Severe water stress constrains, or even halts, water transport in the xylem due to embolism formation. Poplar
trees respond to drought-induced embolisms by accumulating sugars in both xylem apoplast and stem
tissues, thus providing energy and facilitating osmotic adjustment processes. We focused on how the rate of
drought progression modulates energy reserves (sugar accumulation) and affects the recovery process. P.
nigra seedlings were exposed to either fast-developing drought (by ceasing irrigation) or slow-developing
drought (by progressively reducing the amount of water provided to the plants). Once similar level of water
stress was reached, the plants were re-watered and 24 hours amer re-irrigation the recovery rate was
determined thorough x-ray micro-CT analyses. Plants rewatered amer a slowly induced stress, recovered
approximately 25% of embolized vessels, whereas in fast-stressed plants, over 50% of vessels restored their
hydraulic functionality. The reduced growth and lower accumulation of soluble sugars in slowly stressed
stems, combined with the measurements of embolism, indicate that slow developing drought reduced the
hydraulic ability to recover compared to fast developing stressed plants. This suggests that embolism removal
requires the accumulation of sugars. Image analyses indicated that embolism formation and its removal are
spatially coordinated process, with embolism formation occurring from the inside out and recovery from the
outside in, underlining the importance of xylem proximity to phloem (carbohydrates source). To better
understand the effects of stress progression rate, we are now focusing on expression levels of genes related
to carbohydrates reserves mobilization and transport.
