Linking chlorophyll fluorescence signal and vegetation indices to photosynthetic activity under nutrient deficiency, heat, and drought stresses; laboratory and field measurements in monocot and dicot crops

Abstract

Abstract Only a small amount of the light absorbed by the photosynthetic pigments including chlorophylls and carotenoids is de-excited via emission as heat or red and far-red chlorophyll fluorescence under normal physiological conditions. Most of the energy is utilized for photosynthetic quantum conversion. In contrast, photosynthetic performance decreases under numerous stress effects, which is accompanied by a rise in the steady-state levels of chlorophyll fluorescence. Field crops in Hungary are increasingly exposed to extreme weather conditions. Therefore, the main objective of our field study in wheat and sunflower crops was to investigate the effects of heat and drought stress and heterogeneous nutrient availability on the vegetation by quantifying the spatial and temporal variability of photosynthetic efficiency and fluorescence. In a parallel laboratory experiment we attempted to create a pool of plants developing under controlled environment, to meet similar appearance as under field conditions. We found that simultaneous observation of multiple spectral domains and an approach based on field and laboratory measurements were adequate to assess stress and its severity for individual plants and for vegetation canopy. Vegetation indices were good tools to separate the healthy state from the stressed state, and, further combined with fluorescence parameters, we could even draw some conclusion about stress severity. Indices linked to anthocyanin and carotenoid were found to be higher in the already damaged plants, while steady-state fluorescence was higher for leaves with still functioning tissues. Above all, individual species differences were much larger than expected.