Steady-state chlorophyll fluorescence (Fs) measurements as a tool to follow variations of net CO₂ assimilation and stomatal conductance during water-stress in C₃ plants

The steady-state chlorophyll fluorescence (Fs) presented different diurnal variation patterns in irrigated and water-stressed plants, the latter showing a midday depression, which was more pronounced as more severe was the stress. Here we address the possible causes of such variations and discuss the applicability of Fs for stress assessment. In water stress experiments with several C₃ plants, the ratio of Fs normalised to dark-adapted intrinsic fluorescence (Fo) was negatively and exponentially correlated with non-photochemical quenching (NPQ). The relationship was abolished by treatment with DTT, an inhibitor of xanthophyll de-epoxidation. These and other evidences suggest that increased non-radiative dissipation under drought was responsible for Fs variations. Interestingly, the ratio Fs/Fo fixed at a given high light intensity directly correlated with CO₂ assimilation in air, with electron transport rate and with stomatal conductance. Therefore, the ratio Fs/Fo, which can be measured with a remote sensing system, provides a good method for the early detection of water stress.