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Roots in flooded soils experience hypoxia, with the least O2 in the vascular cylinder. Gradients in CO2 across roots had not previously been measured. The respiratory quotient (RQ; CO2 produced : O2 consumed) is expected to increase as O2 availability declines. A new CO2 microsensor and an O2 microsensor were used to measure profiles across roots of chickpea seedlings in aerated or hypoxic conditions. Simultaneous, nondestructive flux measurements of O2 consumption, CO2 production, and thus RQ, were taken for roots with declining O2. Radial profiling revealed severe hypoxia and c. 0.8 kPa CO2 within the root vascular cylinder. The distance penetrated by O2 into the roots was shorter at lower O2. The gradient in CO2 was in the opposite direction to that of O2, across the roots and diffusive boundary layer. RQ increased as external O2 was lowered. For chickpea roots in solution at air equilibrium, O2 was very low and CO2 was elevated within the vascular cylinder; the extent of the severely hypoxic core increased as external O2 was reduced. The increased RQ in roots in response to declining external O2 highlighted the shift from respiration to ethanolic fermentation as the severely hypoxic/anoxic core became a progressively greater proportion of the root tissues.

Responses of wheat (Triticum aestivum) to complete submergence are not well understood as research has focused on waterlogging (soil flooding). The aim of this study was to characterize the responses of 2 wheat cultivars differing vastly in submergence tolerance to test if submergence tolerance was linked to shoot carbohydrate consumption as seen in rice. Eighteen-day-old wheat cultivars Frument (intolerant) and Jackson (tolerant) grown in soil were completely submerged for up to 19 days while assessing responses in physiology, gene expression, and shoot metabolome. Results revealed 50% mortality after 9.3 and 15.9 days of submergence in intolerant Frument and tolerant Jackson, respectively, and significantly higher growth in Jackson during recovery. Frument displayed faster leaf degradation as evident from leaf tissue porosity, chlorophylla , and metabolomic fingerprinting. Surprisingly, shoot soluble carbohydrates, starch, and individual sugars declined to similarly low levels in both cultivars by day 5, showing that cultivar Jackson tolerated longer periods of low shoot carbohydrate levels than Frument. Moreover, intolerant Frument showed higher levels of phytol and the lipid peroxidation marker malondialdehyde relative to tolerant Jackson. Consequently, we propose to further investigate the role of ethylene sensitivity and deprivation of reactive O2 species in submerged wheat.

Roots in flooded soils experience hypoxia, with the least O

Responses of wheat (Triticum aestivum) to complete submergence are not well understood as research has focused on waterlogging (soil flooding). The aim of this study was to characterize the responses of 2 wheat cultivars differing vastly in submergence tolerance to test if submergence tolerance was linked to shoot carbohydrate consumption as seen in rice. Eighteen-day-old wheat cultivars Frument (intolerant) and Jackson (tolerant) grown in soil were completely submerged for up to 19 days while assessing responses in physiology, gene expression, and shoot metabolome. Results revealed 50% mortality after 9.3 and 15.9 days of submergence in intolerant Frument and tolerant Jackson, respectively, and significantly higher growth in Jackson during recovery. Frument displayed faster leaf degradation as evident from leaf tissue porosity, chlorophyll

Floods can completely submerge terrestrial plants but some wetland species can sustain O2 and CO2 exchange with the environment via gas films forming on superhydrophobic leaf surfaces. We used high resolution synchrotron X-ray phase contrast micro-tomography in a novel approach to visualise gas films on submerged leaves of common cordgrass (Spartina anglica). 3D tomograms enabled a hitherto unmatched level of detail regarding the micro-topography of leaf gas films. Gas films formed only on the superhydrophobic adaxial leaf side (water droplet contact angle, Φ=162°) but not on the abaxial side (Φ=135°). The adaxial side of the leaves of common cordgrass is plicate with a longitudinal system of parallel grooves and ridges and the vast majority of the gas film volume was found in large ∼180μm deep elongated triangular volumes in the grooves and these volumes were connected to each neighbouring groove via a fine network of gas tubules (∼1.7μm diameter) across the ridges. In addition to the gas film retained on the leaf exterior, the X-ray phase contrast micro-tomography also successfully distinguished gas spaces internally in the leaf tissues, and the tissue porosity (gas volume per unit tissue volume) ranged from 6.3% to 20.3% in tip and base leaf segments, respectively. We conclude that X-ray phase contrast micro-tomography is a powerful tool to obtain quantitative data of exterior gas features on biological samples because of the significant difference in electron density between air, biological tissues and water.

