Internal transport of gases is crucial for vascular plants inhabiting aquatic, wetland or flood-prone environments. Diffusivity of gases in water is approximately 10 000 times slower than in air; thus direct exchange of gases between submerged tissues and the environment is strongly impeded. Aerenchyma provides a low-resistance internal pathway for gas transport between shoot and root extremities. By this pathway, O2 is supplied to the roots and rhizosphere, while CO2, ethylene, and methane move from the soil to the shoots and atmosphere. Diffusion is the mechanism by which gases move within roots of all plant species, but significant pressurized through-flow occurs in stems and rhizomes of several emergent and floating-leaved wetland plants. Through-flows can raise O2 concentrations in the rhizomes close to ambient levels. In general, rates of flow are determined by plant characteristics such as capacity to generate positive pressures in shoot tissues, and resistance to flow in the aerenchyma, as well as environmental conditions affecting leaf-to-air gradients in humidity and temperature. O2 diffusion in roots is influenced by anatomical, morphological and physiological characteristics, and environmental conditions. Roots of many (but not all) wetland species contain large volumes of aerenchyma (e.g. root porosity can reach 55%), while a barrier impermeable to radial O2 loss (ROL) often occurs in basal zones. These traits act synergistically to enhance the amount of O2 diffusing to the root apex and enable the development of an aerobic rhizosphere around the root tip, which enhances root penetration into anaerobic substrates. The barrier to ROL in roots of some species is induced by growth in stagnant conditions, whereas it is constitutive in others. An inducible change in the resistance to O2 across the hypodermis/exodermis is hypothesized to be of adaptive significance to plants inhabiting transiently waterlogged soils. Knowledge on the anatomical basis of the barrier to ROL in various species is scant. Nevertheless, it has been suggested that the barrier may also impede influx of: (i) soil-derived gases, such as CO2, methane, and ethylene; (ii) potentially toxic substances (e.g. reduced metal ions) often present in waterlogged soils; and (iii) nutrients and water. Lateral roots, that remain permeable to O2, may be the main surface for exchange of substances between the roots and rhizosphere in wetland species. Further work is required to determine whether diversity in structure and function in roots of wetland species can be related to various niche habitats.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3040.2003.00846.x

Long-distance transport of gases in plants: a perspective on internal aeration and radial oxygen loss from roots

There is considerable variability in salt tolerance amongst members of the Triticeae, with the tribe even containing a number of halophytes. This is a review of what is known of the differences in salt tolerance of selected species in this tribe of grasses, and the potential to use wild species to improve salt tolerance in wheat. Most investigators have concentrated on differences in ion accumulation in leaves, describing a desirable phenotype with low leaf Na+ concentration and a high K+/Na+ ratio. Little information is available on other traits (such as "tissue tolerance" of accumulated Na+ and Cl-) that might also contribute to salt tolerance. The sources of Na+ "exclusion" amongst the various genomes that make up tetraploid (AABB) durum wheat (Triticum turgidum L. ssp. durum), hexaploid (AABBDD) bread wheat (Triticum aestivum L. ssp. aestivum), and wild relatives (e.g. Aegilops spp., Thinopyrum spp., Elytrigia elongata syn. Lophopyrum elongatum, Hordeum spp.) are described. The halophytes display a capacity for Na+ "exclusion", and in some cases Cl- "exclusion", even at relatively high salinity. Significantly, it is possible to hybridize several wild species in the Triticeae with durum and bread wheat. Progenitors have been used to make synthetic hexaploids. Halophytic relatives, such as tall wheatgrass spp., have been used to produce amphiploids, disomic chromosome addition and substitution lines, and recombinant lines in wheat. Examples of improved Na+ "exclusion" and enhanced salt tolerance in various derivatives from these various hybridization programmes are given. As several sources of improved Na+ "exclusion" are now known to reside on different chromosomes in various genomes of species in the Triticeae, further work to identify the underlying mechanisms and then to pyramid the controlling genes for the various traits, that could act additively or even synergistically, might enable substantial gains in salt tolerance to be achieved.

