A review is presented for prospects of germplasm improvement for waterlogging tolerance in wheat, barley and oats using a mechanistic approach based on adaptive physiological traits. In `The waterlogged environments for crop production' section, the extent of waterlogging is reviewed commencing with determination of environmental factors which may limit plant growth and development in waterlogging prone regions. This highlights that different types of waterlogging may exist, there may be large spatial and temporal variation in waterlogging, and that waterlogging may be confounded in field experiments with additional environmental factors. Environmental characterisation is therefore a key step to using mechanistic approaches for germplasm improvement for target environments, for extrapolation to other environments, and for development of screening protocols under controlled conditions that accurately reflect the field environment. In the `Information on key components required for germplasm improvement' section, the genetic diversity in wheat, barley and oats for waterlogging tolerance is confirmed. Physiological mechanisms for waterlogging tolerance are diverse and can be grouped into adaptive traits relating to (1) phenology, (2) morphology and anatomy, (3) nutrition, (4) metabolism including anaerobic catabolism and anoxia tolerance, and (5) post anoxic damage and recovery. For wheat and barley, there is some genetic diversity for waterlogging tolerance at the germination stage, however the full potential seems yet to be exploited. Varietal differences in tolerance at the germination stage often differ from tolerance at later stages of development, and this supports the view that different mechanisms of tolerance exist at the whole plant and tissue level. Limited work from genetic studies indicates a high heritability for waterlogging tolerance. It is concluded that the best opportunities for germplasm improvement are for further exploration and utilisation of genetic diversity by improving selection criteria including the use of marker assisted selection. Additional opportunities are described for increasing genetic diversity using wide hybridisations and development of transgenic plants.

Review of prospects for germplasm improvement for waterlogging tolerance in wheat, barley and oats

The response of the ascorbate-glutathione cycle was investigated in roots of young wheat (Triticum aestivum L.) seedlings that were deprived of oxygen either by subjecting them to root hypoxia or to entire plant anoxia and then re-aerated. Although higher total levels of ascorbate and glutathione were observed under hypoxia, only the total amount of ascorbate was increased under anoxia. Under both treatments a significant increase in the reduced form of ascorbate and glutathione was found, resulting in increased reduction states. Upon the onset of re-aeration the ratios started to decline rapidly, indicating oxidative stress. Hypoxia caused higher activity of ascorbate peroxidase, whereas activities of monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase were diminished or only slightly influenced. Under anoxia, activities of ascorbate peroxidase and glutathione reductase decreased significantly to 39 and 62%, respectively. However, after re-aeration of hypoxically or anoxically pretreated roots, activity of enzymes approached the control levels. This corresponds with the restoration of the high reduction state of ascorbate and glutathione within 16 to 96 h of re-aeration, depending on the previous duration of anoxia. Apparently, anoxia followed by re-aeration more severely impairs entire plant metabolism compared with hypoxia, thus leading to decreased viability.

Re-Aeration following Hypoxia or Anoxia Leads to Activation of the Antioxidative Defense System in Roots of Wheat Seedlings

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.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/pce.12676

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

Cell wall polysaccharides in developing barley coleoptiles were examined using acetic acid-nitric acid extraction, alditol acetate and methylation analyses and enzymatic digestion. The coleoptile cell wall from imbibed grain was rich in pectic polysaccharides (30 mol%), arabinoxylan (25 mol%), cellulose (25 mol%) and xyloglucan (6 mol%), but contained only low levels of (1-->3,1-->4)-beta-D-glucan (1 mol%). During 5 days of coleoptile growth, pectic polysaccharides decreased steadily to about 9 mol%, while (1-->3,1-->4)-beta-D-glucan increased to 10 mol%. Following the cessation of growth of the coleoptiles at about 5 days, (1-->3,1-->4)-beta-D-glucan content rapidly decreased to 1 mol%. The cellulose content of the walls remained at about 35-40 mol% throughout coleoptile growth. Similarly, arabinoxylan content remained essentially constant at 25-30 mol% during growth, although the ratio of substituted to unsubstituted 4-linked xylosyl units decreased from about 4:1 to 1:1. Xyloglucan content ranged from 6 mol% to 10 mol% and the oligosaccharide profile determined using a xyloglucan-specific endoglucanase and MALDI-TOF mass spectrometry indicated that the oligosaccharides XXGG and XXGGG were the principal components, with one and two acetyl groups, respectively, Thus, dramatic changes in wall composition were detected during the growth of barley coleoptiles, both with respect to the relative abundance of individual wall constituents and to the fine structure of the arabinoxylans.

Changes in cell wall polysaccharides in developing barley (Hordeum vulgare) coleoptiles

We investigated the effects of hypoxia, anoxia and reaeration on enzymatic activity and expression of superoxide dismutase (SOD) isoforms in wheat roots (Triticum aestivum L.). Neither hypoxia nor subsequent re-aeration caused significant changes in SOD isoenzyme pattern compared with aerated controls. However, anoxia led to the appearance of additional activity bands of SOD in native gels resulting in an increase in total activity. Additional isoformic bands remained also apparent in the following recovery period. Re-aeration following both hypoxia and anoxia resulted in an increased content of hydrogen peroxide in roots. SOD transcript and protein levels were only slightly altered in response to hypoxia. Although SOD mRNA levels were diminished, protein content of different SOD isoforms increased with duration of anoxia. Incubation of roots with cycloheximide revealed that the additional activity bands and higher SOD protein content under anoxia were not due to de novo synthesis. Crude subcellular fractionation experiments implied that the anoxia-responsive SOD isoforms might be plastid-associated. We suggest that SOD is a very stable enzyme which, under anoxia, accumulates relative to total protein content and remains active even after protein modification under severe environmental stress conditions.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3040.2000.00542.x

Expression and activity of isoenzymes of superoxide dismutase in wheat roots in response to hypoxia and anoxia

Changes in the Root System of Wheat Seedlings Following Root Anaerobiosis I. Anatomy and Respiration in Triticum aestivum L.

Morphological and Anatomical Characterization of the Coleoptile of Triticum aestivum with Regard to the Evolution of Forms with Different Ploidy Levels

P. Hoffmann

Papers

Changes in the Root System of Wheat Seedlings Following Root Anaerobiosis I. Anatomy and Respiration in Triticum aestivum L.

Morphological and Anatomical Characterization of the Coleoptile of Triticum aestivum with Regard to the Evolution of Forms with Different Ploidy Levels