JXB Advance Access originally published online on March 28, 2008
Journal of Experimental Botany 2008 59(6):1163-1173; doi:10.1093/jxb/ern043
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© 2008 The Author(s).
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
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RESEARCH PAPER |
Regulation of shoot branching patterns by the basal root system: towards a predictive model
AgResearch Grasslands, Private Bag 11008, Palmerston North, New Zealand
* To whom correspondence should be addressed. E-mail: mike.hay{at}agresearch.co.nz
Received 11 September 2007; Revised 25 November 2007 Accepted 18 January 2008
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Abstract |
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This study aimed to underpin the development of a generic predictive model of the regulation of shoot branching by roots in nodally rooting perennial prostrate-stemmed species using knowledge gained from physiological studies of Trifolium repens. Experiment 1 demonstrated that the net stimulatory influence from the basal rooted region of the plant on growth of newly emerging axillary buds on the primary stem decreased as their phytomeric distance from the basal root system increased. Experiment 2 found that at any one time the distribution of net root stimulus (NRS) to the apical bud on the primary stem and all lateral branches was fairly uniform within a single plant. Thus, although NRS availability was uniform throughout the shoot system at any point in time, it progressively decreased as shoot apical buds grew away from the basal root system. Based on these findings, a preliminary predictive model of the physiological regulation of branching pattern was developed. This model can explain the decline in growth rate of buds on a primary stem as it grows away from its basal root system but not the rapid progressive decline in secondary branch development on successive lateral branches. Thus knowledge of NRS availability to emerging buds is not, by itself, a sufficient basis from which to construct a predictive model. In addition, it seems that the ability of an emerging bud to become activated in response to its local NRS availability is, at least in part, directly influenced by the activation level of its parent apical bud. The experimental testing of this hypothesis, required for continued development of the model, is proceeding.
Key words: Axillary bud outgrowth, branch development, bud activation, intra-plant variation, nodal roots, prostrate clonal herbs, root signals, Trifolium repens
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Introduction |
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It is widely observed that the pattern of branching is similar in many herbaceous species, ranging from both orthotropic annuals and short-lived perennials such as Petunia (Snowden and Napoli, 2003), to nodally rooting procumbent herbs such as Trifolium repens (Thomas et al., 2002) and Glechoma hederacea (Thomas and Hay, 2004). In such species, as the primary stem grows away from its rooted base, or from its youngest rooted node, an early phase, during which axillary buds grow out strongly to form lateral branches, is followed by a period during which the vigour of successively formed branches decreases from phytomer to phytomer along the primary stem until a stage is reached at which newly emerging axillary buds fail to grow out into branches. In T. repens this commonly occurs after the outgrowth of
6–9 branches (Thomas et al., 2002). This observed decrease in vigour of branch development from base to apex along the primary stem is made more marked by the fact that secondary branching of the basalmost lateral branches is commonly much greater than that of later formed ones (Thomas et al., 2002; Thomas and Hay, 2004) and is usually confined to the basalmost few, ranging from the single basalmost branch in Vinca (Thomas and Hay, 2004) to the four or five basalmost branches in T. repens (Fig. 1). Here the focus is on the underlying causes of this widespread pattern of branching decline.
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Previous studies of branching have shown that regulation occurs via apical inhibition (apical dominance), a root- and shoot-derived inhibitor, and a root-derived stimulus (Dun et al., 2006; Ongaro and Leyser, 2008). The relative importance of each of these probably varies from species to species and within an individual plant over time. Studies with mutant genotypes of Petunia, Arabidopsis, and Pisum indicate that, in these species, decreased outgrowth of buds from phytomer to phytomer acropetally along a primary stem axis involves the interaction between an acropetally transported root-derived inhibitory signal and indole acetic acid (IAA) transported basipetally from the primary stem apical bud (Foo et al., 2005; McSteen and Leyser, 2005; Snowden et al., 2005; Dun et al., 2006), with recent studies indicating that the effect of IAA is indirect (McSteen and Leyser, 2005; Tanaka et al., 2006; Aguilar-Martínez et al., 2007). In these erect-stemmed annual species, genetic and physiological evidence points to dominance of common inhibitory influences from roots (see above, Sorefan et al., 2003; and reviews by Leyser, 2005; Beveridge, 2006; Johnson et al., 2006). In contrast, studies with nodally rooting, perennial, prostrate-stemmed species (Thomas et al., 2002, 2003a, b; Thomas and Hay, 2004, 2007a, b) have consistently demonstrated that in such plants, in which the presence or absence of nodal roots can be readily manipulated, the physiological regulation of branching is dominated by fluctuations in stimulatory signal(s) transported acropetally from roots.
