Steady-state chlorophyll florecence (Fs) as an indicator of leaf photosynthesis and stomatal conductance under drought conditons

Abstract

Steady-state chlorophyll fluorescence (Fs) presented distinct patterns of diurnal variation 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. We found that, 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, measured with a remote sensing system, provides a good method for the early detection of water stress, and is a useful guide to irrigation requirements