Complete Submergence Research Articles

Impeded Carbohydrate Metabolism in Rice Plants under Submergence Stress

The detrimental effects of submergence on physiological performances of some rice varieties with special references to carbohydrate metabolisms and their allied enzymes during post-flowering stages have been documented and clarified in the present investigation. It was found that photosynthetic rate and concomitant translocation of sugars into the panicles were both related to the yield. The detrimental effects of the complete submergence were recorded in generation of sucrose, starch, sucrose phosphate synthase and phosphorylase activity in the developing panicles of the plants as compared to those under normal or control (i.e. non-submerged) condition. The accumulation of starch was significantly lower in plants under submergence and that was correlated with ADP-glucose pyrophosphorylase activity. Photosynthetic rate was most affected under submergence in varying days of post-flowering and was also related to the down regulation of Ribulose bisphosphate carboxylase activity. However, under normal or control condition, there recorded a steady maintenance of photosynthetic rate at the post-flowering stages and significantly higher values of Ribulose bisphosphate carboxylase activity. Still, photosynthetic rate of the plants under both control and submerged conditions had hardly any significant correlation with sugar accumulation and other enzymes of carbohydrate metabolism like invertase with grain yield. Finally, plants under submergence suffered significant loss of yield by poor grain filling which was related to impeded carbohydrate metabolism in the tissues. It is evident that loss of yield under submergence is attributed both by lower sink size or sink capacity (number of panicles, in this case) as well as subdued carbohydrate metabolism in plants and its subsequent partitioning into the grains.

Distinct mechanisms for aerenchyma formation in leaf sheaths of rice genotypes displaying a quiescence or escape strategy for flooding tolerance.

Rice is one of the few crops able to withstand periods of partial or even complete submergence. One of the adaptive traits of rice is the constitutive presence and further development of aerenchyma which enables oxygen to be transported to submerged organs. The development of lysigenous aerenchyma is promoted by ethylene accumulating within the submerged plant tissues, although other signalling mechanisms may also co-exist. In this study, aerenchyma development was analysed in two rice (Oryza sativa) varieties, 'FR13A' and 'Arborio Precoce', which show opposite traits in flooding response in terms of internode elongation and survival. The growth and survival of rice varieties under submergence was investigated in the leaf sheath of 'FR13A' and 'Arborio Precoce'. The possible involvement of ethylene and reactive oxygen species (ROS) was evaluated in relation to aerenchyma formation. Cell viability and DNA fragmentation were determined by FDA/FM4-64 staining and TUNEL assay, respectively. Ethylene production was monitored by gas chromatography and by analysing ACO gene expression. ROS production was measured by using Amplex Red assay kit and the fluorescent dye DCFH(2)-DA. The expression of APX1 was also evaluated. AVG and DPI solutions were used to test the effect of inhibiting ethylene biosynthesis and ROS production, respectively. Both the varieties displayed constitutive lysigenous aerenchyma formation, which was further enhanced when submerged. 'Arborio Precoce', which is characterized by fast elongation when submerged, showed active ethylene biosynthetic machinery associated with increased aerenchymatous areas. 'FR13A', which harbours the Sub1A gene that limits growth during oxygen deprivation, did not show any increase in ethylene production after submersion but still displayed increased aerenchyma. Hydrogen peroxide levels increased in 'FR13A' but not in 'Arborio Precoce'. While ethylene controls aerenchyma formation in the fast-elongating 'Arborio Precoce' variety, in 'FR13A' ROS accumulation plays an important role.

Ubuntu: I in You and You in Me (review)

