Excreting and non-excreting grasses exhibit different salt resistance strategies

Abstract

The combination of traits that makes a plant successful under saline conditions varies with the type of plant and its interaction with the environmental conditions. Knowledge about the contribution of these traits towards salt resistance in grasses has great potential for improving the salt resistance of conventional crops. We attempted to identify differential adaptive response patterns of salt-excreting versus non-excreting grasses. More specifically, we studied the growth, osmotic, ionic and nutrient (carbon/nitrogen) relations of two salt-excreting (Aeluropus lagopoides and Sporobolus tremulus) and two non-excreting (Paspalum paspalodes and Paspalidium geminatum) perennial C4 grasses under non-saline and saline (0, 200 and 400 mM NaCl) conditions. Growth and relative growth rate decreased under saline conditions in the order P. geminatum > S. tremulus = A. lagopoides > P. paspalodes. The root-to-shoot biomass allocation was unaffected in salt-excreting grasses, increased in P. paspalodes but decreased in P. geminatum. Salt-excreting grasses had a higher shoot/root Na(+) ratio than non-excreting grasses. K(+), Ca(2+) and Mg(2+) homoeostasis remained undisturbed among test grasses possibly through improved ion selectivity with rising substrate salinity. Salt-excreting grasses increased leaf succulence, decreased ψs and xylem pressure potential, and accumulated proline and glycinebetaine with increasing salinity. Higher salt resistance of P. paspalodes could be attributed to lower Na(+) uptake, higher nitrogen-use efficiency and higher water-use efficiency among the test species. However, P. geminatum was unable to cope with salt-induced physiological drought. More information is required to adequately document the differential strategies of salt resistance in salt-excreting and non-excreting grasses.

Keywords: C–N balance; Na+ flux; compatible solutes; halophytic grasses; ion homoeostasis; nitrogen-use efficiency..

Figure 1.

Relative growth of (A) A. lagopoides, (B) S. tremulus, (C) P. paspalodes and (D) P. geminatum grown under increasing salinity treatments (0, 200 and 400 mM NaCl).

Figure 2.

Percent change in the fresh weight (FW) and dry weight (DW) at shoot, root and whole-plant levels of the salt-excreting (A. lagopoides, S. tremulus) and non-excreting (P. paspalodes, P. geminatum) grasses grown under increasing salinity treatments (0, 200 and 400 mM NaCl). Different letters represent significant differences among species at P < 0.05 (Bonferroni test).

Figure 3.

Percent change in the length and RGR of aboveground and belowground parts of the salt-excreting (A. lagopoides, S. tremulus) and non-excreting (P. paspalodes, P. geminatum) grasses grown under increasing salinity treatments (0, 200 and 400 mM NaCl). Different letters represent significant differences among species at P < 0.05 (Bonferroni test).

Figure 4.

Osmotic potential (ψ_s, MPa) and XPP (MPa) of the salt-excreting (A. lagopoides, S. tremulus) and non-excreting (P. paspalodes, P. geminatum) grasses grown under increasing salinity treatments (0, 200 and 400 mM NaCl). Bars are means (±SE) at each salinity level (n = 3). Different letters represent significant differences among species at P < 0.05 (Bonferroni test).

Figure 5.

Cation (Na⁺, K⁺, Ca²⁺ and Mg²⁺) content (mmol kg⁻¹ dry weight) in shoot and root of the salt-excreting (A. lagopoides, S. tremulus) and non-excreting (P. paspalodes, P. geminatum) grasses grown under increasing salinity treatments (0, 200 and 400 mM NaCl). Symbols indicate means (±SE) at each salinity level (n = 3). Asterisks (*, ** and ***) represent significant differences between cation contents at P < 0.05, 0.01 and 0.001 respectively; NS indicates non-significant differences among salinity treatments by one-way ANOVA.

Figure 6.

Shoot/root ratios of Na⁺ and K⁺ and Na⁺/K⁺ ratios of the salt-excreting (A. lagopoides, S. tremulus) and non-excreting (P. paspalodes, P. geminatum) grasses grown under increasing salinity treatments (0, 200 and 400 mM NaCl). Symbols indicate means (±SE) at each salinity level (n = 3). Asterisks (** and ***) represent significant differences among cation contents at P < 0.01 and 0.001 respectively; NS indicates non-significant differences among salinity treatment by one-way ANOVA.

Figure 7.

Total soluble sugars (TSS), proline (Pro) and glycinebetaine (GB) in mmol kg⁻¹ dry weight, C/N ratio in shoots of the salt-excreting (A. lagopoides, S. tremulus) and non-excreting (P. paspalodes, P. geminatum) grasses grown under increasing salinity treatments (0, 200 and 400 mM NaCl). Bars are means (±SE) at each salinity level (n = 3). Different letters represent significant differences among species at P < 0.05 (Bonferroni test).

Figure 8.

Relationship between C/N ratios and (A) nitrogen (% dry weight) and (B) NUE (g dry weight g⁻¹ shoot N) of the salt-excreting (A. lagopoides = squares; S. tremulus = triangles) and non-excreting (P. paspalodes = circles; P. geminatum = diamonds) grasses grown under increasing NaCl treatments (0 mM = red; 200 mM = green; 400 mM = blue).