Effects of root anaerobiosis on ethylene production, epinasty, and growth of tomato plants

Experiments were performed to determine the source(s) of ethylene-causing epinasty in flooded tomato plants (Lycopersicon esculentum Mill.). Simultaneous measurements were made of ethylene synthesized by the roots and shoots of tomato plants exposed to either aerobic or anaerobic atmospheres in the root zone. When the root zone was made anaerobic by a flowing stream of N₂ gas, petiole epinasty and accelerated ethylene synthesis by the shoots were observed. In soil-grown plants, ethylene synthesis by the root-soil complex increased under anaerobic conditions; but when grown in inert media under the same conditions, ethylene synthesis by roots remained constant or declined during the period of rapid epinastic growth by the petioles. Other characteristic symptoms of flooding, e.g. reduced growth and chlorosis, were also observed in plants with anaerobic roots. Pretreatment of plants with AgNO₃, an inhibitor of ethylene action, completely prevented epinasty, demonstrating that ethylene is the agent responsible for waterlogging symptoms. These results indicate that deprivation of O₂ to the roots is the primary effect of soil flooding, and that this is sufficient to cause increased ethylene synthesis in the shoot. The basis of the observed root-shoot communication is unknown, but root-synthesized hormones or specific ethylene-promoting factors may be involved.     

The effect of waterlogging on the growth and ethylene content of Eucalyptus robusta Sm. (Swamp Mahogany)

Summary When waterlogged over a period of 80 days plants of Eucalyptus robusta Sm. showed symptoms of leaf chlorosis, epinasty and premature abscission, reduction of stem elongation, stem hypertrophy and formation of adventitious shoots; chlorophyll content was reduced and soluble protein content of the upper leaves increased. Waterlogging doubled the rate of release of ethylene from roots and stems within 6 days, but had no effect on the ethylene concentration of leaves.     

Effect of soil flooding on photosynthesis,carbohydrate partitioning and nutrient uptake in the invasive exotic Lepidium latifolium

Lepidium latifolium L. is an invasive exotic crucifer that has spread explosively in wetlands and riparian areas of the western United States. To understand the ecophysiological characteristics of L. latifolium that affect its ability to invade riparian areas and wetlands, we examined photosynthesis, chlorophyll concentration, carbohydrate partitioning and nutrient uptake in L. latifolium in response to soil flooding. Photosynthesis of flooded plants was about 60–70% of the rate of unflooded controls. Chlorophyll concentrations of flooded plants were about 60–70% of the unflooded plants during 15–50 days of flooding. Flooding resulted in an increase in leaf starch concentration, but root starch concentration was not significantly affected. However, concentrations of soluble sugar were significantly higher in both leaves and roots of flooded plants than unflooded controls. On day 50 after initial flooding, the concentrations of N, P, K and Zn in leaves of flooded plants were lower than in control plants. The concentrations of Mn and Fe in leaves of flooded plants were eight and two times those of control plants, respectively. In contrast, N, P, K and Zn concentrations of roots of flooded plants were slightly higher than in unflooded plants. The concentrations of Fe and Mn in roots of flooded plants were 15 and 150 times those of the control plants, respectively. The transport of P, K, and Zn to shoots decreased and that of Mn increased under flooding. The accumulation of N, K and Zn in roots decreased and that of Mn increased in response to flooding. The results suggested that the maintenance of relatively high photosynthesis and the accumulation of soluble sugar in roots of flooded plants are important adaptations for this species in flooded environments. Despite a reduction in photosynthesis and disruption in nutrient and photosynthate allocation in response to flooding, L. latifolium was able to survive 50 days of flooding stress. Overall, L. latifolium performed like a facultative hydrophyte species under flooding.     

