Introduction. With increasing threats to food security due to global warming and climate change, ‎developing ‎new strategies to improve crop production under stressful climates is an immediate ‎demand for ‎sustainability in agriculture. Cultivation tolerant crops to saline and dry climates is one ‎of ‎promising methods to cope with unfavorable conditions ‎Purslane (Portulaca oleracea L.) is a ‎valuable medicinal plant well adapted to saline soils of arid and semi-arid regions of Iran. It also ‎has a high capacity for accumulating certain soil pollutants and has been considered as a beneficial ‎species for phytoremediation programs. Cadmium, as a toxic heavy metal element for plants, is ‎found in some agricultural soils adjacent to industrial areas and can affect the plant’s tolerance to ‎salinity. There is limited knowledge about the combined effects of two simultaneous environmental ‎stressors such as sodium chloride and cadmium on plants‏.‏
Materials and Methods. This experiment was conducted in a completely randomized design with ‎four replications and a two-factor factorial arrangement to investigate some phenotypic traits and ‎element uptake in purslane. The first factor was salinity at four levels of electrical conductivity of ‎the nutrient solution as: 1.34 (control), 3, 8, and 13 dS•m⁻¹. The second factor was external ‎cadmium at five levels including 0 (control), 0.5, 1, 2, and 4 mg•L⁻¹. Salinity and cadmium were ‎applied by adding specified amounts of sodium chloride and/or cadmium nitrate to the half-strength ‎Hoagland solution, respectively. Seeds of a local purslane ecotype were sowed in plastic pots ‎‎(15cm height ×10cm diameter) filled with a Coco-peat/Perlite (2:1 v/v) medium and were irrigated ‎with distilled water until 4-leaf stage. At this time, watering pots with assigned salinity/cadmium ‎solutions started and continued till to the end of experiment (87 days after sowing). Four weeks ‎after the onset of the treatments, leaf area and chlorophyll index were measured. At harvest, shoot ‎and root dry mass as well as shoot content of sodium, potassium, cadmium, phosphorus, iron, zinc ‎and magnesium were measured. The contents of potassium, sodium and cadmium were also ‎measured in roots.‎
Results and discussion. The results showed that increasing salinity gradually reduced the shoot dry ‎weight, being significantly lower than the control at 8 and 13 dS•m⁻¹. In contrast, only the 2 and 4 ‎mg•L⁻¹ cadmium significantly reduced dry matter, with no significant difference between them. ‎Similarly, root dry weight and leaf area per plant generally decreased along with increasing salinity ‎and cadmium levels, with the highest reductions occurring under severe salinity (13 dS•m⁻¹) and 2 ‎or 4 mg•L⁻¹ cadmium levels. Unlike leaf area, the chlorophyll index initially increased with salinity ‎up to 8 dS•m⁻¹ and 2 and 4 mg•L⁻¹cadmium but this trend reversed at 13 dS•m⁻¹, where chlorophyll ‎content decreased substantially with increasing cadmium concentration. Sodium and cadmium ‎uptake were affected by a synergistic effect between the two elements, increasing with the level of ‎either stressor. However, the cadmium and sodium contents in roots were consistently much higher ‎than in shoots. Conversely, the shoot contents of potassium, iron, zinc and manganese decreased ‎with increasing salinity and cadmium, showing a strong and significant correlation with changes in ‎shoot dry weight. Shoot phosphorus content, however, was not markedly affected by either salinity ‎or cadmium and showed no clear association with shoot dry weight or leaf area. ‎
Conclusion. Based on the results, it seems that the main tolerance mechanism of purslane to salinity ‎and cadmium stress is probably accumulation of high amounts of sodium and cadmium in the roots ‎and a marked reduction in their translocation to the aerial tissues‏.‏