Browsing by Person "Lehr, Patrick Pascal"

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Recurrent drought stress in grapevines
(2025) Lehr, Patrick Pascal; Zörb, Christian
Climate change is expected to increase the frequency and intensity of drought, impacting global agricultural production. To maintain food production under these changing conditions, it is crucial to understand how plants respond to drought and the mechanisms they use to cope with water deficit. Drought events frequently occur multiple times during a growing season, potentially leading to stress memory in plants, where responses of primed plants to subsequent droughts are modified. The regulation of transpiration by controlling the stomata is of great relevance under drought stress. Therefore, it is of particular interest to investigate the metabolic processes occurring in guard cells. The role of guard cells in stress memory and the signals involved in stomatal regulation remain under active investigation. A possible signal from root to shoot under drought conditions, leading to stomatal closure via abscisic acid biosynthesis is an increased sulfate concentrations in xylem sap. Therefor three questions were investigated: (i) How does the metabolic acclimation of guard cells in grapevine and maize under recurrent drought stress differ from the acclimation of mesophyll cells? (ii) Can additional sulfate application modify the drought response of these crops? (iii) What are the drought stress strategies of grapevine and maize, and how can agricultural production utilise these strategies? The analysis of metabolites of grapevine and maize mesophyll cells showed that the metabolite profiles of plants that have been subjected to repeated drought stress showed less alteration than those of unprimed plants, indicating that primed plants were less stressed. The metabolome of grapevine and maize guard cells was less affected by drought stress than that of mesophyll cells. This suggests that plants prioritize the stability of guard cell metabolomes to maintain stomatal function during stress. In contrast, grapevine and maize guard cells showed a similar increase in sugar concentrations during drought compared to mesophyll cells. It is debated whether sugars like sucrose, glucose, and fructose have an osmotic effect on guard cells or if they are sensed by hexokinases within the guard cells, which may trigger stomatal closure, thus coordinating sugar levels and photosynthesis with transpiration. Phosphorylated hexoses can be metabolized to pyruvate, which can feed into the citrate cycle and provide energy in the guard cells. They can also be used for the synthesis of metabolites such as malate, which influence stomatal opening. The distinct regulation of sugar concentrations in guard cells of grapevine and maize, in contrast to other metabolite classes, like amino acids, highlights that sugar concentrations in guard cells play a pivotal role during drought stress. In conclusion, the acclimation of the guard cell metabolomes in grapevine and maize differs from that of the mesophyll cells. Sulfate is discussed as a xylem-derived chemical signal for abscisic acid-dependent stomatal closure during early drought stress. Therefore, additional sulfate application may improve sulfate availability under drought conditions, enhancing the drought response, as investigated in this study. The results show that both grapevine and maize leaf sulfate concentrations were increased under drought stress, but only when additional sulfate was applied. This increase in leaf sulfate with supplemental sulfate suggests that increased sulfate availability enhances the drought response, leading to improved metabolic acclimation in leaves. This underscores the importance of adequate sulfate supply for optimal drought stress response and suggests that sulfate fertilization could enhance drought acclimation in crops. The results also show that changes in sulfate availability have a faded impact on the metabolome of guard cells compared to mesophyll cells. This, combined with the reduced metabolic acclimation of guard cells under drought conditions, suggests that guard cells maintain higher metabolic stability against external stress factors. The comparison between grapevine and maize drought stress response revealed that maize shows a more intense metabolic reorganization in response to drought stress, which may enhance its stress resilience and improve survival chances during droughts. However, this rapid reorganization comes at a cost, as it requires resources such as energy and nutrients to synthesize stress-defence molecules. These resources are diverted from other plant functions, potentially reducing yield and product quality. Additionally, the process of re-acclimating to well-watered conditions following a drought event also demands energy investment. In some cases, secondary metabolites such as carotenoids or anthocyanins accumulate in plants during drought and can remain even after the drought ends, altering the quality of harvested products, as seen in grapevine. This suggests that genotypes with a reduced response to drought may have advantages for agricultural production, especially in environments with more favourable conditions. Nevertheless, a rapid and intense metabolic response can be beneficial in cases of severe or prolonged drought, or when combined with other stress factors like heat. In such situations, quick acclimation can be vital for crop survival, allowing the plant to resume growth once the drought ends. A cell type-specific reaction, as seen in guard cells, in which only certain cells adapt metabolically, offers the chance of a resource-saving adaptation. The concept of increased acclimation to drought improving fitness during stress but potentially reducing yield applies to priming effects as well. Stress memory, where drought stress induces lasting effects beyond the drought period, may lead to higher costs or lower yields during favourable conditions. However, if another drought occurs, plants with stress memory are better adapted and show increased fitness during the unfavourable period. For crops, priming is advantageous in regions with frequent drought, while reduced stress memory may be beneficial in areas with optimal conditions.