Using T. repens as a model species representative of this group of plants, it had previously been shown that decreased axillary bud growth at phytomers more distant from the basal root system was primarily not the result of increasing apical inhibition: rather, it occurred despite the absence of an apical bud on decapitated stems (Thomas et al., 2003b). It therefore seems possible that, in this system, decreased axillary bud growth results from a decrease in effectiveness of a root-derived stimulus at later-emerging axillary bud sites. Decreased effectiveness may arise, however, as a consequence of a decrease in the level of the stimulatory signal exported from the basal root system, an increase in supply of root-derived inhibitor, decreasing sensitivity of axillary buds to stimulatory signals with distance from the root system, or a combination of these factors. With regard to the root-supplied stimulatory and inhibitory signals, the strength of the stimulatory signal in the T. repens system is the resultant of the two. Bearing in mind that in other model systems a network of shoot and root feedback and interacting signals is known to operate (Beveridge, 2006; Dun et al., 2006; Simons et al., 2007; Ongaro and Leyser, 2008), this resultant is referred to as the net root stimulus (NRS) for convenience throughout this report.
The aim of the present investigation was to begin to develop a generic predictive model of the physiological regulation of the shoot branching pattern by roots based on the knowledge gained from previous studies using T. repens. As a basis for constructing this model, it was necessary to establish, first, whether the strength of the net positive stimulus from the roots at an axillary bud site at the time of its emergence from its parent apical bud is constant during a plant's development, and, secondly, whether it is uniformly distributed to all emerging axillary buds throughout a plant. Two testable hypotheses were therefore set up regarding the mechanisms by which root-derived signals might regulate the branching pattern. Hypothesis 1 proposes that the supply of NRS to an axillary bud as it emerges from its parent apical bud decreases as the phytomeric distance of the apical bud from the nearest root system increases during its growth away from the basal root system. This would explain the observation that the vigour of bud outgrowth into lateral branches declines from phytomer to phytomer acropetally. Hypothesis 2 proposes that the availability of NRS to emerging buds on a lateral branch is higher in a branch that is closer to a root system than one that is more distanced. If correct, this would provide a possible explanation of the observation that growth of axillary buds to form secondary branches is much greater on the basalmost branches.
In designing experiments to test these hypotheses, it was important to overcome the possibility that the decline in growth response of emerging axillary buds in different positions in the shoot system might result from variation in their sensitivity to NRS. Such variation seems quite possible in the light of observations that axillary buds emerging on vigorously growing branches tend to grow out more readily into secondary branches than those on more weakly growing branches (Salemaa and Sievänen, 2002; Thomas and Hay, 2007a). To eliminate possible variation in the sensitivity of axillary buds to NRS that might be associated with their position in the shoot system, the knowledge was used that a newly emerging bud can be activated by an associated nodal root and that it retains the activation level attained even after excision of the nodal root, whereupon it becomes wholly dependent upon the basal root system (Thomas and Hay, 2007a). After using the temporary presence of nodal roots to activate buds highly and equally, thereby standardizing the sensitivity of buds to NRS, the rate of growth of axillary buds at a range of locations within the shoot system could then be tested and any differences in growth rate be ascribed to the availability of NRS, supplied from the proximal region of the plant, at those plant locations.
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Materials and methods |
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Plant material
All experiments were carried out using a glasshouse-grown stock clone of a single genotype of T. repens L. selected from a Spanish ecotype collection (AgResearch accession number C1067) that remains fully vegetative at temperatures above 12 °C irrespective of photoperiod (Thomas, 1982), shows a pattern of branching in response to nodal root treatments that is typical for the species (Thomas et al., 2002), and, as for the species in general, has a branching pattern that remains consistent under the glasshouse conditions used irrespective of the time of year (Thomas et al., 2003a; Thomas and Hay, 2007a).
