The benefits of recycling: how photosynthetic bark can increase drought tolerance

Woody tissue photosynthesis reduces stem CO2 efflux by half and remains unaffected by drought stress in young Populus tremula trees

“…In P. tremula , woody tissue photosynthesis followed a similar daily pattern among treatments (Figure 5c,d) with no differences in daily cumulative and maximum P wt (Table ), demonstrating that (a) periodic covering and uncovering of the cuvette did not induce chlorophyll degradation, despite prolonged light exclusion of woody tissue is known to reduce chlorophyll content (Bloemen et al, ), and (b) P wt performance is independent of stem water status. This remarkable observation confirms the often suggested hypothesis that P wt is less affected by drought than leaf photosynthesis (Bloemen et al, ; Cernusak & Cheesman, ; Steppe et al, ; Vandegehuchte et al, ). Contrary to photosynthesis in leaves, P wt does not depend on the uptake of atmospheric CO 2 but can instead rely on internal CO 2 pools, making it less vulnerable to dehydration because it will not be affected by stomatal closure and will, thus, increase water‐use efficiency (Bloemen et al, ; Cernusak & Marshall, ; Nilsen, ; Wittmann & Pfanz, ).…”

“…Maximum P wt rate was on average 0.43 ± 0.03 μmol m −2 s −1 , which is at the lower side of reported values (0.1–9 μmol m −2 s −1 ; reviewed by Ávila et al, ). Nevertheless, available literature has mostly focused on twigs and saplings in which P wt is expected to be higher than in older trees given their relatively smooth and thin bark that allows solar radiation to reach bark chlorophyll more easily (Aschan, Wittmann, & Pfanz, ; Ávila et al, ; Cernusak & Cheesman, ; Wittmann et al, ). Tarvainen et al () observed lower refixation rates in mature (90‐year‐old) Scots pine trees ranging between 0.1 and 0.4 μmol m −2 s −1 .…”

“…Assimilation of CO 2 within woody tissues (P wt ) is an important but often overlooked process in plant carbon budgets (Ávila Herrera, & Tezara, ; Cernusak & Cheesman, ; Tarvainen et al, ; Vandegehuchte, Bloemen, Vergeynst, & Steppe, ). High internal CO 2 concentration within the stem (from <1% to 26%, Teskey, Saveyn, Steppe, & McGuire, ), permeability of smooth bark for sunlight (Cernusak & Cheesman, ), and bark chlorophyll to photo‐reduce CO 2 (Berveiller, Kierzkowski, & Damesin, ; Pfanz, Aschan, Langenfeld‐Heyser, Wittmann, & Loose, ) facilitate photosynthetic processes in woody tissues. Whereas this recycling mechanism can offset between 7% and 98% of carbon respiratory losses in stem and branches (Ávila et al, ; Tarvainen et al, ; Teskey et al, ), positive net assimilation is however rarely reached (Wittmann, Aschan, & Pfanz, ).…”

“…Woody tissue photosynthetic rates range between 0.5 and 9 μmol CO 2 m −2 s −1 , with median values of 1.9 μmol CO 2 m −2 s −1 , and are generally lower than those in leaves, with maximum values being up to 75% of leaf photosynthesis (Ávila et al, ; Pfanz et al, ). Nevertheless, P wt is an important local carbon source for plant functioning, especially during drought stress when leaf photosynthesis and phloem transport are limited following stomatal closure and phloem dysfunction, respectively (Cernusak & Cheesman, ; Hubeau et al, ; Steppe, Sterck, & Deslauriers, ; Vandegehuchte et al, ).…”

“…Enhanced xylem hydraulic performance via P wt might be partially explained by promoted bark water uptake and nonstructural carbohydrate accumulation favouring vessel refilling, as observed in branches of Salix matsudana (Liu et al, ). Furthermore, P wt is less sensitive to drought than leaf photosynthesis (Ávila et al, ; Bloemen, McGuire, Aubrey, Teskey, & Steppe, ; Cernusak & Cheesman, ; Teskey et al, ). Remarkably, water‐use efficiency of P wt was found to be 50 times higher than that of leaf photosynthesis in Western white pine (Cernusak & Marshall, ).…”