Utilizing hydrothermal time models to assess the effects of temperature and osmotic stress on maize (Zea mays L.) germination and physiological responses

Abstract

The application of germination models in economic crop management makes them extremely useful for predicting seed germination. Hence, we examined the effect of varying water potentials (Ψs; 0. - 0.3, - 0.6, - 0.9, - 1.2 MPa) and temperatures (Ts; 20, 25, 30, 35, 40 °C) on maize germination and enzymatic antioxidant mechanism. We observed that varying Ts and Ψs significantly influenced germination percentage (GP) and germination rate (GR), and other germination parameters, including germination rate index (GRI), germination index (GI), mean germination index (MGI), mean germination time (MGT), coefficient of the velocity of germination (CVG), and germination energy (GE) (p ≤ 0.01). Maximum (87.60) and minimum (55.20) hydro-time constant (θH) were reported at 35 °C and 20 °C, respectively. In addition, base water potential at 50 percentiles was highest at 30 °C (15.84 MPa) and lowest at 20 °C (15.46 MPa). Furthermore, the optimal, low, and ceiling T (To, Tb and Tc, respectively) were determined as 30 °C, 20 °C and 40 °C, respectively. The highest θT1 and θT2 were reported at 40 °C (0 MPa) and 20 °C (- 0.9 MPa), respectively. HTT has a higher value (R2 = 0.43 at 40 °C) at sub-optimal than supra-optimal temperatures (R2 = 0.41 at 40 °C). Antioxidant enzymes, including peroxidase (POD), catalase (CAT), superoxide dismutase (SOD), ascorbate peroxidase (APX), and glutathione peroxidase (GPX), increased with decreasing Ψs. In contrast, CAT and POD were higher at 20 °C and 40 °C but declined at 25, 30, and 35 °C. The APX and GPX remained unchanged at 20, 25, 30, and 40 °C but declined at 35 °C. Thus, maintaining enzymatic activity is a protective mechanism against oxidative stress. A decline in germination characteristics may result from energy diverting to anti-stress tools (antioxidant enzymes) necessary for eliminating reactive oxygen species (ROS) to reduce salinity-induced oxidative damage. The parameters examined in this study are easily applicable to simulation models of Z. mays L. germination under extreme environmental conditions characterized by water deficits and temperature fluctuations.

Keywords: Abiotic stress; Antioxidant mechanism; Germination; Hydrothermal time model; Physiological responses.

Fig. 1

Changes in cumulative germination for maize at (a) 15 °C (b) 20 °C (c) 25 °C (d) 30 °C (e) 35 °C and (f) 40 °C with different ψ. The symbols indicate the water potential and the lines indicate the cumulative germination rate

Fig. 2

Effects of varying T and ψ on (a) germination index (b) Timson Germination Index (c) germination rate index and (d) mean germination time of maize using HTT

Fig. 3

Effects of varying T and ψ on (a) mean germination rate (b) germination energy, (c) the coefficient of germination velocity and (d) Timson germination index of maize based on the HTT model

Fig. 4

Effects of varying T and ψ on (a) CAT (b) POD (c) SOD (d) APX and (e) GPX activities in maize seedlings

Fig. 5

Correlation of germination features and activities of antioxidant enzymes in maize seedlings under varying T and ψ

Fig. 6

Heatmap correlation of germination features and activities of antioxidant enzymes in maize seedlings under varying T and ψ

Fig. 7

Loading Plot of Principal component analysis (PCA) on germination features and activities of antioxidant enzymes in maize seedlings under varying T and ψ