Timothy D. Colmer

Improving salt tolerance of wheat and barley: future prospects

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.

Flooding tolerance in halophytes

Flooding is a common environmental variable with salinity. Submerged organs can suffer from O(2) deprivation and the resulting energy deficits can compromise ion transport processes essential for salinity tolerance. Tolerance of soil waterlogging in halophytes, as in glycophytes, is often associated with the production of adventitious roots containing aerenchyma, and the resultant internal O(2) supply. For some species, shallow rooting in aerobic upper soil layers appears to be the key to survival on frequently flooded soils, although little is known of the anoxia tolerance in halophytes. Halophytic species that inhabit waterlogged substrates are able to regulate their shoot ion concentrations in spite of the hypoxic (or anoxic) medium in which they are rooted, this being in stark contrast with most other plants which suffer when salinity and waterlogging occur in combination. Very few studies have addressed the consequences of submergence of the shoots by saline water; these have, however, demonstrated tolerance of temporary submergence in some halophytes.

Sodium chloride toxicity and the cellular basis of salt tolerance in halophytes

BACKGROUND Halophytes are the flora of saline soils. They adjust osmotically to soil salinity by accumulating ions and sequestering the vast majority of these (generally Na(+) and Cl(-)) in vacuoles, while in the cytoplasm organic solutes are accumulated to prevent adverse effects on metabolism. At high salinities, however, growth is inhibited. Possible causes are: toxicity to metabolism of Na(+) and/or Cl(-) in the cytoplasm; insufficient osmotic adjustment resulting in reduced net photosynthesis because of stomatal closure; reduced turgor for expansion growth; adverse cellular water relations if ions build up in the apoplast (cell walls) of leaves; diversion of energy needed to maintain solute homeostasis; sub-optimal levels of K(+) (or other mineral nutrients) required for maintaining enzyme activities; possible damage from reactive oxygen species; or changes in hormonal concentrations. SCOPE This review discusses the evidence for Na(+) and Cl(-) toxicity and the concept of tissue tolerance in relation to halophytes. CONCLUSIONS The data reviewed here suggest that halophytes tolerate cytoplasmic Na(+) and Cl(-) concentrations of 100-200 mm, but whether these ions ever reach toxic concentrations that inhibit metabolism in the cytoplasm or cause death is unknown. Measurements of ion concentrations in the cytosol of various cell types for contrasting species and growth conditions are needed. Future work should also focus on the properties of the tonoplast that enable ion accumulation and prevent ion leakage, such as the special properties of ion transporters and of the lipids that determine membrane permeability.

Differential Solute Regulation in Leaf Blades of Various Ages in Salt-Sensitive Wheat and a Salt-Tolerant Wheat x Lophopyrum elongatum (Host) A. Love Amphiploid

Leaf blades of different ages from a salt-tolerant wheat x Lophopyrum elongatum (Host) A. Love (syn. Agropyron elongatum Host) amphiploid and its salt-sensitive wheat parent (Triticum aestivum L.cv Chinese Spring) were compared for their ionic relations, organic solute accumulation, and sap osmotic potential ([pi]sap). The plants were grown for 18 d in nonsaline (1.25 mM Na+) and salinized (200 mM NaCl) nutrient solutions. The response of leaf blades to NaCl salinity depended greatly on their age or position on the main stem. Na and proline levels were highest in the oldest leaf blade and progressively lower in younger ones. Glycine betaine and asparagine levels were highest in the youngest blade. The [pi]sap was similar for corresponding leaf blades of both genotypes, but contributions of various solutes to the difference in [pi]sap between blades from control and 200 mM NaCl treatments differed greatly. The NaCl-induced decline in [pi]sap of the youngest leaf blade of Chinese Spring was predominately due to the accumulation of Na and to a lesser extent asparagine; in the amphiploid, it was due to a combination of glycine betaine, K, Na, and asparagine. Proline contributed little in the youngest blade of either genotype. In the older blades Na was the major solute contributing to the decline in [pi]sap. Thus, the maintenance of low Na and high K levels and the accumulation of glycine betaine in the young leaf tissues contributed to the NaCl tolerance of the amphiploid. No such role was evident for proline.

Changes in physiological and morphological traits of roots and shoots of wheat in response to different depths of waterlogging

The growth reduction of wheat (Triticum aestivum L.) during and after waterlogging stress depends on the depth of water from the soil surface. In a pot experiment with 3-week-old plants, soil was waterlogged for 14 d at the surface, or at 100 or 200 mm below the surface, and pots were then drained to assess recovery. A fully drained treatment kept at field capacity served as control. During waterlogging, the relative growth rate of roots decreased more than that of shoots (by 6-27% for shoots, by 15-74% for roots), and plant growth was reduced proportionally as the water level was increased. Light-saturated net photosynthesis was reduced by 70-80% for the two most severe waterlogging treatments, but was little affected for plants in soil waterlogged at 200 mm below the surface. The number of adventitious roots formed per stem in plants grown in waterlogged soil increased up to 1.5 times, but the number of tillers per plant was reduced by 24-62%. The adventitious roots only penetrated 85-116 mm below the water level in all waterlogging treatments. Adventitious root porosity was enhanced up to 10-fold for plants grown in waterlogged soil, depending on water level and position along the roots. Porosity also increased in basal zones of roots above the water level when the younger tissues had penetrated the waterlogged zone. Fourteen days after draining the pots, growth rates of plants where the soil had been waterlogged at 200 mm below the surface had recovered, while those of plants in the more severely waterlogged treatments had only partially recovered. These findings show that the depth of waterlogging has a large impact on the response of wheat both during and after a waterlogging event so that assessment of recovery is essential in evaluating waterlogging tolerance in crops.

