A predictive method for selecting the most appropriate level of water potential for Thymus daenensis ssp. daenensis Cleak seed priming with respect to the severity of salinity or drought stress

Abstract

Thymus daenensis ssp. daenensis Cleak is an aromatic medicinal plant belonging to Lamiaceae family. Seed priming is a practical method to alleviate salinity and drought stress over germination stage. The objective of this study was to determine the most appropriate water potential level of seed priming solution according to the level of the salinity or drought stress of the medium which T. daenensis seeds will be sown. A factorial completely randomized design with four replications was conducted, one for salinity stress by using NaCl and another for drought stress by using PEG 6000. The first factorial design consisted of five seed medium water potential levels including 0, − 0.3, − 0.6, − 0.9 and − 1.2 MPa and the second was six seed priming treatments, including unprimed (UP), hydroprimed (HP, 0 MPa), − 0.3, − 0.6, − 0.9 and − 1.2 MPa water potential solutions. Priming efficacy (PE) is suggested in this study as a new characteristic for evaluation of the efficiency of priming. The PE values revealed that priming increases the germination percentage in all treatments; based on the obtained values of PE, the most effective priming treatment was the hydropriming in − 9 MPa of PEG. The final percentage germination (FPG), germination speed (GS), and root and shoot length (RL and SL) were reduced by decreasing the water potential levels of NaCl and PEG iso-osmotic solutions at 0, − 0.3, − 0.6, − 0.9 and − 1.2 MPa. All the traits were higher in NaCl than in PEG at the same water potential. Priming was helpful in overcoming the negative effects imposed by reduced water potential upon seed germination. The severity of salinity or drought stress determined the level of water potential would be better suited for seed priming. The highest germination percentage for the seed medium with Ψ-0.3 and Ψ-0.9 solutions was achieved by the priming solution of − 0.9 (96%) and − 0.3 (56%) MPa for NaCl treatments, respectively; and − 0.6 (94%) and 0 (35%) MPa for PEG treatments. There were second or third-order polynomial relationships between water potentials in seed germination medium and priming solutions according to the germination characteristics. Consequently, predictive equations were derived for each characteristic to show the suitable priming treatment regarding to the seed germination water stress level. The results showed that higher water potential treatments should be chosen for priming if germination is expected to occur under lower water potential and vice versa. This achievement can be used in rehabilitation of the degraded lands of T. daenensis by seed dispersal or when irrigating with saline water is required under cultivated fields.