Assessment of Germination Response to Salinity Stress in Castor through the Hydrotime Model

Germination of castor seeds from five dwarf hybrid genotypes, compared to a 'Local' genotype selected by the University of Catania from a Tunisian population well adapted to the Mediterranean environment, was studied at five different salt levels (0, -0.3, -0.6, -0.9, and -1.2 MPa) in order to assess seed germination performance under stress conditions. The results confirmed that optimum moisture (0 MPa) ensured 100% of germination; on the contrary, salt concentration negatively influenced the final germination percentage (FGP) and radicle elongation, causing severe consequences for plant establishment. At a level of -1.2 MPa, no germination occurred, while a level of -0.3 MPa slightly affected the seed germination of the dwarf genotypes, which achieved 77.3% of germination, contrary to the 'Local' genotype, in which germination was kept stable. Higher levels of salt (-0.6 and -0.9 MPa) caused a linear decrease in FGP and radicle elongation. Overall, the dwarf hybrid 'C1019' performed better at higher salt impositions, as did 'C857', confirming these genotypes were the most tolerant among the dwarf hybrids. Conversely, 'C1013' turned out to be the most susceptible genotype, followed by 'C1008'. On the other hand, the 'Local' castor genotype was the best-performing genotype at -0.3 MPa and the most tolerant genotype in terms of salt concentration and germination time, which were accurately predicted by the hydrotime model, validating it as a valid method of assessing the germination response of castor seeds to Psi.