A. R. Yeo

Breeding for Salinity Resistance in Crop Plants: Where Next?

Soil salinity is widely reported to be a major agricultural problem, particularly in irrigated agriculture, and research on salinity in plants has produced a vast literature. However, there are only a handful of instances where cultivars have been developed which are resistant to saline soils. Reasons for the lack of success in developing salt-resistant genotypes, and for the low impact that plant physiological research has made, are explored. We conclude that soil salinity has not yet become a sufficient agricultural problem, other than on a local scale, to make salt resistance a high priority objective for plant breeders. The limited success of simple selection, where this has been practised in breeding programs, can be accounted for by the fact that research has consistently shown salt resistance is a complex character controlled by a number of genes or groups of genes and involves a number of component traits which are likely to be quantitative in nature. We also conclude that the results of physiological research have been poorly marketed by physiologists and, understandably, have failed to impress plant breeders. We anticipate that the importance of salinity as a breeding objective will increase in the future. Our assessment of reports of the degradation of irrigation systems, together with projections of the future demands of irrigated agriculture, is that enhancing the salt resistance of at least some crops will be necessary. Salinity resistance will both help provide stability of yield in subsistence agriculture and, through moderating inputs, help limit salinisation in irrigation systems with inadequate drainage. It is emphasised that plant improvement and drainage engineering should be seen as partners and not alternatives. We conclude with a personal view of one way forward for developing salt-resistant genotypes, through the pyramiding of physiological characters.

The effects of selection for sodium transport and of selection for agronomic characteristics upon salt resistance in rice (Oryza sativa L.)

A multiple cross was constructed with the aim of combining component traits for the complex salinity resistance character. The aim was to combine donors for physiological traits with the agronomically desirable semidwarf/intermediate plant type and with the overall salinity resistance of the traditional tall land races. We report a study of selection strategies in the resulting breeding population. The effects of early selection for agronomic traits and early selection for low sodium transport were compared with a control population in which minimal selection was practised. Conventional selection for agronomic characters at early generations selected against low sodium-transporting (and thus potentially salt-tolerant) genotypes. In contrast, mild early selection for low sodium transport enriched the population in potentially salt-resistant genotypes but did not select against agronomic (semi-dwarf/intermediate) genotypes. It is concluded that selection for agronomic traits should be made after selection for salt resistance and, ideally, should be delayed until the population has reached near-homozygosity.

Some effects of sodium chloride on cells of rice cultured in vitro

Rice (Oryza sativa L.) varieties differ in the salt resistance of their cells in vivo; that is, different degrees of cellular damage are associated with similar concentrations of NaCl in the leaf tissue in different rice varieties (Yeo and Flowers, Physiol. Plant., 59 (1983) 189). Cells of two varieties differing in this respect were cultured in a liquid medium and the rate of oxygen uptake of cells which had been grown for up to 5 weeks in NaCl and in concentrations as high as 600 mol m−3 measured by conventional manometry. Sodium chloride at up to 200 mol m−3 had little effect on oxygen uptake by either variety although cells of one variety were eventually killed by 600 mol m−3 NaCl. The results are discussed in relation to the value of tissue-culture in screening plants for resistance to salinity and to the effects of salinity on cells in the intact plant.

Additive and antagonistic effects of ozone and salinity on the growth, ion contents and gas exchange of five varieties of rice (Oryza sativa L.)