Culture of experimental plants
Plants were grown from stem tip cuttings planted in a commercially obtained potting mix (Thomas et al., 2002) in 2.2 l planter bags. After 3 weeks, the two or three basalmost branches that had formed by this time were trimmed off each plant to leave a single primary stem growing away from its basal root system. All lateral branches that grew out subsequently from this primary stem were retained. The oldest phytomer on the primary stem that retained its branch was termed phytomer 1 (P 1) and later-formed ones termed P 2, P 3, etc. Outgrowth of nodal roots from the branches that developed was prevented by growing shoot systems out over a dry plastic mesh. Throughout the investigation, plants were grown in a heated glasshouse in natural photoperiods at average maximum day/minimum night temperatures of 30/15 °C for Experiment 1 and 25/12 °C for Experiment 2.
Where required, outgrowth of a nodal root on a primary stem or a lateral branch was stimulated by firmly pinning the youngest newly emerged node to a mound of moist potting mix contained in a 200 ml plastic pot.
Experiment 1
This experiment aimed to test Hypothesis 1: that the availability of NRS to axillary buds in the distal region of the plant decreases as plants grow larger and away from their rooted base. Treatments only differed as to the size of the shoot system between the basal root system and the distal region where NRS availability was assessed using a standard procedure (see Fig. 2a). All lateral branches that developed from P 1 onwards between the basal roots and the rooted node at the distal end of the plant were retained throughout the experiment and allowed to develop fully and unhindered. In Fig. 2, the short broken-shafted arrows indicate the positions of elongating lateral branches along the primary stems, but not their lengths. On 17 December, lateral branches of plants in treatment E were similar to those shown in Fig. 1, with those in treatments D to A being successively shorter.
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Thirty plants were grown from five groups of six shoot tip cuttings from the same stock plants taken at
10 d intervals from 1 October to 16 November 2004 so that at the start of experimental treatments (A–E) on 17 December 2004 the youngest phytomer emerged from its apical bud on the primary stem of each was the sixth (A), ninth (B), 12th (C), 15th (D), or 18th (E) from the basal root system (Fig. 2a). The youngest node at the distal end of each primary stem was, at this time, stimulated to form a nodal root. In response to this, axillary buds on each primary stem distal to the rooted node, that otherwise would not have grown out, were stimulated to do so (Fig. 2b). Growth rates of the lateral branches developed at the rooted nodes, at P 6, 9, 12, 15, and 18, were recorded during the period 15–32 d after the onset of stimulation of nodal root outgrowth up to the time when the sixth node on each was newly emerged (Fig. 2b). Previous experience indicated that by this time the apical buds on these branches would have been equally and fully activated (Thomas and Hay, 2007a). When individual branches reached this developmental stage, their associated nodal roots were excised, leaving them, together with the rest of the plant shoot system, dependent upon the root system at the base of the primary stem. Subsequent differences in the rates of growth of the two youngest emerged axillary buds, all of which had acquired equal potentials for growth, could then be ascribed to the effects of the different sizes of the basal portions of the plant on the availability of NRS to them. To maximize the expression of the responses in these two buds, hereafter referred to as test buds, all tissues distal to the rooted node on each primary stem were excised so as to channel NRS in the distal region of the primary stem into the branch that bore the buds (Fig. 2c). Also, to restrict any effects of treatments solely to influences of basal origin, the apical bud just distal to the newly emerged sixth node on each experimental branch was excised. Immediately after the excision procedures, dry weights of nodal roots were determined. Measurements of subsequent axillary test bud growth on the experimental branches (counts of emerged leaves and bud stem length) were made at 4 d intervals for the next 3 weeks.
Experiment 2
This experiment aimed to test Hypothesis 2: that the availability of NRS within a plant, at any given point in time, is greater nearer the basal root system and less at branches more distant from it. Thus the experiment was designed to compare the rate of growth of test buds simultaneously at several different locations within an individual plant after their potential for growth had been standardized using the temporary presence of a nearby nodal root as described in Experiment 1 (Fig. 3). To maximize the expression of differences in NRS availability at the test buds, as in Experiment 1, some shoot tissues were excised, as described below, so as to channel the available NRS to the test buds on the treated branches and to remove any possible influence of variations in intensity of apical inhibition from branch to branch (Fig. 3c).
