Ionic and osmotic relations in quinoa (Chenopodium quinoa Willd.) plants grown at various salinity levels

Abstract

Ionic and osmotic relations in quinoa (Chenopodium quinoa Willd.) were studied by exposing plants to six salinity levels (0-500 mM NaCl range) for 70 d. Salt stress was administered either by pre-mixing of the calculated amount of NaCl with the potting mix before seeds were planted or by the gradual increase of NaCl levels in the irrigation water. For both methods, the optimal plant growth and biomass was achieved between 100 mM and 200 mM NaCl, suggesting that quinoa possess a very efficient system to adjust osmotically for abrupt increases in NaCl stress. Up to 95% of osmotic adjustment in old leaves and between 80% and 85% of osmotic adjustment in young leaves was achieved by means of accumulation of inorganic ions (Na(+), K(+), and Cl(-)) at these NaCl levels, whilst the contribution of organic osmolytes was very limited. Consistently higher K(+) and lower Na(+) levels were found in young, as compared with old leaves, for all salinity treatments. The shoot sap K(+) progressively increased with increased salinity in old leaves; this is interpreted as evidence for the important role of free K(+) in leaf osmotic adjustment under saline conditions. A 5-fold increase in salinity level (from 100 mM to 500 mM) resulted in only a 50% increase in the sap Na(+) content, suggesting either a very strict control of xylem Na(+) loading or an efficient Na(+) removal from leaves. A very strong correlation between NaCl-induced K(+) and H(+) fluxes was observed in quinoa root, suggesting that a rapid NaCl-induced activation of H(+)-ATPase is needed to restore otherwise depolarized membrane potential and prevent further K(+) leak from the cytosol. Taken together, this work emphasizes the role of inorganic ions for osmotic adjustment in halophytes and calls for more in-depth studies of the mechanisms of vacuolar Na(+) sequestration, control of Na(+) and K(+) xylem loading, and their transport to the shoot.

Fig. 1.

Quinoa plants (genotype 5206) grown at various NaCl levels for 70 d.

Fig. 2.

Agronomic characteristics of quinoa plants grown at various salinity levels. NaCl salt was pre-mixed with the soil before seeds were planted (Method 1). (A) Root and shoot length; (B) root and shoot fresh weight. Mean ±SE (n=6). (C) Soil electrical conductivity (EC) data at the end of the experiment using Method 1. Mean ±SE (n=6). Although some salt was removed from the pot (both as a result of accumulation in plants and due to some minor leaching), the EC values remained sufficiently high.

Fig. 3.

Percentage of germinated quinoa seeds after 7 d of treatment with various concentrations of NaCl, isotonic mannitol, or PEG solutions. Mean ±SE (n=4). Different lowercase letters indicate a significant difference at P <0.05.

Fig. 4.

Ion content in the shoot sap (measured by squeezing frozen and thawed leaf samples) from young and old leaves from plants grown at various NaCl levels. Mean ±SE (n=5). The difference between old and young leaves is significant at any time at P <0.01.

Fig. 5.

Correlation between shoot K⁺ and Na⁺ sap concentration in young and old quinoa leaves grown at various NaCl levels in glasshouse experiments. Mean ±SE (n=5).

Fig. 6.

The dose dependency of the leaf sap osmolality in old and young quinoa leaves exposed to various levels of NaCl. Mean ±SE (n=5). * indicates a significant difference between young and old leaves at P <0.05.

Fig. 7.

Typical transient kinetics of net K⁺ and H⁺ fluxes measured from roots of 4-day-old quinoa seedlings in response to 100 mM NaCl treatment. Mean ±SE (n=5). For all MIFE experiments, the sign convention is ‘influx positive’.

Fig. 8.

The dose dependency of NaCl-induced net ion fluxes in 4-day-old quinoa roots. Steady-state K⁺ and H⁺ fluxes were measured 1 h after onset of acute salt stress. Mean ±SE (n=5).