Pistachio Salinity Studies
University of California
Pistachio Salinity Studies

Overview

Louise Ferguson & Blake Sanden

What We Know: The Short of It

We know that roots do most of the work in protecting the plant from excessive uptake of salts, and filter out most of the salt in the soil while taking up water. But frankly, the fundamental processes governing the relationship between water and ion flow through roots are complex. Na, Cl and other ions do not move passively with the transpiration stream, neither are their movements entirely independent of it.

In addition to these root processes, we recognize that salt sensitivity is related to mechanisms within the plant tissue which minimize the effects of toxic ions.

We continue to pursue an understanding of these strategies which underlie a plant's ability to grow and thrive under saline conditions; the successful management of the pistachio and other tree crops depends on it.

What We Know: The Long of It

Salinity & the Soil

Salinity occurs through natural or human-induced processes that result in the accumulation of dissolved salts in the soil water. Salinity in soil is produced two ways:

  • The weathering of parent materials containing soluble salts and from deposition of oceanic salt carried in rain. (primary salinity)
  • Human activities that change the water balance of the soil between water applied (irrigation or rainfall) and water used by crops (transpiration). This is most common for soils irrigated with salt-rich water or with poor drainage. (secondary salinization) (Ghassemi et al., 1995)

When Applied Research is Too Successful...

Dr. Louise Ferguson discusses the research and  challenges ahead:

Developing Pistachios under Saline Conditions (8-minute video)

The Terminology of Salinity

Ions

Salinity in based on the presence of charged ions, which can have either a positive charge (cations) or a negative charge (anions). These ions can be toxic to plants, depending on the plant and the concentration. Milligrams per liter (mg/L) is the typical unit of measure for ions. Examples of common ions affecting salinity:

cations: calcium (Ca++), sodium (Na+), magnesium (Mg++)
anions: chloride (Cl-), bicarbonate (HCO3)
Monovalent ions have an ionic charge of 1; divalent ions have an ionic charge of 2.

Boron (B) can be toxic to plants, but does not contribute to salinity. Boron is measured in parts per million (ppm).

Electrical Conductivity (EC)

The ions bind together in soil, based on their opposing charges, but these bonds are broken in water. The resulting presence of charged ions controls the electrical conductivity of the water. EC is a measure used to determine total salinity of water. Deciseimens/meter (dS/m) is the unit of measure for EC. ECe= EC for a soil-water . ECw = EC for irrigation water.

SAR

SAR is measured in irrigation water to evaluate sodium levels, called sodicity. It is the ratio of soluble cations: Na: (Ca+ Mg)1/ and is more useful than sodium values alone, because more sodium can be tolerated when calcium is present.

1/The formula: Na+/sqrt(Ca+++Mg++)/2

Sodicity

Sodicity is a secondary result of salinity where leaching through either natural or human-induced processes has washed soluble salts into the subsoil, and left sodium (Na+) bound to the particles of the topsoil. Sodicity typically occurs in clay soils. The clay soil complex contains negatively charged particles which are held together by binding to divalent cations. Sodicity rises when sodium, a monovalent cation lacking in these binding properties, displaces the divalent cations. This creates a situation in which the clay particles disperse, degrading the soil structure. Sodicity is defined in terms of the threshold ESP that causes this degradation. A sodic soil has an ESP greater than 15.

Over time, leaching of the soil, either with rainwater or with irrigation, washes the dispersed clay particles deeper into the profile where they block pores and hinder infiltration of water. Thus, sodic soil is slow to drain, and is readily waterlogged. Saline/sodic soils are widespread in arid and semi-arid lands of the world. Water infiltration is slow, and salts derived from irrigation accumulate in saturated zones in the subsoil.

The Effect of Salinity on Plants

Salts in the soil water may inhibit plant growth for two reasons:

  • The salts reduce the ability of the plant to take up water. When salts in the soil solution inhibit water uptake by the roots, the effect is the same as drought stress. That is, a decrease in stomatal conductance and resulting decrease in photosynthesis. This is called the osmotic effect of salinity.
  • The salts that enter the plant in the transpiration stream ultimately injure cells in the transpiring leaves. This is called the ion-specific effect of salinity.

Salts themselves do not build up in the growing tissues at concentrations that inhibit growth: new leaves are fed largely by the phloem from which salt is effectively excluded, and young cells can accommodate the salt that arrives in the transpiration stream within their storage vacuoles. So, the salt taken up by the plant does not directly inhibit the growth of new leaves. These salts, however, enhance the senescence of old leaves. The continual transport of salt into leaves eventually results in high Na and Cl concentrations, and they die.

