., 1992) and frequency of lateral flowering on 1-yr-old wood (Lauri & Lespinasse, 2001). For a given genotype, branching and lateral flowering frequency may be modulated by environmental factors such as temperature, which affects dormancy completion of buds (Cook & Jacobs, 1999; Labuschagnéet al., 2003; Naor et al., 2003), or orientation of growth of the parent shoot, such as vertical vs horizontal positioning (Lauri & Lespinasse, 2001; Naor et al., 2003).
The growth potential of a branch is generally considered to be positively related to the number of primordia in the overwintering bud to which it belongs (Kozlowski, 1973). However, at shoot level the physiological mechanisms underlying branching frequency (ratio of latent vs growing laterals, whether vegetative or floral) and the type (latent, vegetative, floral) and final length of the lateral are still not understood, even though the interplay of genetic programmes with the environment has been investigated intensively in model plants, both dicots and monocots (McSteen & Leyser, 2005). The involvement of the water-transport system (hydraulic architecture; Tyree & Ewers, 1991) in shoot growth and branching has been investigated in relation to growth in height (Koch et al., 2004; Woodruff et al., 2004), transport efficiency (McCulloh & Sperry, 2005) and mechanical properties (Rosner et al., 2007) but, to our knowledge, scarcely in relation to bud morphogenesis. A recent attempt has been made by Cochard et al. (2005), on Fagus sylvatica, showing a positive correlation between the number of leaf primordia in the bud before bud burst and the hydraulic conductance (Kx) of the xylem vascular system connected to this bud, whether in the lateral or terminal position. The study was conducted on the whole shoot, thus including a variability in potential bud development related to the topological position along the shoot, specifically a lower growth potential of laterals situated in proximal position compared with buds situated more distally along the parent shoot (Fisher, 1984; Nicolini, 1997; Le Bris et al., 1998). Therefore this hydraulic study on F. sylvatica does not permit us to separate the effects on hydraulic conductance of the topological position along the shoot from the intrinsic size and composition of the buds. Indeed, small buds are frequent in the proximal part of the shoot, and this positional effect might be related more to the lower Kx of the xylem vascular system connected to these buds, than to bud size and composition per se.
Our study was developed in apple trees, for which shoot architecture (the distribution of lateral bud types and growth along the parent shoot) has been investigated extensively (Costes et al., 2006). Long shoots of apple trees are usually defined by a high frequency of latent buds in the proximal zone, and a distal zone with a high frequency of vegetative and floral buds (Greene & Autio, 1994; Guédon et al., 2001; Brunel et al., 2002; Costes & Guédon, 2002; Renton et al., 2006; Lauri, 2007). Buds, usually latent, in the proximal zone are small, whereas buds in the distal zone, which are usually vegetative or floral, are large (Brunel et al., 2002). Latent buds can also be present in the distal zone, although less frequently (Lauri & Térouanne, 1998; Costes & Guédon, 2002; Lauri, 2007). In addition to these bud types, a proportion of buds that would otherwise develop as vegetative or floral laterals physiologically abort within their first year of development (Lauri & Térouanne, 1998). This physiological abortion of laterals is known as lateral extinction in a horticultural context (Lauri et al., 1995, 1997). The heterogeneity of bud potential within the distal zone offers a unique opportunity to validate the relationships between the hydraulic conductance of the xylem vascular system connected to the lateral bud, hereafter referred to as KLAT, and bud size and composition independent of the topological position.
Our analysis was developed on lateral buds in the distal zone of shoots before bud burst for a range of apple cultivars. Our objectives were: (1) to search for differences in bud size and composition that would predict actual development of the lateral in spring, and especially the existence of latent and aborted buds; (2) to assess relationships between bud size and KLAT and to analyse the possible differential effects of cataphylls and green-leaf primordia on these relationships; and (3) to examine whether these relationships are affected by the cultivar.
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- Materials and Methods
It has been hypothesized that the organogenesis of the lateral bud, which accurately forecasts the type and growth of the branch deriving from it, is influenced by the structural proportions of the 1-yr-old parent shoot: length, proximal and distal diameters, and slenderness (ratio of length to diameter) (Lauri & Trottier, 2004). In apple trees, as in most temperate species, bud organogenesis occurs in the summer and autumn preceding actual spring growth (Bijhouwer, 1924; Pratt, 1988). Cochard et al. (2005) suggested a positive relationship between cambial activity in the year of parent shoot growth and the primary growth of buds, whether in terminal or lateral position, in the following year. Our study showed a high variability of apple bud development within the distal branching zone, independent of position along the shoot, and documented relationships with hydraulics taking into account the number and types of appendage included in the bud.
Bud size and composition variability within the distal zone
Over a large range of shoot sizes, there is a positive relationship between the size (e.g. diameter) of a shoot and the size of its laterals (Marcelis-van Acker, 1994). However, at a smaller scale, our results clearly showed a great variability of bud size and composition within the distal zone of a same shoot, although shoot diameter did not vary significantly along the shoot (data not shown). These differences between buds were therefore independent of a positional effect. Differences could be observed between adjacent buds without a clear pattern, for example, no phyllotactic pattern could be demonstrated from our data compatible with the leaf hydraulic sectoriality hypothesis (Orians et al., 2005; data not shown). The variability in bud development within the distal zone mirrored the architectural pattern of the whole 1-yr-old shoot: small – potentially latent – buds have a lower total number of appendages and a higher percentage of cataphylls than large – potentially vegetative and floral – buds (Brunel et al., 2002; Puntieri et al., 2007).