Maintenance of photosynthetic capacity in flooded tomato plants with reduced ethylene sensitivity

Ethylene is considered one of the most important plant hormones orchestrating plant responses to flooding stress. However, ethylene may induce deleterious effects on plants, especially when produced at high rates in response to stress. In this paper, we explored the effect of attenuated ethylene sensitivity in the Never ripe (Nr) mutant on leaf photosynthetic capacity of flooded tomato plants. We found out that reduced ethylene perception in Nr plants was associated with a more efficient photochemical and non-photochemical radiative energy dissipation capability in response to flooding. The data correlated with the retention of chlorophyll and carotenoids content in flooded Nr leaves. Moreover, leaf area and specific leaf area were higher in Nr, indicating that ethylene would exert a negative role in leaf growth and expansion under flooded conditions. Although stomatal conductance was hampered in flooded Nr plants, carboxylation activity was not affected by flooding in the mutant, suggesting that ethylene is responsible for inducing non-stomatal limitations to photosynthetic CO₂ uptake. Upregulation of several cysteine protease genes and high protease activity led to Rubisco protein loss in response to ethylene under flooding. Reduction of Rubisco content would, at least in part, account for the reduction of its carboxylation efficiency in response to ethylene in flooded plants. Therefore, besides its role as a trigger of many adaptive responses, perception of ethylene entails limitations in light and dark photosynthetic reactions by speeding up the senescence process that leads to a progressive disassembly of the photosynthetic machinery in leaves of flooded tomato plants.     

Formation and function of secondary aerenchyma in hypocotyl, roots and nodules of soybean (Glycine max) under flooded conditions

Flooding is a major problem in many areas of the world and soybean is susceptible to the stress. Understanding the morphological mechanisms of flooding tolerance is important for developing flood-tolerant genotypes. We investigated secondary aerenchyma formation and function in soybean (Glycine max) seedlings grown under flooded conditions. Secondary aerenchyma, a white and spongy tissue, was formed in the hypocotyl, tap root, adventitious roots and root nodules after 3 weeks of flooding. Under irrigated conditions aerenchyma development was either absent or rare and phellem was formed in the hypocotyl, tap root, adventitious roots and root nodules. Secondary meristem partially appeared at the outer parts of the interfascicular cambium and girdled the stele, and then cells differentiated to construct secondary aerenchyma in the flooded hypocotyl. These morphological changes proceeded for 4 days after the initiation of the flooding. After 14 days of treatment, porosity exceeded 30% in flooded hypocotyl with well-developed secondary aerenchyma, while it was below 10% in hypocotyl of irrigated plants that had no aerenchyma. When Vaseline was applied to the hypocotyl of plants from a flooded treatment to prevent the entry of atmospheric oxygen into secondary aerenchyma, plant growth, especially that of roots, was sharply inhibited. Thus secondary aerenchyma might be an adaptive response to flooding.     

Physiological,morphological, and growth responses ofPlatanus occidentalis seedlings to flooding

Summary Flooding ofPlatanus occidentalis L. seedlings for up to 40 days induced several changes including early stomatal closure, greatly accelerated ethylene production by stems, formation of hypertrophied lenticels and adventitious roots on submerged portions of stems, and marked growth inhibition. Poor adaptation ofPlatanus occidentalis seedlings to soil inundation was shown in stomatal closure during the entire flooding period, inhibition of root elongation and branching, and death of roots. Some adaptation to flooding was indicated by (1) production of hypertrophied lenticels which may assist in exchange of dissolved gases in flood water and in release of toxic compounds, and (2) production of adventitious roots on stems which may increase absorption of water. These adaptations appeared to be associated with greatly stimulated ethylene production in stems of flooded plants. The greater reduction of root growth over shoot growth in flooded seedlings will result in decreased drought tolerance after the flood waters recede. The generally low tolerance to flooding of seedlings of species that are widely rated as highly flood tolerant is emphasized.     

Xylem Transport of 1-Aminocyclopropane-1-carboxylic Acid, an Ethylene Precursor, in Waterlogged Tomato Plants

Waterlogging is known to cause an increase in ethylene synthesis in the shoot which results in petiole epinasty. Evidence has suggested that a signal is synthesized in the anaerobic roots and transported to the shoot where it stimulates ethylene synthesis. Experimental data are presented showing that 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene, serves as the signal. Xylem sap was collected from detopped tomato plants (Lycopersicon esculentum Mill. cv. VFN8). ACC in the sap was quantitated by a sensitive and specific assay, and its tentative chemical identity verified by paper chromatography. ACC levels in both roots and xylem sap increased markedly in response to waterlogging or root anaerobiosis. The appearance of ACC in the xylem sap of flooded plants preceded both the increase in ethylene production and epinastic growth, which were closely correlated. Plants flooded and then drained showed a rapid, simultaneous drop in ACC flux and ethylene synthesis rate. ACC supplied through the cut stem of tomato shoots at concentrations comparable to those found in xylem sap caused epinasty and increased ethylene production. These data indicate that ACC is synthesized in the anaerobic root and transported to the shoot where it is readily converted to ethylene.     

