Interactive effect of flooding and grazing on the growth of Serengeti grasses

Summary Grazing and flooding may potentially interact in particular habitats of many grassland regions around the world. We tested the hypothesis that grazing and flooding induce different and largely opposed allocation responses in individual plants. As a result, their combined effect on plant growth would be negative. We studied the response of three grass species from the Serengeti ecosystem (Tanzania) to the effects of flooding and clipping. Plants under the combined effect of flooding and clipping had lower growth rates than plants growing under the effect of either of the two factors individually. Plants under flooding grew taller and allocated more resources to stem growth than controls; for two of the three species, flooded plants also generated a new root system above soil level. All these morphological and physiological responses conflict with the ability of a plant to respond to defoliation with minimum reduction in growth rates. The three species showed a response to flooding reflecting their distribution ranges in the field: the species from the most flood-prone habitat showed a positive effect of flooding on growth, whereas the species from dry uplands showed a strong negative effect of flooding. Flood-tolerant species were taller and less tolerant of clipping than flooding sensitive species. Our results suggest that, in ecological time, individuals subjected to both flooding and grazing have their growth reduced to a greater extent than by either of the two factors acting individually, whereas in evolutionary time, species adapted to flooding are poor grazing tolerators and species adapted to grazing are poor flooding tolerators.     

Evolution and mechanisms of plant tolerance to flooding stress

Background

In recognition of the 200th anniversary of Charles Darwin''s birth, this short article on flooding stress acknowledges not only Darwin''s great contribution to the concept of evolution but also to the study of plant physiology. In modern biology, Darwin-inspired reductionist physiology continues to shed light on mechanisms that confer competitive advantage in many varied and challenging environments, including those where flooding is prevalent.

Scope

Mild flooding is experienced by most land plants but as its severity increases, fewer species are able to grow and survive. At the extreme, a highly exclusive aquatic lifestyle appears to have evolved numerous times over the past 120 million years. Although only 1–2% of angiosperms are aquatics, some of their adaptive characteristics are also seen in those adopting an amphibious lifestyle where flooding is less frequent. Lowland rice, the staple cereal for much of tropical Asia falls into this category. But, even amongst dry-land dwellers, or certain of their sub-populations, modest tolerance to occasional flooding is to be found, for example in wheat. The collection of papers summarized in this article describes advances to the understanding of mechanisms that explain flooding tolerance in aquatic, amphibious and dry-land plants. Work to develop more tolerant crops or manage flood-prone environments more effectively is also included. The experimental approaches range from molecular analyses, through biochemistry and metabolomics to whole-plant physiology, plant breeding and ecology.Key words: Abiotic stress, adaptation, anoxia, Charles Darwin, environmental stress, evolution, flooding, hypoxia, rice, submergence, wetlands     

Physiological integration enhanced the tolerance of Cynodon dactylon to flooding

Many flooding‐tolerant species are clonal plants; however, the effects of physiological integration on plant responses to flooding have received limited attention. We hypothesise that flooding can trigger changes in metabolism of carbohydrates and ROS (reactive oxygen species) in clonal plants, and that physiological integration can ameliorate the adverse effects of stress, subsequently restoring the growth of flooded ramets. In the present study, we conducted a factorial experiment combining flooding to apical ramets and stolon severing (preventing physiological integration) between apical and basal ramets of Cynodon dactylon, which is a stoloniferous perennial grass with considerable flooding tolerance. Flooding‐induced responses including decreased root biomass, accumulation of soluble sugar and starch, as well as increased activity of superoxide dismutase (SOD) and ascorbate peroxidase (APX) in apical ramets. Physiological integration relieved growth inhibition, carbohydrate accumulation and induction of antioxidant enzyme activity in stressed ramets, as expected, without any observable cost in unstressed ramets. We speculate that relief of flooding stress in clonal plants may rely on oxidising power and electron acceptors transferred between ramets through physiological integration.     