Flash floods can submerge paddy field rice (Oryza sativa), with adverse effects on internal aeration, sugar status and survival. Here, we investigated the in situ aeration of roots of rice during complete submergence, and elucidated how underwater photosynthesis and floodwater pO(2) influence root aeration in anoxic soil. In the field, root pO(2) was measured using microelectrodes during 2 d of complete submergence. Leaf gas films that formed on the superhydrophobic leaves were left intact, or experimentally removed, to elucidate their effect on internal aeration. In darkness, root pO(2) declined to very low concentrations (0.24 kPa) and was strongly correlated with floodwater pO(2). In light, root pO(2) was high (14 kPa) and primarily a function of the incident light determining the rates of underwater net photosynthesis. Plants with intact leaf gas films maintained higher underwater net photosynthesis relative to plants without gas films when the submerged shoots were in light. During complete submergence, internal aeration of rice in the field relies on underwater photosynthesis during the day and entry of O(2) from the floodwater during the night. Leaf gas films enhance photosynthesis during submergence leading to improved O(2) production and sugar status, and therefore contribute to the submergence tolerance of rice.

Flooding of fields after sudden rainfall events can result in crops being completely submerged. Some terrestrial plants, including wheat (Triticum aestivum L.), possess superhydrophobic leaf surfaces that retain a thin gas film when submerged, and the gas films enhance gas exchange with the floodwater. However, the leaves lose their hydrophobicity during submergence, and the gas films subsequently disappear. We tested gas film retention time of 14 different wheat cultivars and found that wheat could retain the gas films for a minimum of 2 days, whereas the wild wetland grass Glyceria fluitans (L.) R.Br. had thicker gas films and could retain its gas films for a minimum of 4 days. Scanning electron microscopy showed that the wheat cultivars and G. fluitans possessed high densities of epicuticular wax platelets, which could explain their superhydrophobicity. However, G. fluitans also had papillae that contributed to higher hydrophobicity during the initial submergence and could explain why G. fluitans retained gas films for a longer period of time. The loss of gas films was associated with the leaves being covered by an unidentified substance. We suggest that leaf gas film is a relevant trait to use as a selection criterion to improve the flood tolerance of crops that become temporarily submerged.

Floods and salinization of agricultural land adversely impact global rice production. We investigated whether gas films on leaves of submerged rice delay salt entry during saline submergence. Two-week-old plants with leaf gas films (+GF) or with gas films experimentally removed (-GF) were submerged in artificial floodwater with 0 or 50 mm NaCl for up to 16 d. Gas films were present9 d on GF plants after which gas films were diminished. Tissue ion analysis (Na

Gas films on hydrophobic surfaces of leaves of some wetland plants can improve O2 and CO2 exchange when completely submerged during floods. Here we investigated the in situ aeration of rhizomes of cordgrass (Spartina anglica) during natural tidal submergence, with focus on the role of leaf gas films on underwater gas exchange. Underwater net photosynthesis was also studied in controlled laboratory experiments. In field experiments, O2 microelectrodes were inserted into rhizomes and pO2 measured throughout two tidal submergence events; one during daylight and one during night-time. Plants had leaf gas films intact or removed. Rhizome pO2 dropped significantly during complete submergence and most severely during night. Leaf gas films: (1) enhanced underwater photosynthesis and pO2 in rhizomes remained above 10 kPa during submergence in light; and (2) facilitated O2 entry from the water into leaves so that rhizome pO2 was about 5 kPa during darkness. This study is the first in situ demonstration of the beneficial effects of leaf gas films on internal aeration in a submerged wetland plant. Leaf gas films likely contribute to submergence tolerance of S. anglica and this feature is expected to also benefit other wetland plant species when submerged.