Reviewed by: Ubuntu: I in You and You in Me Barbara Newman (bio) Ubuntu: I in You and You in Me. By Michael Battle. New York: Seabury Books, 2009. viii + 166 pp. $18.00. I think, therefore I am? Think again. According to Kenyan theologian John Mbiti, a more apt formulation would be "we are, therefore I am" (36). His axiom is an expression of [End Page 136] Ubuntu, the indigenous African concept explored in this slim volume by Michael Battle. An African American theologian and Episcopal priest ordained by Archbishop Desmond Tutu, Battle offers his book as an introduction to his mentor's thought and a contribution to theological dialogue between Africa and the West. In a nutshell, Ubuntu is a worldview that derives identity from community, rather than individual rights. As the subtitle suggests, it harmonizes well with the Christian view of personhood encapsulated by the mysterious "in" of Paul's letters and John's gospel. If "a person depends on other persons to be a person," this is no more or less than the divine image, for "the three persons of God are so interdependent that all three . . . have one nature" (3). Hence Battle's discussion toggles between communal values and Trinitarian doctrine, which he takes in part from Russian Orthodox theologian Vladimir Lossky. The technical term for the mutual indwelling of Father, Son, and Holy Spirit is perichoresis, which approximates the meaning of Ubuntu. Loving relationships based on the principle of "I in you, you in me" can and must occur among us because this is both the nature of God and the command of Jesus. Western theologians have rarely developed theologies of personhood along these lines, as Tutu has done in South Africa. One of the few exceptions, the Anglican Charles Williams—unfortunately missing from this book—used the English term coinherence, an even more precise synonym for Ubuntu. Desmond Tutu's turbulent life has given him no leisure to write systematic theology. So the profundity of his thought, less familiar to Westerners than the heroism of his life, is among the gifts afforded by Battle's work. Culling passages from the bishop's countless sermons, speeches, and essays, Battle offers a sampler of Tutu's thinking on topics from apartheid to the Incarnation, from evolution to the flaws of Western individualism. He emerges as not only an inspiring leader, but a deeply prayerful man. Battle recalls that during the two eventful years when he served as his chauffeur, the more tumultuous political affairs became, the more insistently Tutu prayed the Divine Office from memory on the road. One feature that distinguishes his version of liberation theology is the deep conviction, rooted in Ubuntu, that an end to the oppression of apartheid would benefit whites no less than blacks. It was Tutu who chaired South Africa's extraordinary Truth and Reconciliation Commission, a public forum in which victims and perpetrators confronted each other face to face so that oppressors might confess their atrocities in the presence of the oppressed. The goal was not merely to bring wrongdoers to justice, but to effect repentance, forgiveness, and reconciliation among people who would, after all, have to go on living together. How different are the "victim statements" offered in American courts, the purpose of which is to impress a jury with the enormity of some crime. Such statements may at times lead an offender to contrition, but this is hardly their goal or anyone's realistic expectation. I offer this contrast not to critique American justice, but to highlight the remarkable depth of Christian faith—and trust in Ubuntu—behind Tutu's great experiment in social transformation. Battle provides complementary chapters on "How Ubuntu Can Be a Gift to the West" and "How the West Can Be a Gift to Ubuntu." The first includes some familiar critiques of Western rationalism, secularism, and individualistic spirituality. For instance, American evangelicals will ask, "do you have a personal relationship with Jesus?" but never "do you have a communal relationship with Jesus?" (88). The next chapter addresses African tendencies toward dictatorship and the [End Page 137] complete submergence of the individual in the community. Tutu has insisted on the value of Western...

Tolerance of longer-term partial stagnant flooding is independent of the SUB1 locus in rice

Longer-term partial stagnant flooding, particularly when it occurs following transient complete submergence causes severe damage to modern rice varieties. Progress was made in developing varieties tolerant of complete submergence through transfer of the Submergence-1 ( SUB1) gene into popular varieties. However, SUB1 may not be effective under partial stagnant flooding (SF), as the new varieties may not elongate and continue growth when fully or partially submerged because of the SUB1-mediated suppression of elongation. We tested a set of rice genotypes, including a pair of near-isogenic lines (NILs), Swarna and Swarna-Sub1, under either SF or SF following complete submergence of 12 d. Swarna-Sub1 had higher survival and yield than Swarna following 12 d of submergence, but survival and grain yield of all lines decreased substantially when SF of 15–30 cm followed complete submergence, with the sensitive lines experiencing greater reductions in growth and yield. This suggests the importance of combining SUB1 with tolerance of SF in areas where both stresses are expected during the season. Swarna and Swarna-Sub1 are more sensitive to long-term partial SF than IR49830 and IR42 because of their short stature, and Swarna-Sub1 showed slightly higher reduction in tillering than did Swarna when subjected to deeper SF alone, possibly because of further inhibition of elongation by SUB1 if induced in submerged tissue. The results suggest that tolerance of these genotypes to SF depends less on SUB1 introgression and more so on the genetic background of the recipient genotype, with better performance of the genotypes that are inherently taller, such as IR42 and IR49830. The SUB1 donor landrace FR13A and its derivative breeding line IR49830 had better survival and relatively less reduction in grain yield under SF following complete submergence and under longer-term partial SF, indicating that these genotypes may have genes other than SUB1 for submergence and SF tolerance. For better adaptation to prolonged SF, SUB1 should be introgressed into genotypes that tolerate partial SF through better tillering ability and taller shoots, because SUB1 may not be effective in shorter genotypes, as it promotes survival of submerged plants by hindering shoot elongation to conserve energy reserves. Varieties combining tolerance of prolonged SF with SUB1 will have broader adaptation in flood-prone areas and greater impacts on yield stability.

