Wheat leaves embolised by water stress do not recover function upon rewatering.

New techniques now make it possible to precisely and accurately determine the failure threshold of the plant vascular system during water stress. This creates an opportunity to understand the vulnerability of species to drought, but first, it must be determined whether damage to leaf function associated with xylem cavitation is reparable or permanent. This question is particularly relevant in crop plants such as wheat, which may have the capacity to repair xylem embolism with positive root pressure. Using wheat (Triticum aestivum, Heron), we employed non-invasive imaging to find the water potential causing 50% xylem embolism (-2.87 +/- 0.52 MPa) in leaves. Replicate plants were water-stressed to varying degrees to induce embolism ranging from minimal to substantial. Plants were then rewatered to determine the reversibility of xylem damage and photosynthetic inhibition in glasshouse conditions. Rewatering after drought-induced xylem cavitation did not induce visible refilling of embolized xylem, and embolized leaves showed photosynthetic impairment upon rewatering. This impairment was significant even after only 10-20% of leaf veins were embolized, and leaves accumulating >20% embolism died upon rewatering in 7/10 individuals. Photosynthetic damage and hydraulic decline occurred concurrently as wheat leaves dehydrated, and leaf shrinkage during drying was the best predictor of photosynthetic recovery.