Effect of Salinity and Low Osmotic Potential on the Germination and Seedling Growth of Rapeseed Cultivars with Different Stress Tolerance

In mediterranean environments, rapeseed is grown in fall-winter with late-summer sowing and this implies that germination and initial seedling growth may occur in soils with low water content and/or high salinity. Lab experiments were carried out to study the effect of the level and timing of salt and osmotic stress application on the germination and seedling growth of rapeseed (Brassica napus var. oleifera Del.) cultivars with different stress tolerance. After a preliminary screening on eighteen cultivars, four hybrids with different stress tolerance during germination were chosen to study germination on Petri dishes at 14 increasing concentrations of PEG 6000 (ψs up to -1.5 MPa) and 12 increasing concentrations of NaCl (up to 600 mM). Seedlings from seeds germinated at 0 and 250 mM of NaCl were then grown into boxes for the slant test containing NaCl solutions 0 mM and 100 mM, while seedlings from seeds germinated -0.8 and -0.03 MPa (PEG) were grown in pots containing siliceous sand with a water content equivalent to field capacity or half of field capacity. In both experiments seedlings were grown for 11 days under controlled light/temperature conditions. Therefore, for both salt and osmotic stress experiments, the following four combinations for “germination-growth” conditions were obtained for each cultivar: optimum-optimum; optimum-stress; stress-optimum; stress-stress. The following determinations were performed: final percentage germination, time to 50% germination (T50), and base water potential of seeds; dry and fresh weights, and shoot and root lengths of seedlings at the end of the growth period (i.e. 11 days after plant transfer to the growth medium), and correspondent relative growth rates between the start and the end of the growth period. The different degree of stress tolerance observed among rapeseed cultivars for germination was in the order of 150 mMol for NaCl concentration and 0.2-0.3 MPa for osmotic potential. Nonetheless, cultivars did not differ substantially for growth parameters of seedlings exposed to stressing conditions, and in some cases cultivars with higher tolerance during germination resulted even more sensitive during growth, especially under salt stress. The severe stresses applied reduced seedling growth, and root more than shoot growth. Agronomic implications for the different stress tolerance of cultivars are discussed.