We aimed to study the change in the spectral signature associated with both the development of photoinhibition and protective mechanisms during water stress in durum wheat. Seedlings were cultivated in pots in a green-house during summer time. In order to provide water stress, water was not supplied for 8 days. Photosynthetic and transpirative gas exchange, chlorophyllmodulated fluorescence and spectral changes in transmitted light were measured in fully and recently expanded leaves every 2-3 days. As the stress due to drought arose, the photosynthetic efficiency of the plants progressively decreased, whereas their non-photochemical quenching (Qnp) increased. Thus indicating a higher amount of incoming energy dissipating through processes other than photosynthesis. Such changes in photosynthetic efficiency and Qnp were associated with a decrease in the spectroradiometrical index PRI, whose ability to monitor the photosynthetical efficiency of leaves under drought was tested.