Drought limits harvest in numerous crops worldwide and climatic models predicts that this issue will grow in the next decade. Water stress has a negative impact at different levels of the plant: limiting growth and photosynthesis, creating oxidative stress... New methods are investigated including early water stress detection method for a better water management in field and greenhouses in order to save water and prevent drought. Among the new phenotyping methods, low field NMR spectroscopy appears promising because this technique has been shown to be able to follow leaf development and to detect nitrogen deprivation in oilseed rape. This method provides an unique way for the characterization of the leaf water status at the tissue level through T2 relaxation spectra. Therefore the goal of this study is to demonstrate the sensitivity of the low field NMR relaxation to monitor the water stress of plant. Using tobacco plant as model, this study focuses on the early detection of mild water stress on leaves as this organ is very sensitive to stress. The effect of foliar biostimulant against water stress has also been studied. Three treatments have been examined: control, stressed (80 % of the field capacity), and stressed plants treated with foliar biostimulant (TIMAC SL28). For each treatment, two leaves from different ranks (4 and 8), depicting two developmental stages, were characterized through NMR and physiological measurements (relative water content, osmotic potential, Chlorophyll content...). The stress was applied from 28 days after sowing to harvest and measurements were done 42 and 57 DAS. The two leaf ranks studied had different NMR T2 spectra, confirming the ability of NMR to describe developmental stage of the tobacco leaf. For the 4th leaf rank, at 42 DAS physiological measurements were the same whatever the treatments while NMR signals associated to water in vacuoles were already affected by the slight water deficit. The result proved the sensitivity of NMR relaxation to detect slight water stress induced changes occurred in leaf tissues. Moreover, NMR results showed that foliar application of the biostimulant limited the negative consequences of drought.