How important is woody tissue photosynthesis in poplar during drought stress?

Key message

Woody tissue photosynthesis might play a key role in maintaining plant carbon economy and hydraulic function under unfavourable conditions such as drought stress.

Abstract

Within trees, a portion of respired CO₂ is assimilated by bark and woody tissue photosynthesis, but its physiological role remains unclear, in particular under unfavour able conditions like drought stress. We hypothesised that woody tissue photosynthesis will contribute to overall tree carbon gain both under sufficient water supply and during drought, and plays a role in maintaining the hydraulic function. We subjected half of the trees to a stem and branch light-exclusion treatment to prevent bark and woody tissue photosynthesis. Then, we measured leaf gas exchange and stem growth in Populus deltoides x nigra ‘Monviso’ trees both under well-watered and dry conditions. We additionally measured cavitation using acoustic emission in detached control and light-excluded branches to illustrate the role of woody tissue photosynthesis in xylem embolism repair. Under well-watered conditions, light exclusion resulted in reduced stem growth relative to control trees by 30 %. In response to drought, stem shrinkage of light-excluded trees was more pronounced as compared to control trees. During drought stress also maximum photosynthesis and transpiration rate tended to decrease more rapidly in light-excluded trees compared to control trees. Leaf fall in light-excluded branches together with the larger number of acoustic emissions in control branches indicates that in the latter more xylem vessels were still hydraulically functional under drought. Therefore, our study highlights that photosynthesis at branch and stem level might be a key factor in the resilience of trees to drought stress by maintaining both the plant carbon economy and hydraulic function.

     

Root xylem CO<Subscript>2</Subscript> flux: an important but unaccounted-for component of root respiration

Key message

In tree roots, a large fraction of root-respired CO ₂ remains within the root system rather than diffusing into the soil. This CO ₂ is transported in xylem sap into the shoot, and because respiration is almost always measured as the flux of CO ₂ into the atmosphere from plant tissues, it represents an unaccounted-for component of tree root metabolism.

Abstract

Root respiration has been considered a large component of forest soil CO₂ efflux, but recent findings indicate that it may be even more important than previous measurements have shown because a substantial fraction of root-respired CO₂ remains within the tree root system and moves internally with the transpiration stream. The high concentration of CO₂ in roots appears to originate mainly within the root. It has been suggested that plants can take up dissolved inorganic carbon (DIC) from soil, but under most conditions uptake from soil is minimal due to the root-to-soil diffusion gradient, which suggests that most of the CO₂ in root xylem is derived from root respiration. Estimates of the internal flux of CO₂ through root xylem are based on combined measurements of sap flow and internal [CO₂]. Results quantifying root xylem CO₂ flux, obtained for a limited number of species, have raised important concerns regarding our understanding of tree respiration. Taken together, the results of these studies call into question the partitioning of ecosystem respiration into its above- and belowground components, and redefine the energetic costs of tree root metabolism and hence estimates of belowground carbon allocation. Expanding our observations of root xylem CO₂ flux to more species and at longer time scales, as well as improving the techniques used to study this process, could be fruitful avenues for future research, with the potential to substantially revise our understanding of root respiration and forest carbon cycles.

     

Seasonal variation of photosynthesis and photosynthetic efficiency in <Emphasis Type="Italic">Phalaenopsis</Emphasis>

