Drought stress events are becoming more frequent and intense worldwide, leading to significant yield losses in crops like barley, which is a major concern for farmers. Drought stress is a complex and strongly quantitative, influenced by factors such as the timing, severity, and duration of the stress. Therefore, identifying traits responsible for drought stress tolerance of specific barley genotypes is essential. However, traits that are advantageous for tolerance to late drought stress may not be the same as those for early drought stress and vice versa. Some traits might even have disadvantageous effects at other developmental stages. While most drought stress studies have focused on late-stage drought stress, this thesis aims to investigate physiological, metabolic, and yield-related traits in a diverse set of spring barley genotypes to identify trait inter-relationships and those most associated with early drought stress tolerance. First, eleven genotypes were screened for their root behavior under drought stress in a hydroponic system for four weeks using polyethylene glycol (PEG) as an osmotic stressor. One main result was that PEG consistently increased root diameter compared to the control treatment. When comparing PEG-treated roots with those grown in sand-filled pots where drought stress was induced by reduced watering, the drought-stressed roots had a significantly (P < 0.05) smaller root diameter than those in the control treatment. Furthermore, the shoot biomass of plants grown in sand pots showed a stronger correlation with field conditions than that of PEG-treated plants. Thus, a key finding of this study was that PEG is unsuitable for screening barley root traits related to early drought stress tolerance. The second part of this work was the analysis of a diverse set of 40 spring barley genotypes for their early drought stress performance (BBCH 13 - BBCH 53) in a greenhouse experiment. Based on their trait performances, the genotypes were classified into different drought stress tolerance groups (sensitive, neutral, tolerant). Thereafter, the traits that best characterized the drought-tolerant group were identified. Thereby, drought stress tolerance was defined by a high grain biomass (i.e., yield). In addition to a higher grain biomass, the drought-tolerant genotypes were characterized by a significantly (P < 0.05) higher water uptake during the recovery phase and a lower accumulation of proline and soluble sugars in the flag leaf as well as a lower osmolality compared to the neutral and sensitive genotypes. In addition to a significantly (P < 0.05) increased accumulation of the aforementioned metabolites, the sensitive genotypes also showed a significant (P < 0.05) higher water use efficiency (WUE) during drought stress and recovery phase. Afterwards, ten genotypes, representing each drought tolerance group, were selected for further investigations under field conditions to verify the trait performances and inter-relationships observed in the greenhouse. In this respect, significant positive correlations of the trait under drought stress between greenhouse and field conditions were observed for yield-related traits such as full ears (r = 0.58, P < 0.05), grains per ear (r = 0.84, P < 0.05), and grain biomass (r = 0.41, P < 0.05). Surprisingly, proline content was significantly negative correlated to grain biomass under greenhouse condition and field conditions, respectively (r = -0.66, P < 0.05, and r = -0.55, P < 0.05), although proline is often mentioned in context with drought stress tolerance. These results emphasize the importance of continuing early drought stress experiments until maturity as a specific trait performance during the early drought stress period could only be properly assessed in this way. The third part of this work investigated a new and much-debated approach: the use of "biologicals", which are products of natural origin (e.g., microorganisms, plants) that are discussed to improve drought stress tolerance. Here, four biologicals were tested in detail regarding their effect under drought stress on molecular, physiological, yield and quality level. A first important result was that the effect of a biological highly (P < 0.001) depended on the genotype in all environments. Drought stress-responsive genes Dhn1 and HvA1 associated with abscisic acid pathways were downregulated by biologicals in those genotypes, which showed a significant (P < 0.05) yield increase for the same biologicals under drought stress. Interestingly, trait inter-relationships were influenced by the application of biologicals. Regarding the physiological and yield-related traits, the effect of biologicals on drought stress was strongly dependent on the environment and the biological itself. Thereby, most significant effects were observed for Cropcover and Giant knotweed. There, higher and more significant (P < 0.05) effects among all genotypes on yield were observed in the greenhouse (30.5%) than under field conditions (14.1%). This work provides: 1) a deeper understanding of the effects of early drought stress in spring barley, and 2) new insights into the effects and modes of action of biologicals in improving early drought stress tolerance.
Veronic Töpfer (Thu,) studied this question.