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Six plants were grown from shoot tip cuttings until 17 phytomers had emerged from the apical buds of their primary stems. At this stage, on 19 May 2005, nodal root outgrowth was stimulated from the youngest emerged node on the primary stem (at P 17) and on the youngest emerged nodes of the lateral branches at P 1, 4, 7, and 10 along the primary stem (Fig. 3a). Lateral branches intercalated between these remained unrooted and grew unhindered throughout the experiment. As in Fig. 2, short broken-shafted arrows indicate only the positions of elongating branches along the primary stem but not their lengths.
The lateral branch at P 17 and the secondary branches stimulated to grow from the axillary buds at the rooted nodes on the lateral branches at P 1, 4, 7, and 10 were allowed to grow until six nodes had emerged on each (Fig. 3b), and their rate of growth over this period determined. Once their sixth nodes had emerged, the nodal roots, together with all tissues distal to the root at P 17 on the primary stem and distal to the roots on the lateral branches at P 1, 4, 7, and 10, were excised (Fig. 3c) and nodal root dry weights measured. At the same time, each root-induced branch was decapitated distal to its youngest emerged node and its four oldest axillary buds excised to leave only the two youngest emerged buds remaining. The rates of growth of these two test buds were then determined at weekly intervals over 21 d by measuring bud stem lengths and number of leaves emerged from them.
Analysis of data
For the axillary bud at any phytomer position within the plant, means and standard errors for the rates of leaf emergence and stem elongation were all calculated independently (Genstat, 2001). For Experiment 1, regression analyses were used to examine the slopes of the lines in Fig. 4 for evidence of difference from zero slope and to test the linear and non-linear relationships shown in Fig. 5 (Genstat, 2001). For all plants in Experiment 2, the axillary bud characteristics of rates of leaf emergence and stem elongation were regressed (Genstat, 2001) on the phytomer position of the bearing branch (Figs 6, 7). The measurements for phytomer positions within each plant represent data with correlated random effects, as in repeated measures. A straight line regression grouped by plant is an ad hoc, though reasonably robust, way of modelling the data. The correlation of errors is essentially assumed to be the same across plants, and is treated as residual. An overall regression was then done to see if the elevation of the line and its slope are significantly different from zero for the plants in general, which is equivalent to modelling the means across plants at each phytomer position.
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Results |
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Experiment 1
The establishment of uniform growth rates of axillary buds in response to nodal roots
Growth rates of axillary buds at the distal ends of primary stems decreased markedly with increasing phytomeric distance from the basal root systems (Table 1). During the 23 d period after the onset of stimulated nodal root outgrowth, the rate of leaf emergence per day on buds immediately proximal to, and therefore uninfluenced by, a nodal root (Thomas and Hay, 2004, 2007a) ranged from 0.27 at a distance of five phytomers (P 5) from the basal root system down to 0.10 at 17 phytomers. Likewise, stem elongation rate per day on branches developing from these same axillary buds decreased from 7.57 mm at P 5 to 0.08 mm at P 17.
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A similar trend in growth rates was also observed in the time taken by axillary buds to respond to nodal roots at P 6, 9, 12, 15, and 18, those at P 6 and P 9 responding sooner than those at P 12, 15, and 18. As a result, there was an
8 d delay in the emergence of the sixth leaf from the axillary buds at the rooted phytomers at P 12, 15, and 18 compared with that at P 6 and P 9. During the 8 d period immediately preceding the emergence of the sixth leaf and the excision of the nodal root, the growth rates were all similar, however, as seen in Fig. 4. Despite the apparently higher rate of bud stem elongation at P 6 and P 9, regression analyses of the data shown in Fig. 4 found no significant slope with the phytomer position of the buds, relative to the basal roots, for leaf emergence rate (P > 0.089) or stem elongation rate (P > 0.156).