Submergence invokes a range of stressors to plants with impeded gas exchange between tissues and floodwater being the greatest challenge. Many terrestrial plants including wheat (Triticum aestivum L.), possess superhydrophobic leaf cuticles that retain a thin gas film when submerged, and the gas films enhance gas exchange with the floodwater. However, leaf hydrophobicity is lost during submergence and the gas films disappear accordingly. Here, we completely submerged wheat (with or without gas films) for up to 14 days and found that plants with gas films survived significantly longer (13 days) than plants without (10 days). Plants with gas films also had less dead tissue following a period of recovery. However, this study also revealed that reflections by gas films resulted in a higher light compensation point for underwater net photosynthesis for leaves with gas films compared with leaves without (IC = 52 vs 35 µmol photons m–2 s–1 with or without gas films, respectively). Still, already at ~5% of full sunlight the beneficial effect of gas films overcame the negative under ecologically relevant CO2 concentrations. Our study showed that dryland crops also benefit from leaf gas films during submergence and that this trait should be incorporated to improve flood tolerance of wheat.

Terrestrial saltmarsh plants inhabiting flood-prone habitats undergo recurrent and prolonged flooding driven by tidal regimes. In this study, the role of internal plant aeration in contrasting hypoxic/anoxic conditions during submergence was investigated in the two halophytes Limonium narbonense Mill. and Sarcocornia fruticosa (L.) A.J. Scott. Monitoring of tissue O2 dynamics was performed in shoots and roots using microelectrodes under drained conditions, waterlogging, partial and complete submergence, in light or darkness. For both species, submergence in darkness resulted in significant declines in tissue O2 status and when in light, in rapid O2 increases first in shoot tissues and subsequently in roots. During partial submergence, S. fruticosa benefitted from snorkelling and efficiently transported O2 to roots, whereas the O2 concentration in roots of L. narbonense declined by more than 90%. Significantly thinner leaves and articles were recorded under high degree of flooding stress and both species showed considerably high tissue porosity. The presence of aerenchyma seemed to support internal aeration in S. fruticosa whereas O2 diffusion in L. narbonense seemed impeded, despite the higher porosity (up to 50%). Thus, the results obtained for L. narbonense, being well adapted to flooding, suggests that processes other than internal aeration could be involved in better flooding tolerance e.g. fermentative processes, and that traits resulting in flooding tolerance in plants are not yet fully understood.

Summary The flood-tolerant genotype FR13A retains leaf gas films and its capacity for underwater net photosynthesis, whereas gas films are lost faster and photosynthesis declines markedly in sensitive genotypes., Floods can completely submerge some rice (Oryza sativa L.) fields. Leaves of rice have gas films that aid O2 and CO2 exchange under water. The present study explored the relationship between gas film persistence and underwater net photosynthesis (PN) as influenced by genotype and submergence duration. Four contrasting genotypes (FR13A, IR42, Swarna, and Swarna-Sub1) were submerged for 13 days in the field and leaf gas films, chlorophyll, and the capacity for underwater PN at near ambient and high CO2 were assessed with time of submergence. At high CO2 during the PN assay, all genotypes initially showed high rates of underwater PN, and this rate was not affected by time of submergence in FR13A. This superior photosynthetic performance of FR13A was not evident in Swarna-Sub1 (carrying the SUB1 QTL) and the declines in underwater PN in both Swarna-Sub1 and Swarna were equal to that in IR42. At near ambient CO2 concentration, underwater PN declined in all four genotypes and this corresponded with loss of leaf gas films with time of submergence. FR13A retained leaf gas films moderately longer than the other genotypes, but gas film retention was not linked to SUB1. Diverse rice germplasm should be screened for gas film persistence during submergence, as this trait could potentially increase carbohydrate status and internal aeration owing to increased underwater PN, which contributes to submergence tolerance in rice.