The potential for developing fodder plants for the salt-affected areas of southern and eastern Australia: an overview

This paper reviews the major issues that impact upon the development of improved fodder species for saline environments across temperate Australia. It describes past and present research that has been, or is being, undertaken towards improvements in salt tolerance in forage species within Australia in relation to the principal regions where salinity occurs. It includes a discussion on the mechanisms of salt tolerance in plants. An extensive list of known or potential salt-tolerant fodder species is provided and the key opportunities for advancement within each of the 4 major forage groups: grasses, legumes, herbs and shrubs are discussed. Constraints to developing new salt and waterlogging tolerant fodder species are identified. A number of recommendations are made for research that should ensure that Australian producers have access to a new array of productive fodder species suited to saline environments.

Neglecting legumes has compromised human health and sustainable food production

The United Nations declared 2016 as the International Year of Pulses (grain legumes) under the banner ‘nutritious seeds for a sustainable future’. A second green revolution is required to ensure food and nutritional security in the face of global climate change. Grain legumes provide an unparalleled solution to this problem because of their inherent capacity for symbiotic atmospheric nitrogen fixation, which provides economically sustainable advantages for farming. In addition, a legume-rich diet has health benefits for humans and livestock alike. However, grain legumes form only a minor part of most current human diets, and legume crops are greatly under-used. Food security and soil fertility could be significantly improved by greater grain legume usage and increased improvement of a range of grain legumes. The current lack of coordinated focus on grain legumes has compromised human health, nutritional security and sustainable food production.

Oxygen dynamics in submerged rice (Oryza sativa)

Complete submergence of plants prevents direct O(2) and CO(2) exchange with air. Underwater photosynthesis can result in marked diurnal changes in O(2) supply to submerged plants. Dynamics in pO(2) had not been measured directly for submerged rice (Oryza sativa), but in an earlier study, radial O(2) loss from roots showed an initial peak following shoot illumination. O(2) dynamics in shoots and roots of submerged rice were monitored during light and dark periods, using O(2) microelectrodes. Tissue sugar concentrations were also measured. On illumination of shoots of submerged rice, pO(2) increased rapidly and then declined slightly to a new quasi-steady state. An initial peak was evident first in the shoots and then in the roots, and was still observed when 20 mol m(-3) glucose was added to the medium to ensure substrate supply in roots. At the new quasi-steady state following illumination, sheath pO(2) was one order of magnitude higher than in darkness, enhancing also pO(2) in roots. The initial peak in pO(2) following illumination of submerged rice was likely to result from high initial rates of net photosynthesis, fuelled by CO(2) accumulated during the dark period. Nevertheless, since sugars decline with time in submerged rice, substrate limitation of respiration could also contribute to morning peaks in pO(2) after longer periods of submergence.

Enhanced formation of aerenchyma and induction of a barrier to radial oxygen loss in adventitious roots of Zea nicaraguensis contribute to its waterlogging tolerance as compared with maize (Zea mays ssp. mays)

Enhancement of oxygen transport from shoot to root tip by the formation of aerenchyma and also a barrier to radial oxygen loss (ROL) in roots is common in waterlogging-tolerant plants. Zea nicaraguensis (teosinte), a wild relative of maize (Zea mays ssp. mays), grows in waterlogged soils. We investigated the formation of aerenchyma and ROL barrier induction in roots of Z. nicaraguensis, in comparison with roots of maize (inbred line Mi29), in a pot soil system and in hydroponics. Furthermore, depositions of suberin in the exodermis/hypodermis and lignin in the epidermis of adventitious roots of Z. nicaraguensis and maize grown in aerated or stagnant deoxygenated nutrient solution were studied. Growth of maize was more adversely affected by low oxygen in the root zone (waterlogged soil or stagnant deoxygenated nutrient solution) compared with Z. nicaraguensis. In stagnant deoxygenated solution, Z. nicaraguensis was superior to maize in transporting oxygen from shoot base to root tip due to formation of larger aerenchyma and a stronger barrier to ROL in adventitious roots. The relationships between the ROL barrier formation and suberin and lignin depositions in roots are discussed. The ROL barrier, in addition to aerenchyma, would contribute to the waterlogging tolerance of Z. nicaraguensis.

Ion transport in seminal and adventitious roots of cereals during O2 deficiency

O(2) deficiency during soil waterlogging inhibits respiration in roots, resulting in severe energy deficits. Decreased root-to-shoot ratio and suboptimal functioning of the roots, result in nutrient deficiencies in the shoots. In N(2)-flushed nutrient solutions, wheat seminal roots cease growth, while newly formed adventitious roots develop aerenchyma, and grow, albeit to a restricted length. When reliant on an internal O(2) supply from the shoot, nutrient uptake by adventitious roots was inhibited less than in seminal roots. Epidermal and cortical cells are likely to receive sufficient O(2) for oxidative phosphorylation and ion transport. By contrast, stelar hypoxia-anoxia can develop so that H(+)-ATPases in the xylem parenchyma would be inhibited; the diminished H(+) gradients and depolarized membranes inhibit secondary energy-dependent ion transport and channel conductances. Thus, the presence of two transport steps, one in the epidermis and cortex to accumulate ions from the solution and another in the stele to load ions into the xylem, is important for understanding the inhibitory effects of root zone hypoxia on nutrient acquisition and xylem transport, as well as the regulation of delivery to the shoots of unwanted ions, such as Na(+). Improvement of waterlogging tolerance in wheat will require an increased capacity for root growth, and more efficient root functioning, when in anaerobic media.