Adaptations to Flooding Stress: From Plant Community to Molecule

Abstract: This review highlights four major topics in plant flooding research: the processes underlying vegetation zonation in the floodplain, the challenges of using model species to reveal adaptive responses in shoots and roots, the role of micro-organisms in flooded soils in relation to plant growth, and the molecular regulation of the hormone ethylene which is heavily involved in the adaptation reaction of flood-resistant plants. Model species and vegetation strategies are used to unravel mechanisms of vegetation zonation in the river flood-plain. In the case of woodlands, hydrological conditions determine to a large extent their zonation patterns under natural conditions. For softwood species, such as Salicaceae, the interaction between water levels and timing of seed dispersal is the dominating process determining their establishment success on river banks. Their strategy is well adapted to irregular, high and prolonged floods. Hardwood species, Quercus, Fraxinus, UImus and Acer, are flood-sensitive and inhabit the higher sites. They mainly have heavy seeds and germinate under shaded conditions. The most shade-tolerant hardwood species are the least well adapted to flooding. Anthropogenically influenced parts of the floodplain are characterized by grasslands with elevation level and management practices determining the species composition. Low-lying grasslands have flood-tolerant species; elevated zones are seldom flooded and have flooding-sensitive species. Following Grime (1998^[59]), plant species of major vegetation types within the floodplain zone can be divided into three categories–dominants, subordinates and transients–illustrating the diversity in plant species in relation to environmental properties. Model species that are indicative of the different conditions in the various zones are chosen to help in the understanding of morphological and physiological adaptations at the plant level. The formation of aerenchymatous roots and the capacity to elongate shoot parts upon submergence are among the main responses of surviving plants. The role of hormones in the adaptation reaction is emphasized. Owing to high porosities in roots of flood-tolerant plants, radial oxygen loss greatly influences nitrification and denitrification processes in the flooded soil. Nutrient cycles are restored by root-derived oxygen and the oxygenated rhizosphere is detoxified. A new development in flooding ecology is the unravelling of the molecular regulation of hormonally controlled processes. The expression of an ethylene receptor gene in Rumex palustris is highlighted. This paper ends with some suggestions for future flooding research.     

Ethylene – and oxygen signalling – drive plant survival during flooding

Flooding is a widely occurring environmental stress both for natural and cultivated plant species. The primary problems associated with flooding arise due to restricted gas diffusion underwater. This hampers gas exchange needed for the critical processes of photosynthesis and respiration. Plant acclimation to flooding includes the adaptation of a suite of traits that helps alleviate or avoid these stressful conditions and improves or restores exchange of O₂ and CO₂. The manifestation of these traits is, however, reliant on the timely perception of signals that convey the underwater status. Flooding‐associated reduced gas diffusion imposes a drastic change in the internal gas composition within submerged plant organs. One of the earliest changes is an increase in the levels of the gaseous plant hormone ethylene. Depending on the species, organ, flooding conditions and time of the day, plants will also subsequently experience a reduction in oxygen levels. This review provides a comprehensive overview on the roles of ethylene and oxygen as critical signals of flooding stress. It includes a discussion of the dynamics of these gases in plants when underwater, their interaction, current knowledge of their perception mechanisms and the resulting downstream changes that mediate important acclimative processes that allow endurance and survival under flooded conditions.     

Salt stress tolerance; what do we learn from halophytes?

Abiotic environmental stresses can give rise to morphological, biochemical and molecular changes that negatively affect plant growth and productivity. Among these stresses, soil salinity is the major threat. To deal and control effects of high salinity on plants, it is important to understand their responses to salt stress that disturbs the homeostatic equilibrium at cellular and molecular levels. In this regard halophytes (salt tolerant plants) can provide superior models for the study of salt stress defense parameters compared to salt sensitive species (glycophytes). Halophytes use highly developed, complex systems to tolerate salinity by maintaining a low cytosolic Na⁺/K⁺ ratio, sequestration of Na⁺ into vacuoles that then provides the osmotic potential sustaining water influx. Under low intensity stress conditions that moderately and/or transiently affect ion imbalance, the set of responses all plants initiate will be mostly to engage measures that assure ion balance. High salinity, especially over a prolonged time period, will challenge plant survival, which then requires different strategies that employ a variety of mechanisms. Plasticity and connectivity of these diverse mechanisms is engrained in species- and family-specific evolutionary history and their genetic complexity. Highlighting differences in the genetic and biochemical makeup between glycophytes and halophytes allows for comparisons between their approaches towards high salinity. This review provides a brief overview about different strategies and mechanism used by plants to avoid or confine adverse effects of high salinity.     

