Whole-plant versus leaf-level regulation of photosynthetic responses after partial defoliation in Eucalyptus globulus saplings

Abstract

Increases in photosynthetic capacity (A1500) after defoliation have been attributed to changes in leaf-level biochemistry, water, and/or nutrient status. The hypothesis that transient photosynthetic responses to partial defoliation are regulated by whole-plant (e.g. source–sink relationships or changes in hydraulic conductance) rather than leaf-level mechanisms is tested here. Temporal variation in leaf-level gas exchange, chemistry, whole-plant soil-to-leaf hydraulic conductance (KP), and aboveground biomass partitioning were determined to evaluate mechanisms responsible for increases in A1500 of Eucalyptus globulus L. potted saplings. A1500 increased in response to debudding (B), partial defoliation (D), and combined B&D treatments by up to 36% at 5 weeks after treatment. Changes in leaf-level factors partly explained increases in A1500 of B and B&D treatments but not for D treatment. By week 5, saplings in B, B&D, and D treatments had similar leaf-specific KP to control trees by maintaining lower midday water potentials and higher transpiration rate per leaf area. Whole-plant source:sink ratios correlated strongly with A1500. Further, unlike KP, temporal changes in source:sink ratios tracked well with those observed for A1500. The results indicate that increases in A1500 after partial defoliation treatments were largely driven by an increased demand for assimilate by developing sinks rather than improvements in whole-plant water relations and changes in leaf-level factors. Three carbohydrates, galactional, stachyose, and, to a lesser extent, raffinose, correlated strongly with photosynthetic capacity, indicating that these sugars may function as signalling molecules in the regulation of longer term defoliation-induced gas exchange responses.

Impact of defoliation in temperate eucalypt plantations: Physiological perspectives and management implications

“…Eyles et al (2009b) demonstrated that the contribution of other photosynthetic tissues, such as green stem in young seedlings, may also represent a mechanism to minimise the impact of defoliation. Increases in photosynthetic rate following defoliation may be related to whole-plant responses such as changes in source-sink relationships, whereby remaining leaves need to fix more carbon to meet demand from zones of active growth (Turnbull et al, 2007;Pinkard et al, 2011b;Eyles et al, 2013). Reductions in foliar starch and soluble carbohydrate concentration have been observed in remaining leaves, which is consistent with increased sink demand (Eyles et al, 2009b;Quentin et al, 2010;Pinkard et al, 2011b; but see Turnbull et al, 2007).…”

“…Further research into the interaction of defoliation and abiotic stresses is important if we are to build capacity to model forest productivity into the future. Recent studies linking source-sink relationships to defoliation responses Barry et al, 2012;Eyles et al, 2013) may provide a useful conceptual framework for such research.…”

Multifunctional fructans and raffinose family oligosaccharides

“…Gol and Raf are now recognized as signaling molecules during biotic stress responses (Kim et al, 2008) and a similar role during abiotic stress responses has been suggested for RFOs (Valluru and Van den Ende, 2011; Eyles et al, 2013) and for fructans (Van den Ende et al, 2004). This led to the hypothesis that both RFOs and small fructans might act as endogenous, phloem-mobile stress signals.…”

Carbon source–sink limitations differ between two species with contrasting growth strategies

“…Despite this well‐developed ecological theory, we do not currently know the extent to which slower growth in perennials than annuals arises from greater source to sink limitation. Experimental manipulations of the source:sink ratio provide insights into the relative contributions of source and sink processes to growth rate and may be achieved through a variety of techniques including sink removal (Arp ), genetic modification (Ainsworth et al , Weichert et al ; Zuther et al ), source removal (von Caemmerer & Farquhar , Bryant et al ; Rogers et al ; Eyles et al ), inhibiting resource export from the source (Ainsworth & Bush ) and increasing source activity using elevated CO 2 (Kinsman et al ; Masle ), reviewed by White et al (). Here, we alter the atmospheric CO 2 concentration ([CO 2 ]) to non‐invasively manipulate the source:sink ratio in barley – elevated [CO 2 ] to increase the source strength and sub‐ambient [CO 2 ] to decrease it – with current [CO 2 ] as a reference against which to compare the source manipulations.…”