Sodium Metabolism in Plants

Effects on C4 Species

Sodium was reported to be necessary for the growth of some halophyte species (32,141–143); notably, bladder saltbush (Atriplex vesicaria Heward, Chenopodiaceae). Sodium specifically stimulates the growth of Joseph’s coat (Amaranthus tricolor L., Amaranthaceae) (144), possibly by an effect on nitrate uptake and assimilation (145,146). Sodium appears to be essential for the C4 grasses such as proso millet (Panicum miliaceum L.), kleingrass (P. coloratum L.) and saltgrass (Distichlis spicata Greene) (20,147,148) and has been found to stimulate the growth of grasses such as marsh grass (Sporobolus virginicus Kunth) and alkali sacaton (S. airoides Torr.) in some studies (149–151). Subsequent work showed that this requirement was linked with the C4 pathway of photosynthesis (141,142,152–157) and specifically with pyruvate–Na< co-transport into mesophyll chloroplasts (158–163), a step that is necessary for the regeneration of phosphoenolpyruvate and the fixation of CO₂. Not all C4 plants require sodium for photosynthesis or grow better when it is present (161). The C4 species of the NADP<-malic enzyme (ME) type have a different co-transport system for pyruvate that uses protons rather than sodium ions.

In sorghum species (Sorghum L.), there is a specific effect of higher concentrations of sodium (and low concentrations of lithium) on the kinase that regulates the activity of phosphoenolpyruvate (PEP) carboxylase, the primary carbon-fixing enzyme in C4 and crassulacean acid metabolism (CAM) plants (164). The kinase also seems to be linked to the responses of PEP carboxylase to nitrate in C3 and C4 Alternanthera Forssk. species (165). There was a report that sodium was required for CAM in Chandlier plant (Kalanchoe tubiflora Hamet) (166), but little further work has been published on this aspect, and no relationship occurs between CAM and halophytism (167). On the other hand, salinity and other stresses are known to induce CAM photosynthesis in the facultative CAM species, ice plant (Mesembryanthemum crystallinum L., Aizoaceae) (168,169).

Toxicity of Sodium

Application of sodium to recently transplanted seedlings or cuttings runs the risk of uncontrolled bypass flow of water and sodium to the shoots through damaged roots. Hence sodium is often applied in the laboratory, greenhouse, or growth-chamber experiments after the plants have become established in the growing medium. For such situations, Munns (24,25,33) has described a series of events that occurs in most plants. At its simplest, these effects start with the initial osmotic stress caused by making the external (medium) water potential more negative. Subsequently, external inorganic ions are taken up and organic solutes synthesized for osmotic adjustment of the plant cells. Failure to properly control the influx of inorganic salts results in the direct toxicity of high intracellular (particularly cytoplasmic) concentrations of ions or to osmotic imbalances within tissues such as the accumulation of salts in the apoplast of species like rice (12,13). Although this description has been challenged in detail regarding the implications for stress-resistance breeding (11) and the point at which specific ion effects become evident (170), it is still the best model of physiological responses to applied salinity. The same concepts, with modifications of timescale and phenology, can be useful in the crop field and in natural environments, although in both cases the severity of salinity (and other stresses) is subject to fluctuations that the laboratory experiment is designed to avoid.

Important questions are what, when, and why salts are toxic to plants. The question of whether sodium or chloride is a toxic ion is still difficult to answer in most plants, though of course, this action is not important if the problem is primarily osmotic. The question of when inorganic salts (mainly sodium chloride) become toxic is a little easier to answer, at least in theory. Accumulation of salts is required for osmotic adjustment, as cellular dehydration may make a contribution, but generally perturbs metabolism by changing the concentrations of critical intermediates and signaling molecules in the cytoplasm. If salts accumulate much in excess of the concentrations needed for osmotic adjustment of plant cells, it is likely that they will become inhibitory to metabolism and growth, although this may depend on the intracellular location of the salts (see below). The cytoplasm of eukaryotic cells has evolved to work best within a limited range of concentrations of solutes, and particularly of certain ions. Exceeding these ranges for inorganic (and some organic) ions (including potassium) creates problems for macromolecular structures, and hence enzyme activities and nucleic acid metabolism (171,172).