Identification of SUB1A alleles from wild rice Oryza rufipogon Griff.

Submergence stress is a major constraint to rice production in South and Southeast Asia. Most rice (Oryza sativa L.) cultivars die within a week of complete submergence, while a small number of accessions are submergence-tolerant for up to 2 weeks or more. These cultivars have the tolerant allele of the SUB1A gene, one of three ERF genes at this locus on rice chromosome 9. In all O. sativa varieties studied, the SUB1A gene is limited to a subset of indica accessions of O. sativa. Thus far, there has been no published report of the SUB1A gene in wild rice species. Here we report evidence of the SUB1A gene found in wild species of O. rufipogon Griff. accessions by the use of degenerate primers corresponding to the most highly conserved regions of the SUB1 locus. The results indicated that two SUB1A-like alleles, e.g. OrSub1A-1 and OrSub1A-2, were identified from two O. rufipogon accessions. Submergence treatment shows that both of the accessions with SUB1A-like genes were submergence-intolerant. This preliminary study provides insight into the origin and allelic variation of SUB1A, an agronomically important gene that is rapidly being introduced into widely-grown rice cultivars.

Antioxidative enzymes in the response of buckwheat (Fagopyrum esculentum Moench) to complete submergence

Oxidative stress and antioxidative defense system activity were studied in buckwheat leaves after complete submergence and re-aeration. The levels of H2O2 and lipid peroxidation were found to be significantly higher in stressed than in untreated buckwheat leaves. Enzymes catalyzing the degradation of H2O2 and peroxides were shown to participate actively, whereas superoxide dismutase did not take part in the buckwheat leaf response to flooding stress. The most prominent increase in antioxidative enzyme activities was noticed upon return to air, when the strongest oxidative stress occurred and the need for antioxidative defense was the greatest.

A perspective on underwater photosynthesis in submerged terrestrial wetland plants.

Wetland plants inhabit flood-prone areas and therefore can experience episodes of complete submergence. Submergence impedes exchange of O(2) and CO(2) between leaves and the environment, and light availability is also reduced. The present review examines limitations to underwater net photosynthesis (P(N)) by terrestrial (i.e. usually emergent) wetland plants, as compared with submerged aquatic plants, with focus on leaf traits for enhanced CO(2) acquisition. Floodwaters are variable in dissolved O(2), CO(2), light and temperature, and these parameters influence underwater P(N) and the growth and survival of submerged plants. Aquatic species possess morphological and anatomical leaf traits that reduce diffusion limitations to CO(2) uptake and thus aid P(N) under water. Many aquatic plants also have carbon-concentrating mechanisms to increase CO(2) at Rubisco. Terrestrial wetland plants generally lack the numerous beneficial leaf traits possessed by aquatic plants, so submergence markedly reduces P(N). 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 CO(2) entry. Submergence inhibits P(N) 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 P(N) is enhanced. Underwater P(N) provides sugars and O(2) to submerged plants. Floodwaters often contain dissolved CO(2) above levels in equilibrium with air, enabling at least some P(N) by terrestrial species when submerged, although rates remain well below those in air.

