Heat and drought priming induce tolerance to subsequent heat and drought stress by regulating leaf photosynthesis, root morphology, and antioxidant defense in maize seedlings

Novel stress tolerance strategies, such as heat and drought priming, are widely used in plants to increase tolerance to subsequent stresses. To date, it is still not known whether heat, drought, and combined stress priming can improve the tolerance of maize plants to heat, drought, and combined stresses by improving the morphology and distribution of roots and antioxidant defense mechanisms in maize seedlings. Here, seedlings of maize were firstly twice exposed to heat, drought, and combined stress at 33 °C/25 °C and 50–60% of soil water holding capacity (SWHC) followed by temperature and water recovery, and then subjected to more severe heat, drought, and combined stress at 36 °C/28 °C and 45–55% of SWHC. Under individual heat or drought stress, healthy values of gas exchange and chlorophyll fluorescence parameters, leaf relative water content, leaf water potential, and leaf cooling were significantly reduced, while superoxide anion and MDA contents in roots were significantly increased. Combined heat and drought stress usually exhibit a typical superimposed effect for the abovementioned parameters. Plants with heat, drought, and combined stress priming improved the redox balance of maize roots by enhancing the activities of superoxide dismutase, peroxidase, and catalase, initiated increases in root vitality as well as osmoregulatory substance contents as evidenced by increased soluble sugar, soluble protein, and proline contents. Principal component analysis revealed a strong correlation between antioxidant enzyme activities and osmoregulatory substance contents in roots. The combined stress priming effectively increased total root length, total root surface area, total root volume, and total root dry weight when compared with no priming. Combined stress priming optimized root distribution, leading to an increase in root length density and root dry weight density in the 12–18, 18–24, and 24–30 cm soil layers. Additionally, a strong positive correlation was observed between leaf relative water content, leaf water potential, and leaf cooling, indicating that better leaf water relations in the primed plants were conducive to the improvement of leaf cooling capacity, which provides good conditions for the increase in photosynthetic rate to maintain the integrity of the maize plant. The significant increases in leaf area, specific leaf weight, aboveground dry mass, and a significant reduction in leaf rolling in combined stress-primed plants indicated that the negative effects of heat and drought stress can be effectively mitigated. The current data indicated that the beneficial stress memory induced by priming in maize seedlings developed their defense systems to trigger more effective clearance mechanisms against subsequent combined heat and drought stress. These findings provided evidence that heat, drought, and combined stress priming enhanced maize adaptability to subsequent heat, drought, and combined stress by improving the morphology, distribution, and antioxidant capacity of the root system.