Effect of flooding and temperature regime on growth and stomatal resistance ofBetula platyphylla var.japonica seedlings

Summary Both flooding and low temperature reduced height and stem diameter growth; leaf initiation; growth of leaves, stems, and roots; and lowered root-shoot ratios of 112-dayoldBetula platyphylla var.japonica seedlings. Flooding also induced leaf scorching and abscission. Growth was reduced much more by flooding than by low temperature. Interactive effects of flooding and temperature were shown on height growth, leaf initiation and expansion, and dry weight increment of leaves, stems and roots. The amount of growth reduction by flooding and low temperature was greater when based on analysis of dry weight increment of leaves, stems, and roots, than on their relative growth rates. The greater reduction of growth by flooding than by low temperature was associated with fewer and smaller leaves, more leaf injury, more stomatal closure, and greater decay of roots in flooded plants. Flooding and low temperature appeared to reduce growth by somewhat different physiological mechanisms. Research supported by the College of Agricultural and Life Sciences, University of Wisconsin, Madison, WI, USA and by Yamagata University, Tsuruoka, Japan. McIntyre-Stennis Project 2599.     

Some physiological and growth responses of kiwi fruit (Actinidia chinensis) to flooding

The effects of long-term flooding on the growth of six-month-old Actinidia chinensis Planch cv. Abbot plants and some effects on stomatal behaviour and leaf water relations were examined under controlled conditions for 28 days. Flooding caused stomatal closure and decreases in transpiration rate, xylem water potential, osmotic potential and turgor potential. Flooding also caused inhibition of the dry weight increase of leaves plus stems and of roots, chlorosis and necrosis of leaves, production of hypertrophied lenticels and the appearance of a small number of adventitious roots on the submerged portions of the stems. Rapid and partial stomatal closure by flooding may not only be due to the passive mechanical response which follows leaf dehydration, since flooded plants showed an increase in xylem water potential and osmotic potential during the first days of the experiment. The marked intolerance of Actinidia chinensis to flooding has been a serious barrier to its culture in poorly drained soils, hence careful irrigation management is required.     

Growth and chlorophyll a fluorescence in Erythrina crista-galli L. plants under flooding conditions

The present study aimed to analyse the alterations in the growth and chlorophyll content of Erythrina crista-galli plants in response to flooding and to evaluate possible photosystem II (PSII) damage through chlorophyll a fluorescence transient analysis using the JIP test. Plants cultured from seeds were grown in 0.5-L pots in a greenhouse and transferred to 5-L pots. Two treatments were employed: plants were either flooded at the root and maintained with a sheet of water above the soil or not flooded (control). Evaluations were performed at 10, 20, 30 and 40 days after the treatment was initiated. The experiment was set up in a randomised block design, and the results were subjected to analysis of variance (ANOVA). The means were compared by Tukey’s test at a 5 % significance level. The flooding of the plants induced the formation of lenticels, adventitious roots and larger intercellular spaces in the root cortex and inhibited the growth of shoots at early time points compared to control plants. However, growth resumed by the end of the experimental period. The Erythrina crista-galli plants manifested a reduction in chlorophyll content as a function of the duration of exposure to flooding. However, there were changes in the chlorophyll fluorescence parameters, indicating impairment of the structure and function of PSII, and the performance indices were the most representative parameters to describe the effects of flooding. The results showed that the Erythrina crista-galli tolerated flooding by initially investing energy into the formation of morpho-anatomical structures to maintain oxygen diffusion and ATP production. The effect of flooding on the leaves was reflected in increased energy dissipation through chlorophyll a fluorescence, likely as a mechanism to protect the photosynthetic apparatus.     

