Physiological and Germination Responses of Muskmelon (Cucumis melo L.) Seeds to Varying Osmotic Potentials and Cardinal Temperatures via a Hydrothermal Time Model

[Image: see text] Climatic changes have a direct negative impact on the growth, development, and productivity of crops. The water potential (ψ) and temperature (T) are important limiting factors that influence the rate of seed germination and growth indices. To examine how the germination of seed responds to changes in water potential and temperature, the hydrotime model and hydrothermal model (HTT) have been employed. The HTT calculates the concept of germination time across temperatures, between T(b)–T(o), with alteration, and between T(b)–T(c), in supra-optimal ranges. The seeds of Cucumis melo L. were germinated in the laboratory for a hydro-thermal time experiment. Seeds were sown in Petri dishes containing a double-layered filter paper at different osmotic potentials (0, −0.2, −0.4, −0.6, and −0.8 MPa) by providing PEG 6000 (drought stress enhancer) at different temperatures (15, 20, 25, 30, and 35 °C). The controlled replicate was treated with 10 mL of distilled water and the rest with 10 mL of PEG solution. Results indicated that the seed vigor index (SVI–II) was highest at 15 °C with 0 MPa and lowest at 30 °C with −0.2 MPa. However, the highest activity was shown at 15 °C by catalase (CAT) and guaiacol peroxidase (GPX) at (−0.6 MPa), while the lowest values of CAT and GPX were recorded for control at 35 °C with −0.8 MPa at 35 °C, respectively. Germination energy was positively correlated with germination index (GI), germination percentage (G%), germination rate index, seed vigor index-I (SVI–I), mean moisture content (MMC), and root shoot ratio (RSR) and had a negative correlation with mean germination rate, percent moisture content of shoot and root, CAT, superoxide dismutase, peroxidase ascorbate peroxidase, and GPX. In conclusion, thermal and hydrotime models correctly predicted muskmelon germination time in response to varying water potential and temperature. The agronomic attributes were found to be maximum at 30 °C and minimum at 15 °C.