Gustavo G. Striker

Escape from water or remain quiescent? Lotus tenuis changes its strategy depending on depth of submergence.

BACKGROUND AND AIMS Two main strategies that allow plants to cope with soil waterlogging or deeper submergence are: (1) escaping by means of upward shoot elongation or (2) remaining quiescent underwater. This study investigates these strategies in Lotus tenuis, a forage legume of increasing importance in areas prone to soil waterlogging, shallow submergence or complete submergence. METHODS Plants of L. tenuis were subjected for 30 d to well-drained (control), waterlogged (water-saturated soil), partially submerged (6 cm water depth) and completely submerged conditions. Plant responses assessed were tissue porosity, shoot number and length, biomass and utilization of water-soluble carbohydrates (WSCs) and starch in the crown. KEY RESULTS Lotus tenuis adjusted its strategy depending on the depth of submergence. Root growth of partially submerged plants ceased and carbon allocation prioritized shoot lengthening (32 cm vs. 24.5 cm under other treatments), without depleting carbohydrate reserves to sustain the faster growth. These plants also developed more shoot and root porosity. In contrast, completely submerged plants became quiescent, with no associated biomass accumulation, new shoot production or shoot elongation. In addition, tissue porosity was not enhanced. The survival of completely submerged plants is attributed to consumption of WSCs and starch reserves from crowns (concentrations 50-75 % less than in other treatments). CONCLUSIONS The forage legume L. tenuis has the flexibility either to escape from partial submergence by elongating its shoot more vigorously to avoid becoming totally submerged or to adopt a non-elongating quiescent strategy when completely immersed that is based on utilizing stored reserves. The possession of these alternative survival strategies helps to explain the success of L. tenuis in environments subjected to unpredictable flooding depths.

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.

Mechanisms of waterlogging tolerance in wheat – a review of root and shoot physiology

We review the detrimental effects of waterlogging on physiology, growth and yield of wheat. We highlight traits contributing to waterlogging tolerance and genetic diversity in wheat. Death of seminal roots and restriction of adventitious root length due to O2 deficiency result in low root:shoot ratio. Genotypes differ in seminal root anoxia tolerance, but mechanisms remain to be established; ethanol production rates do not explain anoxia tolerance. Root tip survival is short-term, and thereafter, seminal root re-growth upon re-aeration is limited. Genotypes differ in adventitious root numbers and in aerenchyma formation within these roots, resulting in varying waterlogging tolerances. Root extension is restricted by capacity for internal O2 movement to the apex. Sub-optimal O2 restricts root N uptake and translocation to the shoots, with N deficiency causing reduced shoot growth and grain yield. Although photosynthesis declines, sugars typically accumulate in shoots of waterlogged plants. Mn or Fe toxicity might occur in shoots of wheat on strongly acidic soils, but probably not more widely. Future breeding for waterlogging tolerance should focus on root internal aeration and better N-use efficiency; exploiting the genetic diversity in wheat for these and other traits should enable improvement of waterlogging tolerance.

Different strategies of Lotus japonicus, L. corniculatus and L. tenuis to deal with complete submergence at seedling stage.

Two main strategies allow plants to deal with submergence: (i) escape from below water by means of shoot elongation, or (ii) remaining quiescent under the water until water subsides and then resume growth. We investigated these strategies in seedlings of Lotus japonicus, L. corniculatus and L. tenuis subjected to control and submergence for 12 days, with a subsequent 30-day recovery period. All three species survived submergence but used different strategies. Submerged seedlings of L. japonicus exhibited an escape strategy (emerging from water) as a result of preferential carbon allocation towards shoot mass and lengthening, in detriment to root growth. In contrast, seedlings of L. corniculatus and L. tenuis became quiescent, with no biomass accumulation, no new unfolding of leaves and no shoot elongation. Upon de-submergence, seedlings of L. japonicus had the lowest recovery growth (a biomass and shoot height 58% and 40% less than controls, respectively), L. corniculatus was intermediate and L. tenuis showed the greatest recovery growth. Previously submerged seedlings of L. tenuis did not differ from their controls, either in final shoot biomass or shoot height. Thus, for the studied species, quiescence appears to be an adequate strategy for tolerance of short-term (i.e., 12 days) complete submergence, being consistent with field observations of L. tenuis colonisation of flood-prone environments.

