Drought and flooding are the two most important environmental factors limiting maize (Zea mays L.) production globally. This study aimed to investigate the physiological mechanisms and accurate evaluation indicators and methods of maize germplasm involved in drought and flooding stresses. The twice replicated pot experiments with 60 varieties, combined with the field validation experiment with 3 varieties, were conducted under well-watered, drought, and flooding conditions. Most varieties exhibited stronger tolerance to drought than flooding due to higher antioxidant enzyme activities, osmotic adjustment substances, and lower reactive oxygen species. In contrast, flooding stress resulted in higher levels of reactive oxygen species (particularly O2−), ascorbate peroxidase, catalase, peroxidase, and soluble sugars but lower levels of superoxide dismutase, proline, and soluble protein compared with well-watered conditions. Superoxide dismutase, peroxidase, catalase, ascorbate peroxidase, proline, soluble sugars, and protein contents, in addition to plant height, leaf area/plant, and stem diameter, were accurate and representative indicators for evaluating maize tolerance to drought and flooding stresses and could determine a relatively high mean forecast accuracy of 100.0% for the comprehensive evaluation value. A total of 4 principal components were extracted, in which different principal components played a vital role in resisting different water stresses. Finally, the accuracy of the 3 varieties screened by multivariate analysis was verified in the field. This study provides insights into the different physiological mechanisms and accurate evaluation methods of maize germplasm involved in drought and flooding stresses, which could be valuable for further research and breeding.

Flooding and drought are the two most devastating natural hazards limiting maize production. Exogenous glycinebetaine (GB), an osmotic adjustment agent, has been extensively used but there is limited research on its role in mitigating the negative effects of different abiotic stresses. This study aims to identify the different roles of GB in regulating the diverse defense regulation of maize against drought and flooding. Hybrids of Yindieyu 9 and Heyu 397 grown in pots in a ventilated greenhouse were subjected to flooding (2–3 cm standing layer) and drought (40–45% field capacity) at the three-leaf stage for 8 d. The effects of different concentrations of foliar GB (0, 0.5, 1.0, 5.0, and 10.0 mM) on the physiochemical attributes and growth of maize were tested. Greater drought than flooding tolerance in both varieties to combat oxidative stress was associated with higher antioxidant activities and proline content. While flooding decreased superoxide dismutase and guaiacol peroxidase (POD) activities and proline content compared to normal water, they all declined with stress duration, leading to a larger reactive oxygen species compared to drought. It was POD under drought stress and ascorbate peroxidase under flooding stress that played crucial roles in tolerating water stress. Foliar GB further enhanced antioxidant ability and contributed more effects to POD to eliminate more hydrogen peroxide than the superoxide anion, promoting growth, especially for leaves under water stress. Furthermore, exogenous GB made a greater increment in Heyu 397 than Yindieyu 9, as well as flooding compared to drought. Overall, a GB concentration of 5.0 mM, with a non-toxic effect on well-watered maize, was determined to be optimal for the effective mitigation of water-stress damage to the physiochemical characteristics and growth of maize.

Crop tolerance to waterlogging significantly influences survival and productivity under waterlogging conditions. Elucidating the molecular mechanisms underlying waterlogging tolerance could facilitate the development of resilient crop varieties through breeding. This study conducted a comparative analysis of the physiological, transcriptional, and metabolic responses of a waterlogging-tolerant maize genotype Guidan162 (GD) and a waterlogging-sensitive genotype Zhaofeng 588 (ZF) during the grain filling stage. Phenotypic and physiological characteristics indicated that the leaf morphology of maize plants is affected, while the levels of peroxidase (POD) and catalase (CAT) and proline significantly increase under waterlogging stress. Transcriptomic analysis identified 3280 and 2260 differentially expressed genes (DEGs) between normal and waterlogged conditions in GD and ZF, respectively. KEGG enrichment analysis of DEGs revealed that pathways related to plant stress tolerance were enriched, including peroxisome, plant hormone signal transduction, and arginine and proline metabolism. In addition, metabolomic profiling revealed 359 and 209 differentially abundant metabolites (DAMs) in GD and ZF under waterlogging stress. Many of these DAMs participate in arginine and proline metabolism, plant signal transduction, and glutathione metabolism. Integrated transcriptomic and metabolomic analyses highlighted significant enrichment in abscisic acid (ABA) signaling, glutathione metabolism, and proline biosynthesis pathways. Several key candidate genes-including Arginase, PIP, P4H, PYR/PYL, PP2C, SnRK2, ABF, IDH, GPX, GGCT, OXP, and GCL were implicated in conferring waterlogging tolerance. These findings provide new insights into the complex molecular mechanisms of waterlogging tolerance in maize.

Drought and flooding stress alternately and frequently occur in Guangxi, China, and the whole world, which seriously limit maize production. Few studies focus on the different responses and evaluations of maize to drought and flooding stresses. A pot experiment with 40 varieties was conducted under well water, drought and flooding stresses. A multivariate analysis method of principal component analysis, comprehensive evaluation value, correlation analysis, stepwise regression analysis, and cluster analysis was used to evaluate the resistance of maize. Most varieties had stronger drought resistance rather than flooding resistance because of the higher antioxidant enzyme activities, osmotic adjustment substances, less reactive oxygen species, and a greater than 1.0 drought-resistance coefficient. However, there was an increment of reactive oxygen species (especially O ₂ ⁻ ), ascorbate peroxidase, peroxidase, soluble sugar, and the decrement of superoxide dismutase, catalase, soluble protein, and a lower than 1.0 of flooding-resistance coefficient of most maize varieties in flooding stress compared with well water. The superoxide dismutase, peroxidase, catalase, ascorbate peroxidase, proline, soluble sugar and protein, plant height, leaf area/plant, and stem diameter were screened out to be accurate and representative indicators to evaluate the drought and flooding resistance of maize. The study provides an insight to comprehend the different mechanisms of maize in response to drought and flooding stresses and provides a multivariate analysis method for screening the resistance of maize germplasm which could be valuable for further research and breeding of drought and flooding resistances of maize. One-sentence summary: A multivariate analysis method for the screening the resistance of maize germplasm and the different physiological mechanisms of drought and flooding stresses were revealed.