Measurements of leaf reflectance in two narrow wavelength bands centred around 530 and 570 nm were carried out during withering of two contrasting functional and structural types (annual vs. evergreen): Chenopodium quinoa (Willdenow) and Arbutus unedo (Linnaeus). The photochemical reflectance index PRI = (R530 - R570) / (R530 + R 570) was calculated. The results showed that PRI could be used as a reliable index for a non-destructive and low cost assessment of water stress. However, some limits with the PRI have to be emphasised, e.g. the wilting of leaves during the dry periods. If such limits are under control, PRI could be a way in remote sensing applications.