Search

Early heat and drought priming may increase the plant's ability to resist later drought and heat stress. However, it remains unclear whether combined heat and drought priming can enhance the acclimation of plants to later combined stress by improving physiological activities. In this study, maize seedlings were first pre-exposed twice to heat, drought, and a combination of stresses followed by recovery, and then subjected to six days of more severe stresses. A considerable reduction in photosynthetic pigment content, stomatal size, and photosynthesis was observed under heat and drought conditions, and the changes in the above indicators were amplified under combined stress conditions. Stress priming improves antioxidant defense and cellular osmoregulation, as indicated by improved superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), glutathione reductase (GR), and ascorbate peroxidase activities, as well as elevated soluble sugar (SS) and proline (Pro) contents. Lower superoxide anion and malondialdehyde contents and injury index in the primed seedlings demonstrated the mitigation of oxidative stress. ROC analysis revealed that SOD and POD had considerable reliability in determining that maize seedlings were experiencing heat stress (AUC = 0.941–0.971); GR and SS were capable of accurately monitoring drought stress that was being experienced by plants (AUC = 0.919–0.958); and SOD, GR, and Pro had more capability for detecting the combination of heat and drought stress (AUC = 0.907–0.958). Collectively, the primed seedlings exhibited better performance than the non-primed seedlings, exhibiting stronger stress acclimation supported by an effective antioxidant defense system and osmoregulatory function. [ABSTRACT FROM AUTHOR]

More frequent and more intense heat waves and greater drought stress will occur in the future climate environment. Short-term extreme heat and drought stress often occur simultaneously after winter wheat anthesis, which has become the major constraint threatening future wheat yield. In this study, short-term heat, drought and their combination stress were applied to wheat plants after anthesis, and all wheat plants were restored to the outdoor normal temperature and full watering after stress treatment. The aim of the current study was to evaluate the role of nitrogen (N) in modulating the effects of post-anthesis short-term heat, drought and their combination stress on photosynthesis, N metabolism-related enzymes, the accumulation of N and protein and growth, as well as on the yield and water (WUE) and N use efficiency (NUE) of wheat after stress treatment. The results showed that compared with low N application (N1), medium application (N2) enhanced the activities of nitrate reductase (NR) and glutamine synthase (GS) in grains under post-anthesis heat and drought stress alone, which provided a basis for the accumulation of N and protein in grains at the later stage of growth. Under post-anthesis individual stresses, N2 or high application (N3) increased the leaf photosynthetic rate (An), PSII photochemical efficiency and instantaneous WUE compared with N1, whereas these parameters were usually significantly improved by N1 application under post-anthesis combined stress. The positive effect of increased An by N application on growth was well represented in a higher green leaf area, aboveground dry mass and plant height, and the variation in An can be explained more accurately by the N content per unit leaf area. Short-term heat, drought and combined stress after anthesis resulted in a pronounced decrease in yield by reducing grain number per spike and thousand kernel weight. The reduction in NUE under combined stress was higher than that under individual heat and drought stress. Compared with N1, N2 or N3 application significantly prevented the decrease in yield and NUE caused by post-anthesis heat and drought stress alone. However, N1 application was conducive to improving the productivity, WUE and NUE of wheat when exposed to post-anthesis combined stress. The current data indicated that under short-term individual heat and drought stress after anthesis, appropriately increasing N application effectively improved the growth and physiological activity of wheat compared with N1, alleviating the reduction in yield, WUE and NUE. However, under combined stress conditions, reducing N application (N1) may be a suitable strategy to compensate for the decrease in yield, WUE and NUE. [ABSTRACT FROM AUTHOR]

