The effect of salinity and substrates on the growth parameters and antioxidant potential of Tetragonia Decumbens (Dune Spinach) for horticultural applications

Abstract

Climate change, expanding soil salinization and the developing shortages of freshwater supplies indicate the need for a sustainable and sufficient crop production method. Seawater and salinized lands represent potentially cultivable areas for edible salt tolerate plants. Majority of the accessible fields with these qualities in South Africa are located close to coastal areas and suffer from salinity. Therefore, knowledge on propagation, cultivation, fertigation and salt stress responses of the edible halophyte, Tetragonia decumbens (Dune spinach) is needed in developing a viable growth protocol for both home gardeners and commercial farmers for the utilization of saline soils. Hence, this study was conducted to evaluate the effects of salinity, hormone treatment and substrates on the vegetative growth, nutrient uptake and antioxidant potential of T. decumbens Mill.

The study consisted of two separate experiments, with the 1st experiment presented in chapters 3 and the 2nd experiment presented in chapters 4 & 5. The 1st experiment investigated the possibility of root initiation of T. decumbens nodal stem cuttings as influenced by rooting media and hormone. The cuttings were obtained from a single stock plant growing along the Cape Town coast at Granger bay campus (CPUT). Only cuttings taken using homogeneous methods; i.e stem cuttings with about two-thirds of leaves removed, ± 15 cm long with a stem thickness of approx. 8 mm were used for the experiment. The nodal stem cuttings were treated with different concentrations of rooting hormone (Dynaroot 1, 2 and 3 0- Control) and planted into four different rooting media: Sand (S), Sand: Peat (SP) (1:1), Perlite: Peat (PP) (1:1) and Peat:Perlite: Vermiculite (PPV) (1:1:1) and placed on a specialized propagation bed with heating cables/rod underneath at 26 ºC. A completely randomised design was utilized for the experiment. The total number of cuttings used in the experiment was 528 for 16 treatments in three replicates (11 cuttings per treatment x 16 treatments x 3 replicates). The parameters evaluated were percentage survival, root length, number of roots, number of leaves and height of the cuttings. The data was statistically analysed using Analysis of Variance. The significant differences between treatment means at p ≤0.05 were compared using Fisher’s least significant difference. The result indicated significant growth differences in various rooting media, whereas the rooting hormone and its interaction with rooting media were not significant on most assessed variables. Improved rooting was observed from nodal cuttings planted in the sand: peat mixture with or without the use of the rooting hormone. This study showed that dune spinach does not require additional rooting hormones which could be a cost-saving option for potential commercial growers of the species.

The 2nd experiment investigated the effects of salt stress on plant growth, nutrient uptake, chlorophyll content and antioxidant potential of T. decumens grown in soilless culture. The objectives of the study were to measure plant growth, nutrient uptake, SPAD-502 values (chlorophyll content) and metabolite (phenolics) formation in the leaves of dune spinach, grown under various salt concentrations. Rooted cuttings of uniform size were arranged into four treatments each containing fifteen replicates. Salt stress were set up on three treatments by adding increasing NaCl concentrations in the NUTRIFEED™ nutritive solution (50, 100 and 200 mM). A total of 300 mℓ nutrient solutions were prepared for each plant with and/or without NaCl addition, the plants were then watered every three days using a plastic beaker. The control treatment was sustained and irrigated only by the nutritive solution. In all treatments, the pH was maintained at 6.0. SPAD-502 values were measured weekly using a handheld SPAD-502 meter. Determination of nutrient uptake of macronutrients N, K, P, Ca, Mg and Na and micronutrients Cu, Zn, Mn, Al and B were measured from dry plant material at the end of the experiment by Bemlab, 16 Van Der Berg Crescent, Gants Centre, Strand. Plant growth parameters such as shoot length, number of branches, wet and dry weight of shoots, stem and root were measured after harvest. Determination of concentrations of secondary metabolites (phenolic compounds) were assayed and measured spectrophotometrically at the end of the experiment.

The results showed that the use of nutrient solution incorporated with 50 mM and 100 mM of NaCl on plant growth resulted in plants that have higher yields of material with regards to both wet and dry weights of shoots, stem and roots, total weights and number of branches compared to the plants irrigated with nutrient solution only. Moderate (100 mM) to high (200 mM) NaCl increased chlorophyll content in leaves for up to seven weeks, then decreased on week eight and ten. Salt stress also positively affected polyphenolic compounds and antioxidant capacity (FRAP excluding ABTS) in the leaves. Polyphenolic compounds increased with increasing concentration of NaCl with the highest content found in 200 mM (highest NaCl conc.). This was not the case when FRAP capacity was evaluated, where the lower (50 mM) and moderate (100 mM) NaCl concentration proved to strengthen the FRAP capacity more than the highest concentration. As for the ABTS capacity, salt stress did not have any significant effect since the control had a slightly more capacity than all NaCl concentration.

In conclusion, this study found that the use of peat and sand as a rooting media had a positive impact on root initiation and growth of the nodal stem cuttings of dune spinach. On the other hand, salt stress at certain levels positively affected the total wet weight, nutrient uptake and antioxidant activities in the leaves of dune spinach. Based on these results it is evident that there is potential for seawater cultivation of the leafy vegetable T. decumbens. This could be a water-saving option in provinces experiencing the adverse effect of drought and salinity, where seawater or underground saline water could be diluted and used as irrigation water in the production of this vegetable.