Relations with hydraulic conductance: common trends and differences between cultivars
Although shoot sampling lasted 10–15 d for each cultivar, there was no significant change in KLAT over this period (data not shown). This would a posteriori support our presumption that measurements were carried out before new cambial growth occurred. Our study showed, for a range of apple cultivars, a positive relationship between bud size and number of appendages, and a positive relationship between these two variables and the hydraulic conductance of the vascular system connected to the buds. It confirmed and expanded on previous results at the whole-shoot scale on F. sylvatica by Cochard et al. (2005). Based on our results, we suggest a two-step process for the relationships between KLAT and bud development. First, the absence of any exudation (KLAT= 0 in our experimental conditions) was observed mainly for small and medium-sized buds. This would apply well to buds that remain latent, or that burst in spring and soon abort, leaving a scar (Lauri & Térouanne, 1998). Second, for bud scars that exuded, the positive correlation between KLAT and the number of bud appendages would explain the range of sizes reached by vegetative and floral laterals.
According to Marcelis-van Acker (1994), increasing the assimilate supply during lateral bud development increases the mass of the bud and the number of leaf primordia in the bud. Our results showed that KLAT correlated positively with bud size, and not only with green-leaf primordia but also with cataphylls. However, it was shown here that slopes of the relationships with KLAT were higher for green-leaf primordia than for cataphylls, indicating that for a given increase in KLAT, the number of green-leaf primordia increased more than the number of cataphylls. The slopes of the relationships between KLAT and number of appendages were not affected by bud size or cultivar. However, these relationships were differentiated by their allometric constant, meaning that the efficiency of the vascular system for bud organogenesis differed depending on bud size and cultivar: large buds and Gala had higher efficiencies compared with small and medium-sized buds and the other cultivars, respectively.
It has been hypothesized that the formation of cataphylls is a result of the slowing down of the plastochron (in apple trees, cataphylls begin to form when the plastochron becomes longer than 5 d; Crabbé & Escobedo-Alvarez, 1991). These cataphylls are likely to play a key role in bud formation in buffering the apex against a resumption of growth, possibly via the abscisic acid they contain (Abbott, 1970; Crabbé, 1994; Brunel et al., 2002). According to Brunel et al. (2002), proximal (potentially latent) buds have a significantly higher number of cataphylls than distal buds, and both the number of cataphylls and KNAP2 (KN1-like gene family) expression are negatively related to the growth potential of the bud. Our results showed that, in the distal zone, small buds had fewer cataphylls and green-leaf primordia compared with large buds. It was also shown that small buds had a higher percentage of cataphylls than large buds (Table 2), meaning that the differences in the number of appendages between small and large buds were caused by differences in the number of green-leaf primordia (2.8-fold) rather than by differences in the number of cataphylls (1.6-fold). Based on these results, it may be proposed that differences between small and large buds in the distal zone did not result from a higher number of cataphylls of the former, which would hamper further organogenesis within the bud. Rather, the process would begin with the development of the first cataphylls and increase with bud development, and at any time was positively related to KLAT.
A potential role of the whole metamer in the relationships between KLAT and lateral bud development
The acrotony concept is used to interpret the differential branching pattern of the proximal as opposed to the distal zone of the parent shoot (Crabbé, 1985; Bell, 1991; Cook et al., 1998; Wilson, 2000). At this scale, architectural gradients may result from competitive interactions between distal and proximal zones, and also with secondary growth (Lauri, 2007). In our study, secondary growth could hardly be advocated as a main competition factor between buds, because buds of different sizes were mixed in a distal zone with a similar diameter. However, it may be hypothesized that hydraulically mediated competition for assimilates between adjacent territories along the shoot are set as soon as metamers unfold. Relationships between leaf and lateral bud developments have rarely been investigated, and conclude to either a positive (Larson & Pizzolato, 1977) or a negative (Schmitz & Theres, 1999) relationship. However, it is likely that relationships between the development of both the leaf and its axillary bud are more complex and should include the whole metamer (leaf, node and subtending internode), and should be considered in a time scale. First, a minimum leaf size has to be reached to develop a visible axillary meristem (Lauri & Térouanne, 1998). Second, the growth dynamics of the leaf (e.g. in peach; Kervella et al., 1995) or of the whole metamer (e.g. in apple; Lauri & Térouanne, 1995, 1998) plays a crucial role in the axillary bud type, floral vs vegetative. These relationships at the morphological level suggest that, within the distal zone, all factors favouring early and sustained growth of the leaf, compared with the internode, also favour growth of the axillary bud. The involvement of hydraulic conductance in these dynamic relationships is poorly documented. Correlations between hydraulic traits of adjacent territories have generally been established between stem and leaf (Preston & Ackerly, 2003; Edwards, 2006), and scarcely between stem and buds (Cochard et al., 2005). According to Lo Gullo et al. (2004), apical dominance, the absence of branching during growth of the terminal bud, and hydraulic dominance, higher leaf hydraulic conductance for distal leaves, are positively related. Considering adjacent metamers within the same branching zone, however, the involvement of hydraulic conductance in the cross-relationships between metamer components (internode, leaf and axillary meristem) is an interesting avenue for further studies.