Interaction with Other Nutrients
The role of potassium in generating turgor can be fulfilled by sodium and to some extent, by calcium
and magnesium, particularly at low concentrations of potassium
(38-41). The estimated extent
to which potassium can be replaced by sodium in the edible portions of crops varies from 1% in
wheat (Triticum aestivum L.) and rice (Oryza sativa L.) to 90% in red beet (Beta vulgaris L.)
(42).
The interactions among cations in terms of uptake and accumulation rates are complex. The ability
of low concentrations (
-500 �M) of sodium to stimulate potassium uptake when potassium concentrations
are low does not appear to be of importance outside the laboratory
(43). The extensive
literature on the physiology and genetics of potassium-sodium interactions, especially related to
membrane transport, is beyond the scope of this section and has been reviewed comprehensively by
other researchers
(44-50). Some evidence suggests that shoot sodium concentrations (altered by
spraying sodium onto leaves) affects the transport of potassium to the shoots, or at least leaf potassium
concentrations
(51).
Interactions between sodium and other nutrients have been observed
(52-54). Excessive
sodium inhibits the uptake of potassium
(43,55), calcium
(56-67), and magnesium
(53). A deficiency
of calcium, or a high sodium/calcium ratio, results in enhanced sodium uptake. For most species, this
calcium requirement is satisfied at a few moles per cubic meter of calcium in solution and is rarely
detected in soils. It can become a problem in hydroponics if the calcium concentration in the nutrient
solution is low, and no extra calcium is added. Maintaining low sodium/calcium ratios (as a
general rule, not
+10:1 for dicots and 20:1 for monocots) will prevent this problem. Similar considerations
apply to silicon
(68-75).
Nitrogen nutrition modifies the effects of sodium on Chenopodiaceae such as goosefoot
(Suaeda salsa L.)
(76). Plants of this family accumulate large amounts of nitrogen in the form of
nitrate and glycinebetaine
(30,77-80). The interactions among salinity, nitrogen, and sulfur nutrition
have been investigated in relation to the accumulation of different organic solutes in the halophytic
grasses of the genus Spartina
(81-83). Generally, adequate nitrogen nutrition is necessary to
minimize the inhibition of growth caused by excess salt, but with some differences between the
ammonium- and nitrate-fed plants
(84-94).
Salinity may interfere with nitrogen metabolism in a number of ways, starting with the uptake
of nitrate and ammonium
(87,95). Under nonsaline conditions, nitrate is an important vacuolar
solute in many plants, including members of the Chenopodiaceae and Gramineae. Under saline
conditions, much of the vacuolar nitrate may be replaced by chloride, possibly releasing some
nitrate-nitrogen for plant growth and metabolism. On the other hand, salinity can result in the synthesis
of large amounts of nitrogen-containing compatible solutes such as glycinebetaine (and in a
few cases, proline) and lead to the accumulation of amides and polyamines. Changes may occur at
the site of nitrate reduction from the leaves to the roots, and hence changes in nitrate transport to
the shoots. Since the latter is linked to potassium recirculation
(96,97) and long-range signaling
mechanisms controlling growth and resource allocation
(98), the implications of such changes are
wide ranging. The activity of nitrate reductase may also be affected by salinity. Although toxic ions
can affect all aspects of nitrogen metabolism, little evidence suggests that nitrogen supply directly
limits the growth of plants under conditions of moderate salinities
(99).
In comparison with the other nutrients, the interactions between salinity and phosphorus have
received relatively little attention
(100) and depend to a large extent on the substrate
(52,53). When
investigating interactions between salinity and nutrients, one has to be aware of the effects of the
substrate, the environment, the genotype-nutrient balances, the nutrient and salt concentrations, the
time of exposure to salinity, and the phenology of the plant. These interactions are complex and cannot
be comprehended adequately from one or two experiments.