Natural variation of submergence tolerance among Arabidopsis thaliana accessions

• The exploitation of natural variation in Arabidopsis thaliana (Arabidopsis) provides a huge potential for the identification of the molecular mechanisms underlying this variation as a result of the availability of a vast array of genetic and genomic resources for this species. Eighty-six Arabidopsis accessions were screened for natural variation in flooding tolerance. This forms the first step towards the identification and characterization of the role of candidate genes contributing to flooding tolerance. • Arabidopsis accessions at the 10-leaf stage were subjected to complete submergence in the dark. Survival curves were plotted to estimate median lethal times as a measure of tolerance. Flooding-associated survival parameters, such as root and shoot oxygen content, initial carbohydrate content and petiole elongation under water, were also measured. • There was a significant variation in submergence tolerance among Arabidopsis accessions. However, the order of tolerance did not correlate with root and shoot oxygen content or initial amounts of shoot starch and total soluble sugars. A negative correlation was observed between submergence tolerance and underwater petiole elongation. • Arabidopsis accessions show considerable variation in the ability to tolerate complete submergence, making it a good species in which to identify and characterize genes and to study mechanisms that contribute to survival under water.

On the Relevance and Control of Leaf Angle

Plants can have constitutive leaf angles that are fixed and do not vary much among different growth environments. Several species, however, have the ability to actively adjust their leaf angles. Active leaf repositioning can be functional in avoiding detrimental environmental conditions, such as avoidance of heat stress and complete submergence, or can be, for example, utilized to maximize carbon gain by positioning the leaves relative to the incoming radiation. In recent years, major advances have been made in the understanding of the molecular mechanisms, and the underlying hormonal regulation of a particular type of leaf movement: hyponastic growth. This differential petiole growth-driven upward leaf movement is now relatively well understood in model systems such as Rumex palustris and Arabidopsis thaliana. In the first part of this review we will discuss the functional consequences of leaf orientation for plant performance. Next, we will consider hyponastic growth and describe how exploitation of natural (genetic) variation can be instrumental in studying the relevance and control of leaf positioning.

Submergence Tolerant Rice: SUB1’s Journey from Landrace to Modern Cultivar

Rice landraces tolerant of up to 2 weeks of complete submergence were collected from farmers’ fields in the 1950s. Success in fine mapping of SUBMERGENCE 1 (SUB1), a robust quantitative trait locus from the submergence tolerant FR13A landrace, has enabled marker-assisted breeding of high-yielding rice capable of enduring transient complete submergence. At the molecular level, SUB1 is a variable polygenic locus encoding two or three ethylene responsive factor (ERF) DNA binding proteins. All Oryza sativa accessions encode SUB1B and SUB1C at this locus. An additional ERF, SUB1A, is present at SUB1 in FR13A and other tolerant accessions. The induction of SUB1A expression by ethylene during submergence disrupts the elongation escape strategy typical of lowland and deepwater rice, by limiting ethylene-induced gibberellic acid-promoted elongation. Microarray and metabolite studies confirm that SUB1A orchestrates its effects on metabolism and growth in a submergence-dependent manner. Due to the conditional activity of SUB1A, new “Sub1” mega-varieties effectively provide submergence tolerance without apparent ill effect on development, productivity, or grain quality.

Effects of simulated submergence on survival and recovery growth of three species in water fluctuation zone of the Three Gorges reservoir

Paspalum distichum, Cynodon dactylon and Hemarthria altissima distribute widely in natural water fluctuation zone of the Three Gorges region. To investigate whether they are suitable for growing in the artificial water fluctuation zone, which has longer submergence time and different submergence rhythm, of the Three Gorges reservoir, three complete submergence depths (0.5, 1 and 2 m) were conducted for about 6 months (from 12 November 2007 to 30 April 2008), and the survival and recovery growth of the three species were recorded after re-emergence for two weeks. The three species could start recovery growth within one week and more than 50% plants could survive. Among the three species, P. distichum had the largest increments in branch number and maximum stem length, and the smallest root shoot ratio. C. dactylon, however, had the smallest maximum stem length increment, and its survival and branch number increment were both larger than those of H. altissima. For C. dactylon and H. altissima, the survival and branch number increment significantly increased, while maximum stem length increment tended to decrease when submergence depth went higher. For P. distichum, the survival and the shoot mass were the lowest after 2 m submergence depth, but the other parameters were not different among different submergence treatments. Compared with control plants, submergence increased root shoot ratio of C. dactylon and H. altissima, but did not affect that of P. distichum. These results demonstrated that the three species are submergence-tolerant and can be applied in vegetation reconstruction in water fluctuation zone of the Three Gorges reservoir. Meanwhile, the results also suggested that the three species developed different survival tactics during the long-term submergence.