The role of ethylene during flooding of Phaseolus vulgaris

Wilting symptoms in Phaseolus vulgaris L. cv. bruine Noord-Hollandse were observed after a few hours of flooding. They were well correlated with an accumulation of ethylene. The ethylene level in the leaves started to increase after 2 h of flooding and reached a 3–4 fold rise after 4–6 h. From then on throughout the next two days it gradually returned to control values. On the day when plants were flooded, a positive correlation was found between the ethylene concentration and the degree of wilting. During this day the time course of abscisic acid (ABA) level, diffusion resistance and water potential was measured. The effect of ethylene on stomatal aperture was investigated by spraying Ethrel on the leaves. In control plants, Ethrel treatment had no influence on the diffusion resistance. Spraying plants with ABA resulted in a significant, dose-dependent increase in diffusion resistance. When Ethrel was added to the ABA-containing solutions only 62% of this increase was observed. Ethrel (pre-) treatment of plants that were to be flooded had a similar effect; the increase in diffusion resistance was only 70% of what was observed in untreated flooded plants. It is concluded that ethylene may interfere with the regulation of stomatal aperture by abscisic acid.     

Stomatal Closure in Flooded Tomato Plants Involves Abscisic Acid and a Chemically Unidentified Anti-Transpirant in Xylem Sap

We address the question of how soil flooding closes stomata of tomato (Lycopersicon esculentum Mill. cv Ailsa Craig) plants within a few hours in the absence of leaf water deficits. Three hypotheses to explain this were tested, namely that (a) flooding increases abscisic acid (ABA) export in xylem sap from roots, (b) flooding increases ABA synthesis and export from older to younger leaves, and (c) flooding promotes accumulation of ABA within foliage because of reduced export. Hypothesis a was rejected because delivery of ABA from flooded roots in xylem sap decreased. Hypothesis b was rejected because older leaves neither supplied younger leaves with ABA nor influenced their stomata. Limited support was obtained for hypothesis c. Heat girdling of petioles inhibited phloem export and mimicked flooding by decreasing export of [14C]sucrose, increasing bulk ABA, and closing stomata without leaf water deficits. However, in flooded plants bulk leaf ABA did not increase until after stomata began to close. Later, ABA declined, even though stomata remained closed. Commelina communis L. epidermal strip bioassays showed that xylem sap from roots of flooded tomato plants contained an unknown factor that promoted stomatal closure, but it was not ABA. This may be a root-sourced positive message that closes stomata in flooded tomato plants.     

Antioxidative responses and morpho-anatomical adaptations to waterlogging in Sesbania virgata

Sesbania virgata (Leguminosae) is tolerant of long periods of soil inundation. However, its morphological adaptations to anoxia and its response to possible damage from oxidative stress are still unknown. Here, we provide new information that helps to explain the ability of S. virgata plants to grow in flooded environments. Plants containing six expanded leaves were placed in masonry tanks and were subjected to the following conditions: control (well watered), soil waterlogging (water to the setup level of 1 cm above the soil surface—roots and parts of the stems flooded), and complete submergence (whole plant flooded). Plants exposed to flooding (soil waterlogging and complete submergence) significantly increased their production of hydrogen peroxide (H₂O₂), indicating the extent of oxidative injury posed by stress conditions. We demonstrate that plants exposed to flooding develop an efficient scavenger of ROS (generated during stress) in the roots through the coordinated action of nonenzymatic ascorbic acid (Asc) and dehydroascorbate (DHA) as well as the enzymatic antioxidants superoxide dismutase (SOD), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), and glutathione reductase (GR) that are present in the tissues. Moreover, we observed the development of morpho-anatomical structures such as adventitious roots, lenticels, and cracks in the stem of plants under soil waterlogging. The secondary root of plants under soil waterlogging showed a thinner cortex and larger number of elements of small diameter vessels. Numerous aerenchymas were observed in the newly formed in the adventitious roots. We conclude that these antioxidative responses and morpho-anatomical adaptations in the roots are part of a suite of adaptations that allow S. virgata plants to survive long periods of flooding, notably under waterlogged conditions.     

