http://nhjy.hzau.edu.cn/kech/ssyy/qysd/njsl/21.pdf

Potential biochemical indicators of salinity tolerance in plants

Accumulation of high levels of salts in the soil is characteristic of arid and semi-arid regions. Although different curative and management measures are being used to render salt-affected soils fit for agriculture, they are extremely expensive and do not provide permanent solutions to overcome the salinity problem. In contrast, a biotic approach for overcoming salinity stress has gained considerable recognition within the past few decades in view of the vast experimental evidence from what has happened in nature concerning the evolution of highly salt-tolerant ecotypes of different plant species, and also from the remarkable achievements that have been made in improveing different agronomic traits through artificial selection. Considerable improvements in salt tolerance of important crop species have been achieved in the past 2 decades using barley, rice, pearl millet, maize, sorghum, alfalfa, and many grass species. Such achievements relied solely on assessment of the phenotypic expression of t...

Breeding for Salinity Tolerance in Plants

Salt toxicity comprises osmotic and ionic components both of which can severely affect root and shoot growth. Uptake of Na+ across the plasma membrane is very fast resulting in physiological effects on extracellular as well as intracellular sites. Sodium reduces binding of Ca2+ to the plasma membrane, inhibits influx while increasing efflux of Ca2+, and depletes the internal stores of Ca2+ from endomembranes. These changes in the cell Ca2+ homeostasis are suggested here to be the primary responses to salt stress that are perceived by root cells. Salt would almost instantly reduce the amount of Ca2+ being transferred to the leaf cells, with Ca2+ activity dropping and Na+ activity rising in the apoplasm of leaf cells. This Ca2+ signal would be transported to leaves together with, if not preceding, the signal of limited water supply. Hormonal signals are likely to be secondary in nature and caused by the Na+-related disturbance of the root cell Ca2+ homeostasis. Ameliorative effects of supplemental Ca2+ on salt stress are exerted through preventing Na+-related changes in the cell Ca2+ homeostasis.

The role of calcium in salt toxicity

Agricultural losses caused by salinity are difficult to assess but estimated to be substantial and expected to increase with time. Secondary salinization of agricultural lands is particularly widespread in arid and semiarid environments where crop production requires irrigation chemes. At least 20% of all irrigated lands are salt-affected, with some estimates being as high as 50%. Whereas the world’s population continues to rise, the total land area under irrigation appears to have leveled off. The need for increased food production therefore needs to be met by increases in yield per land area. To reach this goal, genetic engineering of crop plants for enhanced salt tolerance will be a very important approach. In dry regions where fresh water becomes a scarce commodity, irrigation of moderately salt tolerant crops with brackish water is feasible. Transgenic lines of some crop species have been generated which can grow and develop at fairly high salinity levels in controlled environments. These transgenics must be tested vigorously for yield potential under field conditions.

Global Impact of Salinity and Agricultural Ecosystems

Publisher Summary The subject of salt tolerance has received a tremendous amount of attention during the past two decades. The concept of developing salt-tolerant plants, even to the degree that they can be grown with seawater, captures the imagination of both the scientific and the public sector. There is a wide range of salt tolerance in vascular plants. Several genera include species that are extreme halophytes and are ecologically competitive only under saline conditions. Crop species primarily are glycophytes, salt-sensitive, and grow well only under cultivated conditions. Domesticated species are provided optimum conditions to obtain high yields. There is currently a need to develop new direction and cohesive impetus in the area of salt-tolerant crops development. Realistic short- and long-range goals need to be established and continually pursued. The information that is known is extensive but fragmented. This chapter outlines some of the issues and strategies concerning selection and breeding for plant salt tolerance, highlights crucial advances in knowledge in recent years, and identifies some of the fundamental gaps in our understanding.

