Resource limitation can constrain a plant’s ability to respond to herbivores (Coley et al., 1985; Herms & Mattson, 1992; Steppuhn et al., 2008) and recently we have begun to appreciate how grazed plants mobilize these resources to or from attack sites. Resource flow can increase, stop or reverse within hours in response to attack or elicitation (Babst et al., 2005, 2008; Schwachtje et al., 2006; Newingham et al., 2007; Frost & Hunter, 2008; Gómez et al., 2010) and depends on the source–sink dynamics of individual plant modules, their proximity and their integration via vascular architecture (Orians, 2005). Timely adjustments in resource allocation are needed to support a plant’s ability to respond to herbivore and pathogen attack (Matyssek et al., 2005).
Long-distance carbohydrate (CHO) transport can be influenced by wounding. However, the prevailing direction of CHO flow, towards or away from wounded plant tissues, is an issue of debate, as both have been observed to occur. In some species the import of CHOs via the phloem can be critical for the production of chemical defences in wounded tissues (Kleiner et al., 1999; Arnold & Schultz, 2002; Arnold et al., 2004). Such responses probably depend on a rise in cell wall invertase (CWI; EC 126.96.36.199) activity at the attack site, which facilitates phloem unloading at sinks (Sturm, 1999; Arnold & Schultz, 2002). Indeed wound-induced sink strength is a common plant response, triggered by grazing, mechanical wounding, infection, natural systemic signals and artificial elicitors in tomato (Ohyama et al., 1998), carrot (Sturm & Chrispeels, 1990), goosefoot (Ehness et al., 1997), pea (Zhang et al., 1996) and hybrid poplar trees (Arnold & Schultz, 2002; Arnold et al., 2004; Philippe et al., 2010), among others (Roitsch et al., 2003; Roitsch & González, 2004). For example, we previously demonstrated that biochemical responses to herbivory and jasmonate (JA) by young hybrid poplar leaves depend on long-distance CHO transport, which is associated with elevated CWI activity in sink leaves (Arnold & Schultz, 2002; Arnold et al., 2004). Disrupting CHO import reduced or eliminated CHO-based responses by elicited leaves. Polyphenols also accumulated in unelicited young leaves when normal CHO transport from sources was blocked by girdling, suggesting that export from those leaves is also an important defence regulator (Arnold et al., 2004; Steele et al., 2005). However, there have also been reports of the opposite response: the flow of CHOs away from wound sites, often to storage organs (Babst et al., 2005, 2008; Schwachtje et al., 2006; Newingham et al., 2007; Frost & Hunter, 2008; Gómez et al., 2010). For example, Schwachtje et al. (2006) found that simulated grazing and the removal of sink leaf tissues both resulted in an increased flow of carbon to root tissues (‘bunkering’) in native tobacco, Nicotiana attenuata, within 5 h. These C stores eventually prolonged flowering and led to increased seed capsule production in these plants. Gómez et al. (2010) recently observed induced export in tomato, showing that within 4 h of methyl jasmonate exposure the export of newly fixed C11 from leaves increased from 27% to 36%, with some of the label being transported to roots.
Nitrogen can also be involved in responses to wounding because it forms a part of defence metabolites or because of its requirement as part of enzymes necessary for metabolism or tissue repair. But the translocation of N per se from sources to sinks to support defence responses has not been well studied. Expression of wound-induced genes in young poplar leaves has been linked to N supply. Rennenberg et al. (2010) and Gómez et al. (2010) reported an increase in short-term nitrogen export from MeJA-elicited poplar leaves, from 12.5% to 22.7%. In other species, such as native tobacco, the nitrogenous defences themselves are transported via the vascular system in response to attack (Schwachtje et al., 2006).
In the study reported here, we used 13C and 15N labels simultaneously to determine whether nitrogen and CHO translocation from sources to wound sites is increased by treatment with the wound hormone JA, and the degree to which their movement is constrained by cell wall invertase activity, stem girdling and the presence of potentially competing sinks in branches. Our aim was to determine whether or not increased import of C and/or N resources is a component of the wound response in poplar foliage.