Nowadays open field and protected vegetable cultivation practices require and use genotypes which are precisely tailored to their intended growth environments. Variability of this kind provides a rich source of material to uncover molecular mechanisms supporting the necessarily divergent physiological traits. In this study, typical field-optimized and glasshouse-cultivated cucumber F1 hybrids were investigated, and displayed slower growth (‘Joker’) and faster growth (‘Oitol’) in seedlings. Antioxidant capacity was lower in ‘Joker’ and higher in ‘Oitol’, pointing to a potential redox regulation of growth. The growth response of seedlings to paraquat treatment indicated stronger oxidative stress tolerance in the fast-growing ‘Oitol’. To test whether protection against nitrate-induced oxidative stress was also different, fertigation with increasing potassium nitrate content was applied. This treatment did not change growth but decreased the antioxidant capacities of both hybrids. Bioluminescence emission revealed stronger lipid peroxidation triggered by high nitrate fertigation in the leaves of ‘Joker’ seedlings. To explore the background of the more effective antioxidant protection of ‘Oitol’, levels of ascorbic acid (AsA), as well as transcriptional regulation of relevant genes of the Smirnoff–Wheeler biosynthetic pathway and ascorbate recycling, were investigated. Genes related to AsA biosynthesis were strongly upregulated at an elevated nitrate supply in ‘Oitol’ leaves only, but this was only reflected in a small increase in total AsA content. High nitrate provision also triggered expression of ascorbate–glutathion cycle genes with stronger or exclusive induction in ‘Oitol’. AsA/dehydro–ascorbate ratios were higher in ‘Oitol’ for all treatments, with a more pronounced difference at high nitrate levels. Despite strong transcriptional upregulation of ascorbate peroxidase genes (APX) in ‘Oitol’, APX activity only increased significantly in ‘Joker’. This suggests potential inhibition of APX enzyme activity specifically in ‘Oitol’ at a high nitrate supply. Our results uncover an unexpected variability in redox stress management in cucumbers, including nitrate inducibility of AsA biosynthetic and recycling pathways in certain genotypes. Possible connections between AsA biosynthesis, recycling and nitro-oxidative stress protection are discussed. Cucumber hybrids emerge as an excellent model system for studying the regulation of AsA metabolism and the roles of AsA in growth and stress tolerance.

Cucumber (Cucumis sativus L.) F1 hybrids are grown commercially in open-field or greenhouse conditions. Hybrids are well adapted to these settings due to directed breeding. In protected cultivation systems, a small rhizosphere volume and intensive, continuous fertigation predispose the roots to waterlogging (WL) conditions and potentially to hypoxia. However, high productivity is expected and achieved under these conditions. The aim of this study was to identify traits that play a role in this surprising behavior. Initial observations revealed the presence of a significantly higher number of adventitious roots (ARs) in three greenhouse (7–14) vs. three open-field cultivars (less than two) grown under normal conditions. Further on, two contrasting representative hybrids typically grown in open-field and in greenhouse conditions were subjected to WL stress. Declining oxygen levels in the media and increased alcohol dehydrogenase activity (ADH) in the roots were experienced during the WL treatment in both hybrids, with anaerobic metabolism triggered less intensively (~4-fold less ADH activity) in the greenhouse-type ‘Oitol’. The induction characteristics of cysteine oxidase (CysOx) genes, key components of the hypoxia sensing pathway, were used to confirm the hypoxic stress experienced by the roots. The lower extent of upregulation in CysOx genes expression agreed with the milder level of hypoxic stress in the roots of ‘Oitol’ than in ‘Joker’. The more efficient induction of AR formation with a ~50% increase upon waterlogging stress was found to be a prominent trait in ‘Oitol’, apparently helping root internal aeration and mitigating hypoxia. The shoot growth of neither hybrid was set back by hypoxic root conditions. ‘Joker’ plants maintained the same growth rate as that of the control, while the growth of ‘Oitol’ accelerated when its root system was flooded with nutrient solution. Acclimation processes to hypoxia were proposed to explain the lack of growth retardation in both varieties. This corresponded well with a general abundance of AR development in greenhouse-type (slicing) cucumbers that are typically cultivated in soilless systems.

The role of endogenous salicylic acid (SA), a major signaling molecule, was addressed in relation to the waterlogging (WL) stress response, including redox homeostasis and senescence. Wild-type and salicylate hydroxylase-expressing (NahG) tobacco plants were studied to reveal the stress-related effects of the transgene, which is known to deplete the endogenous SA pool. In control conditions, SA levels of the top leaves of NahG plants were moderately lower than those of wild-type, while SA was considerably reduced in the bottom leaves. WL conditions triggered a rise in H₂O₂ concentrations in young leaves, which was exaggerated in NahG plants, pointing to a mitigating effect of SA against the stress-associated oxidative burden. The NahG transgenic leaves displayed lower protein levels than their wild-type counterparts, indicating a role of SA in protein retention. In non-stressed NahG plants, young (top) leaves showed an increased level of protein nitration. WL treatment triggered decreased protein contents in the leaves of both genotypes. This coincided with the high H₂O₂ content of old leaves exceeding that of young leaves in most cases. The expression of the senescence marker gene Cysteine protease 1 was upregulated in WL-stressed bottom leaves. According to this marker, senescence progressed faster in NahG leaves. Links between SA, protein nitration, and leaf senescence were discussed. Additionally, a stimulating effect of the NahG transgene was confirmed on adventitious roots (AR) formation, which may have helped root functions and thus probably contributed to maintaining the growth of the WL-stressed plants. Our results have implications for how endogenous SA levels influence plants in a WL stress situation. According to our findings, the depletion of SA may trigger protein loss and tyrosine nitration, but at the same time accelerates AR formation in WL-stressed tobacco.