Adaptation of Plants to Salinity

Advances in and prospects for the development of salt tolerant crops are discussed. The genetic approach to the salinity problem is fairly new, but research has become quite active in a short span of time. Difficulties and opportunities are outlined. Salinity varies spatially, temporally, qualitatively, and quantitatively. In addition, the responses of plants to salt stress vary during their life cycle. Selection and breeding, including the use of wide crosses, are considered the best short-term approaches to the development of salt tolerant crops, but the new biotechnological and molecular biological techniques will make increasingly important contributions. Cooperation is called for among soil and water scientists, agronomists, plant physiologists and biochemists, cytologists, and plant geneticists, breeders, and biotechnologists. Given such cooperation and adequate support for these endeavors, the potential for increasing productivity in salt-affected areas can be realized.

Advances in salt tolerance

Effects of N source and media-N and P levels were examined on growth, N uptake, and N2 fixation ofAzolla pinnata withAnabaena azollae association (azolla) at two inoculum-P concentrations. Each expeiment was conducted for 7 days in a growth chamber using azolla at a predetermined inoculum-P concentration and the growth media containing a combination of four levels of P (0, 15, 75, and 200 μM) and three levels (0, 1, and 5 mM) of either15N-enriched NH4+ as ammonium sulfate or15N-enriched NO3− as potassium nitrate. Nitrogen uptake and N2 fixation were measured by15N isotopic dilution method. Tissue P and N, N uptake, and N2-fixation increased with increasing P concentration in the media regardless of the inoculum-P level of azolla. Increasing P concentration in the media increased growth of azolla at low inoculum P, but the effect on high inoculum-P azolla was either small or absen. High inoculum-P concentration resulted in increased growth, tissue-N and P concentrations, N uptake, and N2 fixation by azolla. Ammonium in the growth media caused larger increase in tissue-N and greater repression of N2 fixation than equimolar concentration of NO3−. In the presence of NH4+ or NO3−, in the growth media, N uptake by azolla exceeded the corresponding decrease in N2 fixation, resulting in an overall increase in tissue-N concentration. Phosphorus in the media tended to negate the inhibitory effect of NH4+ or NO3− on N2 fixation. A multiple regression model showed that the effect of tissue-N on N2 fixation was negative while that of tissue-P was positive. Therefore, a relative change in tissue-N and P appeared to regulate N2 fixation. Tissue-N and P had similar effects on relative growth rate of azolla also. Inoculum-P level of azolla was important in determining the response to media-P.

Interactive effects of exogenous combined nitrogen and phosphorus on growth and nitrogen fixation by azolla

Etude de l'induction et de la cinetique du transport du nitrate chez Azolla: effet de la lumiere et du taux de CO 2 ambiant

Effects of Light on NO3- Transport by Azolla pinnata

An isotopic dilution method that overcomes the drawbacks of commonly used methods for measuring N2 fixation by aquatic N‐fixers such as Azolla pinnata‐Anabaena azollae association (Azolla) is presented. The method was compared with 15N2 gas (while maintaining CO2) and the difference methods of measuring N2 fixation. The isotopic dilution method was used for two conditions: a. For 15N‐free growth medium, Azolla was pre‐enriched with 15N, and N2 fixation was determined by measuring the dilution of 15N in the tissue. b. For the growth medium containing N, N2 fixation was determined by providing 15N enriched ammonium sulfate in the growth medium and measuring 15N to 14N ratio in the tissue. An airtight chamber, necessary for 15N2 gas and acetylene reduction methods, was not representative of the growing environment of Azolla. Temperature in the airtight chamber was far from uniform and CO2 was rapidly depleted. The isotopic dilution method is simpler, relatively inexpensive, subject to fewer errors a...

D. W. Rains

Papers

Advances in salt tolerance

Interactive effects of exogenous combined nitrogen and phosphorus on growth and nitrogen fixation by azolla

Effects of Light on NO3- Transport by Azolla pinnata

Evaluation of isotopic dilution method for measuring N2 fixation in Azolla: Comparison with other methods 1