Abscisic acid has contrasting effects on salt excretion and polyamine concentrations of an inland and a coastal population of the Mediterranean xero-halophyte species Atriplex halimus

Abstract

Background and aims: Different populations of the Mediterranean xerohalophyte species Atriplex halimus exhibit different levels of resistance to salt and osmotic stress depending on the nature of the osmocompatible solute they accumulate. There is, however, no conclusive description of the involvement of abscisic acid (ABA) in the plant response to NaCl or osmotic stress in this species.

Methods: Seedlings issued from an inland water-stress-resistant population (Sbikha) and from a coastal salt-resistant one (Monastir) were exposed in nutrient solutions to NaCl (40 or 160 mm) or to 15 % PEG for 1 d and 10 d in the presence or absence of 50 microm ABA.

Key results: Plants from Sbikha accumulated higher amounts of ABA in response to osmotic stress than those of Monastir, while an opposite trend was recorded for NaCl exposure. Exogenous ABA improved osmotic stress resistance in Monastir through an improvement in the efficiency of stomatal conductance regulation. It also improved NaCl resistance in Sbikha through an increase in sodium excretion through the external bladders. It is suggested that polyamines (spermidine and spermine) are involved in the salt excretion process and that ABA contributes to polyamine synthesis as well as to the conversion from the bound and conjugated to the free soluble forms of polyamine. Proline accumulated in response to osmotic stress and slightly increased in response to ABA treatment while glycinebetaine accumulated in response to salinity and was not influenced by ABA.

Conclusions: It is concluded that ABA is involved in both salt and osmotic stress resistance in the xerohalophyte species Atriplex halimus but that it acts on different physiological cues in response to those distinct environmental constraints.

Fig. 1.

Shoot and root dry weights in seedlings of Atriplex halimus issued from a coastal saline site (Monastir) or an inland semi-arid area (Sbikha) and exposed for 10 d to control nutrient solution or to nutrient solution containing 40 or 160 mm NaCl or 15 % PEG in the absence or presence of 50 µm ABA in the nutrient solution. Initial dry weights were 2·53 ± 0·13 g and 1·97 ± 0·11 g for shoots and roots, respectively. Each value is the mean of 12 replicates and vertical bars are s.e. of the means. Values sharing a common letter are not significantly different at P < 0·05.

Fig. 2.

Leaf abscisic acid concentration (ABA, in nmol g⁻¹ f. wt) in seedlings of Atriplex halimus issued from a coastal saline site (Monastir) or an inland semi-arid area (Sbikha) and exposed for 1 d and 10 d to control nutrient solution or to nutrient solution containing 40 or 160 mm NaCl or 15 % PEG. Measurements were performed on fully expanded leaves. Each value is the mean of 12 replicates and vertical bars are s.e. of the means. Values sharing a common letter are not significantly different at P < 0·05.

Fig. 3.

Leaf stomatal conductance (g_leaf, in cm s⁻¹) in seedlings of Atriplex halimus issued from a coastal saline site (Monastir) or an inland semi-arid area (Sbikha) and exposed for 10 d to control nutrient solution or to nutrient solution containing 40 or 160 mm NaCl or 15 % PEG. Measurements were performed at 1100 h on a fully expanded leaf located at the middle part of the stem. Each value is the mean of 12 replicates and vertical bars are s.e. of the means. Values sharing a common letter are not significantly different at P < 0·05.

Fig. 4.

Sodium (A), chloride (B) and potassium (C) concentrations in the leaves of Atriplex halimus issued from a coastal saline site (Monastir) or an inland semi-arid area (Sbikha) and exposed for 10 d to control nutrient solution or to nutrient solution containing 40 or 160 mm NaCl or 15 % PEG in the absence or presence of 50 µm ABA in the nutrient solution. Measurements were performed on fully expanded leaves. Each value is the mean of 12 replicates and vertical bars are s.e. of the means. Values sharing a common letter are not significantly different at P < 0·05.

Fig. 5.

Scanning electron micrography of the leaf surface of Atriplex halimus showing bladders (A) consisting of vesiculated balloon-like hairs attached to a stalk and playing a significant role in removing salt from the reminder of the leaf thus preventing accumulation of toxic salts in the parenchyma and vascular tissues. Accumulation of electrolytes leads to the bladder shrinking (B) and then bursting, depositing crystals at the leaf surface (C).

Fig. 6.

Leaf polyamine (Putrescine, Spermidine and Spermine) concentration in seedlings of Atriplex halimus issued from a coastal saline site (Monastir; M) and an inland semi-arid area (Sbikha; S) and exposed for 10 d to control nutrient solution or to nutrient solution containing 40 or 160 mm NaCl or 15 % PEG in the absence or presence of 50 µm ABA in the nutrient solution. Measurements were performed on fully expanded leaves. Each value is the mean of 12 replicates. In the columns, concentrations are given for the free, conjugated and bound fractions of each polyamine, as indicated.

Fig. 7.

Leaf proline and glycinebetaine concentrations in seedlings of Atriplex halimus issued from a coastal saline site (Monastir) and an inland semi-arid area (Sbikha) and exposed for 10 d to control nutrient solution or to nutrient solution containing 40 or 160 mm NaCl or 15 % PEG in the absence or presence of 50 µm ABA in the nutrient solution. Measurements were performed on fully expanded leaves. Each value is the mean of 12 replicates. Values sharing a common letter are not significantly different at P < 0·05.