Flooding Effects on Plants Recovering from Defoliation in Paspalum dilatatum and Lotus tenuis

BACKGROUND AND AIMS Flooding and grazing are major disturbances that simultaneously affect plant performance in many humid grassland ecosystems. The effects of flooding on plant recovery from defoliation were studied in two species: the grass Paspalum dilatatum, regrowing primarily from current assimilation; and the legume, Lotus tenuis, which can use crown reserves during regrowth. METHODS Plants of both species were subjected to intense defoliation in combination with 15 d of flooding at 6 cm water depth. Plant recovery was evaluated during a subsequent 30-d growth period under well-watered conditions. Plant responses in tissue porosity, height, tiller or shoot number and biomass of the different organs were assessed. KEY RESULTS Flooding increased porosity in both P. dilatatum and L. tenuis, as expected in flood-tolerant species. In P. dilatatum, defoliation of flooded plants induced a reduction in plant height, thus encouraging the prostrated-growth response typical of defoliated plants rather than the restoration of contact with atmospheric oxygen, and most tillers remained submerged until the end of the flooding period. In contrast, in L. tenuis, plant height was not reduced when defoliated and flooded, a high proportion of shoots being presented emerging above water (72 %). In consequence, flooding plus defoliation did not depress plant recovery from defoliation in the legume species, which showed high sprouting and use of crown biomass during regrowth, whereas in the grass species it negatively affected plant recovery, achieving 32 % lower biomass than plants subjected to flooding or defoliation as single treatments. CONCLUSIONS The interactive effect of flooding and defoliation determines a reduction in the regrowth of P. dilatatum that was not detected in L. tenuis. In the legume, the use of crown reserves seems to be a key factor in plant recovery from defoliation under flooding conditions.

CONSTITUTIVE AND PLASTIC ROOT TRAITS AND THEIR ROLE IN DIFFERENTIAL TOLERANCE TO SOIL FLOODING AMONG COEXISTING SPECIES OF A LOWLAND GRASSLAND

Natural flooding is a major component of the disturbance regime in many grassland ecosystems. The objective of this study was to analyze the relationship among constitutive and plastic root traits and tolerance to flooding in coexisting perennial species of the flooding pampa grasslands (Argentina). A mesocosm experiment was designed for five native species (Paspalidium paludivagum, Paspalum dilatatum, Bothriochloa laguroides, Eryngium ebracteatum, and Eclipta bellidioides) and two exotic ones (Mentha pulegium and Plantago lanceolata). Across species, constitutive root porosity was positively correlated with the tolerance to soil flooding. Moreover, the generation of additional aerenchyma was larger in species with intermediate values of constitutive root porosity and lower in species with low or high constitutive root porosity. This differential increase in the root porosity of each species, combined with the values of constitutive root porosity, resulted in a stronger correlation between final root porosity and tolerance under flooding conditions. Native grasses increased the proportion of root aerenchyma, showing a small change in the number of lysed cells but a significant increase in the cortex proportion and diameter of roots. Exotic dicots generated lysigenous aerenchyma throughout their cortex; in contrast, native dicot species maintained the cell layers adjacent to the stele. A lag in the development of secondary growth during flooding was detected in both groups of dicots, a response that was particularly evident in the exotic species, contrasting with their prominent growth under nonflooded conditions. In general, our results indicate that constitutive and plastic root traits are very significant in terms of the effects of periodic flooding on the abundance of coexisting species of the flooding pampa grasslands.

Flooding tolerance of forage legumes.

We review waterlogging and submergence tolerances of forage (pasture) legumes. Growth reductions from waterlogging in perennial species ranged from >50% for Medicago sativa and Trifolium pratense to <25% for Lotus corniculatus, L. tenuis, and T. fragiferum. For annual species, waterlogging reduced Medicago truncatula by ~50%, whereas Melilotus siculus and T. michelianum were not reduced. Tolerant species have higher root porosity (gas-filled volume in tissues) owing to aerenchyma formation. Plant dry mass (waterlogged relative to control) had a positive (hyperbolic) relationship to root porosity across eight species. Metabolism in hypoxic roots was influenced by internal aeration. Sugars accumulate in M. sativa due to growth inhibition from limited respiration and low energy in roots of low porosity (i.e. 4.5%). In contrast, L. corniculatus, with higher root porosity (i.e. 17.2%) and O2 supply allowing respiration, maintained growth better and sugars did not accumulate. Tolerant legumes form nodules, and internal O2 diffusion along roots can sustain metabolism, including N2 fixation, in submerged nodules. Shoot physiology depends on species tolerance. In M. sativa, photosynthesis soon declines and in the longer term (>10 d) leaves suffer chlorophyll degradation, damage, and N, P, and K deficiencies. In tolerant L. corniculatus and L. tenuis, photosynthesis is maintained longer, shoot N is less affected, and shoot P can even increase during waterlogging. Species also differ in tolerance of partial and complete shoot submergence. Gaps in knowledge include anoxia tolerance of roots, N2 fixation during field waterlogging, and identification of traits conferring the ability to recover after water subsides.

