Climate changes such as erratic rainfall patterns and unpredictable temperatures have brought about environmental stresses, leading to reduction in crop, horticulture, and forage productivities and natural resources in agriculture worldwide. In addition, legacies of anthropognic activities such as contamination and salinisation cause abiotic stress that consequently induces decrease in ariguclaural production. Among the different environmental stresses, salt stress has been an important factor affecting plant growth, which constrains water availability in the soil. As a consequence, high saline soil causes poor seed germination and early seedling growth as well as damages plant cells by inducing generation of reactive oxygen species (ROS) such as superoxide radical (O2•−), singlet oxygen (1O2), hydroxyl radical (OH•) and hydrogen peroxide (H2O2).

It could be expected that New Zealand pasture lands for dairy farming are steadily salinised due to increase in drought duration and excessive irrigation management. This would greatly influence yields of the dairy products and the New Zealand economy. Therefore, in order to maintain sustainable agricultural production, it is necessary to study the resistance of pastoral species to salt tolerance. Italian ryegrass (Lolium multiflorum) as a biennial species has been considered to be moderately salt stress resistant and useful for dairy pasture required by cows in New Zealand.

When considering plant response to salinity, seed priming has been used for improving germination performance, uniformity, and stress tolerance. Understanding cellular mechanisms of seed priming would advance seed germination under saline soil. Various materials are used for seed priming, but there is little information relating to seed priming with amino acids under salt stress. Plants utilise synthesis of amino acids for growth and development as well as defence under environmental stress conditions.

Hence, the aims of the present PhD research were to investigate the effect of priming L. multiflorum seeds with twenty L-amino acids applied singly as well as casein hydrolysate mainly as a mixture of amino acids on germination and seedling growth under salt stress. When the seedlings were exposed to salt stress, those developed from seeds primed with casein hydrolysate exhibited higher germination percentage, activities of antioxidant enzymes including superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD), proline level, and biomass than those from control (non-seed priming) and hydro-priming. The hydrogen peroxide (H2O2) and malondialdehyde (MDA) contents were markedly lower in the roots of the seedlings from seeds primed with casein hydrolysate when the seedlings were grown under salt stress compared to those from the control and hydro-priming. Histochemical staining with diaminobenzidine (DAB) and nitro-blue tetrazolium (NBT) revealed less staining in the roots of the seedlings from seeds primed with the casein hydrolysate than control, hydro-priming, and L- and D-methionine priming when the seedlings were grown under salt stress. The beneficial effects of seed priming with casein hydrolysate shown in the laboratory experiments were validated in pot experiments when the plants from the different seed priming treatments were grown in soil added with NaCl solutions under glasshouse conditions. Moreover, in additional glasshouse experiments, there was evidence that seed priming with casein hydrolysate was also beneficial for the plants exposed to another stressor, namely water stress.

Therefore, the beneficial effect of casein hydrolysate priming could potentially enhance functions of physiological and biochemical response under soil salinity. It is essential that priming of L. multiflorum seeds with casein hydrolysate might be potentially considered as an effective priming method leading to improved plant performance under stress conditions.