Effect of soil O₂ deficiency (hypoxia) on biomass (g DW) of poplar plants.

<p>During harvest the trees were divided into leaf, petiole, bark, wood, fine roots and coarse roots. The shoot was separated into the top 40 cm, middle 40 cm and bottom 50 cm Data shown are means ± SD of 22–24 biological replicates. The differences between plants exposed to normal O₂ supply (normoxia) and reduced soil O₂ supply (hypoxia) were tested by Student’s t-test at p<0.05; significant differences are indicated by bold.</p><p>Effect of soil O₂ deficiency (hypoxia) on biomass (g DW) of poplar plants.</p

N contents [mmol g^-1 DW] in poplar plants exposed to normoxia or hypoxia for 14 days.

<p>During harvest the trees were divided into leaf, petiole, bark, wood, fine roots and coarse roots. The shoot was separated into the top 40 cm, middle 40 cm and bottom 50 cm Data shown are means ± SD of 22–24 biological replicates. The differences between plants exposed to either hypoxia or normoxia were tested by Student’s <i>t-test</i> at p< 0.05; significant differences are indicated by bold.</p><p>N contents [mmol g^-1 DW] in poplar plants exposed to normoxia or hypoxia for 14 days.</p

Effect of soil O₂ deficiency on biomass accumulation and daily transpiration of young poplar trees.

<p>Four months old, hydroponically grown poplar trees were exposed to either normoxic or hypoxic conditions. After 14 days of treatment the plants were harvested, oven dried and the dry weights determined. Root-to-shoot ratios were calculated for each plant. In addition, daily transpiration rates were determined. Data shown are means ± SD of 10–12 biological replicates per treatment. Statistically significant differences at p< 0.05 between hypoxic and normoxic plants were calculated by Student’s t-test and are shown by asterisk.</p

Effect of hypoxia on ¹⁵N allocation rates of ¹⁵N derived from ¹⁵NH₄⁺ in young poplar trees.

<p>Poplar plants were treated as described in legend of <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136579#pone.0136579.g003" target="_blank">Fig 3</a>. ¹⁵N contents in all plant organs in different plant parts (top 40 cm, middle 40 cm, lowest 50 cm, fine and coarse roots) were determined and data used to calculate ¹⁵N allocation rates to these organs. The color codes indicate the magnitude of the allocation rates to the organs. Data shown are means ± SD of 10–12 biological replicates. Statistically significant differences between plants exposed to normoxia or hypoxia were tested by Student’s t-test and are indicated in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136579#pone.0136579.s001" target="_blank">S1 Fig</a>.</p

Effect of hypoxia on ¹⁵N allocation rates of ¹⁵N derived from ¹⁵NO₃^- in young poplar trees.

<p>Poplar plants were treated as described in legend of <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136579#pone.0136579.g003" target="_blank">Fig 3</a>. ¹⁵N contents in all plant organs in different plant parts (top 40 cm, middle 40 cm, lowest 50 cm, fine and coarse roots) were determined and data used to calculate ¹⁵N allocation rates to these organs. The color codes indicate the magnitude of the allocation rates to the organs. Data shown are means ± SD of 10–12 biological replicates. Statistically significant differences between normoxic and hypoxic plants were tested by Student’s t-test and are indicated in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0136579#pone.0136579.s001" target="_blank">S1 Fig</a>.</p

Distribution of absorbed ¹⁵N (% of total ¹⁵N taken up per plant) in poplar plants kept under normoxia or hypoxia.

<p>Data shown are means ± SD of 10–12 biological replicates; statistically significant differences at p<0.05 between normoxic controls and hypoxically treated plants were calculated by Student’s <i>t-test</i> and are indicated by bold.</p><p>Distribution of absorbed ¹⁵N (% of total ¹⁵N taken up per plant) in poplar plants kept under normoxia or hypoxia.</p

Hypoxia Affects Nitrogen Uptake and Distribution in Young Poplar (<i>Populus</i> × <i>canescens</i>) Trees

