Canopy imaging spectroscopy reveals a stepwise redshifted energy redistribution in the antenna under drought stress

There is a consensus on the role of protein conformational changes within the photosynthetic antenna that alter the spectral properties of the embedded pigments during regulated heat dissipation, but despite this, the molecular mechanisms involved are still poorly understood. The mechanisms, associated with the quenching of excessive energy, are however commonly seen in vitro as 'red spectral forms' of Chlorophyll a or red-shifted and broadened absorbance behaviour. Similar mechanisms are expected to occur in vivo, but so far, the spectral absorbance changes have not been described in detail at the whole plant canopy level. Here we derive the dynamic changes in surface absorbance features from canopy reflectance of tomato plants (Solanum lycopersicum L.), under increasing light exposure and drought. Specific features in the green (520 nm-peak) and the red-edge (695 nm-peak) region could indicate the quick activation of quenched conformational states under low light conditions, for all plant canopies. Under additional drought stress, further red-shifted and broadened absorbance changes appear, suggesting another conformational change. The latter changes disappeared upon drought recovery. Observing these antenna-related mechanisms from proximal sensing demonstrates the promising potential of imaging spectroscopy to detect the stepwise tuning of regulated energy dissipation of plants in a non-destructive way.