The rise and fall of photosynthesis: hormetic dose response in plants

The recent recognition that low doses of herbicides, human and veterinary antibiotics, metallic elements, micro/nano-plastics, and various other types of environmental pollutants widely enhance chlorophylls in the framework of hormesis created the need to further evaluate the response of photosynthetic pigments and gas exchange to low doses of stresses. An analysis of about 370 values of maximum stimulatory response (MAX; percentage of control response, %) of chlorophylls in higher plants, algae and duckweeds, and other photosynthesizing organisms, mined from published literatures, revealed a greater MAX for higher plants (median = 139.2%) compared to algae and duckweeds (median = 119.6%). However, an analysis of about 50 mined values of MAX of carotenoids revealed no significant difference in the median MAX between higher plants (median = 133.0%) and algae-duckweeds (median = 138.1%). About 70 mined values of MAX were also concentrated for photosynthetic rate (median MAX = 129.2%) and stomatal conductance (median MAX = 124.7%) in higher plants. Within higher plants, there was no significant difference in the median MAX among chlorophylls, carotenoids, photosynthetic rate, and stomatal conductance. Similarly, there was no significant difference in the median MAX between chlorophylls and carotenoids of pooled algae and duckweeds. The results suggest that the MAX is typically below 160% and as a rule below 200% of control response, and does not differ among chlorophylls, carotenoids, photosynthetic rate, and stomatal conductance. New research programs with improved experimental designs, in terms of number and spacing of doses within the "low-dose zone" of the hormetic dose-response relationship, are needed to study the molecular/genetic mechanisms underpinning the low-dose stimulation of photosynthesis and its ecological implications.