Long-distance transport of gases in plants: a perspective on internal aeration and radial oxygen loss from roots

Internal transport of gases is crucial for vascular plants inhabiting aquatic, wetland or flood‐prone environments. Diffusivity of gases in water is approximately 10 000 times slower than in air; thus direct exchange of gases between submerged tissues and the environment is strongly impeded. Aerenchyma provides a low‐resistance internal pathway for gas transport between shoot and root extremities. By this pathway, O₂ is supplied to the roots and rhizosphere, while CO₂, ethylene, and methane move from the soil to the shoots and atmosphere. Diffusion is the mechanism by which gases move within roots of all plant species, but significant pressurized through‐flow occurs in stems and rhizomes of several emergent and floating‐leaved wetland plants. Through‐flows can raise O₂ concentrations in the rhizomes close to ambient levels. In general, rates of flow are determined by plant characteristics such as capacity to generate positive pressures in shoot tissues, and resistance to flow in the aerenchyma, as well as environmental conditions affecting leaf‐to‐air gradients in humidity and temperature. O₂ diffusion in roots is influenced by anatomical, morphological and physiological characteristics, and environmental conditions. Roots of many (but not all) wetland species contain large volumes of aerenchyma (e.g. root porosity can reach 55%), while a barrier impermeable to radial O₂ loss (ROL) often occurs in basal zones. These traits act synergistically to enhance the amount of O₂ diffusing to the root apex and enable the development of an aerobic rhizosphere around the root tip, which enhances root penetration into anaerobic substrates. The barrier to ROL in roots of some species is induced by growth in stagnant conditions, whereas it is constitutive in others. An inducible change in the resistance to O₂ across the hypodermis/exodermis is hypothesized to be of adaptive significance to plants inhabiting transiently waterlogged soils. Knowledge on the anatomical basis of the barrier to ROL in various species is scant. Nevertheless, it has been suggested that the barrier may also impede influx of: (i) soil‐derived gases, such as CO₂, methane, and ethylene; (ii) potentially toxic substances (e.g. reduced metal ions) often present in waterlogged soils; and (iii) nutrients and water. Lateral roots, that remain permeable to O₂, may be the main surface for exchange of substances between the roots and rhizosphere in wetland species. Further work is required to determine whether diversity in structure and function in roots of wetland species can be related to various niche habitats.     

Acclimation of a terrestrial plant to submergence facilitates gas exchange under water

Flooding imposes stress upon terrestrial plants since it severely hampers gas exchange rates between the shoot and the environment. The resulting oxygen deficiency is considered to be the major problem for submerged plants. Oxygen microelectrode studies have, however, shown that aquatic plants maintain relatively high internal oxygen pressures under water, and even may release oxygen via the roots into the sediment, also in dark. Based on these results, we challenge the dogma that oxygen pressures in submerged terrestrial plants immediately drop to levels at which aerobic respiration is impaired. The present study demonstrates that the internal oxygen pressure in the petioles of Rumex palustris plants under water is indeed well above the critical oxygen pressure for aerobic respiration, provided that the air‐saturated water is not completely stagnant. The beneficial effect of shoot acclimation of this terrestrial plant species to submergence for gas exchange capacity is also shown. Shoot acclimation to submergence involved a reduction of the diffusion resistance to gases, which was not only functional by increasing diffusion of oxygen into the plant, but also by increasing influx of CO₂, which enhances underwater photosynthesis.     

Do tropical wetland plants possess convective gas flow mechanisms?

? Internal pressurization and convective gas flow, which can aerate wetland plants more efficiently than diffusion, are common in temperate species. Here, we present the first survey of convective flow in a range of tropical plants. ? The occurrence of pressurization and convective flow was determined in 20 common wetland plants from the Mekong Delta in Vietnam. The diel variation in pressurization in culms and the convective flow and gas composition from stubbles were examined for Eleocharis dulcis, Phragmites vallatoria and Hymenachne acutigluma, and related to light, humidity and air temperature. ? Nine of the 20 species studied were able to build up a static pressure of > 50 Pa, and eight species had convective flow rates higher than 1 ml min(-1). There was a clear diel variation, with higher pressures and flows during the day than during the night, when pressures and flows were close to zero. ? It is concluded that convective flow through shoots and rhizomes is a common mechanism for below-ground aeration of tropical wetland plants and that plants with convective flow might have a competitive advantage for growth in deep water.     