Community recommendations on terminology and procedures used in flooding and low oxygen stress research

Apart from playing a key role in important biochemical reactions, molecular oxygen (O2) and its by-products also have crucial signaling roles in shaping plant developmental programs and environmental responses. Even under normal conditions, sharp O2 gradients can occur within the plant when cellular O2 demand exceeds supply, especially in dense organs such as tubers, seeds and fruits. Spatial and temporal variations in O2 concentrations are important cues for plants to modulate development (van Dongen & Licausi, 2015; Considine et al., 2016). Environmental conditions can also expand the low O2 regions within the plant. For example, excessive rainfall can lead to partial or complete plant submergence resulting in O2 deficiency in the root or the entire plant (Voesenek & Bailey-Serres, 2015). Climate change-associated increases in precipitation events have made flooding a major abiotic stress threatening crop production and food sustainability. This increased flooding and associated crop losses highlight the urgency of understanding plant flooding responses and tolerance mechanisms. Timely manifestation of physiological and morphological changes triggering developmental adjustments or flooding survival strategies requires accurate sensing of O2 levels. Despite progress in understanding how plants sense and respond to changes in intracellular O2 concentrations (van Dongen & Licausi, 2015), several questions remain unanswered due to a lack of high resolution tools to accurately and noninvasively monitor (sub)cellular O2 concentrations. In the absence of such tools, it is therefore critical for researchers in the field to be aware of how experimental conditions can influence plant O2 levels, and thus on the importance of accurately reporting specific experimental details. This also requires a consensus on the definition of frequently used terms. At the 15th New Phytologist Workshop on Flooding stress (Voesenek et al., 2016), community members discussed and agreed on unified nomenclature and standard norms for low O2 and flooding stress research. This consensus on terminology and experimental guidelines is presented here. We expect that these norms will facilitate more effective interpretation, comparison and reproducibility of research in this field. We also highlight the current challenges in noninvasively monitoring and measuring O2 concentrations in plant cells, outlining the technologies currently available, their strengths and drawbacks, and their suitability for use in flooding and low O2 research.

Flooding Stress on Plants: Anatomical, Morphological and Physiological Responses

Flooding is a natural disturbance affecting crop and forage production worldwide due to the detrimental effects that it provokes on most terrestrial plants (Bailey-Serres & Voesenek, 2008; Colmer & Vosenek, 2009). Over the last years, the Intergovernmental Panel on Climate Change (http://www.ippc.ch) has informed that man-induced world climate change will increase the frequency of precipitations of higher magnitude as well as tropical cyclone activity. As a result, the occurrence of flooding events on flood plains (i.e. lowlands) and cultivated lands is expected to be higher (Arnell & Liu, 2001). On the other hand, the increasing world population, along with the intensification of agriculture have provoked a reduction in the arable land per capita, which has decreased over the last five decades from 0.32 ha to 0.21 ha, and it is expected to be further diminished up to 0.16 ha per capita by 2030 (FAO 2006 as cited in Mancuso & Shabala, 2010). As a consequence, marginal lands are being incorporated into production to cope with the rising food demand. These issues lead to the necessity to get highly productive crops in arable lands subjected to periodic events of water excess, and to introduce new (or improved) flood-tolerant forage species in floodprone pastures (and grasslands) devoted to livestock production. So, the understanding of plant functioning under flooding conditions is crucial in order to achieve these goals.

Phenomic networks reveal largely independent root and shoot adjustment in waterlogged plants of Lotus japonicus

Waterlogging imposes severe stress to the plant, and the interplay between root and aerial organs in the adjustment to this stress is poorly understood. A set of recombinant inbred lines (RILs) of Lotus japonicus (Gifu B-129 × Miyakojima MG-20) was subjected to control and waterlogging conditions for 21 d, and 12 traits related to leaf physiological functioning, root aerenchyma formation, shoot and root morphology, and dry mass accumulation were assessed to generate phenomic networks. The phenomic network became more complex under waterlogging as a result of the incorporation of root aerenchyma and dark-adapted Fv/Fm. Significant waterlogging-induced variation was found for stomatal conductance, dark-adapted Fv/Fm and aerenchyma. The RILs with stronger induction of aerenchyma in response to waterlogging tended to show reduced negative impact of this stress on root growth but suffered average impact on shoot growth. The RILs that retained higher stomatal conductance under waterlogging tended to retain higher dark-adapted Fv/Fm and shoot growth under waterlogging conditions but showed average impact on root traits. We propose a model where, although the stress experienced by the roots during waterlogging is transmitted to the shoot, shoots and roots deal with waterlogging in a less interdependent manner than often assumed.