Population-based threshold models: A reliable tool for describing aged seeds response of rapeseed under salinity and water stress

Rapeseed (Brassica napus L.; Brassicaceae) is the second largest source of oilseed crop which is used for food and industrial applications globally. The production of this plant will be affected by climate change, increasing heat, salt and water stresses, in the future. Therefore, this study aimed to quantify the seed germination responses of two spring (‘Dalgan’ and ‘Hyola 50′) and one winter (‘Ahmadi’) rapeseed cultivars with a range of seed vigor (artificially aged at 42 °C) under water and salt stresses using hydrotime and halotime models, respectively. Germination time courses were recorded and seed germination characteristics were calculated. Seed moisture content and electrical conductivity were measured and also osmotic adjustment value associated with accumulation of salt ions was estimated. Rapeseed germination was negatively influenced by the accelerated aging period (AAP), water potential (ψ) and salinity (NaCl concentration) in all cultivars (P < 0.01). The hydrotime and halotime models accurately describe rapeseed germination at a range of ψs and NaCls at each AAP, with R² values of 0.80–0.96. The estimated median threshold water potential (ψb(50)) and salinity (NaClb(50)) were −1.291, −1.170 and −1.104 MPa and 373.3, 326.7 and 304.4 mM for Ahmadi, Dalgan and Hyola 50 cultivars at AAP = 0, respectively. With increasing AAP, the ψb(50) increased (become more positive) and NaClb(50) decreased (linearly in both), reached zero at 130, 85.5 and 77.3 h AAP. The ψb(50) estimated by the hydrotime model was higher after converting the NaCl concentrations to ψ as salt ions absorbed by the seeds in response to salt stress (–1.707, –1.455, –1.361 MPa, respectively). The highest osmotic adjustment value observed in Ahmadi (∼21−54 %, depending on AAP) than those obtained in other cultivars, suggesting that the seed mass/size is an effective factor for improving seed germination of rapeseed under saline conditions (r ≥ 0.86, osmotic adjustment value vs. seed mass). However, this value decreased linearly with AAP in all cultivars, reflecting the cell membrane damage and loss of seed vigor. It was confirmed by the increasing conductivity values at higher AAP. In conclusion, the models used in this study were able to accurately predict the rapeseed germination behavior at different levels of water and saline conditions.