Thorsten Knipfer

The impact of drought on the rhizosphere is largely unknown, but understanding and leveraging soil-plant responses to climatic stress could represent a key strategy to increase plant resilience, adapt agricultural systems, and protect ecosystems in a changing climate. Root exudation, the release of organic molecules from roots into rhizosphere soil, is a critical process that mediates plant-soil-microbe interactions, shaping plants’ stress response, carbon (C) allocation, and survival. The effect of soil water stress on root exudation is not well understood across crop species; however, it appears that moderate stress stimulates exudation rates, increases root C allocation, and shifts exudate composition. Our greenhouse experiment investigates the rate of root exudation in hazelnut plants—an emerging woody perennial crop globally—under both well-watered and drought conditions. First, we adapted the Phillips et al. (2008) exudation collection methodology to sample exudates from hazelnut saplings in a greenhouse setting. This thorough investigation made clear that different methodological collection parameters greatly impact exudate results. With this information, we decided on a collection method that balanced scientific best practices and practical considerations. We then began the water stress experiment by randomly assigning 12 hazelnut saplings to either water-stressed or well-watered treatments. We measured exudate concentration using a Shimadzu TOCL. Additionally, we took images of the measured roots to analyze the relationship between root physiological parameters and exudation. We observed that water stress increased exudation rates. Combined with plant physiological and water stress data, increased exudation indicates a greater C allocation to below-ground root systems during water stress. Our results show a strong relationship between root parameters (i.e., number of root tips) and exudation. This research helps improve our understanding of root exudation as a potential mechanism for enhancing plant drought resilience, offering insights into plants’ adaptive strategies that could support sustainable agricultural systems in a changing climate.

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Increasing frequency and severity of drought events pose significant risks to native North American tree species such as balsam poplar (Populus balsamifera L.). Drought induces excessive xylem tension in the main stem, leading to air embolism that disrupts long-distance water transport. Although embolism repair following soil rehydration has been observed in some woody species, its duration and mechanism remain unclear in balsam poplar. This study investigated xylem refilling over time in intact P. balsamifera L. saplings using X-ray microtomography (microCT) to visualize and quantify the water- and air-filled status of xylem vessels and fibers. Two experiments were conducted: (1) a single-scan experiment comparing droughted saplings and those 7 days after rehydration, and (2) a time-series experiment in which individual saplings were repeatedly imaged over 50 hours.In the single-scan experiment, embolized vessel counts decreased by an average of 76%, accompanied by reductions in air-filled vessel and fiber areas (83% and 50%, respectively), and recovery of stem water potential (Ψstem) from approximately −15 to −5 bar. In contrast, the time-series experiment showed increases in embolized vessel counts (51%) and air-filled areas of vessels and fibers (39% and 16%, respectively) over 50 hours despite recovery in Ψstem. Water droplets and short water columns were initially observed within embolized vessels (~5 hours) but did not expand and instead disappeared over time. Anatomical analysis showed that approximately 40% of vessel wall area consisted of vessel-associated cell (VAC) pits across vessels of varying diameter and radial position, suggesting that initial water entry into embolized vessels may be linked to metabolic activity and solute export from adjacent living cells. However, the failure of embolism repair and formation of air pockets near the cambial region in time-series scans indicate that repeated X-ray exposure likely compromised cellular integrity and interfered with the repair process.Overall, these results provide evidence that embolized xylem vessels and fibers can refill following soil rehydration in intact P. balsamifera L. saplings, supporting recovery of water transport and storage over longer timescales. However, frequent microCT imaging under the conditions used here disrupts embolism repair and is not suitable for resolving its spatiotemporal dynamics.

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Drought-induced soil water limitations are a limiting factor in wheat yield; therefore, the expected increased frequency in drought events make it a vital topic of research. For wheat, sustained crop yield at limited soil water availability (i.e., drought tolerance) has been linked to osmotic adjustment (OA) as the main driver to minimize drought-induced reductions in leaf hydration status and growth. Hard red spring wheat (HRSW) cultivars are typically grown in rainfed areas of western Canada with milder climates, but ongoing climate change has increased the frequency and intensity of drought events during the summer months, which raises questions about how successful HRSW cultivars are in tolerating drought. The extent of OA and its relation to stomatal behavior, leaf rolling, and kernel development under periods of drought remains unknown for HRSW. For several commercially used cultivars (‘Superb’, ‘Stettler’, ‘AAC Viewfield’), data indicates that OA does not contribute to drought tolerance. In contrast, I found that sustained kernel weight during periods of relatively low soil water content was linked to ‘tight’ stomatal behavior (i.e., efficient transition from onset to full stomatal closure) and ‘early’ leaf rolling (i.e., reductions in flag leaf width). Among cultivars, ‘Superb’ was most successful in employing these strategies which also prolonged the onset of severe leaf dehydration under drought to a soil relative water content (i.e., % of field capacity) as low as 36% (defined as threshold ΘRWC); ‘Stettler’ at a ΘRWC of 48%, and ‘AAC Viewfield’ at a ΘRWC of 51%. Moreover, ΘRWC marked the onset of drought-induced losses in kernel weight in all three cultivars. Leaf epicuticular waxes exhibited differences in chemical composition between cultivars, which is discussed in the context of leaf water loss beyond stomatal regulation under drought. In conclusion, hard red spring wheat lacks OA but both leaf stomatal behavior and leaf rolling aid in securing leaf hydration status and kernel weight under drought. 

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