Control of sodium transport in durum wheat

Abstract

In many species, salt sensitivity is associated with the accumulation of sodium (Na(+)) in photosynthetic tissues. Na(+) uptake to leaves involves a series of transport steps and so far very few candidate genes have been implicated in the control of these processes. In this study, Na(+) transport was compared in two varieties of durum wheat (Triticum turgidum) L. subsp. durum known to differ in salt tolerance and Na(+) accumulation; the relatively salt tolerant landrace line 149 and the salt sensitive cultivar Tamaroi. Genetic studies indicated that these genotypes differed at two major loci controlling leaf blade Na(+) accumulation (R. Munns, G.J. Rebetzke, S. Husain, R.A. James, R.A. Hare [2003] Aust J Agric Res 54: 627-635). The physiological traits determined by these genetic differences were investigated using measurements of unidirectional (22)Na(+) transport and net Na(+) accumulation. The major differences in Na(+) transport between the genotypes were (1) the rate of transfer from the root to the shoot (xylem loading), which was much lower in the salt tolerant genotype, and (2) the capacity of the leaf sheath to extract and sequester Na(+) as it entered the leaf. The genotypes did not differ significantly in unidirectional root uptake of Na(+) and there was no evidence for recirculation of Na(+) from shoots to roots. It is likely that xylem loading and leaf sheath sequestration are separate genetic traits that interact to control leaf blade Na(+).

Figure 1.

Plant at the three-leaf stage that was used for most experiments. Leaves are clearly divided into sheath and blades by the ligule. The sheath of leaf 1 is about 40 mm in length. The stem of the seedling is comprised of leaf sheaths, the shoot apex, and leaf meristems. The coleoptile was dead at this stage and was removed before measurements.

Figure 2.

Increase in Na⁺ concentrations in the leaf sheaths (A) and leaf blades (B) of leaf 1 (squares) and leaf 2 (circles), and roots (C), of salt tolerant line 149 (black symbols) and salt sensitive Tamaroi (white symbols) during 10 d exposure to 50 mm NaCl and 2 mm CaCl₂ in half-strength modified Hoagland. D, Na⁺ content of leaves 1 and 2 after 10 d. Data represent mean ± sem (n = 4).

Figure 3.

Na⁺ content of leaves 1 (A–C) and 2 (D–F) after 5 d exposure to different concentrations of NaCl (all solutions contained half-strength modified Hoagland and CaCl₂ to maintain a Na⁺:Ca²⁺ ratio below 15:1). A, B, D, and E, Na⁺ concentrations in the leaf blade (white symbols) and sheath (black symbols). C and F, Ratio of sheath Na⁺ content to total leaf Na⁺ (blade + sheath). Data represent mean ± sem (n = 4).

Figure 4.

Leaf sheath and blade Na⁺ partitioning in four durum varieties and a bread wheat (C) differing in leaf blade Na⁺ (low versus high). A to E, Increase in Na⁺ concentrations in the leaf sheaths (black symbols) and leaf blades (white symbols) of leaf 2 during 10 d exposure to 50 mm NaCl and 2 mm CaCl₂ in half-strength modified Hoagland. F, Ratio of leaf sheath Na⁺ content to total leaf Na⁺ content in line 149 (black circles), Wollaroi (black diamonds), Janz (black squares), Tamaroi (white circles), and line 141 (white diamonds). Data represent mean ± sem (n = 4).

Figure 5.

Root Na⁺ uptake measured as root ²²Na⁺ accumulation in wheat seedlings pretreated for 5 d in 25 mm NaCl and 2 mm CaCl₂ (+ modified half-strength Hoagland). Root uptake measured over 0 to 10 min (A); 15 to 450 min (B); 0.3 to 24 h (C); 0.3 to 24 h with 8-h-dark period subtracted (D). Uptake rates were estimated by fitting linear regressions using least squares method; results are presented in Table II. Black symbols represent salt tolerant line 149; white symbols represent salt sensitive Tamaroi; mean ± sem (n = 4).

Figure 6.

Shoot Na⁺ uptake measured as shoot ²²Na⁺ accumulation per root FW in wheat seedlings pretreated for 5 d in 25 mm NaCl and 2 mm CaCl₂ in modified half-strength Hoagland. A, Shoot uptake over 4 h showing delay in initial uptake. B, Shoot uptake over 24 h. Black symbols represent salt tolerant line 149; white symbols represent salt sensitive Tamaroi; mean ± sem (n = 4).

Figure 7.

Efflux of ²²Na⁺ from excised roots expressed as decrease in tissue ²²Na⁺ after transfer to nonradioactive solution (black circles). Data were fitted with a double exponential equation (lines) to estimate plasma membrane and tonoplast efflux rates. Half-times for exchange of the cytoplasm (k1) and tonoplast (k2) in min⁻¹ were 0.060 and 0.0012 (line 149) and 0.0726 and 0.0007 (Tamaroi), respectively. Initial contents of the cytoplasm (Q_c) and vacuole (Q_v) in nmol g FW⁻¹ were 5,676 and 4,224 (line 149) and 1,494 and 4,956 (Tamaroi), respectively. Data are presented as root Na⁺ (calculated using the specific activity of the loading solution) and are equivalent to ²²Na⁺ remaining in the tissue. A, Salt tolerant line 149. B, Salt sensitive Tamaroi. Data represent mean ± sem (n = 5).

Figure 8.

Unidirectional uptake of ²²Na⁺ (A–C) and ⁸⁶Rb⁺ (D) into leaf blades and sheaths after 2 h root exposure to radioactively labeled solution (all solutions contained 25 mm NaCl and 2 mm CaCl₂ in half-strength modified Hoagland with K⁺ replaced with Rb⁺ in D). Uptake is expressed as the concentration of Na⁺ or Rb⁺ per leaf organ (nmol g FW⁻¹) and as the ratio of sheath to total leaf uptake (measured as nanomole content). A to C, Uptake of Na⁺ into salt tolerant line 149 (A and C) and salt sensitive Tamaroi (B and C). D, Uptake of Rb⁺ into roots, total shoot, and sheath and blade of leaf 1, and ratio of sheath to total leaf Rb⁺ in leaf 1. Data represent mean ± sem (n = 5 [A–C]; n = 6 [D]).

Figure 9.

Recirculation of Na⁺ from shoot to root measured as decrease in tissue ²²Na⁺ after transfer to nonradioactive solution. Plants were loaded with ²²Na⁺ for 24 h then transferred to unlabeled solution for up to 4 d. A and B, Salt tolerant line 149. C and D, Salt sensitive Tamaroi. Note that root data are divided by 10. Data are presented as tissue Na⁺ and are equivalent to tissue ²²Na⁺ content. Data represent mean ± sem (n = 8).