Stem hypertrophic lenticels and secondary aerenchyma enable oxygen transport to roots of soybean in flooded soil

Aerenchyma provides a low-resistance O(2) transport pathway that enhances plant survival during soil flooding. When in flooded soil, soybean produces aerenchyma and hypertrophic stem lenticels. The aims of this study were to investigate O(2) dynamics in stem aerenchyma and evaluate O(2) supply via stem lenticels to the roots of soybean during soil flooding. Oxygen dynamics in aerenchymatous stems were investigated using Clark-type O(2) microelectrodes, and O(2) transport to roots was evaluated using stable-isotope (18)O(2) as a tracer, for plants with shoots in air and roots in flooded sand or soil. Short-term experiments also assessed venting of CO(2) via the stem lenticels. The radial distribution of the O(2) partial pressure (pO(2)) was stable at 17 kPa in the stem aerenchyma 15 mm below the water level, but rapidly declined to 8 kPa at 200-300 microm inside the stele. Complete submergence of the hypertrophic lenticels at the stem base, with the remainder of the shoot still in air, resulted in gradual declines in pO(2) in stem aerenchyma from 17.5 to 7.6 kPa at 13 mm below the water level, and from 14.7 to 6.1 kPa at 51 mm below the water level. Subsequently, re-exposure of the lenticels to air caused pO(2) to increase again to 14-17 kPa at both positions within 10 min. After introducing (18)O(2) gas via the stem lenticels, significant (18)O(2) enrichment in water extracted from roots after 3 h was confirmed, suggesting that transported O(2) sustained root respiration. In contrast, slight (18)O(2) enrichment was detected 3 h after treatment of stems that lacked aerenchyma and lenticels. Moreover, aerenchyma accelerated venting of CO(2) from submerged tissues to the atmosphere. Hypertrophic lenticels on the stem of soybean, just above the water surface, are entry points for O(2), and these connect to aerenchyma and enable O(2) transport into roots in flooded soil. Stems that develop aerenchyma thus serve as a 'snorkel' that enables O(2) movement from air to the submerged roots.

A kinetic analysis of hyponastic growth and petiole elongation upon ethylene exposure in Rumex palustris.

Complete submergence is an important stress factor for many terrestrial plants, and a limited number of species have evolved mechanisms to deal with these conditions. Rumex palustris is one such species and manages to outgrow the water, and thus restore contact with the atmosphere, through upward leaf growth (hyponasty) followed by strongly enhanced petiole elongation. These responses are initiated by the gaseous plant hormone ethylene, which accumulates inside plants due to physical entrapment. This study aimed to investigate the kinetics of ethylene-induced leaf hyponasty and petiole elongation. Leaf hyponasty and petiole elongation was studied using a computerized digital camera set-up followed by image analyses. Linear variable displacement transducers were used for fine resolution monitoring and measurement of petiole growth rates. We show that submergence-induced hyponastic growth and petiole elongation in R. palustris can be mimicked by exposing plants to ethylene. The petiole elongation response to ethylene is shown to depend on the initial angle of the petiole. When petiole angles were artificially kept at 0 degrees, rather than the natural angle of 35 degrees, ethylene could not induce enhanced petiole elongation. This is very similar to submergence studies and confirms the idea that there are endogenous, angle-dependent signals that influence the petiole elongation response to ethylene. Our data suggest that submergence and ethylene-induced hyponastic growth and enhanced petiole elongation responses in R. palustris are largely similar. However, there are some differences that may relate to the complexity of the submergence treatment as compared with an ethylene treatment.

Vagility of Aquatic Salamanders: Implications for Wetland Connectivity

Research on landscape connectivity for amphibians that use isolated wetlands has focused on terrestrial and semiterrestrial species. Although aquatic species are commonly encountered in isolated wetlands, their dispersal capability and mode of dispersal has yet to be conclusively determined. For these salamander species, temporary waterways formed during heavy rains may provide transient dispersal opportunities among otherwise terrestrially isolated wetland patches and large contiguous sources (e.g., river swamps, lake systems). We assessed the vagility of two aquatic salamanders, the Greater Siren (Siren lacertina) and Two-Toed Amphiuma (Amphiuma means), under three simulated environmental conditions: terrestrial (damp but no standing water); shallow standing water (1 cm of water); and complete submergence (approximately 5 cm of water). Salamanders were placed inside a modified Living Stream container and stimulated into moving through each treatment. Both species demonstrated a trend toward exhaustion for all treatments and failed to move more than 8 m in the terrestrial or shallow water treatments. As expected, animals in the fully submerged treatment were able to disperse the farthest. Physical characteristics of salamanders did not affect vagility. To disperse, these species likely rely on the formation of aquatic corridors during flooding events. Therefore, successful dispersal among isolated wetlands depends on the ability of the surrounding landscape either to be periodically inundated with water or to form temporary waterways during heavy rains. Human activities that alter flooding events and watershed connectivity, such as flood control regimes and roads, may have important implications for wetland connectivity and, thus, metapopulation viability of aquatic salamanders.