Plants have mechanisms for coping with the delivery of salt to the leaves. The most important of these is the compartmentalization of salts in the vacuoles of the leaf cells. This strategy allows plants to minimize or delay the toxic effects of ions on cytoplasmic processes. The rate at which leaves die is the rate at which salts accumulate to toxic levels, so plants that have a poor ability to sequester salt in cell vacuoles, have a greater rate of leaf death. If the rate of leaf death exceeds the rate at which new leaves are produced, then the plant may not survive. For perennial species like trees, the plant may enter a state of dormancy, and survive the stress.

In his experiments with wheat, by Rana Munns demonstrated that two effects occur sequentially, giving rise to a two-phase growth response to salinity (1995). 'Phase 1' of growth reduction was quickly apparent, probably due to the salt outside the roots (the osmotic effect). The second phase of growth reduction, which takes time to develop, resulted from internal injury. This suggests that the 'Phase 2' response is due to genotypic differences in coping with the Na or Cl ions in the soil, as distinct from osmotic stress. What distinguishes a salt-sensitive plant from a more tolerant one is the inability to prevent salt from reaching toxic levels in the transpiring leaves, which takes time.

To Survive, the Plant Must Exclude Salt

A plant can only grow or survive in a saline soil if it can both continue to take up water and exclude a large proportion of the salt in the soil solution. Research based on the
study of plant water use efficiency indicate that the plant retains only 2% of the water transpired. In order to prevent the salt concentration in the plant from increasing above that in the soil, then only 2% of the salt should be allowed into the shoot, i.e. 98% should be excluded.

A soil salinity of 100 mM NaCl or 10 dS m-1 is about as high as most crops will tolerate without a significant reduction in growth or yield, and a concentration of 100 mM NaCl in leaf tissueis about as high as is desirable because it will include some old leaves with much higher salt concentrations, as well as younger leaves with lower concentrations.

In Summary

We know that roots do most of the work in protecting the plant from excessive uptake of salts, and filter out most of the salt in the soil while taking up water. But frankly, the fundamental processes governing the relationship between water and ion flow through roots are complex. Na, Cl and other ions do not move passively with the transpiration stream, neither are their movements entirely independent of it.

In addition to these root processes, we recognize that salt sensitivity is related to mechanisms within the plant tissue which minimize the effects of toxic ions.

We continue to pursue an understanding of these strategies which underlie a plant's ability to grow and thrive in various soil conditions and irrigation regimes--the successful management of the pistachio and other tree crops in California depends on it.

Top of page

References

  • This review in greater detail: Salinity_in_pistachio (pdf)
  • Garcia A, Rizzo C.A., Ud-Din J., Bartos S.L., Senadhira D., Flowers T.J., Yeo A.R. 1997. Sodium and potassium transport to the xylem are inherited independently in rice, and the mechanism of sodium:potassium selectivity differs between rice and wheat. Plant Cell Environ. 20:1167-1174.
  • Ghassemi F.,  Jakeman A.J., Nix H.A. 1995. Salinization of land and water resources: Human causes, extent, management and case studies. UNSW Press, Sydney, Australia, and CAB International, Wallingford, UK.
  • Greenway H., Munns R. 1980. Mechanisms of salt tolerance in nonhalophytes. Annu. Rev. Plant Physiol. 31:149-190.
  • Munns, R. 1985. Na+, K+ and C1- Xylem sap flowing to shoots of NaCl-treated barley. J. Exp. Bot. 36:1032-1042.
  • Munns R. 1993. Physiological processes limiting plant growth in saline soil: some dogmas and hypotheses. Plant Cell Environ. 16:15-24.
  • Munns R (2002) Comparative physiology of salt and water stress. Plant Cell Environ. 25, 239-250.
  • Munns. R., Schachtman D.P. Condon A.G. 1995). The significance of a two-phase growth response to salinity in wheat and barley. Aust. J. Plant Physiol. 22:561-569.
  • Munns, R., Husain, S..Rivelli, A.R James, R.A. Condon, A.G. (Tony) Lindsay, M.P. Lagudah, E.S. Schachtman, D.P. Hare R.A. 2002. Avenues for increasing salt tolerance of crops, and the role of physiologically based selection traits. Plant and Soil 247:93-105).
  • Rengasamy P 2002. Transient salinity and subsoil constraints to dryland farming in Australian sodic soils: an overview. Aust. J. Exp. Agric. 42:351-361.
Page Last Updated: August 22, 2011
Webmaster Email: jmzalom@ucdavis.edu