Riparian plant community responses to increased flooding: a meta‐analysis

A future higher risk of severe flooding of streams and rivers has been projected to change riparian plant community composition and species richness, but the extent and direction of the expected change remain uncertain. We conducted a meta‐analysis to synthesize globally available experimental evidence and assess the effects of increased flooding on (1) riparian adult plant and seedling survival, (2) riparian plant biomass and (3) riparian plant species composition and richness. We evaluated which plant traits are of key importance for the response of riparian plant species to flooding. We identified and analysed 53 papers from ISI Web of Knowledge which presented quantitative experimental results on flooding treatments and corresponding control situations. Our meta‐analysis demonstrated how longer duration of flooding, greater depth of flooding and, particularly, their combination reduce seedling survival of most riparian species. Plant height above water level, ability to elongate shoots and plasticity in root porosity were decisive for adult plant survival and growth during longer periods of flooding. Both ‘quiescence’ and ‘escape’ proved to be successful strategies promoting riparian plant survival, which was reflected in the wide variation in survival (full range between 0 and 100%) under fully submerged conditions, while plants that protrude above the water level (>20 cm) almost all survive. Our survey confirmed that the projected increase in the duration and depth of flooding periods is sufficient to result in species shifts. These shifts may lead to increased or decreased riparian species richness depending on the nutrient, climatic and hydrological status of the catchment. Species richness was generally reduced at flooded sites in nutrient‐rich catchments and sites that previously experienced relatively stable hydrographs (e.g. rain‐fed lowland streams). Species richness usually increased at sites in desert and semi‐arid climate regions (e.g. intermittent streams).     

Time is on our side: the importance of considering a recovery period when assessing flooding tolerance in plants

There is wide consensus about the significance of monitoring plant responses during flooding when evaluating specific tolerance. Nonetheless, plant recovery once water recedes has often been overlooked. This note highlights the importance of registering plant performance during a recovery phase. Two opposite types of plant growth responses, during and after flooding, are discussed. It is shown that an apparently poor performance during flooding does not necessarily involve a reduced tolerance, as plants can prioritize saving energy and carbohydrates for later resumption of vigorous growth during recovery. Conversely, maintenance of positive plant growth during flooding can imply extensive depletion of reserves, consequently constraining future plant growth. Therefore, to accurately estimate real tolerance to this stress, plant performance should be appraised during both flooding and recovery periods.     

三峡水库消落区不同海拔高度的植物群落多样性差异

本文对三峡水库消落区不同海拔间植物多样性和植物群落结构差异进行了研究。从α多样性上来看,上部和中部消落区物种丰富度和均匀度差异不显著,下部消落区丰富度指数明显低于中上部。下部消落区植物种间相遇几率较大,植物种间相互依存性较强。从β多样性上来看,由上部到中部再到下部,随着海拔下降,水库消落区植物物种的替代性减少是均质的;不同地区间β多样性没有显著性差异,但不同海拔间差异显著。从群落结构及稳定性上来看,目前水库消落区植物群落结构稳定性中部<上部<下部,上部消落区水淹胁迫较小,植物物种多为竞争种(C-对策种),竞争力较强的杂草偏向形成优势群落;下部消落区水淹胁迫最强,植物物种多为耐胁迫种(S-对策种),能忍受高强度水淹环境的物种形成了植物群落;中部消落区,处于物种定居和水淹胁迫的双重压力下,竞争种和耐胁迫种间竞争明显,更偏向于形成共优群落,其群落稳定性较差。在目前情况下,消落区下部的植物群落组成比较单一,但是随着水库蓄水高程稳定在175 m,估计消落区上中部群落组成也会逐渐趋于单一化。     