Nodal root dry weights
Because the axillary buds at P 6 and P 9 grew out into branches faster than those at P 12, 15, and 18, the sixth leaf emerged from the buds at P 6 and P 9 sooner (after 23 d) than from those at P 12, 15, and 18 (after 32–34 d). Nodal roots at P 6 and P 9 were therefore younger and smaller at the time of their excision than those at P12, 15, and 18. This is reflected in the nodal dry weights being greater in the larger older plants (rooted at P 12, 15, or 18), than in the smaller younger plants rooted at P 6 and P 9 (Table 1).
The effect of lateral branch position on growth rate of axillary test buds
Activation of axillary buds by nodal roots resulted in their development into lateral branches at P 6, 9, 12, 15, and 18 on each of which there were two axillary test buds (Fig. 2). The rates of growth of these test buds following nodal root excision (Fig. 2c) varied greatly with phytomeric position of the branches on which they were borne (Fig. 5). Buds on branches nearer the basal root system, at P 6 and P 9, grew out much faster than those further away. Regression analyses found that the relationship between rate of leaf emergence per bud and phytomeric distance (Fig. 5a) was of linear form, y=a + bx, where a=9.67±0.717, b = –0.430±0.0541, P < 0.001, whereas the rate of bud stem elongation (Fig. 5b) showed an exponential relationship of form y=a + brx, where a=3.64±3.581, b=730±340, r=0.683±0.0532, P < 0.001.
Experiment 2
The establishment of uniform growth rates of axillary buds in response to nodal roots
In this experiment the initial stimulation of axillary bud growth in response to a nodal root was slower on the lateral branch at P 10 and on the primary stem at P 17, being delayed by 4 d compared with buds at P 1 and P 4. As a result of this, emergence of six leaves from these axillary buds took an average of 41.75 d from the start of nodal rooting in the former compared with 37 d in the latter. However, it is clear from Fig. 6 that buds at all rooted nodes were uniformly activated during the 10 d immediately preceding emergence of the sixth leaf and application of excision procedures (Fig. 3c). Regression analyses showed that the rates of leaf emergence (P > 0.816) and stem elongation (P > 0.995) of all these buds were similar during this period.
Nodal root dry weights
In this experiment, in which the nodal roots were all excised over a time span of 37–42 d after the onset of stimulated root outgrowth, the position of the roots on the plant had no statistically significant effect on their dry weight, which ranged only from 292 mg to 317 mg.
The effect of the location within a plant on growth rate of axillary test buds
Following the excision procedures imposed once the sixth leaf had emerged from buds at the rooted nodes, the growth of the two axillary test buds (see Fig. 3c) formed from uniformly activated apical buds on secondary branches at P 1, 4, 7, and 10, or on a nodal root-induced lateral branch at P 17, varied little with position on the plant (Fig. 7). Regression analysis indicated there was no statistically significant trend over the phytomer position of buds within plants for either rate of leaf emergence (P > 0.210) or rate of bud stem elongation (P > 0.124). Importantly, with regard to Hypothesis 2, there was no decline in test bud growth with increasing phytomeric distance from the basal root system on which they were now dependent. Though not significant statistically, the trend was in the opposite direction.
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Discussion |
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It has been shown previously (Thomas and Hay, 2007a) that in T. repens the outgrowth of a nodal root activates the axillary bud at the same node and that the rate of growth of the bud, as measured by the rate of leaf emergence from it, reflects its level of activation. In the present study, the possibility that the responses of test axillary buds were the result of differences in their initial activation levels, rather than in the level of NRS available, was overcome by comparing responses of buds on branches which had all their apical buds raised to a similar activation level (Figs 4, 6). In addition, any possible variation in inhibitory influence of the apical bud was avoided by its excision immediately distal to the activated test buds. Thus any observed difference in the growth rate of axillary buds at different positions can be ascribed solely to variations in the net stimulatory influence of the proximal regions of the plants.
It is therefore concluded that the results of Experiment 1 (Fig. 5) show that the stimulatory influence of the proximal region of the plant on the rate of growth of axillary buds decreases as their phytomeric distance from the basal root system increases. Hypothesis 1 is, thereby, strongly supported.