Cross-Kingdom Comparison of Transcriptomic Adjustments to Low-Oxygen Stress Highlights Conserved and Plant-Specific Responses

High-throughput technology has facilitated genome-scale analyses of transcriptomic adjustments in response to environmental perturbations with an oxygen deprivation component, such as transient hypoxia or anoxia, root waterlogging, or complete submergence. We showed previously that Arabidopsis (Arabidopsis thaliana) seedlings elevate the levels of hundreds of transcripts, including a core group of 49 genes that are prioritized for translation across cell types of both shoots and roots. To recognize low-oxygen responses that are evolutionarily conserved versus species specific, we compared the transcriptomic reconfiguration in 21 organisms from four kingdoms (Plantae, Animalia, Fungi, and Bacteria). Sorting of organism proteomes into clusters of putative orthologs identified broadly conserved responses associated with glycolysis, fermentation, alternative respiration, metabolite transport, reactive oxygen species amelioration, chaperone activity, and ribosome biogenesis. Differentially regulated genes involved in signaling and transcriptional regulation were poorly conserved across kingdoms. Strikingly, nearly half of the induced mRNAs of Arabidopsis seedlings encode proteins of unknown function, of which over 40% had up-regulated orthologs in poplar (Populus trichocarpa), rice (Oryza sativa), or Chlamydomonas reinhardtii. Sixteen HYPOXIA-RESPONSIVE UNKNOWN PROTEIN (HUP) genes, including four that are Arabidopsis specific, were ectopically overexpressed and evaluated for their effect on seedling tolerance to oxygen deprivation. This allowed the identification of HUPs coregulated with genes associated with anaerobic metabolism and other processes that significantly enhance or reduce stress survival when ectopically overexpressed. These findings illuminate both broadly conserved and plant-specific low-oxygen stress responses and confirm that plant-specific HUPs with limited phylogenetic distribution influence low-oxygen stress endurance.

Responses of the antioxidative enzymes in Malaysian rice (Oryza sativa L.) cultivars under submergence condition

The potential involvement of activated oxygen species by submergence stress was studied in two Malaysian rice cultivars, MR219-4 and MR219-9, and cultivar FR13A that is known to be tolerant to submergence. Seedlings of these three rice cultivars were subjected to different submergence periods (4, 8, and 12 days). Under 8 days of complete submergence, FR13A cultivar showed higher lipid peroxidation in terms of malondialdehyde level and activities of antioxidative enzymes, superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR) when compared to the MR219-4 and MR219-9 cultivars. MR219-9 showed higher SOD, APX, and GR activities after 12 days of submergence. The levels of SOD activity indicated that detoxification of O 2 ·− to H2O2 was maintained at a stable level throughout the submergence stress until up to 8 days and increased rapidly at 12 days of submergence. The results indicated that tolerance to submergence in rice is associated until 8 days submergence for MR219-4 and FR13A cultivars. These findings suggested that tolerance to submergence stress in rice might be proven by increased the capacity of antioxidative system. In addition, CAT activity has much higher affinity for scavenges H2O2 than APX. Therefore, ascorbate glutathione cycle might be more efficient to scavenge H2O2.