植物水淹适应与碳水化合物的相关性

水淹会对陆生植物存活造成本质影响, 特别是完全水淹对陆生植物的影响更为明显。水淹对陆生植物最为主要的影响是氧气不足, 这主要是由氧气在水中的扩散速率较低引起的。同时, 在水淹胁迫下植物对光和CO₂的获取都会受到限制。所有这些因素都将引起植物生物量减少, 最终导致受淹植物死亡。碳水化合物是植物的能量来源, 与植物在水淹胁迫下存活与否有着密切联系。植物水淹适应性与碳水化合物的相关性主要体现在两大方面: 在生理形态层面, 植物通过伸长生长或抑制伸长生长、地上和地下部分碳水化合物的分配比例不同来应对水淹胁迫; 在另一个层面, 植物通过改变激素、酶和基因的表达, 调整碳水化合物的代谢方式, 从而适应水淹环境。该文结合国内外研究现状, 通过对植物在水淹胁迫下生理形态、激素、酶及基因表达诸方面的变化来认识水淹耐受性与碳水化合物的关系, 并就今后的研究方向提出几点建议。     

Catalase-deficient tobacco plants: tools for in planta studies on the role of hydrogen peroxide

Adequate responses to environmental changes are crucial for plant growth and survival. However, the molecular and biochemical mechanisms involved are poorly understood and the signaling networks remain elusive. The accumulation of active oxygen species (AOS) is a central theme during plant responses to both biotic and abiotic stresses. In both situations, AOS can play two divergent roles: either exacerbating damage or activating multiple defense responses, thereby acting as signal molecules. Such a dual function was first described in pathogenesis, but also recently has been demonstrated during several abiotic stress responses. To allow for these different roles, cellular levels of AOS must be tightly controlled. This control can be attained through a diverse battery of oxidant scavengers. Perturbation of this scavenging capacity can lead to dramatic imbalances of AOS concentrations, leading to a modified redox status. Here, we summarize mainly the work done on plants that are deficient in catalase activity. These plants not only revealed the importance of catalase in coping with environmental stress but also provided us with a powerful tool to investigate the (multiple) roles of H2O2 in an intact plant system.     

Life cycle stage and water depth affect flooding-induced adventitious root formation in the terrestrial species Solanum dulcamara

Background and Aims Flooding can occur at any stage of the life cycle of a plant, but often adaptive responses of plants are only studied at a single developmental stage. It may be anticipated that juvenile plants may respond differently from mature plants, as the amount of stored resources may differ and morphological changes can be constrained. Moreover, different water depths may require different strategies to cope with the flooding stress, the expression of which may also depend on developmental stage. This study investigated whether flooding-induced adventitious root formation and plant growth were affected by flooding depth in Solanum dulcamara plants at different developmental stages.Methods Juvenile plants without pre-formed adventitious root primordia and mature plants with primordia were subjected to shallow flooding or deep flooding for 5 weeks. Plant growth and the timing of adventitious root formation were monitored during the flooding treatments.Key Results Adventitious root formation in response to shallow flooding was significantly constrained in juvenile S. dulcamara plants compared with mature plants, and was delayed by deep flooding compared with shallow flooding. Complete submergence suppressed adventitious root formation until up to 2 weeks after shoots restored contact with the atmosphere. Independent of developmental stage, a strong positive correlation was found between adventitious root formation and total biomass accumulation during shallow flooding.Conclusions The potential to deploy an escape strategy (i.e. adventitious root formation) may change throughout a plant’s life cycle, and is largely dependent on flooding depth. Adaptive responses at a given stage of the life cycle thus do not necessarily predict how the plant responds to flooding in another growth stage. As variation in adventitious root formation also correlates with finally attained biomass, this variation may form the basis for variation in resistance to shallow flooding among plants.     

