Modeling seed germination of unprimed and primed seeds of sweet sorghum under PEG-induced water stress through the hydrotime analysis

The effects of reduced water potential (psi) on seed germination at 25 and 15 degrees C in unprimed (UP) and primed (P) seeds of two cultivars of sweet sorghum (cv. Keller and cv. Makueni local), were analyzed through the hydrotime model. Six psi (from 0 to -1.0 MPa) in polyethylene glycol 6000 (PEG) solutions were used for the tests. Seeds were primed in 250 g/L PEG solution at 15 degrees C for 48 h. Decreasing psi of imbibition solution reduced and delayed germination. At 15 degrees C seeds germinated less and slower than at 25 degrees C at any psi. Seeds of cv. Makueni local exhibited a greater sensitivity to water stress in terms of germination percentage, than seeds of cv. Keller, but they were faster in germination. Osmopriming was beneficial for seed germination, both in terms of final percentage and rate, at any temperature and psi. The hydrotime analysis revealed that predicted theta(H) constant was increased when temperature was reduced to 15 degrees C and at this temperature median base water potential [psi(b(50))] for germination was higher (less negative) than at 25 degrees C. Seed priming shifted psi(b(50)) towards more negative values and reduced theta(H) requirements for germination. At 25 degrees C the two cultivars behaved similarly while at 15 degrees C cv. Keller exhibited a psi(b) more negative but required a greater theta(H) to germinate, indicating a greater water-stress tolerance but a slower germination, than cv. Makueni local. The application of the model allows to identify water stress tolerant cultivars during germination, to include into breeding programs for the selection of well-performing cultivars under stress conditions.