Revealing the bark water uptake of isotopically-enriched water by intact branches in the field and its potential contribution (or consequences) to (or for) transpiration estimates

<p>In plants, the constant demand for water driven by transpiration is supplied by uptake from the soil through the roots. Alternative water-uptake pathways through the leaves and the bark have been demonstrated for some species, mainly conifers. Alternative water-uptake pathways could allow plants to complement their water supply with canopy interception, fog or dew, sources often assumed unavailable as they are lost via evaporation before they can contribute to soil water recharge. Bark water-uptake has been putatively linked to repair of xylem embolism, although this has only been demonstrated in cut branches and/or under artificial conditions. We hypothesized that besides embolism repair, bark water uptake might also contribute to maintaining the transpiration stream in upper canopy branches when the xylem water column is subject to excess negative pressure, either because temperature drops, and water viscosity increases, or under high vapour pressure deficit and low soil water availability. We used a novel labelling methodology combining online measurements of the isotope composition (δ<sup>2</sup>H and δ<sup>18</sup>O) of the transpiration stream with analyses of δ<sup>2</sup>H and δ<sup>18</sup>O from leaf, bark and xylem water in <em>Pinus sylvestris</em> and <em>Fagus sylvatica</em>. We conducted sampling campaigns in two study sites: a boreal (northern Sweden) and a temperate (northern Spain) forest. We applied semi-permeable bandages injected with <sup>2</sup>H-enriched water (0.8% <sup>2</sup>H<sub>2</sub>O), on intact upper canopy branches (7-13 m), and monitored δ<sup>2</sup>H and δ<sup>18</sup>O of the transpiration stream with a Cavity Ring-Down Spectrometer (CRDS) in three branches (only <em>P. sylvestris</em> in Sweden) for 24 h and then sampled branch segments 2 cm upstream and downstream of the bandage. We determined δ<sup>2</sup>H and δ<sup>18</sup>O of leaf, bark and xylem water from sampled segments with a CRDS after cryogenic extraction. Xylem, bark and leaf water from segments downstream of the bandage were enriched in δ<sup>2</sup>H with respect to their corresponding upstream segments. The δ<sup>2</sup>H and δ<sup>18</sup>O from leaf, bark and xylem water from upstream segments were similar to those of control branches (no bandages). Results were similar for both study species, sites and campaigns, indicating that bark water uptake is not restricted to gymnosperms and may be more ubiquitous than previously considered. Enrichment in δ<sup>2</sup>H in the transpiration stream was also detected in one out of the three continuously monitored pine branches within the 12 h following the bandage application. Our results demonstrate that water taken up through the bark may be incorporated into the transpiration stream and that transpiration might not solely rely on water absorbed through the roots and transported through the main stem. This could imply, for example, that sapflux measurements would underestimate canopy transpiration. Combining empirical flux measurements with stable isotopes and/or other atmospheric tracers could render more realistic estimates of transpiration and help constrain partitioning of evaporation and transpiration and its coupling to gross primary productivity.</p>

Transpiration and transporters: teasing apart passive and active transport of plant silicon

<p>All plants contain some silicon (Si), but some species take it up passively through the transpiration stream while others additionally actively accumulate Si by producing transporters. Here, we review the literature, both qualitatively and quantitatively, to investigate the importance of transpiration for Si uptake across diverse plant groups with different accumulation capacities.  We will use variation among species in terms of phylogeny, habitat (e.g. aquatic vs. terrestrial), and environmental conditions (e.g. water or nutrient stress) to tease apart the roles of transporters and transpiration in controlling rates of Si accumulation, and make use of published manipulative experiments to explore how Si availability impacts the importance of these two uptake mechanisms.</p>

Environmental Factors Influence Plant Vascular System and Water Regulation

Developmental initiation of plant vascular tissue, including xylem and phloem, from the vascular cambium depends on environmental factors, such as temperature and precipitation. Proper formation of vascular tissue is critical for the transpiration stream, along with photosynthesis as a whole. While effects of individual environmental factors on the transpiration stream are well studied, interactive effects of multiple stress factors are underrepresented. As expected, climate change will result in plants experiencing multiple co-occurring environmental stress factors, which require further studies. Also, the effects of the main climate change components (carbon dioxide, temperature, and drought) on vascular cambium are not well understood. This review aims at synthesizing current knowledge regarding the effects of the main climate change components on the initiation and differentiation of vascular cambium, the transpiration stream, and photosynthesis. We predict that combined environmental factors will result in increased diameter and density of xylem vessels or tracheids in the absence of water stress. However, drought may decrease the density of xylem vessels or tracheids. All interactive combinations are expected to increase vascular cell wall thickness, and therefore increase carbon allocation to these tissues. A comprehensive study of the effects of multiple environmental factors on plant vascular tissue and water regulation should help us understand plant responses to climate change.

Sucrose degradation in sink and source poplar leaves treated with sulfite

Sulfite at concentrations from 0.5 to 5.0 mM was supplied to illuminated, detached poplar (<em>Populus deltoides</em> Bartr. ex Marsh) leaves via the transpiration stream. Concentrations of chlorophyll <em>a</em> and <em>b</em>, carotenoids, glucose, fructose, sucrose and starch, and extractable specific activities of sucrose synthase (SuSy), acid invertase (AI) and neutral invertase (NI) were measured. In sink and source leaves chlorophylls and carotenoids appeared to be increased by sulfite at concentrations ranging from 0.5 to 5.0 mM. In source leaves the application of 5.0 mM and ≥ 2.5 mM sulfite led to a decrease in total nonstructural soluble sugars (sTNC) and starch, respectively. The specific activity of SuSy was enhanced by ≥ 0.5 mM sulfite. There was only a slight decline in the activity of Al and N1.In sink leaves sTNC decreased and starch increased under the influence of sulfite at concentrations ≤ 1.O mM and ≥ 0.5 mM, respectively. The effect of sulfite on the activity of At, NI and SuSy was negligible. However, in both development groups of leaves alterations in sucrose, glucose and fructose contents were observed. On the basis cf the obtained results we postulate that sulfite changes the sucrolytic activities primarily in source leaves and only marginally in sink leaves.