Desiccation of leaves after de-submergence is one cause for intolerance to complete submergence of the rice cultivar IR 42

This paper presents evidence that severe water deficits, following de-submergence after flash flooding of rice, contribute to submergence intolerance of IR 42, a rice cultivar that rapidly elongates during submergence. In glasshouse experiments, 13-day-old rice seedlings were completely submerged for 3–5 days. The main experiments were with IR 42, a cultivar intolerant to transient complete submergence. During submergence the 3rd leaf expanded, and after 5 days submergence its sheath was 4-fold longer than in non-submerged seedlings. After de-submergence, this leaf rapidly desiccated, its water potential dropped below –2 MPa, while the stomatal conductance was very low. Excision experiments showed the water deficits after de-submergence were due mainly to a large reduction in the hydraulic conductivity in the leaf sheath. The water deficits are an important cause in the sequence of events rather than a mere result of injury: when plants were de-submerged at 100% rather than at 50% RH, water potentials remained high. However, when, after another 5 days, these plants were transferred to 50% RH, the 3rd leaf rapidly desiccated, indicating little repair of the lesion causing the low hydraulic conductivity.

Submergence tolerance in Hordeum marinum: dissolved CO2 determines underwater photosynthesis and growth

Floodwaters differ markedly in dissolved CO2, yet the effects of CO2 on submergence responses of terrestrial plants have rarely been examined. The influence of dissolved CO2 on underwater photosynthesis and growth was evaluated for three accessions of the wetland plant Hordeum marinum Huds. All three accessions tolerated complete submergence, but only when in CO2 enriched floodwater. Plants submerged for 7 days in water at air equilibrium (18 µM CO2) suffered loss of biomass, whereas those with 200 µM CO2 continued to grow. Higher underwater net photosynthesis at 200 µM CO2 increased by 2.7- to 3.2-fold sugar concentrations in roots of submerged plants, compared with at air equilibrium CO2. Leaf gas films enhancing gas exchange with floodwater, lack of a shoot elongation response conserving tissue sugars and high tissue porosity (24–31% in roots) facilitating internal O2 movement, would all contribute to submergence tolerance in H. marinum. The present study demonstrates that dissolved CO2 levels can determine submergence tolerance of terrestrial plants. So, submergence experiments should be conducted with defined CO2 concentrations and enrichment might be needed to simulate natural environments and, thus, provide relevant plant responses.

Ethylene-induced hyponastic growth inArabidopsis thalianais controlled by ERECTA

Plants can respond quickly and profoundly to detrimental changes in their environment. For example, Arabidopsis thaliana can induce an upward leaf movement response through differential petiole growth (hyponastic growth) to outgrow complete submergence. This response is induced by accumulation of the phytohormone ethylene in the plant. Currently, only limited information is available on how this response is molecularly controlled. In this study, we utilized quantitative trait loci (QTL) analysis of natural genetic variation among Arabidopsis accessions to isolate novel factors controlling constitutive petiole angles and ethylene-induced hyponastic growth. Analysis of mutants in various backgrounds and complementation analysis of naturally occurring mutant accessions provided evidence that the leucin-rich repeat receptor-like Ser/Thr kinase gene, ERECTA, controls ethylene-induced hyponastic growth. Moreover, ERECTA controls leaf positioning in the absence of ethylene treatment. Our data demonstrate that this is not due to altered ethylene production or sensitivity.

Germination and anaerobic metabolism of seeds of Tabebuia cassinoides (Lam.) DC subjected to flooding and anoxia

Tabebuia cassinoides (Lam.) DC (Bignoniaceae) is an arboreal species common in seasonally or permanently waterlogged areas of the “restinga” forest (a type of forest that occurs on the sandbanks of the coastal plains of southeastern Brazil). The objectives of the present study were to establish seed germination responses of this species to flooding and anoxia and investigate the end products of the anaerobic metabolism of seeds subjected to these conditions, with the goal of understanding the adaptive strategies that enable this species to dominate flood prone areas of “restinga”, as well as determine reserves stored in their seeds. Seeds of T. cassinoides did not germinate under anoxia or complete submergence, but remained viable under these conditions for 15 and 20 days, respectively. Due to their membranaceous wings, the seeds float very well and reached 100% germination in this condition, an important adaptation to overcome the initial stages of development in flooded habitats. In relation to anaerobic metabolism, ethanol is the most important end product, while lactate is produced in lower concentrations. Seeds of T. cassinoides have very little endosperm and the reserves, mainly glycoproteins, lipids and free sugars, accumulate in the cotyledons. Free sugars may provide the substrate for the initial metabolism of seed germination, but the level of these reserves was not enough to allow germination under oxygen deprivation. Possibly, carbohydrate reserves were enough only to maintain seed viability for a relative short period under this condition.