Steady-state and maximum chlorophyll fluorescence responses to water stress in grapevine leaves: A new remote sensing system

A new fluorimeter built at Orsay allowed us to measure at a distance of up to 6 m both the steady-state and the maximum chlorophyll fluorescence. This instrument has been applied continuously during 17 days of water stress development to follow the chlorophyll fluorescence parameters of a potted grapevine. Gas-exchange rates for H₂O and CO₂ and chlorophyll fluorescence parameters of the same leaf were recorded concurrently. It was shown that: (1) Under well-watered conditions, before noon, a correlation was found between net photosynthetic rate and the rate of electron transport calculated from fluorescence measurements. After several hours of high light exposure, CO₂ assimilation (A) started to decrease more than the rate of electron transport (ETR). Under drought conditions, the above-mentioned correspondence was lost: when A almost vanished due to high stomatal closure, the ETR was still about 50% of the control value. It is suggested that under these conditions, the ratio of photorespiration to CO₂ assimilation increased. (2) Light response of the quantum yield of ETR became increasingly different between morning and afternoon as water stress progressed, thus serving as a good indicator of plant water status. (3) A simple fluorescence parameter, Fs, accurately reflected the plant physiological state. Over the range of light intensities used in this study, this parameter changed in parallel with irradiance in well-watered plants. With increasing water stress, Fs changed in opposite direction to irradiance changes. The response of Fs to rapid changes in irradiance was fast (within seconds). The potential of this parameter for remote sensing of water stress is discussed. (C) Elsevier Science Inc., 2000.