## 1 Introduction  
Drought is the main abiotic stress that limits the quantity and quality of the harvest of numerous crops worldwide. Climate models indicate that, because of global warming, drought episodes will become more frequent in the coming decades and maintaining stable crop yields under both normal and drought stress conditions will be indispensable for the food security of the growing world population. Water stress has adverse impacts on many aspects of plant physiology, one of the first being photosynthesis [1]. Drought stress can also have a negative impact on plants at other levels, including limiting growth [2], and creating oxidative stress [3]. Water stress thus negatively affects the quantity and quality of the yield of most crops. Plant responses to water deficiency depend on the species [4], the age of the plant [5], the genotype [6], and the intensity of the stress [7]. Severe drought has been extensively studied [8; 9; 10], whereas mild drought stress has not, even though mild stress occurs more frequently in cultivated areas [6]. Due to climate change and policies aimed at reducing the quantities of fertilizer used, new methods for the protection of plants from drought have been investigated. Improved control of plant water status and irrigation is one option, and non-destructive ways to detect and prevent water stress are thus receiving considerable attention [11; 12]. Another option is the use of biostimulants. Plant biostimulants contain substance(s) and/or micro-organisms whose function when applied to plants or the rhizosphere is to stimulate natural processes to enhance/benefit nutrient uptake, nutrient efficiency, tolerance to abiotic stress, and crop quality [13]. The use of foliar biostimulants in agriculture is relatively recent but few studies published to date have shown biostimulants to have a positive effect on yield in several crops, oilseed rape [14], cotton [15], bean [16], and carrot [17]. Using biostimulants makes it possible to reduce the amount of fertilizer thereby reducing the negative impact of the crop on the environment. The use of biostimulants to prevent abiotic stress is less documented, but several studies have highlighted changes in the expression of genes involved in the abiotic stress resistance [18; 19; 20]. The severity of the effect of drought on plants is often assessed through relative water content, leaf hydric potential or photosynthetic capacity [21]. However, these parameters do not account for differences in tissue inside the lamina. Using the insects' feeding particularity, [PERSON] et al. [22] demonstrated that the two main parenchyma of the leaf (palisade and spongy) played different roles in leaf hydraulic resistance. Changes in cell size in response to drought have already been reported in various species [23; 24; 25]. These effects of drought stress can be effectively monitored using NMR (nuclear magnetic resonance) relaxometry. NMR relaxation is a powerful technique to study changes in the status and distribution of water in plant tissues and has also been used to investigate water status in seeds [26], fruit [27], stem [28] leaves [29; 30; 31] or to monitor water relations in trees [32]. In plant tissues, the NMR transverse relaxation (T\({}_{2}\) relaxation) is always described by multi-exponential decay. Each NMR relaxation peak describes a specific water pool in the tissue. In apple fruit, [PERSON] and [PERSON] [33] suggested that each peak of the multi-exponential NMR relaxation signal describes the water in the specific cell compartment (vacuole, cytosol and apoplast). Indeed, transverse relaxation time T\({}_{2}\) is sensitive to the chemical composition of water solution in each compartment, to the size of the water compartment, and to the diffusional process between different water pools. Water proton relaxation times are related to the rotational and translational mobility of water molecules [34].