Fv/Fm can be also very sensitive to drough stress provided that you apply the following protocol (Unfortunately it is a bit costy):

with this method you will be able to track the stomatal responses which is sensitive to drought:

chlorophyll fluorescence imaging under non-photorespiratory condition were used. Leaves were placed in a flow-through cuvette. The temperature in the cuvette was 22±1 °C. The cuvette was placed under a chlorophyll fluorescence imaging system (FuorCam 700MF, PSI, Brno, Czech republic). The imaging measurement was conducted with an atmosphere with 20 mmol mol-1 O2, 380 µmol mol-1 CO2 and the rest N2 (non-photorespiratory condition) in the cuvette. The RH was set to 40±3% via passing the gas mixture through a temperature-controlled column of iron (II)-sulphate heptahydrate (Fluka). The leaf that was placed in the cuvette was exposed to a continuous irradiance of 100 µmol m-2 s-1. Once reaching the steady state, an image of  photosystem II efficiency (ΦPSII) was taken from leaves in water. Then the vial was replaced by another vial with the same volume of an ABA solution (100 µM ABA). Every 30 min the protocol for the FluorCam run and images were taken for 150 min. The average value of ΦPSII per leaf was calculated by using version 5 of FluorCam software. Values for Ft and Fm’ in the generated image were averaged over all pixels per leaf and then the ΦPSII was calculated using the ratio of the difference between (Fm’-Ft) and Fm’. To investigate if all stomata of one leaf responded similarly, frequency distributions were analysed by using the individual values for Ft and Fm’ in the generated image. To ensure that the decreased ΦPSII was due to stomatal closure, at the end of imaging ΦPSII for each treatment an image was taken in an atmosphere with a high CO2 concentration (20 mmol mol-1 O2, 5000 µmol mol-1 CO2) to test the recovery of ΦPSII.

Water stress in the field is aassessed by IR thermometry. Stress induces a shift in leaf energy balance. This principle is applied on thermo images as well by unexpensive IR thermometers. Principles are in Jackson et al http://scholar.google.it/scholar_url?url=http://naldc.nal.usda.gov/download/54608/PDF&hl=it&sa=X&scisig=AAGBfm03osbIC5LAQMdqL2uTF6fagWQb7A&nossl=1&oi=scholarr&ved=0ahUKEwiK08fP7rrMAhXIVhoKHYyIDqsQgAMIHigAMAA. Much less clear is how to detect Hypoxia. This may produce severe effect without apparent changes depending on species and maturity. Leaf shedding after quite a long time is a common trait often after color changes in the 400-500 nm bands due to a decrease in carotenoids and beta carotene. https://www.google.it/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwif0baU8brMAhVDXBoKHS6SDaYQFggqMAE&url=https%3A%2F%2Fwww.researchgate.net%2Fpublication%2F223772775_Sensing_of_tomato_plant_response_to_hypoxia_in_the_root_environment&usg=AFQjCNFisSdw27pIL-YAuK_Dqbn-l_jtZw&sig2=jTLYi14c2xpQeiaU4_MuqA

      Dear Yuan Ge

For a long time, my colleagues and I have been developed optical diagnostic methods for the precise detection of plant stress and the availability of heavy metals in soil in a wide wavelength range 0.4-14mkm in the field conditions and from space.

Drought and over damping are specific kinds of plant stress.    

     At last time for their diagnosis during the vegetative cycle of the crops development  we use an active optical Red/Ired tester. Defined values are NDVI and its variability (CV-NDVI and RMS-NDVI). Forms of these dependencies for this phase and type of impact on planting depended on meteorological conditions. Special calibration of the received data allows us to identify abnormal weather conditions related to drought and over damping (drowing).

 Vladimir Surin  [email protected]

Dear Yuan Ge

I think you can measure plants' drought stress via Proline or carbohydrate determination that increase under stress condition comparing to untreated control. 

Hi,

I'm using filed spectroscopy (see attached files)

Best