Hormones and root-shoot relationships in flooded plants — an analysis of methods and results

Two aspects of root to shoot communication in flooded plants are discussed (i) the formation of porous aerenchyma that enhances the passage of oxygen, and other gases, from shoots to roots and (ii) the movement of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) from roots to shoots in the transpiration stream, and the effect of this on ethylene production and epinastic curvature in the shoots. For aerenchyma studies a highly sensitive photoacoustic laser detector for ethylene was used to avoid interference associated with other methods of ethylene measurement that require tissue excision. ACC concentrations in xylem sap were measured by physico-chemical means to ensure correct identification and account for processing losses. Solute concentrations, e.g., abscisic acid (ABA), in xylem sap are shown to be distorted by temporary contamination caused by the method used to collect sap. Concentrations of solutes in xylem sap (e.g., ACC) are also altered by changes in sap flow brought about by conventional methods of sap collection or by experimental treatments such as flooding the soil. Ways of for overcoming these problems are described together with a summary of preliminary results.     

The Roots of Carnivorous Plants

Carnivorous plants may benefit from animal-derived nutrients to supplement minerals from the soil. Therefore, the role and importance of their roots is a matter of debate. Aquatic carnivorous species lack roots completely, and many hygrophytic and epiphytic carnivorous species only have a weakly devel-oped root system. In xerophytes, however, large, extended and/or deep-reaching roots and sub-soil shoots develop. Roots develop also in carnivorous plants in other habitats that are hostile, due to flood-ing, salinity or heavy metal occurance. Information about the structure and functioning of roots of car- nivorous plants is limited, but this knowledge is essential for a sound understanding of the plants’ physiology and ecology. Here we compile and summarise available information on: (1) The morphology of the roots. (2) The root functions that are taken over by stems and leaves in species without roots or with poorly developed root systems; anchoring and storage occur by specialized chlorophyll-less stems; water and nutrients are taken up by the trap leaves. (3) The contribution of the roots to the nutrient supply of the plants; this varies considerably amongst the few investigated species. We compare nutrient uptake by the roots with the acquisition of nutri-ents via the traps. (4) The ability of the roots of some carnivorous species to tolerate stressful conditions in their habitats; e.g., lack of oxygen, saline conditions, heavy metals in the soil, heat during bushfires, drought, and flooding     

Measuring and interpreting respiratory critical oxygen pressures in roots

BACKGROUND AND AIMS: Respiratory critical oxygen pressures (COPR) determined from O(2)-depletion rates in media bathing intact or excised roots are unreliable indicators of respiratory O(2)-dependency in O(2)-free media and wetlands. A mathematical model was used to help illustrate this, and more relevant polarographic methods for determining COPR in roots of intact plants are discussed. METHODS: Cortical [O(2)] near the root apex was monitored indirectly (pea seedlings) from radial oxygen losses (ROL) using sleeving Pt electrodes, or directly (maize) using microelectrodes; [O(2)] in the root was controlled by manipulating [O(2)] around the shoots. Mathematical modelling of radial diffusive and respiratory properties of roots used Michaelis-Menten enzyme kinetics. KEY RESULTS: Respiration declined only when the O(2) partial pressure (OPP) in the cortex of root tips fell below 0.5-4.5 kPa, values consistent with depressed respiration near the centre of the stele as confirmed by microelectrode measurements and mathematical modelling. Modelling predictions suggested that the OPP of a significant core at the centre of roots could be below the usual detection limits of O(2)-microelectrodes but still support some aerobic respiration. CONCLUSIONS: In O(2)-free media, as in wetlands, the COPR for roots is likely to be quite low, dependent upon the respiratory demands, dimensions and diffusion characteristics of the stele/stelar meristem and the enzyme kinetics of cytochrome oxidase. Roots of non-wetland plants may not differ greatly in their COPRs from those of wetland species. There is a possibility that trace amounts of O(2) may still be present in stelar 'anaerobic' cores where fermentation is induced at low cortical OPPs.     

