## 1 Introduction  
Green technologies such as ultrasound are being increasingly investigated, aiming both at process intensification and meeting the goals of sustainable processes [1, 2, 3]. Extraction of natural products (e.g., bioactive compounds such as essential oil, antioxidants, oil, and dyes) implies the recovery of compounds that are often of low occurrence in a plant matrix [4, 5]. In this field, the understanding of solid-liquid extraction, and how green technologies can improve extraction, appear as key issues for an enhanced control of extraction.  
The effectiveness of ultrasound for natural product extraction has been widely reviewed [5, 6, 7, 8, 9, 10, 11]. In fact, when submitted into a liquid, ultrasonic irradiation can induce the nucleation, growth, and collapse of bubbles filled with solvent vapor and dissolved gas [12]. This phenomenon, known as acoustic cavitation, is actually at the origin of various physical and chemical effects due to the extreme local conditions obtained during the bubble collapse [13, 14]. Applied during extraction, ultrasound reduces extraction duration to 30 min instead of hours, and increases the yield by 5 to 30%, of monitored compounds compared with conventional extraction processes such as maceration.  
Reported mechanisms for this mass-transfer enhancement are generally attributed to cell disruption correlated with implosion of cavitation bubbles on a vegetable surface. A previous research team demonstrated that power ultrasound (20 kHz) specifically impacts cellular structures such as excretion hairs of pot marigold [15]. This research team also highlighted that ultrasound improves the swelling of plants immersed in a solvent [16]. In a more recent review, several mechanisms impacting plant structure were identified (sonoporation, detexturization, erosion, fragmentation, local shear stress, and sonocapillarity) [17]. However, the relationship between the physical impacts of ultrasound on plant matrices and extraction enhancement has rarely been investigated.  
Within this objective, we investigated the impact of ultrasound at 20 kHz on polyphenol extraction, taking the example of olive leaves. Olive leaves contain valuable polyphenols such as oleuropein, tyrosol, and hydroxytyrosol, which can be valorized after an extraction step. The positive impact of ultrasound on polyphenol extraction from olive leaves in an organic solvent (e.g., ethanol/water mixtures) or in olive oil has been reported in previous studies [18; 19; 20; 21].  
To understand further the mechanical action of ultrasound, a multiscale analysis of the leaf correlated with polyphenol extraction yield is needed. For this, we exposed a single leaf to an ultrasonic field. The extraction solvent was analyzed to determine its polyphenol content. The treated leaf was examined by Environmental Scanning Electron Microscopy (ESEM) for surface observation and a histochemical study was conducted to evaluate modifications generated by ultrasound at the tissue level. This study aims to observe how ultrasound affects whole olive leaves from a histological point of view, and assess if those observations are correlated with polyphenol extraction.