Abstract
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.
Highlights
Photosynthesis converts sunlight into energy, driving plant growth and productivity, and contributes in sustaining life on the planet as it uses water to release oxygen in the atmosphere
Studies with multifactorial experimental designs including different levels of co-occurring stresses suggest that lowdose stimulation of photosynthetic pigments and photosynthesis is more likely to occur under “disease conditions” in the framework of conditioning hormesis, i.e. when plants are under adverse stress induced by other factors, but the stimulation ceases as the dose of the concurrent stresses exceeds a specific threshold of adversity (Mostofa et al 2015; Gelioli Salgado et al 2019; Liu et al 2019; Soliman et al 2019; Agathokleous et al 2020b; Dong et al 2020; Gohari et al 2020b; Gohari et al 2020a; Li et al 2020c; Yang et al 2020;)
The low-dose enhancement of chlorophylls and gas exchange (Seth et al 2008; de Carvalho et al 2012; Tang et al 2018; Zhou et al 2018; Zhao et al 2019; Hu et al 2019; Liu et al 2020a) in higher plants and other photosynthetic organisms may be transient. This may indicate that the doses applied may become higher or smaller than those needed to induce maximum stimulatory response (MAX) because considerable research designed to study hormesis shows that the low-dose stimulation of chlorophylls and photosynthesis can persist throughout the plant growing season (Jia et al 2015; Nascentes et al 2018; Yang et al 2020), yet it is known that the maximum stimulation occurs over a defined time window, after which it declines (Agathokleous et al 2020b, c)
Summary
Photosynthesis converts sunlight into energy, driving plant growth and productivity, and contributes in sustaining life on the planet as it uses water to release oxygen in the atmosphere. Hormesis is a dose1–response phenomenon where low doses of stress stimulate and high doses of stress adversely affect plants, producing biphasic dose–response relationships (Cedergreen et al 2007; Belz 2008; Calabrese and Blain 2009) This dose–response relationship indicates that (1) biologically significant effects can occur at stress doses multi-fold smaller than the dose where the traditional toxicological threshold appears (i.e. noobserved-adverse-effect-level, NOAEL), and (2) prediction of effects by extrapolating from considerably higher doses to lower doses based on a linear-no-threshold perspective can generate incorrect estimates of inhibition at doses where even stimulation might occur (Calabrese and Blain 2009; Agathokleous et al 2020c). There is no integrated documentation of hormetic response of photosynthesis to various environmental stresses
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