N(2) fixation by bacteria associated with maize roots at a low partial o(2) pressure

Nitrogen fixation by bacteria associated with roots of intact maize plants was measured by exposing the roots to N(2) at a partial O(2) pressure (pO(2)) of 2 or 10 kPa. The plants were grown in a mixture of Weswood soil and sand and then transferred to plastic cylinders containing an N-free plant nutrient solution. The solution was sparged continuously with a mixture of air and N(2) at a pO(2) of 2 or 10 kPa. Acetylene reduction was measured after the roots were exposed to the low pO(2) overnight. The air-N(2) atmosphere in the cylinders was then replaced with an O(2)-He atmosphere at the same pO(2), and the roots were exposed to 20 kPa of N(2) for 20 to 22 h. Incorporation of N into the roots was 200 times greater at 2 kPa of O(2) than at 10 kPa of O(2). Adding l-malate (1 g of C liter) to the nutrient solution increased root-associated nitrogenase activity, producing a strong N label which could be traced into the shoots. Fixed N was detected in the shoots within 5 days after the plants were returned to unfertilized soil. In a similar experiment with undisturbed plants grown in fritted clay, movement of fixed N into the shoots was evident within 4 days after the roots were exposed to N(2) at 2 kPa of O(2). Inoculation with Azospirillum lipoferum yielded no significant differences in shoot dry weight, total nitrogen content, percent nitrogen, or N enrichment of plant tissues. Inoculated plants did exhibit greater root dry weight than uninoculated plants, however.     

Water relations under root chilling in a sensitive and tolerant tomato species

The shoots of cultivated tomato (Lycopersicon esculentum cv. T5) wilt if their roots are exposed to chilling temperatures of around 5 °C. Under the same treatment, a chilling‐tolerant congener (Lycopersicon hirsutum LA 1778) maintains shoot turgor. To determine the physiological basis of this differential response, the effect of chilling on both excised roots and roots of intact plants in pressure chambers were investigated. In excised roots and intact plants, root hydraulic conductance declined with temperature to nearly twice the extent expected from the temperature dependence of the viscosity of water, but the response was similar in both species. The species differed markedly, however, in stomatal behaviour: in L. hirsutum, stomatal conductance declined as root temperatures were lowered, whereas the stomata of L. esculentum remained open until the roots reached 5 °C, and the plants became flaccid and suffered damage. Grafted plants with the shoots of one genotype and roots of another indicated that the differential stomatal behaviour during root chilling has distinct shoot and root components.     

Cadmium uptake from solution by plants and its transport from roots to shoots

Summary The uptake of cadmium by the roots of plants, and its transport to shoots was examined using solution culture. Uptake by the roots of perennial ryegrass over a period of 4 hours from an aqueous solution containing 0.25 ppm cadmium as CdCl₂ was (i) enhanced by killing the roots and (ii) depressed when Ca²⁺, Mn²⁺ or Zn²⁺ were added to the solution. The distribution of cadmium between the roots and shoots of 23 species was examined at 4 days after a single, 3-day exposure to a nutrient solution containing 0.01 ppm added Cd. In all except 3 species, i.e. kale, lettuce and watercress, more than 50 per cent of that taken up was retained in the roots. The concentration in the roots was always greater than in the shoots, and in fibrous roots of fodder beet, parsnip, carrot and radish it was greater than in the swollen storage roots. When perennial ryegrass was similarly exposed to solutions containing 0.01, 0.05, and 0.25 ppm added cadmium, uptake, as measured at 3 days after adding cadmium, increased with increasing rates of addition, but the proportion retained in the roots was constant (approximately 88 per cent). There was no further transport from roots to shoots during the next 21 days, with the result that the concentration in the shoots decreased progressively with increasing growth. It is concluded that although the roots of several species can take up large quantities of cadmium from solution there are mechanisms which may restrict the movement of cadmium through plants, and thus to animals.     