Involvement of plant growth substances in the alteration of leaf gas exchange of flooded tomato plants

Ethylene, abscisic acid, and cytokinins were tested for their ability to either induce or prevent the changes which occur in gas exchange characteristics of tomato (Lycopersicon esculentum Mill. cv. Rheinlands Ruhm) leaves during short-term soil flooding. Ethylene, which increases in the shoots of flooded plants, had no effect on stomatal conductance or photosynthetic capacity of drained plants. Abscisic acid, which also accumulates in the shoots of flooded plants, could reproduce the stomatal behavior of flooded plants when sprayed on the leaves of drained plants. However, photosynthetic capacity of drained plants was unaffected by abscisic acid sprays. Cytokinin export from the roots to the shoots declines in flooded plants. Spray applications of benzyladenine increased stomatal conductance in both flooded and drained plants. In addition, the decline in photosynthetic capacity during flooding was largely prevented by supplementary cytokinin applications. The possible involvement of these growth substances in modifying leaf gas exchange during flooding is discussed.     

Role of root systems of eastern larch and white spruce in response to flooding

Abstract. Soil flooding causes rapid reductions in transpiration, stomatal conductance and photosynthesis of many woody plants, which can decrease growth and ultimately result in plant death. This study was conducted to determine the role of the root system in the flooding response. Eastern larch ( Larix laricina ) seedlings were grown in Plexiglas tubes in which water uptake by flooded and unflooded roots was measured independently. Further flooding studies were conducted with eastern larch and white spruce ( Picea glauca ) in which stems were girdled. Root hydraulic properties were analysed using pressure-flow relationships. Transpiration rates of partially flooded plants declined more slowly than fully-flooded plants. Water uptake by unflooded roots of partially flooded seedlings increased momentarily with flooding. After lOd, flooding caused little change in root hydraulic conductance, a decrease in root system reflection coefficient, and an increase in osmotic permeability. Stem girdling had little effect on stomatal conductance and transpiration in comparison to flooding effects. The response of plant tops to flooding appears to be xylem-mediated and in proportion to the amount of root system flooded. Root hydraulic conductance appears to be unaffected by flooding except for a possible temporary increase on the first day following flooding treatments.     

Water relations,ethylene production,and morphological adaptation ofFraxinus pennsylvanica seedlings to flooding

Summary Fraxinus pennsylvanica Marsh. seedlings that were 150 days old adapted well to flooding of soil with stagnant water for 28 days. Early stomatal closure, followed by reopening as well as hypertrophy of lenticels and formation of adventitious roots on submerged portions of stems appeared to be important adaptations for flood tolerance. Leaf water potential (₁) was consistently higher in flooded than in unflooded seedlings, indicating higher leaf turgidity in the former. This was the result of (1) early reduction in transpiration associated with stomatal closure, and (2) subsequently increased absorption of water by the newly-formed adventitious roots as stomata reopened and transpiration increased. Waterlogging of soil was followed by large increases in ethylene content of stems, both below and above the level of submersion. Formation of hypertrophied lenticels and adventitious roots on flooded plants was correlated with increased ethylene production. However, the involvement of various compounds other than ethylene in inducing morphological changes in flooded plants is also emphasized.Research supported by the College of Agricultural and Life Sciences, University of Wisconsin, Madison, WI, USA     

Cortical Aerenchyma formation in hypocotyl and adventitious roots of Luffa cylindrica subjected to soil flooding