Use of wild relatives to improve salt tolerance in wheat

Cropping on saline land is restricted by the low tolerance of crops to salinity and waterlogging. Prospects for improving salt tolerance in wheat and barley include the use of: (i) intra-specific variation, (ii) variation for salt tolerance in the progenitors of these cereals, (iii) wide-hybridisation with halophytic ‘wild’ relatives (an option for wheat, but not barley), and (iv) transgenic techniques. In this review, key traits contributing to salt tolerance, and sources of variation for these within the Triticeae, are identified and recommendations for use of these traits in screening for salt tolerance are summarised. The potential of the approaches to deliver substantial improvements in salt tolerance is discussed, and the importance of adverse interactions between waterlogging and salinity are emphasised. The potential to develop new crops from the diverse halophytic flora is also considered.

Improving salt tolerance of wheat and barley: future prospects

The cereals are of enormous importance to mankind. Many of the major cereal species - specifically, wheat, barley, oat, rye, and maize - have large genomes. Early cytogenetics, genome analysis and genetic mapping in the cereals benefited greatly from their large chromosomes, and the allopolyploidy of wheat and oats that has allowed for the development of many precise cytogenetic stocks. In the genomics era, however, large genomes are disadvantageous. Sequencing large and complex genomes is expensive, and the assembly of genome sequence is hampered by a significant content of repetitive DNA and, in allopolyploids, by the presence of homoeologous genomes. Dissection of the genome into its component chromosomes and chromosome arms provides an elegant solution to these problems. In this review we illustrate how this can be achieved by flow cytometric sorting. We describe the development of methods for the preparation of intact chromosome suspensions from the major cereals, and their analysis and sorting using flow cytometry. We explain how difficulties in the discrimination of specific chromosomes and their arms can be overcome by exploiting extant cytogenetic stocks of polyploid wheat and oats, in particular chromosome deletion and alien addition lines. Finally, we discuss some of the applications of flow-sorted chromosomes, and present some examples demonstrating that a chromosome-based approach is advantageous for the analysis of the complex genomes of cereals, and that it can offer significant potential for the delivery of genome sequencing and gene cloning in these crops.

/pdf/chromosome-based-genomics-in-the-cereals-4lkve5gt3r.pdf

Chromosome-based genomics in the cereals.

The paper reviews recent literature on the cytology, cytogenetics, molecular cytogenetics, and gene structure of barley, and on the relationship of the barley and wheat genomes. The information collected demonstrates largely similar genetic contents and collinearity of gene loci in the seven chromosomes of barley and in those of other members of the Triticeae. The high level of synteny indicates that the whole group can be considered as a single gene pool. On this basis, the 7th International Barley Genetics Symposium recommended that (i) the currently used designations of the barley chromosomes (the BURNHAM and HAGBERG system) should become secondary to the use of the Triticeae system; i.e., each of the seven barley chromosomes is designated by a numeral between 1 and 7 according to its homoeologous relationship with the chromosomes of the wheat genomes followed by the genomic symbol H, e.g., 2H; (ii) the genomes of Hordeum vulgare and H. bulbosum are symbolized with the letter H; and (iii) the arms of the barley chromosomes are designated by the capital letters S and L (cf. SINGHand TSUCHIYA 1982b). Further, it was recommended that the barley genome present in the barley cultivar ‘Betzes’ should become the reference genome in the Triticeae.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1601-5223.1997.00001.x

The barley Genome and its Relationship with the Wheat Genomes. A Survey with an Internationally Agreed Recommendation for Barley Chromosome Nomenclature

Wide crosses between different plant species and even between different genera have interested plant breeders and botanists since before the turn of the century. Wheat (Triticum),being the most important food crop of the world, has attracted most attention and it has been hybridized extensively with “alien” species belonging to several neighboring genera (Islam 1980a; Sharma and Gill 1983). The objectives in hybridizing wheat with barley are manifold. The prospect of transferring desirable agronomic characters like tolerance to drought or soil salinity from barley to wheat prompted some early workers to attempt wheat-barley hybridizations. More recent considerations are to transfer nematode and disease resistance genes from barley to wheat. Furthermore, hybrids between wheat and barley are the starting materials for determining the evolutionary and genetical relationship between wheat and barley chromosomes.

A. K. M. R. Islam

Papers

Incorporation of Barley Chromosomes into Wheat