Photochemical reflectance index as a mean of monitoring early water stress

Water stress in plants affects a number of physiological processes such as photosynthetic rate, stomatal conductance as well as the operating efficiency of photosystem II (PSII) and non-photochemical quenching (NPQ). Photochemical reflectance index (PRI) is reported to be sensitive to changes in xanthophyll cycle which occur during stress and could possibly be used to monitor changes in the parameters mentioned before. Therefore, the aim of this study was to evaluate the use of PRI as an early water stress indicator. Water stress treatment was imposed in a greenhouse tomato crop. CO2 assimilation, stomatal conductance, light-adapted and dark-adapted fluorescence as well as PRI and relative water content (RWCs%) of the rooting medium were repeatedly measured. The same measurements were also performed on well-irrigated plants that acted as a reference. The experiment was repeated in four consecutive weeks. Results showed a strong correlation between RWCs% and photosynthetic rate, stomatal conductance, NPQ and operating efficiency of PSII but not with PRI when the whole dataset was considered. Nevertheless, more detailed analysis revealed that PRI gave a good correlation when light levels were above 700 mu mol m-2 s-1. Therefore, the use of PRI as a water stress indicator cannot be independent of the ambient light conditions.