Mercury uptake and phytotoxicity in terrestrial plants grown naturally in the Gumuskoy (Kutahya) mining area,Turkey

This study investigated mercury (Hg) uptake and transport from the soil to different plant parts by documenting the distribution and accumulation of Hg in the roots and shoots of 12 terrestrial plant species, all of which grow naturally in surface soils of the Gumuskoy Pb-Ag mining area. Plant samples and their associated soils were collected and analyzed for Hg content by ICP-MS. Mean Hg values in the soils, roots, and shoots of all plants were 6.914, 460, and 206 µg kg^?1, respectively and lower than 1. The mean enrichment factors for the roots (ECR) and shoots (ECS) of these plants were 0.06 and 0.09, respectively and lower than 1. These results show that the roots of the studied plants prevented Hg from reaching the aerial parts of the plants. The mean translocation factor (TLF) was 1.29 and higher than 1. The mean TLF values indicated that all 12 plant species had the ability to transfer Hg from the roots to the shoots but that transfer was more efficient in plants with higher ECR and ECS. Therefore, these plants could be useful for the biomonitoring of environmental pollution and for rehabilitating areas contaminated by Hg.     

Pressurized gas transport in two Japanese alder species in relation to their natural habitats

Oxygen uptake measurements have shown that pressurized gas transport, resulting from the physical effect of thermo-osmosis of gases, improves oxygen supply to the roots of the seedlings in two alder speciesAlnus japonica (Thunb.) Steud. andAlnus hirsuta (Spach) Rupr., which are both native in Japan. When gas transport conditions were established by irradiation of the tree stems the internal aeration was increased to a level nearly equal to the oxygen demand of the root system in leafless seedlings ofA. hirsuta, but was higher inA. japonica so that excess oxygen was excreted into the environment. An increase of superoxide dismutase (SOD) activity, which protects plants from toxic oxygen radicals and post-anoxic injury, has been observed in root tissues ofA. japonica when the seedlings were flooded for 3 days. The increase of SOD activity, in concert with high gas transport rates, may enable this tree species to grow in wet sites characterized by low oxygen partial pressure in the soil and by varying water tables. A less effective gas transport, flood-induced reduction of SOD activity in root tissues, and reduced height growth in waterlogged soil may be responsible for the fact thatA. hirsuta is unable to inhabit wettland sites.     

The aerenchymatous phellem of Lythrum salicaria (L.): a pathway for gas transport and its role in flood tolerance

While the importance of cortical aerenchyma in flood tolerance is well established, this pathway for gaseous exchange is often destroyed during secondary growth. For woody species, therefore, an additional pathway must develop for oxygen to reach submerged tissues. In this paper we examine the potential for the aerenchymatous phellem (cork) of Lythrum salicaria L. to provide a pathway for gas transport from shoots to roots and assess its importance in flood tolerance. Plants in which the continuity of the aerenchymatous phellem between shoots and roots was broken showed a significant reduction in oxygen levels in roots, but no difference in carbon dioxide levels compared with controls that retained an intact phellem. These plants also had a greater total shoot height and shoot dry weight, and an increase in shoot/root dry mass ratios compared with controls. Total dry weight was not significantly affected by this treatment. This study is the first to show that the aerenchymatous phellem can provide a pathway for gaseous exchange between roots and shoots and can influence plant morphology and patterns of resource allocation. This suggests that this tissue may play a significant role in the flood tolerance of a woody plant.     

Ethylene Evolution from Maize (Zea mays L.) Seedling Roots and Shoots in Response to Mechanical Impedance

