Role of corticular photosynthesis following defoliation in Eucalyptus globulus

Abstract

Defoliation can reduce net fixation of atmospheric CO2 by the canopy, but increase the intensity and duration of photosynthetically active radiation on stems. Stem CO2 flux and leaf gas exchange in young Eucalyptus globulus seedlings were measured to assess the impact of defoliation on these processes and to determine the potential contribution of re-fixation by photosynthetic inner bark in offsetting the effects of defoliation in a woody species. Pot and field trials examined how artificial defoliation of the canopy affected the photosynthetic characteristics of main stems of young Eucalyptus globulus seedlings. Defoliated potted seedlings were characterized by transient increases in foliar photosynthetic rates and concomitant decreases in stem CO2 fluxes (both in the dark and light). Defoliated field-grown seedlings showed similar stem CO2 flux responses, but of reduced magnitude. Despite demonstrating increased re-fixation capability, defoliated potted-seedlings had slowed stem growth. The green stem of seedlings exhibited largely shade-adapted characteristics. Defoliation reduced stem chlorophyll a/b ratio and increased carotenoid concentration. An increased capacity to re-fix internally respired CO2 (up to 96%) suggested that stem re-fixation represents a previously unexplored mechanism to minimize the impact of foliar loss by maximizing the contribution of all photosynthetic tissues, particularly for young seedlings.

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

“…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). Photosynthesis is responsive to the requirements of sinks for assimilated carbon.…”

“…in E. cloeziana and E. pilularis, Alcorn et al, 2008). Light-saturated photosynthesis was increased by 11-57% following partial defoliation of 9-12-month-old E. globulus Turnbull et al, 2007;Eyles et al, 2009b;Quentin et al, 2010Quentin et al, , 2012Pinkard et al, 2011a,b). This response commonly occurs throughout the crown irrespective of foliage age (Pinkard et al, 1998) though the magnitude and duration of the response may vary with severity, growing condition, and tree age.…”

“…Partial defoliation also has a rejuvenating effect on older, undamaged foliage, restoring the photosynthetic capacity to near the value of newly formed leaves (Pinkard et al, 1998;Forrester et al, 2012). 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).…”

“…The duration of this photosynthetic response in eucalypts is generally transient in saplings (e.g. 3-5 weeks in E. globulus seedlings, Turnbull et al, 2007;Eyles et al, 2009b;Quentin et al, 2012) but can last for up to 12 months in 3-year-old trees (Pinkard et al, 1998). Increases in photosynthetic rate following defoliation occur earlier in seedlings than trees (Pinkard et al, 1998), as the capacity to store carbon increases with tree size and age (Cannell and Dewar, 1994).…”

Stable Isotopes Reveal the Contribution of Corticular Photosynthesis to Growth in Branches of Eucalyptus miniata

“…Although many eucalypts maintain smooth-bark surfaces by seasonally shedding a layer of dead bark, little research has been conducted into corticular photosynthesis in these trees (Tausz et al, 2005;Cernusak et al, 2006;Cerasoli et al, 2009;Eyles et al, 2009). Of particular interest from an ecological and evolutionary perspective is the extent to which corticular photosynthesis contributes toward the carbon economy of smooth-barked eucalypts.…”