BACKGROUND AND AIMS: Aerenchyma formation is thought to be one of the important morphological adaptations to hypoxic stress. Although sponge gourd is an annual vegetable upland crop, in response to flooding the hypocotyl and newly formed adventitious roots create aerenchyma that is neither schizogenous nor lysigenous, but is produced by radial elongation of cortical cells. The aim of this study is to characterize the morphological changes in flooded tissues and the pattern of cortical aerenchyma formation, and to analyse the relative amount of aerenchyma formed. METHODS: Plants were harvested at 16 d after the flooding treatment was initiated. The root system was observed, and sections of fresh materials (hypocotyl, tap root and adventitious root) were viewed with a light or fluorescence microscope. Distributions of porosity along adventitious roots were estimated by a pycnometer method. KEY RESULTS: Under flooded conditions, a considerable part of the root system consisted of new adventitious roots which soon emerged and grew quickly over the soil surface. The outer cortical cells of these roots and those of the hypocotyl elongated radially and contributed to the development of large intercellular spaces. The elongated cortical cells of adventitious roots were clearly T-shaped, and occurred regularly in mesh-like lacunate structures. In these positions, slits were formed in the epidermis. In the roots, the enlargement of the gas space system began close to the apex in the cortical cell layers immediately beneath the epidermis. The porosity along these roots was 11-45 %. In non-flooded plants, adventitious roots were not formed and no aerenchyma developed in the hypocotyl or tap root. CONCLUSIONS: Sponge gourd aerenchyma is produced by the unique radial elongation of cells that make the expansigeny. These morphological changes seem to enhance flooding tolerance by promoting tissue gas exchange, and sponge gourd might thereby adapt to flooding stress.     

Photosynthetic responses of citrus trees to soil flooding

Continuous soil flooding reduced leaf photosynthetic rate, stomatal conductance to water vapor, chlorophyll concentration and activity of ribulose bisphosphate carboxylase-oxygenase (Rubisco, EC 4.1.1.39) of sweet orange [ Citrus sinensis (L.) Osbeck cv. Hamlin] trees, grafted onto rough lemon (RL; C. jambhiri Lush.) and sour orange (SO; C. aurantium L.) rootstocks. After 24 days of waterlogging, trees showed senescence, wilting and abscission of leaves, and these symptoms were more evident with flooded Hamlin/SO than Hamlin/RL. Reduction of leaf photosynthetic rate at day 24 was ca 94%, of stomatal conductance, 71%, of chlorophyll, 38% and of Rubisco, 62% for flooded Hamlin/SO, compared with 22, 5, 18 and 33%, respectively, for flooded Hamlin/RL. For both Hamlin/RL and Hamlin/SO, leaf photosynthetic rate and stomatal conductance to water vapor were closely correlated (r²= 0.87). Leaf internal CO₂ concentration of flooded trees, however, was not decreased by reduced stomatal conductance. Dark respiration rates of fibrous roots of flooded trees were greatly reduced, but not in leaf tissues. Total nonstructural carbohydrate concentrations were higher in leaves (50 and 80% increases for Hamlin/SO and Hamlin/RL, respectively), but drastically reduced in roots (60 and 45% reductions for Hamlin/SO and Hamlin/RL, respectively), as a result of flooding. The data indicate that Hamlin grafted onto RL rootstocks was more tolerant to soil flooding than Hamlin grafted onto SO rootstocks.     

Ethylene accumulation in waterlogged Rumex plants promotes formation of adventitious roots

Accumulation of the gaseous plant hormone ethylene is very importantfor the induction of several responses of plants to flooding.However, little is known about the role of this gas in the formationof flooding-induced adventitious roots. Formation of adventitiousroots in Rumex species is an adaptation of these plants to floodedsoil conditions. The large air-spaces in these roots enablesdiffusion of gases between shoot and roots. Application of ethylene to non-flooded Rumex plants resultedin the formation of adventitious roots. In R. palustris Sm.shoot elongation and epinasty were also observed. The numberof roots in R. thyrsiflorus Fingerh. was much lower than inR. palustris, which corresponds with the inherent differencein root forming capacity between these two species. Ethyleneconcentrations of 1.5–2µI I_{– 1} induced a maximumnumber of roots in both species. Quantification of ethylene escaping from root systems of Rumexplants that were de-submerged after a 24 h submergence periodshowed that average ethylene concentrations in submerged rootsreached 1.8 and 9.1 µl I_–1 in R. palustris and R.thyrsiflorus, respectively. Inhibition of ethylene productionin R. palustris by L--(2-aminoethoxyvinyl)-glycine (AVG) or-aminobutyric acid (AIB) decreased the number of adventitiousroots induced by flooding, indicating that high ethylene concentrationsmay be a prerequisite for the flooding-induced formation ofadventitious roots in Rumex species. Key words: Adventitious roots, epinasty, ethylene, flooding, Rumex, shoot elongation