Nowadays, a quest for efficient greenhouse heating strategies, and their related effects on the plant’s performance, exists. In this study, the effects of a combination of warm days and cool nights in autumn and spring on the photosynthetic activity and efficiency of Phalaenopsis were evaluated; the latter, being poorly characterised in plants with crassulacean acid metabolism (CAM) and, to our knowledge, not reported before in Phalaenopsis. 24-h CO₂ flux measurements and chlorophyll (Chl) fluorescence analyses were performed in both seasons on Phalaenopsis ‘Hercules’ exposed to relatively constant temperature regimes, 25.5/24.0°C (autumn) and 30/27°C (spring) respectively, and distinctive warm day/cool night temperature regimes, 27/20°C (autumn) and 36/24°C (spring), respectively. Cumulated leaf net CO₂ uptake of the distinctive warm day/cool night temperature regimes declined with 10–16% as compared to the more constant temperature regimes, while the efficiency of carbon fixation revealed no substantial differences in both seasons. Nevertheless, a distinctive warm day/cool night temperature regime seemed to induce photorespiration. Although photorespiration is expected not to occur in CAM, the suppression of the leaf net CO₂ exchange during Phase II and Phase IV as well as the slightly lower efficiency of carbon fixation for the distinctive warm day/cool night temperature regimes confirms the involvement of photorespiration in CAM. A seasonal effect was reflected in the leaf net CO₂ exchange rate with considerably higher rates in spring. In addition, sufficiently high levels of photosynthetically active radiation (PAR) in spring led to an efficiency of carbon fixation of 1.06–1.27% which is about twice as high than in autumn. As a result, only in the case where a net energy reduction between the temperature regimes compensates for the reduction in net CO₂ uptake, warm day/cool night temperature regimes may be recommended as a practical sustainable alternative.     

Report on non-temperature related variations in CO<Subscript>2</Subscript> efflux rates from young tree stems in the dormant season

Respiration rates are reported to increase exponentially with temperature. Respiration rates of woody tissues are commonly measured as CO₂ efflux rates () from that tissue. However, this paper describes clear variations in stem that were not related to temperature for the case of a young beech (Fagus sylvatica L.) and oak (Quercus robur L.) tree during the dormant season. The CO₂ concentration ([CO₂]) in the xylem of the beech tree showed similar temperature-independent variations. The trees were grown in a growth chamber in which radiation patterns and temperature were kept constant. was measured with an IRGA connected to cuvettes surrounding a stem segment. Xylem [CO₂] was measured in situ using a CO₂ microelectrode. Depressions in and [CO₂] occurred during the light period, despite equal temperatures in the light and dark period. Explanations found in literature for discrepancies in the exponential relationship between temperature and are the influence of (1) sap flow or (2) decreased cell water content. However, (1) the variations were observed in the dormant season, when no sap flow was observed yet, and (2) reduced cell water content was not likely to be apparent as differences in stem transpiration rates between the dark and light period were not significant. Hence, previously formulated theories failed to explain our results. This work therefore provides a new ground for discussion on other possible causes of daytime depressions in . One might be the refixation of respired CO₂ by corticular photosynthesis in the stem parts adjacent to the stem segment enclosed by the cuvette.     

Modelling reveals endogenous osmotic adaptation of storage tissue water potential as an important driver determining different stem diameter variation patterns in the mangrove species Avicennia marina and Rhizophora stylosa

Background

Stem diameter variations are mainly determined by the radial water transport between xylem and storage tissues. This radial transport results from the water potential difference between these tissues, which is influenced by both hydraulic and carbon related processes. Measurements have shown that when subjected to the same environmental conditions, the co-occurring mangrove species Avicennia marina and Rhizophora stylosa unexpectedly show a totally different pattern in daily stem diameter variation.

Methods

Using in situ measurements of stem diameter variation, stem water potential and sap flow, a mechanistic flow and storage model based on the cohesion–tension theory was applied to assess the differences in osmotic storage water potential between Avicennia marina and Rhizophora stylosa.

Key results

Both species, subjected to the same environmental conditions, showed a resembling daily pattern in simulated osmotic storage water potential. However, the osmotic storage water potential of R. stylosa started to decrease slightly after that of A. marina in the morning and increased again slightly later in the evening. This small shift in osmotic storage water potential likely underlaid the marked differences in daily stem diameter variation pattern between the two species.

Conclusions

The results show that in addition to environmental dynamics, endogenous changes in the osmotic storage water potential must be taken into account in order to accurately predict stem diameter variations, and hence growth.     

