Management of dryland salinity in Central-west New South Wales, Australia: remediation trials using salt-tolerant plants: remediation trials using salt-tolerant plants

In this thesis, I present details of the trials made towards the management of dryland salinity in Central-west New South Wales using selected salt-tolerant plants belonging to Fabaceae, Amaranthaceae (Salicornidiodeae), and Poaceae. To provide a comprehensive context, Chapter 1 reviews earlier literature in the areas of (i) salinity in Australia, the state of New South Wales, and Centralwest New South Wales, (ii) salinity-remediation strategies and phytoremediation trialled in Australia, the state of New South Wales, and Central-west New South Wales, besides other parts of the world, (iii) effect of salinity on plant growth and photosynthetic performance, and (iv) physiology of salt-tolerance in plants. Against these contextual notes, I have outlined the purpose and goals of this study. Chapter 2 explains how I evaluated the experimental sites and selected some of the naturally occurring plants in two salt-affected Gumble and Cundumbul sites in Central-west New South Wales, Australia. Gumble and Cundumbul. Cynodon dactylon (Poaceae) and Thinopyrum ponticum (Poaceae) are selected two most abundant naturally occurring populations in both sites. The concentrations of Na+ , K+ , Ca2+, and Mg2+ in roots and shoots of C. dactylon and T. ponticum, bioaccumulation factor (BF), and translocation factor (TF) were also measured to evaluate their phytoremediation capacity. Salinity monitoring results revealed that Cundumbul site is more saline than Gumble besides a positive correlation exists between the EM38 result and soil: water1:5 extraction results. Ion-accumulation results indicated that T. ponticum accumulated greater quantity of Na+ , K+ , and Ca2+ than C. dactylon irrespective of locations. Cynodon dactylon accumulated greater Mg2+ than T. ponticum in Gumble site. The BF values of Ca2+, K+ and Na+ were significantly higher in T. ponticum than C. dactylon while BF value of Mg2+ was significantly higher in C. dactylon than T. ponticum irrespective of locations. At Gumble site, the TF value of Na+ was significantly greater in T. ponticum than C. dactylon whereas, this TF value was opposite in Cundumbul site. The overall salt accumulation (root+ shoot) was greater in T. ponticum than C. dactylon; therefore, I suggest that T. ponticum can be used for restoration of salt-affected land in Central-west New South Wales (Bhuiyan et al. 2016d). Chapter 3 explains the salt-ion accumulation and stress-tolerance capacities of the selected plants, viz., C. dactylon and T. ponticum. I evaluated Na+ , K+ , Mg2+, and Ca2+ levels in their roots and shoots and in the supporting soil. The physiological parameters of these species, including net photosynthetic rate (Pn), stomatal conductance (gs), and intercellular CO2 concentration (Ci) were investigated using LI-COR 6400 XT, a portable photosynthesis measurement system. Increasing salinity levels in the topsoil had a significant influence on Ci and gs, whereas no significant effect occurred on Pn in C. dactylon and T. ponticum. The Pn values in C. dactylon and T. ponticum were greater at Cundumbul than Gumble. The greater Mg2+ concentration facilitated greater Pn in C. dactylon and T. ponticum populations at Cundumbul than Gumble. Accumulation of Na+ , K+ , Mg2+, and Ca2+ was higher in T. ponticum than C. dactylon; therefore, T. ponticum impresses as a better salt accumulator than C. dactylon. Thinopyrum ponticum could be used in the phytoremediation of saltaffected pasture landscape in Central-west New South Wales (Bhuiyan et al. 2015a). Chapter 4 includes results of salt-ion accumulation levels in and physiological responses of M. siculus, T. pergranulata, T. ponticum, and C. dactylon against the saline–waterlogged and saline–dry conditions. A glasshouse-pot trial using four levels of soil moisture (>100%, 90%, 75%, and 60% field capacity) and three levels of salinity (1.0 dS m–1 , 3.5 dS m–1 , and 6.0 dS m–1 ) was done to evaluate Na+ , K+ , and Cl– accumulation capacity and the physiological responses, such as Pn, gs, Ci, maximum yield efficiency of PSII (Fv/Fm), and non-photochemical quenching (NPQ), of the tested species. The combined effect of soil water and salinity had a significant effect on Na+ and Cl– accumulation. Concentrations of Na+ and Cl– increased significantly whereas Pn, gs, and Ci, decreased in the tested plants, because of salinity. Soil moisture had a significant effect on Na+ , K+ , and Cl– accumulation and the physiological responses in the tested plants. No strong positive correlation between Pn and gs was evident in the tested plants. The reduced physiological performance could be because of non-stomatal activities. Na+ and Cl–accumulation capacity in the tested plants was in the following order: T. pergranulata>M. siculus>T. ponticum>C. dactylon. Cynodon dactylon, a C4 plant, accumulated lower salt ions than the other three C3 plants (Bhuiyan et al. 2015b). Chapter 5 pertains to the phytoremediation assessment* of M. siculus, T. pergranulata, and T. ponticum in simulated