Annals of Botany

Salt strength and pulling power put brakes on germination

Salinity is an increasingly important factor in world agriculture because of the higher frequency of flooding of coastal land with seawater and because of side-effects of extensive irrigation. Gaining a fuller understanding of the range of salt tolerance mechanisms is thus a matter of urgency as we seek traits that may be transferred into crop species. One approach to the problem is that adopted by Laura Sosa and colleagues at San Luis and Rio Cuarto, Argentina (pp. 261–267). They have studied the effects of dissolved salts on the germination of Prosopis strombulifera, a spiny shrub found in salinized areas in Central Argentina. Although the plant is salt tolerant, germination is inhibited by higher concentrations of salt and the experiments were designed to ascertain the effects of different salts and of osmotica, in order to separate the effects of specific ions from osmotic effects. The first point to note is that at similar osmotic potentials, different salts had different effects on germination. Thus, inhibition of germination by salts is not due to osmotic effects alone; there are some ion-specific events too. Of the salts tested, Na₂SO₄ was the most inhibitory to germination but this effect was lessened by the addition of NaCl (even the latter was itself somewhat inhibitory). Similarly, KCl ameliorated the effects of K₂SO₄. Comparison of cations revealed that K⁺ was more inhibitory than Na⁺. Further, organic osmotica over the same range of osmotic potential as for the salts had less inhibitory effect, at a specific osmotic potential, than the salts. The effects on germination are thus composed of at least two components, one based on osmotic effects and the other(s) on the effects of specific anions and cations. The challenge now is to identify the underlying mechanisms, an important step along the route to transfer of specific genes to crop plants.