<div><p>The present study with young poplar trees aimed at characterizing the effect of O₂ shortage in the soil on net uptake of NO₃^- and NH₄⁺ and the spatial distribution of the N taken up. Moreover, we assessed biomass increment as well as N status of the trees affected by O₂ deficiency. For this purpose, an experiment was conducted in which hydroponically grown young poplar trees were exposed to hypoxic and normoxic (control) conditions for 14 days. ¹⁵N-labelled NO₃^- and NH₄⁺ were used to elucidate N uptake and distribution of currently absorbed N and N allocation rates in the plants. Whereas shoot biomass was not affected by soil O₂ deficiency, it significantly reduced root biomass and, consequently, the root-to-shoot ratio. Uptake of NO₃^- but not of NH₄⁺ by the roots of the trees was severely impaired by hypoxia. As a consequence of reduced N uptake, the N content of all poplar tissues was significantly diminished. Under normoxic control conditions, the spatial distribution of currently absorbed N and N allocation rates differed depending on the N source. Whereas NO₃^- derived N was mainly transported to the younger parts of the shoot, particularly to the developing and young mature leaves, N derived from NH₄⁺ was preferentially allocated to older parts of the shoot, mainly to wood and bark. Soil O₂ deficiency enhanced this differential allocation pattern. From these results we assume that NO₃^- was assimilated in developing tissues and preferentially used to maintain growth and ensure plant survival under hypoxia, whereas NH₄⁺ based N was used for biosynthesis of storage proteins in bark and wood of the trees. Still, further studies are needed to understand the mechanistic basis as well as the eco-physiological advantages of such differential allocation patterns.</p></div

Changes in the Dynamics of Foliar N Metabolites in Oak Saplings by Drought and Air Warming Depend on Species and Soil Type

<div><p>Climate change poses direct or indirect influences on physiological mechanisms in plants. In particular, long living plants like trees have to cope with the predicted climate changes (i.e. drought and air warming) during their life span. The present study aimed to quantify the consequences of simulated climate change for foliar N metabolites over a drought-rewetting-drought course. Saplings of three Central European oak species (i.e. <i>Quercus robur</i>, <i>Q</i>. <i>petraea</i>, <i>Q</i>. <i>pubescens</i>) were tested on two different soil types (i.e. acidic and calcareous). Consecutive drought periods increased foliar amino acid-N and soluble protein-N concentrations at the expense of structural N in all three oak species. In addition, transient effects on foliar metabolite dynamics were observed over the drought-rewetting-drought course. The lowest levels of foliar soluble protein-N, amino acid-N and potassium cation with a minor response to drought and air warming were found in the oak species originating from the driest/warmest habitat (<i>Q</i>. <i>pubescens</i>) compared to <i>Q</i>. <i>robur</i> and <i>Q</i>. <i>petraea</i>. Higher foliar osmolyte-N and potassium under drought and air warming were observed in all oak species when grown on calcareous versus acidic soil. These results indicate that species-specific differences in physiological mechanisms to compensate drought and elevated temperature are modified by soil acidity.</p></div

Foliar total amino acid-N of three oak species exposed to different climate treatments and soil types during a drought-rewetting-drought course.

<p>Vertical bar charts show mean and standard error and the bar charts were grouped for each species, in each treatment and soil type. Periods are indicated from left to right side: white bar = the first drought period (1^stD), hedged bar = the rewetting period (RW), cyclone bar = the second drought period (2^ndD). Different small letters indicate significant differences across three oak species within each climate treatment, period and soil type. Different capital letters indicate significant differences across different climate treatments for each species, period and soil type. Greek symbols indicate significant differences across the different drought-rewetting-drought periods for each species, climate treatment and soil type. Asterisks indicate significant differences between acidic and calcareous soil within each climate treatment, species and time course (<i>P</i> ≤ 0.050).</p

Main effects table during the experimental periods of 2009.

<p>Difference among species, drought-rewetting-drought periods (1 = at the end of 1^stD period, 2 = after RW period and 3 = at the end of 2^ndD period) or differences versus control (<u>± </u>%) or calcareous versus acidic soil (<u>± </u>%); significant difference:</p><p>(*) <i>P</i> ≤ 0.1,</p><p>* <i>P</i> ≤ 0.05,</p><p>** <i>P</i> ≤ 0.01,</p><p>*** <i>P</i> ≤ 0.001;</p><p>ns = not significant <i>P</i> > 0.05;</p><p>na = not available, because foliar K concentrations were determined only at the end of 2^ndD period.</p><p>Main effects table during the experimental periods of 2009.</p