saline conditions. A glasshouse trial against salinity levels 0.0, 2.5, and 5.0 dS m–1 was done measuring shoot and root water content, shoot–root biomass ratio, and plant height of M. siculus, T. pergranulata, and T. ponticum determining their growth performance. The following indices, viz., total-ion accumulation (TIA), bioaccumulation factor (BF), translocation factor (TF), and bioconcentration factor (BCF) of Na+ and Cl− were measured evaluating remediation capacity of these plants. With increasing salinity in soil, total Na+ and Cl− accumulation increased in the tested plants in the following order: T. pergranulata > M. siculus > T. ponticum. Tecticornia pergranulata displayed the maximal phytoextraction capacity of Na+ and Cl− . The BF and BCF values of Na+ and Cl− were >1 in plants and in varied salinity treatments. The TF value of Cl− was >1 in the tested plants and salinity treatments, whereas the TF value of Na+ was >1 in T. pergranulata and M. siculus, and it was <1 in T. ponticum. Tecticornia pergranulata and M. siculus performed the best, accumulating more of Na+ and Cl− , and therefore they appear to be the candidates-of-choice for phytoremediation of saline sites in Central-west New South Wales (Bhuiyan et al. 2016a). Chapter 6 includes how high concentrations of Na+ and Cl– affect the ionaccumulation capacity, malondialdehyde (MDA) levels, growth and photosyn- thetic performance of the tested plants. I evaluated M. siculus, T. pergranulata, and T. ponticum in a glasshouse-pot trial using salinity levels 0.0, 2.0 (Na+ ), 4.0 (Na+ ), 2.0 (Cl– ), and 4.0 (Cl– ) dS m–1. Cl– accumulation was greater than that of Na+ in the tested plants. The shoot of T. pergranulata accumulated the greatest Na+ and Cl– whereas the root of T. ponticum accumulated the greatest of Na+ and Cl– . Growth performance was the least in M. siculus, T. pergranulata, and T. ponticum at 4.0 (Cl– ) dS m–1 treatment. MDA level is maximum in the tested plants at 4.0 (Cl– ) dS m–1 treatment. Pn and gs were the greatest in M. siculus, T. pergranulata, and T. ponticum at 0.0, 4.0 (Na+ ), and 2.0 (Cl– ) dS m–1 treatments, respectively. The NPQ value was the greatest in M. siculus and T. pergranulata at 4.0 (Cl– ) dS m–1 treatments, and T. ponticum at 2.0 (Na+ ) dS m–1 treatments. Results indicate that high concentration of Cl– is more destructive than Na+ in the tested plants (Bhuiyan et al. 2016c). Chapter 7 pertains how salt accumulation influences proline and glycine betaine levels and photosynthetic performance of M. siculus, T. pergranulata, and T. ponticum under saline conditions in the glasshouse. Na+ and Cl– accumulation was the greatest in T. pergranulata shoots and the least was in T. ponticum shoots. Tecticornia pergranulata and T. ponticum metabolized high levels of glycine betaine, whereas M. siculus metabolized high levels of proline. Thinopyrum ponticum accumulated intermediate levels of these organic osmolytes. Pn and gs values increased and NPQ values decreased in T. pergranulata with increasing salinity and the reverse occurred in M. siculus. A positive correlation between gas-exchange and organic-osmolyte values and a negative correlation between NPQ and organic-osmolyte values occurred in T. pergranulata indicating that T. pergranulata could maintain cell-osmotic balance by synthesizing high levels of organic osmolytes and concurrently showing the most efficient photosynthetic performance. I conclude that glycine betaine is the vital organic osmolyte for T. pergranulata and T. ponticum, and proline is the key organic osmolyte in M. siculus, which enables tolerance of salinity stress in these species (Bhuiyan et al. 2016b). Chapter 8 includes a general discussion and conclusion of the selection of plants used in this study, plant performance under salinity stress, salinity tolerance mechanisms and relevance of the phytoremediation capacity of the tested plants in restoring of dryland salinity in Central-west NSW and elsewhere. Between two grasses tested in this thesis, T. ponticum accumulated greater levels of salts than C. dactylon. However, among the tested plants, T. pergranulata accumulated the greatest levels of salts in shoots with the highest photosynthetic performance. These behaviours indicate that T. pergranulata could be the best taxa for remediation of salinity through phytoextraction. In addition, T. pergranulata accumulated the greatest levels of glycine betaine whereas, M. siculus accumulated proline, indicating that T. pergranulata tolerates salinity by synthesizing glycine betaine whereas M. siculus does so through proline. Phytoremediation indices showed that T. pergranulata would be the best phytoextractor whereas T. ponticum would be a phytostabilizor. This is because T. pergranulata accumulated greater levels of salts in their shoots than in roots whereas T. ponticum accumulated greater levels of salts in their roots than in their shoots. Salt accumulation and retention capacities are critical factor for success of phytoremedation. The plants which adapt salt-excretion mechanism to tolerate salinity would not be suitable candidates for phytoremediation strategy. One such is C. dactylon.