Gas films on hydrophobic surfaces of leaves of some wetland plants can improve O(2) and CO(2) exchange when completely submerged during floods. Here we investigated the in situ aeration of rhizomes of cordgrass (Spartina anglica) during natural tidal submergence, with focus on the role of leaf gas films on underwater gas exchange. Underwater net photosynthesis was also studied in controlled laboratory experiments. In field experiments, O(2) microelectrodes were inserted into rhizomes and pO(2) measured throughout two tidal submergence events; one during daylight and one during night-time. Plants had leaf gas films intact or removed. Rhizome pO(2) dropped significantly during complete submergence and most severely during night. Leaf gas films: (1) enhanced underwater photosynthesis and pO(2) in rhizomes remained above 10 kPa during submergence in light; and (2) facilitated O(2) entry from the water into leaves so that rhizome pO(2) was about 5 kPa during darkness. This study is the first in situ demonstration of the beneficial effects of leaf gas films on internal aeration in a submerged wetland plant. Leaf gas films likely contribute to submergence tolerance of S. anglica and this feature is expected to also benefit other wetland plant species when submerged.

Submergence inhibits photosynthesis by terrestrial wetland plants, but less so in species that possess leaf gas films when submerged. Floodwaters are often supersaturated with dissolved CO2 enabling photosynthesis by submerged terrestrial plants, although rates remain well-below those in air. This important adaptation that enhances survival in submerged conditions is reviewed., Background and aims Wetland plants inhabit flood-prone areas and therefore can experience episodes of complete submergence. Submergence impedes exchange of O2 and CO2 between leaves and the environment, and light availability is also reduced. The present review examines limitations to underwater net photosynthesis (PN) by terrestrial (i.e. usually emergent) wetland plants, as compared with submerged aquatic plants, with focus on leaf traits for enhanced CO2 acquisition. Scope Floodwaters are variable in dissolved O2, CO2, light and temperature, and these parameters influence underwater PN and the growth and survival of submerged plants. Aquatic species possess morphological and anatomical leaf traits that reduce diffusion limitations to CO2 uptake and thus aid PN under water. Many aquatic plants also have carbon-concentrating mechanisms to increase CO2 at Rubisco. Terrestrial wetland plants generally lack the numerous beneficial leaf traits possessed by aquatic plants, so submergence markedly reduces PN. Some terrestrial species, however, produce new leaves with a thinner cuticle and higher specific leaf area, whereas others have leaves with hydrophobic surfaces so that gas films are retained when submerged; both improve CO2 entry. Conclusions Submergence inhibits PN by terrestrial wetland plants, but less so in species that produce new leaves under water or in those with leaf gas films. Leaves with a thinner cuticle, or those with gas films, have improved gas diffusion with floodwaters, so that underwater PN is enhanced. Underwater PN provides sugars and O2 to submerged plants. Floodwaters often contain dissolved CO2 above levels in equilibrium with air, enabling at least some PN by terrestrial species when submerged, although rates remain well below those in air.

† Background and Aims Submersed plants have different strategies to overcome inorganic carbon limitation. It is generally assumed that only small rosette species (isoetids) are able to utilize the high sediment CO2 availability. The present study examined to what extent five species of submersed freshwater plants with different morphology and growth characteristics (Lobelia dortmanna, Lilaeopsis macloviana, Ludwigia repens, Vallisneria americana and Hydrocotyle verticillata) are able to support photosynthesis supplied by uptake of CO2 from the sediment. † Methods Gross photosynthesis was measured in two-compartment split chambers with low inorganic carbon availability in leaf compartments and variable CO2 availability (0 to .8 mmol L 21 ) in root compartments. Photosynthetic rates based on root-supplied CO2 were compared with maximum rates obtained at saturating leaf CO2 availability, and 14 C experiments were conducted for two species to localize bottlenecks for utilization of sediment CO2. † Key Results All species except Hydrocotyle were able to use sediment CO2, however, with variable efficiency, and with the isoetid, Lobelia, as clearly the most effective and the elodeid, Ludwigia, as the least efficient. At a water column CO2 concentration in equilibrium with air, Lobelia, Lilaeopsis and Vallisneria covered .75% of their CO2 requirements by sediment uptake, and sediment CO2 contributed substantially to photosynthesis at water CO2 concentrations up to 1000 mmol L 21 . For all species except Ludwigia, the shoot to root ratio on an areal basis was the single factor best explaining variability in the importance of sediment CO2 .F orLudwigia, diffusion barriers limited uptake or transport from roots to stems and transport from stems to leaves. † Conclusions Submersed plants other than isoetids can utilize sediment CO2, and small and medium sized elodeids with high root to shoot area in particular may benefit substantially from uptake of sediment CO2 in low alkaline lakes.