Extreme temperatures and droughts are considered as the two main factors that limit wheat growth and production. Although responses of wheat plants to heat and drought stress have been extensively investigated, little is known about the extent to which wheat plants can recover after stress relief. In this study, a winter wheat pot experiment was conducted to evaluate the growth, physiological activities, and yield formation responses of wheat to stress and recovery periods under heat stress (36 °C, daily maximum temperature), drought (45–55% of soil water holding capacity), and combined stress conditions. Heat and drought stress significantly reduced photosynthesis, leaf relative water content (LRWC), leaf water potential (LWP noon), and nitrogen metabolism enzyme activities and increased electrolyte leakage. These parameters showed significant interactions between heat and drought stress. Beneficial osmoregulation of membrane stability was observed in stressed plants because of the accumulation of proline and soluble sugars. Within a range of stresses, the abovementioned physiological processes of individual heat- and drought-stressed plants recovered to levels comparable to those of the control. The recovery capacities of the physiological traits decreased gradually with increasing stress duration, particularly under combined stress. The recovery of LWP noon and LRWC contributed to the improved photosynthetic performance after stress relief. The combined stress caused greater yield losses than individual heat and drought stress, which was mainly attributed to low levels of thousand grain weight (TGW), the number of grains per ear, and the grain filling rate. After stress relief, the recovery of proline content, glutamine synthetase activity, photosynthetic rate, and LRWC were closely associated with grain yield and thousand grain weight. Collectively, these findings contribute to a better understanding of the coordinated responses of winter wheat during the combined heat and drought stress and recovery periods. • Combined heat and drought stress is more detrimental to wheat than individual stress. • Most physiological processes of plants were reversible within a range of stress. • Recovery of physiological traits was closely related to wheat yield. [ABSTRACT FROM AUTHOR]

There will be longer and more intense periods of heat and drought stress in the future for terrestrial ecosystems. Although the responses of wheat plants to heat and drought stress alone have been extensively investigated, little is known about the extent to which their recovery can be assured after stress relief. In this study, a winter wheat pot experiment was conducted to investigate the changes in photosynthetic performance, antioxidant activity, osmoregulation, and membrane lipid peroxidation under heat stress (36 °C), drought (45–55% of soil water holding capacity), and combined stress conditions. The results showed that heat and drought stress significantly reduced the photosynthetic rate and the contents of chlorophyll and carotenoid. Superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and glutathione reductase (GR) activities were greatly activated by heat and drought stress to scavenge overproduced superoxide anion (O 2 -). Plants exhibited positive osmoregulation through the synthesis of soluble protein (SP), soluble sugar (SS), and proline (Pro) to improve membrane stability. Within a range of stress, combined heat and drought stress exhibited significant interactive effects in the above mentioned indicators. After stress relief, the majority of physiological processes were reversible, as indicated by the effective recovery of pigment contents, photosynthetic rate, antioxidant enzyme activities, osmoregulatory substance contents, and O 2 - production. Antioxidant enzyme activities tended to increase after recovering from 12 days of combined stress, whereas they were still not effective in mitigating oxidative damage. High levels of O 2 - and malondialdehyde (MDA) and a low relative growth rate during the recovery confirmed the irreversible damage caused by combined heat and drought stress. ROC (receiver operating characteristic) analysis indicated that GR and SS could accurately detect individual heat and drought stress that wheat plants were suffering or had suffered (AUC = 0.812–0.965), while POD and Pro had greater potential for diagnosing combined heat and drought stress (AUC = 0.871–0.958). Physiological indicators of stress tolerance were closely related to the photosynthetic rate during the stress, particularly Pro and GR. Collectively, the physiological processes of plants are reversible within a certain range of stress. POD, GR, Pro, and SS play vital roles in identifying and resisting heat, drought, and combined stress, and the recovery of these indicators contributed to improving photosynthesis and thereby increasing wheat growth. Our research contributes to the understanding of the underlying physiological mechanisms of plants in response to combined heat and drought stress and after stress relief. [Display omitted] • The responses of physiological activity to stress and recovery periods were studied. • Physiological processes of plants are reversible within a range of stresses. • POD, GR, Pro, and SS play vital roles in identifying heat and drought stress. • Physiological traits of stress resistance are closely related to photosynthesis. [ABSTRACT FROM AUTHOR]

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. • The effects of combined heat and drought stress on leaf water relations, photosynthesis, , and root physiological activities were studied. • Combined stress-primed plants acquired a stress imprint. • The enhanced tolerance of primed plants to heat and drought stress may be attributed to improved physiological activities in roots. [ABSTRACT FROM AUTHOR]