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

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.

Ethylenediurea (EDU) spray effects on willows (Salix sachalinensis F. Schmid) grown in ambient or ozone-enriched air: implications for renewable biomass production

“…Data were analyzed with a General Linear Model (GLM) at α = 0.05 limited, and the status of young, top leaves has rarely been studied because commonly the mature/older leaves are studied. Considering that chlorophylls have an Mg atom in the center of closed ring tetrapyroles, our study newly suggests that Mg in young top leaves may be enriched in the framework of a strategy to precondition new leaves so to protect them against O 3 -induced inhibition of chlorophylls; thus, keeping chlorophylls at 'normal' levels (no significant effect on SPAD was found) (Agathokleous 2021). Mg contributes to plant health, and, in addition to its physiological benefits for active growth, is involved in plant resistance to diseases, such as protecting tissues against degradation by pectolytic enzymes of some pathogens (Huber and Jones 2012).…”

Hormesis: Transforming disciplines that rely on the dose response

“…It provides the foundation for understanding biological plasticity, allometric management of biological resources under constitutive and stressed conditions, fundamental strategies for protection against immediate and future threats, the signaling processes for a broad spectrum of pheromones for sexual reproduction, alarm notifications, and other functions 89,91 . The hormesis concept has profoundly important implications for enhancing agricultural productivity via multiple strategies and tactics (e.g., by improving photosynthesis and plant defense, enhancing photosynthates, and increasing crop yields) and public health practices as well as improving therapeutic applications (e.g., enhancing wound healing, accelerating bone fracture repair/reunion, preventing/reversing hearing loss, slowing the onset of cataracts, preventing/reversing baldness, preventing heart attacks and strokes as well as lessening the damage when they do occur, as well as slowing the onset and reducing the severity of a broad spectrum of neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, and ALS) 92–97 …”

“…89,91 The hormesis concept has profoundly important implications for enhancing agricultural productivity via multiple strategies and tactics (e.g., by improving photosynthesis and plant defense, enhancing photosynthates, and increasing crop yields) and public health practices as well as improving therapeutic applications (e.g., enhancing wound healing, accelerating bone fracture repair/reunion, preventing/reversing hearing loss, slowing the onset of cataracts, preventing/ reversing baldness, preventing heart attacks and strokes as well as lessening the damage when they do occur, as well as slowing the onset and reducing the severity of a broad spectrum of neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, and ALS). [92][93][94][95][96][97]…”

Low Levels of Contaminants Stimulate Harmful Algal Organisms and Enrich Their Toxins

“…As a result of the physiological hormetic controls (Figure ), cell density and proliferation, growth rate, as well as biomass exhibit hormetic responses to contaminants. ,− ,,− These hormetic responses appear generally across different stresses, highlighting that low, sub-NOAEL doses of contaminants can act in promoting the growth of the population of harmful algae, forming more robust, dense, and competitive colonies. The broad hormesis literature, including algae, demonstrates that the low-dose enhancement is restricted by the limits of biological plasticity. ,− Thus, the stimulation is modest in amplitude, typically, 30–60%, and rarely exceeding 100%, independently from the organisms, biological mechanisms, and stressors. ,− Similarly modest, but significant, are also the responses of chlorophylls, photosynthesis, and harmful algal growth/densities or proliferation to low doses of contaminants. − ,− ,,− , For instance, the stimulation of M. aeruginosa growth (cell density) by antibiotics was commonly less than 60% and as a rule smaller than 100%. − ,…”