The effect of mechanical impedance on ethylene evolution and growth of preemergent maize (Zea mays L.) seedlings was investigated by pressurizing the growth medium in triaxial cells in a controlled environment. Pressure increased the bulk density of the medium and thus the resistance to growth. The elongation of maize primary roots and preemergent shoots was severely hindered by applied pressures as low as 10 kilopascals. Following a steep decline in elongation at low pressures, both shoots and roots responded to additional pressure in a linear manner, but shoots were more severely affected than roots at higher pressures. Radial expansion was promoted in both organs by mechanical impedance. Primary roots typically became thinner during the experimental period when grown unimpeded. In contrast, pressures as low as 25 kilopascals caused a 25% increase in root tip diameter. Shoots showed a slight enhancement of radial expansion; however, in contrast to roots, the shoots increased in diameter even when growing unimpeded. Such morphological changes were not evident until at least 3 hours after initiation of treatment. All levels of applied pressure promoted ethylene evolution as early as 1 hour after application of pressure. After 1 hour, ethylene evolution rates had increased 10, 32, 70, and 255% at 25, 50, 75, and 100 kilopascals respectively, and continued to increase linearly for at least 10 hours. When intact corn seedlings were subjected to a series of hourly cycles of pressure, followed by relaxation, ethylene production rates increased or decreased rapidly, illustrating tight coupling between mechanical impedance and tissue response. Seedlings exposed to 1 microliter of ethylene per liter showed symptoms similar to those shown by plants grown under mechanical impedance. Root diameter increased 5 times as much as the shoot diameter. Pretreatment with 10 micromolar aminoethoxyvinyl glycine plus 1 micromolar silver thiosulfate maintained ethylene production rates of impeded seedlings at basal levels and restored shoot and root extension to 84 and 90% of unimpeded values, respectively. Our results support the hypothesis that ethylene plays a pivotal role in the regulation of plant tissue response to mechanical impedance.     

Oxygen dynamics during submergence in the halophytic stem succulent Halosarcia pergranulata

This study elucidated O2 dynamics in shoots and roots of submerged Halosarcia pergranulata (Salicornioideae), a perennial halophytic stem succulent that grows on floodprone mudflats of salt lakes. Oxygen within shoots and roots was measured using microelectrodes, for plants when waterlogged or completely submerged, with shoots in light or in darkness, in a controlled environment. Net photosynthesis (PN) when underwater, at a range of dissolved CO2 concentrations, was measured by monitoring O2 production rates by excised stems. The bulky nature and apparently low volume of gas-filled spaces of the succulent stems resulted in relatively high radial resistance to gas diffusion. At ambient CO2, quasi-steady state rates of PN by excised succulent stems were estimated to be close to zero; nevertheless, in intact plants, underwater photosynthesis provided O2 to tissues and led to radial O2 loss (ROL) from the roots, at least during the first several hours (the time period measured) after submergence or when light periods followed darkness. The influence of light on tissue O2 dynamics was confirmed in an experiment on a submerged plant in a salt lake in south-western Australia. In the late afternoon, partial pressure of O2 (pO2) in the succulent stem was 23.2 kPa (i.e. approximately 10% above that in the air), while in the roots, it was 6.2-9.8 kPa. Upon sunset, the pO2 in the succulent stems declined within 1 h to below detection, but then showed some fluctuations with the pO2 increasing to at most 2.5 kPa during the night. At night, pO2 in the roots remained higher than in the succulent stems, especially for a root with the basal portion in the floodwater. At sunrise, the pO2 increased in the succulent stems within minutes. In the roots, changes in the pO2 lagged behind those in the succulent stems. In summary, photosynthesis in stems of submerged plants increased the pO2 in the shoots and roots so that tissues experience diurnal changes in the pO2, but O2 from the H2O column also entered submerged plants.     

