Physiological responses of Zea mays (L.) roots to NaCl stress

Abstract

The effects of two sodium chloride (NaCI) stress protocols, salt acclimation (sa) and salt shock (ss), on growth, water relations, and the contribution of inorganic and organic solutes to osmotic adjustment were studied in the apical zones of roots of intact Zea mays (L.) seedlings. Experiments were conducted on etiolated seedlings grown in a standard nutrient solution to which NaCI was added. At concentrations of 100 mM NaCI, the sa stress did not inhibit root elongation, whereas the ss stress initially inhibited root growth. However, after 12 h, root elongation of salinized roots attained a rate of 2 mm/h, similar to control roots. Seedlings treated with 1 μM fluridone to block synthesis of abscisic acid (ABA) resulted in reduced root elongation in control and NaCI-treated roots. Exogenous ABA at concentrations above 2.0 μM inhibited root elongation. The inhibitory effects of ABA on root growth were stronger than those effects due to 100 mM NaCI. Despite the gradual and sharp declines in root osmotic potential (ψπ) and water potential (ψw) in the 0 to 3 and 3 to 10 mm root zone, root turgor potential (ψp) remained constant in both salt stress protocols. The early- and long-term changes in ψπ in the 0 to 3 mm zone were associated mainly with the accumulation of Na+, K+, Cl-, and sucrose. Their contribution to ψπ (bar) was about -2.1, -4.2, -3.6, and -1.0 bar, respectively. In contrast, in the 3 to 10 mm zone, the contribution to ψπ (bar) was -2.2 (Cl-), -2.0 (Na+), -2.8 (K+), and -1.0 (glucose and fructose). The total contribution of inorganic and organic osmolytes to ψπ ranged between 70 to 90% of the estimated root ψπ. It appears that sucrose and proline are mainly compartmented within the cytoplasm (0 to 3 mm zone), whereas glucose and fructose appear to be compartmented primarily in the vacuole (3 to 10 mm zone).