The results of Experiment 2 (Fig. 7) show there to have been no clear effect of the intra-plant position of axillary buds on their subsequent rate of growth once buds had their potential for growth raised to the same level. Hypothesis 2, that more basally located buds might receive higher levels of NRS at the time of their emergence, is thus not correct. Instead, these results strongly suggest that NRS availability, at any particular point in time, does not decrease with increasing phytomeric distance from the basal root system; rather, NRS is fairly uniformly distributed to the apical buds of the primary stem and all lateral branches. Because previous observations have suggested that stimulatory root signals are transported acropetally in the xylem (Thomas et al., 2002), these results are probably to be expected as the outcome of stimulatory influences being passively supplied to all growing apical buds via the transpiration stream.
Based on these findings, the development of a predictive model was undertaken as an aid to understanding the factors determining the observed pattern of decline in branching that occurs as distance from roots increases (Fig. 8). In constructing this model, the following assumptions were made: (i) the activation level attained by a bud is dependent upon the level of NRS available at the bud site at the time of its emergence (Thomas and Hay, 2007a); (ii) the activation level attained is retained for up to 6 weeks (Thomas and Hay, 2007a); (iii) the level of NRS at the site of an emerging bud on the primary stem decreases as its phytomeric distance from the nearest root increases (Fig. 5; Thomas et al., 2003b); and (iv) at any given point in time, the availability of NRS at each emerging bud within the shoot system distal to a nodal root is uniform (Fig. 7).
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It has also been assumed, based on Fig. 5a, that the level of NRS available at the sites of successively emerging buds decreases by 10%, compounding, from phytomer to phytomer along the primary stem.
The rate of leaf emergence was chosen as the index of bud activation for two reasons. First, there is evidence that stem elongation of a bud may to some extent be directly dependent on its leaf emergence rate (or the activation level of the bud itself) and, therefore, only indirectly influenced by NRS. For instance, internode elongation ceases upon the excision of apical tissues but is partially restored by application of auxin and/or gibberellin to the stump (Thomas, 1972) and it also fails to occur when the leaf appearance rate of a bud remains below 0.06 leaves per day but increases exponentially as the leaf appearance rate increases above this threshold value (Thomas et al., 2002; Thomas and Hay, 2007a). Secondly, the use of the leaf emergence rate is convenient as it has a direct linear relationship with NRS availability (Fig. 5a; Thomas and Hay, 2007a). The evidence that the physiology of regulation of leaf emergence and stem elongation appears to differ in this species is consistent with the suggestions of Cline (1997) concerning the regulation of the differing processes of bud outgrowth.
In constructing the model shown in Fig. 8, the maximum activation level possible has been arbitrarily designated as 100 units. Thus, in Fig. 8a, the level of activation of the primary stem apical bud (AB) was set at 100 units by the initial availability of NRS. The bud at phytomer 1 (P 1) received the same level of NRS and its activation was therefore also set at 100 units. By the time the second bud on the primary stem emerged, however, the level of NRS available was reduced by 10% to 90%, thereby establishing the activation level of that bud at 90 units. At the next bud on the primary stem (at P 3) the NRS level was 10% lower again, at 81%, thereby setting the activation level of that bud at 81 units. The first bud to emerge from the apical bud on the lateral shoot at P 1 did so at the same time as the bud at P 3 on the primary stem and thus achieved the same activation level of 81 units. In Fig. 8b, four more buds have emerged from the apical bud of the primary stem and there are now five lateral branches. Successively emerging buds on the primary stem did so at NRS levels decreasing from 100 to 90, 81, 73, 66, 59, and 53%. On the basis that distribution of NRS to emerging axillary buds throughout the plant is uniform (Experiment 2), the youngest buds emerging on all five lateral branches and on the primary stem are envisaged to do so at equal NRS levels of 53% and will thus all attain an activation level of 53 units.
