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Abstract An extremely important oil crop in the world, Helianthus annuus L. is one of the world's most significant members of the Asteraceae family. The rate and extent of seed germination and agronomic features are consistently affecting by temperature (T) and changes in water potential (ψ). A broad hydrothermal time model with T and ψ components could explain sunflower responses over suboptimal T and ψ. A lab experiment was performed using the HTT model to discover both T and ψ and their interactive effects on sunflower germination and also to figure out the cardinal Ts values. The sunflower seeds were germinated at temperatures (15 °C, 20 °C, 25 °C and 30 °C); each Ts had five constant ψs of 0, 0.3, 0.6, 0.9, and 1.2 MPa via PEG 6000 as osmotic stress inducer. The results revealed that highest germination index was found in seed grown at 20 °C in distilled water (0 MPa) and the lowest at 30 °C with osmotic stress of (− 1.2 MPa). The highest value of germination rate index was found in seed grown at 20 °C in distilled water (0 MPa) and the lowest at 15 °C with an osmotic stress of (− 1.2 MPa). In conclusion, water potential, temperature, and their interactions have a considerable impact on seed germination rate, and other metrics (GI, SVI-I, GRI, GE, SVI-II, and MGT). Seeds sown at 20 °C with zero water potential showed high germination metrics such as GE, GP, GRI, and T50%. The maximum value to TTsub noted at 30 °C in − 0.9 MPa osmotic stress and the minimum value was calculated at 15 °C in − 1.2 MPa osmotic stress. The result of TTsupra recorded highest at 15 °C in controlled group (0 MPa). Moreover, θH was highest at 30 °C in controlled condition (0 MPa) and minimum value was observed at 20 °C under − 1.2 MPa osmotic stress. The value of θHTT were maximum at 30 °C in controlled group (0 MPa) and minimum value was recorded at 15 °C under − 1.2 MPa osmotic potential. The base, optimum and ceiling temperatures for sunflower germination metrics in this experiment were noted 6.8, 20 and 30 °C respectively. |
