134 Moore: Root Tip Growth and the Presence of Leaves Affect Epicormic and Lignotuberous Shoot Development in a diverse group of tree species. Lignotubers (basal burls) are modified stem structures that are reservoirs of large numbers of dormant buds (Carr et al. 1984; Burrows 2002). They occur in many members of the Myrtaceae, and in eucalypts, lignotubers develop from axillary buds, usually at the first three leaf nodes at the base of seedlings at or just below the soil sur- face (Chattaway 1958; Carr et al. 1984; Molinas and Verdaguer 1993; Mibus and Sedgley 2000; Clarke et al. 2013). Subsequent divisions are downwards, and as the stem of the tree increases in diameter, the ligno- tuber is incorporated into the trunk largely below- ground (Mullette 1978; Moore 1982), where it is well-protected by the insulating properties of soil from the high temperatures that occur during forest fires (Jacobs 1955; Chattaway 1958; Parsons 1968; Carrodus and Blake 1970; Mullette 1978; Moore 1982; Clarke et al. 2013). Lignotubers contain many bud traces (Carrodus and Blake 1970), and trees subjected to multiple epi- sodes of stress may produce hundreds of lignotuber- ous shoots over several decades (Chattaway 1958; Burrows 2002). Lignotuberous buds, which exhibit developmental plasticity, can also form roots that replace damaged root systems (Clarke et al. 2013). When the upper parts of the tree are destroyed, the potential of the lignotuber to produce shoots is real- ised as part of recovery processes (Colak et al. 2009). Lignotubers may act as storage organs when they contain large amounts of carbohydrates (Carrodus and Blake 1970; Bamber and Mullette 1978; Varner et al. 2009). The production of epicormic or lignotuberous shoots does not guarantee survival of a tree, as the mortality of the shoots is very high. For Eucalyptus obliqua, there is an average mortality rate of up to 50% for epicormic shoots, and between 50% and 100% for lignotuberous shoots, depending on the level of stress imposed (Moore 2015). These responses are relevant to arborists whose role in post-fire management in peri-urban parts of cities has expanded (Moore 2011). However, they also provide insight into shoot pro- duction after other stresses, such as insect defoliation or the severance of tree trunks by vehicles. In some ways, the effect of fire and heat on plants is to cause an effective defoliation and/or decapita- tion, which subsequently causes a reduction in root carbohydrate reserves when starch is converted to sugar (Parker 1970; Wargo et al. 1972; Clarke et al. ©2021 International Society of Arboriculture 2013). High temperatures can cause the degradation of cellular constituents such as lipids and plant pig- ments (Bär et al. 2019), and there is the possibility of translocated heat injury, where heat damage to one part of a plant produces inhibitors that affect develop- ment in other parts (Yarwood 1975; Bär et al. 2019). The production of phaeophorbide (pheide) from the degradation of chlorophyll has been known for a long time (Aubry et al. 2020), but more recently its effects on other metabolic moderators such as jasmonic acid, ethylene, and abscisic acid (ABA) have become clearer (Kim et al. 2018; Kuai et al. 2018). These hor- mones and inhibitors have a role in senescence but may also inhibit shoot initiation after stress (Aubry et al. 2020). Decapitation and defoliation also reduce root dry weight and growth but enhance leaf produc- tion (Bruce 1956; Madgwick 1975; Hodkinson and Baas Becking 1978; Moore 2015). These responses can impact the production and survival of shoots pro- duced subsequent to fire and other stresses. Both epicormic and lignotuberous buds usually have access to a store of carbohydrates and a supply of water and nutrients from an established root sys- tem. This is a rapid response system that allows the restoration of photosynthetic capacity in the case of serious damage to the foliage and trunk. In warmer parts of the world, with longer growing seasons, epi- cormic and lignotuberous shoots can grow at rates exceeding 4 m per annum under good conditions (Moore 2015). The production of shoots from dormant buds is often explained in terms of access to carbohy- drate reserves moderated by hormones, particularly auxins and cytokinins (Tworkoski et al. 2006; Ras- mussen et al. 2009; Rasmussen and Hunt 2010; Meier et al. 2012; Clarke et al. 2013; Aloni 2015). The disruption in hormone concentration may be triggered by defoliation, fire, pruning, or sudden expo- sure to higher levels of light (Moore 1998, 2015; Meier et al. 2012; Aloni 2015). During wildfires, there is an effective defoliation or decapitation that removes some or all of the aboveground parts of the plant, impacting hormone levels (Meier et al. 2012; Clarke et al. 2013; Aloni 2015). Often, however, the root system remains undamaged due to the insulating properties of soil, and so root-sourced hormones are still present (Clarke et al. 2013). Root tip involvement in overall plant coordination and response is well-established (Fromm and Eschrich 1993; Baluska et al. 2004; Aloni 2015; Karban 2015), but the involvement of the root system
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