Effect of salinity on growth and physiology of Tetragonia tetragonioides (Pall.) Kuntze.
The challenges connected with salinization of soils and water bodies, as well as increased competition for restricted freshwater resources are increasing. Current efforts to adapt to these conditions through sustainable agriculture include the hunt for novel highly salt-tolerant crops and wild species that have potential as saline crops being particularly suitable. Hence, this study aimed to assess the effect of different salinity gradient on growth, physiological characteristics, and mineral concentrations on Tetragonia tetragonioides. T. tetragonioides is an extensively farmed crop around the world for its use as a food, ground cover, and medicinal plant; however, its salt tolerance has not been determined. The plant was subjected under different salinity gradient (0, 5, 10, and 20% NaCl solutions) twice a week during the course of the experiment. The height of Tetragonia tetragonioides plants in this experiment was improved by salinity particularly at 10% NaCl treatment. The 10% NaCl treatment was found to have the tallest plant with the average length of 58 cm when compared to 46 cm, 39 cm and 35 cm of 5%, 0% and 20% treatments respectively. However, the number of leaves per plant was significantly decreased at the highest salt treatment (20% NaCl treatment) (p > 0.05), whereas at 0% and 5% NaCl treatments the decrease was not significant (p < 0.05). The highest number of leaves per plant was obtained at 10% NaCl (with the average of 57 leaves) followed by 0% (control) (with the average of 48 leaves). In this study it was also observed that 10% NaCl treatment had higher biomass compared to other treatments used (0%, 5% and 20% NaCl treatments), meaning that Tetragonia tetragonioides plants were able to tolerate salinity of up to 10%. However, there was no statistical significance between 0% and 5% treatments (p <0.05) and from these findings 10% salt treatment was found to be the most favourable condition for the growth of this plant species. Root length of Tetragonia tetragonioides plants were drastically reduced with the increasing salinity levels, 0% was found to have the longest length of 23 cm (average) compared to 18 cm, 15.5 cm and 13 cm of 5%, 10% and 20% NaCl treatments respectively. The presence of potassium, phosphorus, sodium, calcium, magnesium, iron and zinc substances may add to the nutritional and health benefits of T. teragonioides leaves. Potassium was found to be abundant (100.15±o.27, 83.02±0.3, 45.23±0.23 and 33.34±1.35) at 0%, 5%, 10% and 20% NaCl treatments respectively, followed by Phosphorus being higher at 0% treatment (control) and lower at 20% NaCl treatment and Zinc was found to be the less present and constant mineral in all the treatments. This findings therefore encourages adequate consumption of T. tetragonioides leaves. The maximum average CO2 uptake (A) of 17.66 μmol m-2 s -1 was recorded at 12h00 PM for 10% NaCl treatment compared to values of 15.58, 14.28, and 13.07 μmol m-2 s -1 for 0%, 5% and 20% respectively. Leaf conductance and transpiration followed trends similar to CO2 uptake. Maximum average stomatal leaf conductance and transpiration values were 1.389 mol m-2 s -1 and 0.007 mmol m-2 s -1 , respectively, at 12h00 PM. An increased in stomatal conductance at higher temperatures particularly at midday may have assisted Tetragonia tetragonioides to increase rates of photosynthesis. The lack of light saturation of photosynthetic CO2 with increases in photosynthetic photon flux density may have allowed maximum carbon gain and contributing to an increased growth of T. tetragonioides at 10 % NaCl treatment. Therefore, the photosynthetic traits may be significant to the success of spreading and growth of Tetragonia tetragonioides even in harsh conditions.