Change in plants as bicarbonate rises Freshwater plants can be broadly divided into two major categories according to their photosynthetic traits: Some use carbon dioxide as their carbon source, whereas others use bicarbonate. Iversen et al. found that the relative concentrations of these two inorganic carbon forms in water determine the functional composition of plant communities across freshwater ecosystems (see the Perspective by Marcé and Obrador). They created global maps revealing that community composition is structured by catchment geology and not climate (in contrast to the terrestrial realm, where the trait composition is structured by temperature and rainfall). Anthropogenic influences from land-use change are causing large-scale increases in bicarbonate concentrations in freshwater catchments and are thus leading to wholesale changes in the composition of their aquatic plant communities. Unlike in land plants, photosynthesis in many aquatic plants relies on bicarbonate in addition to carbon dioxide (CO2) to compensate for the low diffusivity and potential depletion of CO2 in water. Concentrations of bicarbonate and CO2 vary greatly with catchment geology. In this study, we investigate whether there is a link between these concentrations and the frequency of freshwater plants possessing the bicarbonate use trait. We show, globally, that the frequency of plant species with this trait increases with bicarbonate concentration. Regionally, however, the frequency of bicarbonate use is reduced at sites where the CO2 concentration is substantially above the air equilibrium, consistent with this trait being an adaptation to carbon limitation. Future anthropogenic changes of bicarbonate and CO2 concentrations may alter the species compositions of freshwater plant communities. One sentence summary: The widespread photosynthetic trait of freshwater plants, bicarbonate use, has a global biogeography controlled by catchment characteristics

Aquatic plants are never short of water but instead they are challenged with low light and slow movement of oxygen (O₂) and carbon dioxide (CO₂). In the present paper, we focus on CO₂ limitation of underwater photosynthesisand the various strategies to overcome the limitation resulting from evolutionary adaptation to growth under water. Knowledge of such strategies helps you toselect the right CO₂ environment and thereby maximize the chances that your favorite plants flourish.

Limonium narbonense Mill. and Sarcocornia fruticosa (L.) A.J. Scott are two perennial saltmarsh species sharing distribution in the Mediterranean coastal region. L. narbonense has basal leaves in rosettes and a deep root system connected to a vertical rhizome. S. fruticosa is instead a semi-woody plant with modified photosynthetic stems and superficial roots. The distribution of these two species, ranging from elevated saltmarsh sites to low and daily inundated areas, arose the interest around the flooding tolerance mechanisms involved and the possible adaptation to different levels of flooding stress. Field collected plants were conditioned in a controlled tidal system in a greenhouse, imposing two treatments: (1) changing from drained to waterlogged conditions and (2) changing from waterlogged to complete submergence. Internal tissue aeration was monitored using O2 microelectrodes during waterlogging, partial and complete submergence in darkness as well as in light. Underwater photosynthesis and dark respiration were measured by measuring the O2 balance from excised green stems and leaves while porosity measurements were performed in different epigeous and hypogeal plant tissues. O2 dynamics in soil showed a clear and rapid increase of O2 during drainage condition (1) and steady anoxic conditions in the submerged treatment (2), which adversely affected L. narbonense leaf production but stimulated the development of new articles in S. fruticosa. In both species, internal O2 declined upon partial and complete submergence in darkness: L. narbonense showed values closed to zero already during partial submergence, both in petioles (9.1 μmol∙l-1 in average) and in roots (5.6 μmol∙l-1), while S. fruticosa displayed better root aeration during partial submergence (44.4 μmol∙l-1). In light, internal O2 concentrations of the articles of S. fruticosa and the petioles of L. narbonense increased, entailing also a gradual increase in O2 in roots, especially during the re-establishment of waterlogging conditions (over 50 μmol∙l-1 in both species). At air equilibrium of CO2, rates of underwater photosynthesis and dark respiration differ significantly between treatments only when based on dry mass, highlighting a morphological acclimation in leaf thickness of the plants during submergence (higher SLA). Despite the low photosynthetic rates, intact plants were able to cover the O2 requirement during inundation under light conditions (up to 179.7 and 73.0 μmol∙l-1 in S. fruticosa and L. narbonense, respectively). High tissue porosity is a common trait providing a low-resistance internal pathway for O2 diffusion to roots. S. fruticosa displayed high tissue porosity both in lignified stems (up to 25%) and in roots (up to 27%) but for L. nabonense tissues had even higher porosity (in petioles and leaves up to 63%). These findings suggest that the low internal O2 recorded during submergence of L. nabonense must be due to unidentified bottlenecks for O2 diffusion in e.g. root-rhizome junctions or rhizome-petiole junctions, restricting internal aeration. Our findings demonstrate the importance of waterlogging and submergence tolerance in these two species inhabiting saltmarshes, providing additional knowledge to flooding tolerance of halophytes.