Five varieties of rice (Oryza sativa L.) of varying salinity resistance were grown in non-saline and in saline conditions, with and without a repeated exposure to ozone at a concentration of 83 nmol mol(-1) giving an AOT40 (cumulative exposure above 40 nmol mol(-1)) of 3600 nmol mol(-1) h. Salinity caused a substantial reduction in shoot and root dry weight in all varieties, but the effect on root growth was proportionately less than on shoot growth. Ozone reduced root dry weight but the treatment used did not significantly affect shoot dry weight. Both salinity and ozone reduced plant height. The potassium concentration in the leaves of all five varieties was reduced by salinity, and by ozone in both saline and non-saline treatments. Ozone reduced the sodium concentration in plants grown at 50 mM NaCl but had no effect upon the chloride concentration. Carbon dioxide assimilation, transpiration and stomatal conductance were all reduced by salinity and by ozone and there was close quantitative similarity between the effects of ozone and/or salinity upon assimilation, stomatal conductance and transpiration. There were some antagonistic effects but there were additive effects of salinity and of ozone on root dry weight, plant height, shoot potassium concentration, photosynthesis, transpiration and stomatal conductance. The possible basis of the additive effects of salinity and ozone on gas exchange and mineral uptake are discussed.

Uptake and localisation of rubidium in the halophyte Suaeda maritima

Plants of Suaeda maritima grown in culture solution containing rubidium chloride accumulate rubidium, which can be estimated by flame emission spectrophotometry. This accumulation of rubidium is correlated with the appearance, under the electron microscope, of electron dense deposits in the cells. These deposits are restricted to the vacuoles and only seen after growing plants in the presence of rubidium. The procedures necessary to visualise the rubidium are described and the significance of the localisation discussed in relation to the distribution of ions in the cells of halophytes.

Solution culture for screening rice varieties for sodicity tolerance

Sodic soils are widespread, especially in the Indo–Gangetic plain. Amelioration with gypsum is effective, especially when combined with growing a crop of rice. However, it has proved difficult to generate new varieties of sodic-tolerant rice, because of the difficulties of screening – other than in the field, where spatial variation of sodicity is notoriously high. Growing plants at high pH in solution culture in a controlled environment is problematic, because of the effects of pH on nutrient availability. This study evaluated a system of growing rice in a nutrient solution with added Tris but without minor nutrients, which were supplied in a foliar spray. Performance was evaluated from the change in weight and in appearance and correlated with performance in soil. Growing rice in the presence of Tris (4 mM, pH 6) provides a means of evaluating genotypic differences in response to sodicity.

Salt Tolerance in the Halophyte Suaeda maritima (L.) Dum.: Interaction between Aluminium and Salinity

Growth of Suaeda maritima was stimulated by low aluminium concentrations in saline solution culture with an increase in the number and extent of lateral roots. Under non-saline conditions the same Al concentrations inhibited growth and led to an abnormal lateral root initiation. Increasing the level of Al led to growth inhibition under both culture conditions. Salinity reduced the uptake of A1 into plant tissue, and there was no evidence that A1 was tolerated internally. Although short-term 32P influx was increased by A1 there were no long-term effects of significance on levels of Na, K, Ca nor P in the shoots. The results are discussed in relation to proposed mechanisms of A1 toxicity and the interaction between A1 and salt toxicities. An explanation is proposed for both stimulatory and inhibitory effects of A1 as a quantitative expression of a single primary effect upon the root system.

Rice cultivation in saline soils

Lines from a cross between two accessions of rice (IR4630-22-2-5-1-3 and IR15324-117-3-2-2) were subjected to salinity in tanks which were filled with sand and flooded with quarter strength Hoagland solution containing 50 mM NaCI (ECiw 6 dS.m−1). Salinity generally reduced vigor, stunted growth, reduced straw and grain weight, induced panicle sterility with large variations between lines. From these studies, ten lines have been identified which possess the potential of satisfactory yield under saline conditions.

Transport from Root to Shoot

The movement of solutes into and across the root described in the last chapter suggested little interaction between the flows of solute and solvent. From the root to the leaves, however, the flow of water and solutes is closely coupled. The xylem has evolved to transport the large volumes of water needed to replace transpirational losses and presents a system of large capacity and speed which provides a ‘free ride’ for anything that will dissolve in water. The xylem, however, is a one-way pathway from the roots to the leaves, and its capacity and exact destination are determined by how fast particular parts of the shoot are losing water to the atmosphere, not by the requirements of the shoot for solutes. As a system for solute transport this does have shortcomings.