Regulatory network of microRNA399 and PHO2 by systemic signaling

Recently, we showed that microRNA399s (miR399s) control inorganic phosphate (Pi) homeostasis by regulating the expression of PHO2 encoding a ubiquitin-conjugating E2 enzyme 24. Arabidopsis (Arabidopsis thaliana) plants overexpressing miR399 or the pho2 mutant overaccumulate Pi in shoots. The association of Pi translocation and coexpression of miR399s and PHO2 in vascular tissues suggests their involvement in long-distance signaling. In this study, we used reciprocal grafting between wild-type and miR399-overexpressing transgenic plants to dissect the systemic roles of miR399 and PHO2. Arabidopsis rootstocks overexpressing miR399 showed high accumulation of Pi in the wild-type scions because of reduced PHO2 expression in the rootstocks. Although miR399 precursors or expression was not detected, we found a small but substantial amount of mature miR399 in the wild-type rootstocks grafted with transgenic scions, which indicates the movement of miR399 from shoots to roots. Suppression of PHO2 with miR399b or c was less efficient than that with miR399f. Of note, findings in grafted Arabidopsis were also discovered in grafted tobacco (Nicotiana benthamiana) plants. The analysis of the pho1 mutant provides additional support for systemic suppression of PHO2 by the movement of miR399 from Pi-depleted shoots to Pi-sufficient roots. We propose that the long-distance movement of miR399s from shoots to roots is crucial to enhance Pi uptake and translocation during the onset of Pi deficiency. Moreover, PHO2 small interfering RNAs mediated by the cleavage of miR399s may function to refine the suppression of PHO2. The regulation of miR399 and PHO2 via long-distance communication in response to Pi deficiency is discussed.     

Regulation of root anaerobiosis and carbon translocation by light and root aeration in Isoetes alpinus

The activity of alcohol dehydrogenase (ADH) was measured in corms and roots of the submerged freshwater macrophyte Isoetes alpinus Kirk. growing in situ, and related to its capacity for internal oxygen transport and to carbohydrate translocation. ADH activity was present in roots but not corms at uniform activity (0.15–0.35 × 10^?6 mol g^?1 fresh weight s^?1) over the entire plant depth range (3–7 m depth), and was intermediate to that developed in excised roots after 1‐week exposure to either dissolved oxygen at air‐saturation or to anoxia. Responses of photosynthesis and root oxygen release to light intensity confirmed that shoot‐to‐root oxygen transport saturated at similar light intensities to photosynthetic oxygen evolution, but was positive in the dark and at irradiances below the compensation point for photosynthesis, due to contributions to transport by oxygen diffusion from the external medium. Transport of ¹⁴C‐labelled photo‐assimilates to roots nevertheless ceased when intact plants were exposed to a combination of leaf darkness and root external anoxia, even when high ¹⁴C concentrations were present in shoots, but remained high when the roots were provided with external oxygen. The lack of any control over permeability of the root surface to gases in this species suggested that ADH activity and reduced translocation is most likely caused by development of hypoxic tissues in the apical tissue. These results suggest that reductions in ambient light intensity may have indirect effects on I. alpinus viability by increasing the degree of root hypoxia and impairing carbon partitioning.     

Export of Abscisic Acid, 1-Aminocyclopropane-1-Carboxylic Acid,Phosphate, and Nitrate from Roots to Shoots of Flooded Tomato Plants (Accounting for Effects of Xylem Sap Flow Rate on Concentration and Delivery)

We determined whether root stress alters the output of physiologically active messages passing from roots to shoots in the transpiration stream. Concentrations were not good measures of output. This was because changes in volume flow of xylem sap caused either by sampling procedures or by effects of root stress on rates of whole-plant transpiration modified concentrations simply by dilution. Thus, delivery rate (concentration x sap flow rate) was preferred to concentration as a measure of solute output from roots. To demonstrate these points, 1-aminocyclopropane-1-carboxylic acid (ACC), abscisic acid, phosphate, nitrate, and pH were measured in xylem sap of flooded and well-drained tomato (Lycopersicon esculentum Mill., cv Ailsa Craig) plants expressed at various rates from pressurized detopped roots. Concentrations decreased as sap flow rates were increased. However, dilution of solutes was often less than proportional to flow, especially in flooded plants. Thus, sap flowing through detopped roots at whole-plant transpiration rates was used to estimate solute delivery rates in intact plants. On this basis, delivery of ACC from roots to shoots was 3.1-fold greater in plants flooded for 24 h than in well-drained plants, and delivery of phosphate was 2.3-fold greater. Delivery rates of abscisic acid and nitrate in flooded plants were only 11 and 7%, respectively, of those in well-drained plants.     