In Fig. 8c, the plant has reached the same stage of development as that illustrated in Fig. 1. All newly emerging buds have a low activation level of 19 units in response to a uniform NRS availability of only 19%. Comparison of these two figures reveals a slight discrepancy between the observed pattern of branching in Fig. 1 and that expected from the model in Fig. 8c. At the basalmost six phytomers (P 1–P 6) in Fig. 8c, the predicted number of buds emerged on successive lateral branches is similar to that observed in Fig. 1 but the number emerged at successive phytomers in Fig. 1 declined more rapidly thereafter than the number expected from the model, as seen below:
Observed number of emerged buds/lateral branch15 15 13 11 10 9 7 5 4 2 1 0 0 0 0 (Fig. 1)
Expected number of emerged buds/lateral branch
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 (Fig. 8c)
In addition, the observed growth of secondary branches from lateral branches at phytomers distal to P 2 on the primary stem in Fig. 1 is less than would be predicted from the model. For example, in the model (Fig. 8c), the axillary bud forming the lowermost secondary branch on the lateral branch at P 4 emerged at the same time, and therefore the same level of NRS availability (59%), as the axillary bud at P 6 on the primary stem. The model therefore predicts that their activation levels would be the same (59 units), and, as a result, they would be predicted to have produced the same number of emerged phytomers by the time the primary stem had produced 17 phytomers: in other words, they would both have 10 emerged phytomers. This is not so, however; in the plant shown in Fig. 1, nine new phytomers emerged on the lateral branch at P 6 while only two emerged on the lowermost secondary branch at P 4. Similarly, the model predicts that if an activation level of 48 units is high enough to stimulate growth of a bud into a secondary branch at the sixth node on the lateral branch at P 1, as did occur in the plant in Fig. 1, then bud growth to form a secondary branch would also be expected at the lowermost node on the lateral branch at P 6; but the latter did not occur on the typical plant represented in Fig. 1.
The present findings can explain in large part the decline in the growth of axillary buds along the primary stem as it grows away from its basal root system, but not the very rapid progressive decline observed in secondary branch development on successive lateral branches along the primary stem (Thomas et al., 2002). Bearing in mind the apparently uniform distribution of stimulatory root-derived signals throughout the plant observed in Experiment 2, the present observations imply that the sensitivity to stimulatory signals that is possessed by axillary buds at the time of their emergence on lateral branches located near the basal roots is greater than that of emerging buds on more distal branches. The possibility that the potential for activation is lower in buds forming on more distal lateral branches is supported by observations made in both of the experiments. In Experiment 1, newly emerged axillary buds at P 12, 15, and 18 were slower to respond to stimulation by a nodal root than those at P 6 and P 9. Similarly, in Experiment 2, in which the availability of NRS throughout the plant appeared to be uniform (Fig. 7), the time taken for newly emerged axillary buds to respond to the presence of newly forming nodal roots was greater at phytomers that were more distanced from the basal root system. This suggests that the decreasing strength of NRS throughout the plant, as the shoot system grows larger, is not the only factor influencing the rate of growth of buds.
It has been shown that knowledge of NRS availability to emerging buds on lateral branches formed at increasing distances from the basal root system (Experiment 1) is not, by itself, a sufficient basis from which to construct a predictive model. However, findings from this study suggest a possible additional factor to include in the model to improve its predictability. From the results of Experiment 1, it would be expected that activation levels of the apical buds of successively formed lateral branches would decrease as a result of decreasing NRS availability at the time of their emergence from the primary stem apical bud, and the results of Experiment 2 indicate that despite all buds that emerge at any one time receiving the same level of NRS, the growth of those on branches more distanced from the basal root system is slower. It thus seems possible that the lower activation level of the apical bud on a branch that is further from the basal root system in turn leads to a decreased ability of an axillary bud emerging from it to become activated in response to its local NRS availability. In other words, the ability of such an emerging bud to respond to a given level of local NRS might be, at least in part, directly influenced by the activation level of the apical bud from which it emerges. It is therefore suggested that the level of activation attained by a bud as it emerges from its parent apical bud might be determined by a combination of both the level of NRS available to it and its ability to respond (a characteristic determined by the activation level of the apical bud from which it emerges). The experimental testing of this hypothesis will be the next stage in development of a predictive model of branching regulation for perennial prostrate-stemmed species.
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Acknowledgements |
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We thank Jocelyn Tilbrook and Rachael Sheridan, AgResearch Grasslands, for technical assistance, and Fred Potter for statistical advice. The constructive comments from two anonymous referees greatly benefited this manuscript. This work was funded by the MeriNet programme, New Zealand Foundation for Research, Science and Technology, contract C10X0404.
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References |
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