Competition Is a Strong Driving Factor in Wetlands,Peaking during Drying Out Periods

The aim of the study is to investigate the relative importance of plant-plant interactions with regard to flooding and drought effect on perennial plant performances in wetlands. Flooding is expected to be the major driver and, accordingly, the importance of drought is hardly if ever taken into account. Focusing on five widespread species, the growth, the survival and the competitive ability of plants were monitored on permanent plots spread along two elevation gradients. Flooding duration and drought intensity were found to vary substantially along the ~ 0.5 meter range elevation gradient. Flooding and drought alternate over the hydrological year and the pin-point surveys were thus conducted over the course of one year. The data were modeled taking into account survival, recruitment and competitive growth throughout flooding and drying out periods. Flooding and drought both directly impacted the plant performances and their competitive effect, with the effect of drought being much more general among species and of higher magnitude than flooding. The importance of competition was found to be high for all species, particularly during the drying out period. It varied more along the flooding gradient than along the drought gradient. The higher flooding tolerance shown by the studied species compared to drought may be related to species specific growth timing together with efficient response traits. These results offer new insights into the filters operating over the species pools. This suggests that the drying out period and drought conditions may be even more important for species’ relative success and the importance of competition than the flooding pattern. The general applicability of this result, obtained in mild Atlantic climate and fertile wetlands, remains to be studied.     

Adaptive mechanisms of medicinal plants along altitude gradient: contribution of proteomics

Medicinal plants are a rich source of secondary metabolites extensively used in traditional health care systems. High altitude biodiversity encompasses the diversified and valuable medicinal plant species. The extreme environmental conditions of high altitude region viz. fluctuating temperatures, high UV radiation, salinity, low oxygen concentration, and high wind velocity limits the plant growth and distribution. Yet, how medicinal plants respond to these extreme conditions is not sufficiently understood. Therefore, addressing plant acclimation to different stresses presents an opportunity to unravel adaptive mechanism of medicinal plants along altitude gradient. This article reviews the recently published research that highlights the major role of proteins in plant adaptation to extreme environmental conditions. In the last few decades, climate change has made a profound impact on high altitude plants. Stress conditions alter cellular homeostasis of plants. With the advent of proteomics, it has become evident that stresses induce changes in proteome by synthesis/expression of novel stress responsive proteins. These proteins constitute a highly organized, complex network that leads to changes in the molecular, biochemical, physiological, and morphological responses of plants. Herein, we comprehensively discuss the proteomics of medicinal plants and its role in adaptation along altitude gradient. This review aims to provide impetus to current research in medicinal plants ranging from developmental to stress biology and to generate basis for genetic engineers and plant breeders to produce next-generation medicinal plants.     

Variations among Woody Angiosperms in Response to Flooding

Effects of flooding on young Populus deltoides, Salix nigra, Eucalyptus camaldulensis, E. globulus, Ulmus americana, Quercus rubra and Fraxinus pennsylvanica plants were studied. Flooding variously induced several sequential physiological disturbances, with stomatal closure among the earliest responses. Subsequent responses included inhibition of root growth, alterations in root and stem morphology, formation of adventitious roots, and leaf senescence. In amphistomatous species (Populus deltoides, Salix nigra, Eucalyptus camaldulensis) flooding rapidly induced stomatal closure on the adaxial leaf surface. It also significantly induced stomatal closure on the abaxial epidermis of Populus deltoides but not that of Salix nigra or Eucalyptus camaldulensis. In hypostomatous species (Eucalyptus globulus, Ulmus americana) flooding significantly induced stomatal closure on the abaxial surface within 3 days. Stomatal responses to flooding were not correlated with leaf water stress. In both long- and short-term experiments, flooding did not significantly increase plant water stress. These results deemphasized the importance of plant water stress in inducing plant responses to flooding. The importance of various hormones in inducing flooding symptoms is discussed.