Chill-induced decrease in capacity of RuBP carboxylation and associated H2O2 accumulation in cucumber leaves are alleviated by grafting onto figleaf gourd

BACKGROUND AND AIMS: Chilling results in a significant decrease in Rubisco content and increased generation of reactive oxygen species (ROS) in cucumber (Cucumis sativus), a chilling-sensitive species. The role of roots in the regulation of the tolerance is unknown. Here, cucumber plants grafted onto figleaf gourd (Cucurbita ficifolia), a chilling-tolerant species were used to study the role of roots in the regulation of shoot functioning and the associated root-to-shoot communication. METHODS: Gas exchange and chlorophyll fluorescence were measured using an infrared gas analyser combined with a pulse amplitude fluorimeter during chilling at 14 degrees C or 7 degrees C and subsequent recovery. At the same time, Rubisco content and activity and ROS generation were spectrophotometrically assayed. Abscisic acid and cytokinin concentrations in xylem sap were also determined by enzyme-linked immunosorbent assay. KEY RESULTS AND CONCLUSIONS: Grafted plants showed a significantly higher light-saturated rate of CO(2) assimilation (A(sat)) than own-rooted plants when roots were gradually cooled, but no differences were detected when shoots were cooled. Chill at 7 degrees C irreversibly reduced A(sat), and significantly decreased maximum carboxylation activity, Rubisco content and initial Rubisco activity. However, grafted plants showed weaker inhibition, together with decreased electron flux in the water-water cycle. Higher activity of antioxidant enzymes with less ROS production was found in grafted plants. In addition, ABA concentration increased by 48.4-fold whilst cytokinin concentration decreased by 91.5% in the xylem sap of own-rooted plants after exposure to a 7 degrees C chill. In comparison, ABA and cytokinin concentrations increased by 10.5-fold and 36.9%, respectively, for the grafted plants. Improved plant growth was also observed in grafted plants after the chill. These results suggest that some signals coming from chilling-resistant roots (i.e. ABA and cytokinins) protect leaf photosynthesis in shoots of chilling-sensitive plants.     

Fermentation metabolism in roots of wheat seedlings after hypoxic pre-treatment in different anoxic incubation systems

A hypoxic pre-treatment (HPT) can improve the anoxic survival of flooding sensitive plants. Here, we tested whether a 4-d HPT of wheat plants (Triticum aestivum L.) would improve their anoxic resistance, and if so, why. We found that the metabolic adjustment during prolonged HPT involved an increased lactate excretion rate, the up-regulation of glycolytic and fermentative enzymes as well as the accumulation of various sugars. Therefore, HPT wheat roots could sustain a 3 times higher ethanolic fermentation rate during an anoxic period compared to non-pre-treated (NHPT) roots. Nevertheless, the enhanced fermentation rate provided temporary relief to the energy crisis only, and both NHPT and HPT plants died after 5d of anoxia in illumination. Comparison of different low oxygen incubation systems using excised roots or roots of intact plants revealed striking differences. The benefits of intact shoots, oxygen transport as well as additional sugar supply enabled a more stable energy supply of anoxia-treated NHPT and HPT roots. However, the height of the fermentation rate was correlated with a high ATP content during dark anoxic incubation, but not in illumination.     

Anatomical patterns of aerenchyma in aquatic and wetland plants

A well-developed aerenchyma is a major characteristic of aquatic plants. However, because such tissues are also found in wetland and terrestrial plants, it is not always possible to use their presence or absence to distinguish aquatic species. Whereas patterns of aerenchyma in roots have been studied in detail, those of the shoots have not. We collected and tested 110 species of various aquatic and wetland plants, including ferns (5), basal angiosperms (5), monocots (65), and eudicots (35). Three common and two rare types of aerenchyma were observed in their roots (three schizogeny and two lysigeny), plus five types of schizogeny in their shoots. We re-confirmed that, although a well-developed aerenchyma is more common in most organs of aquatic plants than in wetland plants, this presence cannot be used as strict evidence for the aquatic quality of vascular plants. Here, aerenchyma patterns were stable at the genus level, and the consistency of pattern was stronger in the roots than in the shoots. Furthermore, significant trends were verified in several higher taxa, and those consistencies of patterns partially coincided with their phylogeny.