Elusive but not hypothetical: axillary meristems in Wollemia nobilis

Abstract

Background

The branches of Wollemia nobilis are unbranched; however, it has been noted that new branches can form from the distal end of damaged ones, and branches can grow from axillary structures once a terminal strobilus has fallen. Tomlinson and Huggett (2011, Annals of Botany 107: 909–916) have recently investigated the formation of these reiterative branches and stated in the title of their paper that ‘Partial shoot reiteration in Wollemia nobilis (Araucariaceae) does not arise from “axillary meristems”’. They go on to state ‘Further research may reveal the presence of these elusive, but still only hypothetical, axillary meristems’.

Response

In this Viewpoint, I argue that Tomlinson and Huggett do not refer to previously published information that indicates that axillary meristems are present in Wollemia nobilis branch leaf axils, and that their anatomical methods were probably not optimal for locating and examining these minute structures. Thus, whilst I would agree that the axillary meristems in branch leaf axils of Wollemia nobilis are elusive, I contend that they are not hypothetical.

INTRODUCTION

The branches of Wollemia nobilis display several botanical peculiarities. They are plagiotropic, unbranched, relatively short-lived, terminated by male or female strobili, shed as single units with all leaves still attached, and have a well-developed basal abscission zone (Burrows et al., 2007). Apparently associated with several of these features is a remarkable constriction of the branch base vascular supply (Burrows et al., 2007). As noted the branches are unbranched, but it has been noted that damaged branches can form new branches (Offord et al., 1999; Tomlinson and Murch, 2009) and branches can grow from axillary structures once a strobilus has fallen (P. F. Meagher and C. A. Offord, unpubl. res.). Tomlinson and Huggett (2011) have recently investigated the formation of these reiterative branches.

Their findings are summarized in the title of their paper: ‘Partial shoot reiteration in Wollemia nobilis (Araucariaceae) does not arise from “axillary meristems”’. I have two concerns with this statement. Firstly, that Tomlinson and Huggett (2011) do not refer to previously published information that indicates that axillary meristems are present in Wollemia branch leaf axils, and secondly, their anatomical methods were probably not optimal for locating and examining these minute structures.

EVIDENCE

As noted, Tomlinson and Huggett (2011) did not refer to published information (both text and photomicrographs) that demonstrate the presence of axillary meristems in branch leaf axils of Wollemia nobilis (Burrows, 1999). Tomlinson and Huggett state that the suggestion [from information in Burrows (1999) and Burrows et al. (2003)] that all leaf axils in the Araucariaceae were potential sites for future bud development ‘was made only with reference to orthotropic shoots’. In contrast, Burrows (1999) illustrated axillary meristems in the leaf axils of plagiotropic shoots of Wollemia nobilis with both juvenile (figs 1, 3, 4) and adult leaf morphology (figs 5, 6). Interestingly, in an earlier paper, Tomlinson and Murch (2009) did mention that Burrows (1999) had found axillary meristems in plagiotropic branches of Wollemia. Tomlinson and Huggett (2011) is not the first paper to report a failure to locate araucarian axillary meristems. Holthusen (1940) did not find meristems or buds in the apparently blank leaf axils of 22 species of conifers in 11 genera, including four species of Araucariaceae. Holthusen (1940) collected leaf axils from side shoots of various orders. Commenting on Holthusen's results Burrows (1987) noted that axillary meristems in this type of material ‘are reduced in size and relatively inconspicuous’.

In addition, Burrows (1986) included photomicrographs of axillary meristems in the axils of leaves on first-, second- and third-order branches of 2-year-old Araucaria cunninghamii plants (figs 26–28). He noted that as the branch order increased the structural complexity of the leaf axil and size of the axillary meristem decreased. The meristems were <100 µm in diameter, indicating they were progressively more difficult to locate in smaller diameter shoots. Axillary meristems were also recorded in plagiotropic branches of another four species of Araucaria and six species of Agathis (Burrows, 1987). This indicates that in the Araucariaceae axillary meristems have been widely recorded in leaf axils of plagiotropic branches.

Tomlinson and Huggett found no evidence of axillary meristems in untreated (non-decapitated) branches, but found clusters of cells with enlarged nuclei in approximate axillary positions in decapitated material. As Burrows (1999) and Tomlinson and Huggett (2011) studied different plants there is potential for differences in material to give different results. However, the failure to detect axillary meristems by Tomlinson and Huggett could be related to sectioning techniques. Burrows (1999) used plastic-embedded material, sectioned at 1–4 µm and stained with toludine blue. Tomlinson and Huggett used unembedded material, sectioned at 60–90 µm, and initially examined unstained sections. Previous studies have shown araucarian axillary meristems in branch leaf axils are small structures (<100 µm diameter and often much smaller: 45–75 µm in Wollemia) and are generally smaller than those in the orthotropic mainstem (Burrows, 1986, 1999). When searching for such structures, plastic-embedded, semi-thin sections will provide high-quality sections where a single axillary meristem will appear sequentially over several sections, increasing the chance of detection. While serial sections 50–100 µm thick are suited for following the path of vascular tissue and resin canals they are not ideal for locating small, infrequent structures (a whole axillary meristem can fit on a single section). In addition, when a meristem is found it will be several cell layers in thickness which can hamper accurate observations. For example, the unembedded sliding microtome sections of Tomlinson and Murch (2009, fig. 2L) gave a rather vague image of a Wollemia orthotropic axillary meristem that did not show diagnostic features such as the surface layer and adjoining lacuna.

As they did not find axillary meristems in intact material, but found that the buds formed in an axillary position in decapitated shoots, Tomlinson and Huggett deduced the buds ‘are initiated by cell dedifferentiation from a group of cells at a precise axillary location’. Tomlinson and Huggett reported that in other plants there can be a phase of partial differentiation of an axillary bud or meristem before the cells become meristematic again. Other references not given by Tomlinson and Huggett support this as an infrequent mode of development (Fink, 1984; Evert, 2006). However, in the only detailed study in the Araucariaceae, Burrows (1986) serially sectioned an estimated 700–900 leaf axils in ten orthotropic apices from 2- to 3-year-old Araucaria cunninghamii plants. It was shown that the axillary meristems were ‘detached’ from the shoot apical meristem (figs 1–8) and continuously maintained with a meristematic appearance for many years (Burrows, 1986). While the ‘vacuolation’ of axillary meristem or bud cells, followed by a later differentiation back into meristematic cells, has been recorded, maintenance of a meristematic appearance would appear to be typical of axillary meristems in the Araucariaceae.

While many leaf axils of conifers are generally considered to be ‘blank’ or ‘empty’ (i.e. devoid of specialized bud-forming cells) (Fink, 1984; Burrows, 1986) the reality for several genera is much more interesting, with reports of relatively short-lived detached meristems in several genera (Fink, 1984), to the long-lived axillary meristems of the Araucariaceae. The ontogeny of reiterative axillary structures in the Araucariaceae continues to be of scientific interest, with different modes of origin now reported by Fink (1983), Burrows (1986, 1987) and Tomlinson and Huggett (2011).

Tomlinson and Huggett note that the suggestion that the buds arise from differentiated cells ‘is certainly open to question’ which appears to be at odds with the definitive statement made by the paper's title. Tomlinson and Huggett state that ‘Further search may reveal the presence of these elusive, but still only hypothetical, axillary meristems’. I would agree that the axillary meristems in branch leaf axils are elusive, but contend that they are not hypothetical.