Gas films on superhydrophobic leaves of terrestrial plants have been shown to enhance gas exchange when the plants are submerged, both in terms of CO2 uptake for photosynthesis during the day and entry of O2 for respiration during the night. Hence, the gas films contribute to improved internal aeration and survival during temporary submergence, which is highly relevant for some crops. Waterlogging or submergence of winter wheat seedlings after torrential rainfall is becoming an increasing problem in many countries where precipitation patterns are changing due to climate change.The present study explored gas film retention during submergence of 14 wheat genotypes and compared them to the wild species Catabrosa aquatica. The plants were completely submerged for up to 10 days and gas film thickness was measured and related to the loss of hydrophobicity measured as contact angle. Underwater photosynthesis measured at 200 μM CO2 declined with time of submergence and this corresponded to the loss of gas films. The surfaces of the leaves were investigated using Scanning Electron Microscopy (SEM) in order to determine why the leaves lose their hydrophobicity. It is concluded that the loss of gas film is associated with the leaves being covered by an unknown substance.

I Danmark vil klimaforandringerne resulterer i ændrede nedbørsmønstre, som medfører oversvømmelse af brednære arealer omkring søer og vandløb. Den terrestriske vegetation vil i mange tilfælde blive udsat for den ultimative abiotiske stressfaktor nemlig fuldstændig oversvømmelse, hvor hele planten i en periode neddykkes.Vi har for år tilbage opdaget en egenskab hos landplanter, som øger deres oversvømmelsestolerance når de neddykkes. Visse plantearter, f.eks. langt de fleste græsser, har blade som tilbageholder en tynd gasfilm når planterne oversvømmes. Gasfilmen dannes fordi bladenes kutikula er superhydrofobisk, og den tynde gasfilm fungerer på mange måder som en plastron hos visse arter af vandlevende insekter, f.eks. dybvandstægen og de vandlevende biller som Elmis og Limnius.Gasfilmen øger markant udvekslingen af O2 og CO2 med det omgivende vand og sikrer endvidere at spalteåbningerne kan forblive åbne og funktionelle hos de oversvømmede blade. Dermed kan landplanter med gasfilm opretholde deres metabolisme under vand og på den måde overleve en oversvømmelseshændelse.Gasfilmen forsvinder imidlertid efter nogle dage hos langt de fleste arter, og vi ved ikke præcis hvorfor. Vi har fulgt de ultrastrukturelle ændringer i kutikulaen hos forskellige arter – bl.a. hos tæppegræs, Catabrosa aquatica – og vi har ligeledes afbildet gasfilmens 3D struktur ved hjælp af røntgen-tomografi. Vi kan se en klar sammenhæng imellem kutikulaens 3D struktur og hydrofobiciteten, og vi har vist hvor vigtig den er for planternes oversvømmelsestolerance. På det seneste har vi også isoleret et enkelt gen (foreløbigt navngivet som GF1), som styrer kutikulaens hydrofobicitet hos ris. Sidstnævnte åbner store muligheder for at forbedre vores afgrøders oversvømmelsestolerance, og vi arbejder for en fremtid hvor man kan lukke ned for markdrænene og genskabe den naturlige hydrologi i landskabet til glæde for dyr og planter i søer og vandløb.