Acclimation effects of heat waves and elevated [CO2] on gas exchange and chlorophyll fluorescence of northern red oak (Quercus rubra L.) seedlings

Heat wave frequency and intensity are predicted to increase. We investigated whether repeated exposure to heat waves would induce acclimation in Quercus rubra seedlings and considered [CO₂] as an interacting factor. We measured gas exchange and chlorophyll fluorescence of seedlings grown in 380 (C _A) or 700 (C _E) μmol CO₂ mol^?1, and three temperature treatments (ambient, ambient +3 °C, and an ambient +12 °C heat wave every fourth week). Measurements were performed during the third and fourth +12 °C heat waves (July and August 2010) at Whitehall Forest, GA, USA. Additionally, previously unexposed seedlings were subjected to the August heat wave to serve as a control to determine acclimation of seedlings which were previously exposed. Seedlings with a history of heat wave exposure showed lower net photosynthesis (A _net) and stomatal conductance (on average ?47 and ?38 %, respectively) than seedlings with no such history, when both were subjected to the same +12 °C heat wave. During both heat waves, A _net significantly declined in the +12 °C treatment compared with the other treatments. Additionally, the A _net decline during the August compared with the July heat wave was stronger in C _E than in C _A, suggesting that elevated [CO₂] might have had a negative effect on acclimation capacity. We conclude that seedlings subjected to consecutive heat waves will moderate stomatal conductance outside the heat wave, to reduce water usage at lower temperatures, increasing survival at the expense of carbon assimilation.     

Fate of xylem‐transported 11C‐ and 13C‐labeled CO2 in leaves of poplar

In recent studies, assimilation of xylem‐transported CO₂ has gained considerable attention as a means of recycling respired CO₂ in trees. However, we still lack a clear and detailed picture on the magnitude of xylem‐transported CO₂ assimilation, in particular within leaf tissues. To this end, detached poplar leaves (Populus × canadensis Moench ‘Robusta’) were allowed to take up a dissolved ¹³CO₂ label serving as a proxy of xylem‐transported CO₂ entering the leaf from the branch. The uptake rate of the ¹³C was manipulated by altering the vapor pressure deficit (VPD) (0.84, 1.29 and 1.83 kPa). Highest tissue enrichments were observed under the highest VPD. Among tissues, highest enrichment was observed in the petiole and the veins, regardless of the VPD treatment. Analysis of non‐labeled leaves showed that some ¹³C diffused from the labeled leaves and was fixed in the mesophyll of the non‐labeled leaves. However, ¹³C leaf tissue enrichment analysis with elemental analysis coupled to isotope ratio mass spectrometry was limited in spatial resolution at the leaf tissue level. Therefore, ¹¹C‐based CO₂ labeling combined with positron autoradiography was used and showed a more detailed spatial distribution within a single tissue, in particular in secondary veins. Therefore, in addition to ¹³C, ¹¹C‐based autoradiography can be used to study the fate of xylem‐transported CO₂ at leaf level, allowing the acquisition of data at a yet unprecedented resolution.     

Identifying the pathways for foliar water uptake in beech (Fagus sylvatica L.): a major role for trichomes

Foliar water uptake (FWU), the direct uptake of water into leaves, is a global phenomenon, having been observed in an increasing number of plant species. Despite the growing recognition of its functional relevance, our understanding of how FWU occurs and which foliar surface structures are implicated, is limited. In the present study, fluorescent and ionic tracers, as well as microcomputed tomography, were used to assess potential pathways for water entry in leaves of beech, a widely distributed tree species from European temperate regions. Although none of the tracers entered the leaf through the stomatal pores, small amounts of silver precipitation were observed in some epidermal cells, indicating moderate cuticular uptake. Trichomes, however, were shown to absorb and redistribute considerable amounts of ionic and fluorescent tracers. Moreover, microcomputed tomography indicated that 72% of empty trichomes refilled during leaf surface wetting and microscopic investigations revealed that trichomes do not have a cuticle but are covered with a pectin‐rich cell wall layer. Taken together, our findings demonstrate that foliar trichomes, which exhibit strong hygroscopic properties as a result of their structural and chemical design, constitute a major FWU pathway in beech.