Plant Cuttings

This image of an aerosol spray can by PiccoloNamek is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license. Subject to disclaimers.

We start today’s post with this text in the Holy Bible, from the Gospel according to Matthew Chapter 6, verse 28, “Consider the lilies of the field, how they grow; they toil not, neither do they spin”. Taken at face value – and I’m not a Bible scholar* – this statement could be interpreted as meaning that lilies, and, by extension, all plants, ‘toil not’, i.e. they don’t undertake ‘work’. Well, I am a Botanist, and I tell you that plants do work. Compared to people, they work jolly hard. But, don’t just take my word for it. The next paragraph provides the justification for that assertion (i.e. the evidence/sources to support that statement).

Many (most? all?) humans with a job can’t wait for the end of the working week and the weekend (Andrew Bryce) when they can stop working and do something more to their liking. Which is fine. But, when they do, and are enjoying their ‘downtime’, maybe they should spare a thought for the plants. Indeed, even during the working day they should salute their hard-working green neighbours. Why? Because, and unlike most people nowadays, when they are working**, they do so from sun-up to sun-down. Whilst that’s not ‘24/7’, it’s considerably more than a person’s typical ‘9 to 5’. And the plant’s hours of work are only restricted in that way because its gainful employment is photosynthesis (Summer Beach). Since, photosynthesis is the manufacture/synthesis of organic compounds using sunlight as an energy source, by its very definition it can only take place when it’s sunny, i.e. between sunrise and sunset. So, in terms of available hours in which to work, plants are still considerably more active than their human ‘competitors’. If one was keeping score, that’s definitely, a case of Plants 1, People 0.

But, let’s talk more about the bit at the beginning of the previous paragraph, the weekend. As long as the sun shines, plants will continue to work on Saturday and Sunday [isn’t every day potentially a ‘sun’ day for a plant..?] whilst humans are having a couple of days off. Not only that, but in some parts of the world they actually work harder at the weekend than during the week (when we’ve already established they’re putting in the hours more so than human labourers). This notion comes from a study by Liyin He et al. (Proceedings of the National Academy of Sciences (2023) 120(49) e2306507120; doi: 10.1073/pnas.2306507120). Interpreted for, and communicated to, the general public by Chen Ly with the eye-catching title “Plants are more productive on weekends thanks to cleaner air”, the research concerns the response of plants to atmospheric pollution.

At the start, it’s important to state that He et al. didn’t actually measure photosynthesis (Summer Beach) – uptake of CO₂, formation of sugars, and release of O₂. Instead, they recorded the fluorescence (Joe Schwarcz) from chlorophyll, which “solar-induced fluorescence (SIF)*** strongly correlates with photosynthesis” (He et al.). The advantage of that remote sensing approach over measuring inputs to, or outputs from, photosynthesis up close to the plants on the ground, is that it is can be performed using man-made satellites (Pawan Gupta & Melanie Follette-Cook) that are stationary above, or orbiting, the Earth. Which means that large areas of the planet’s surface can be scanned to give data that is more meaningful than the limited number of measurements that human workers can make directly upon plants in very small areas within the same time period.

When the SIF data were examined in connection with another set of data that showed the pollution in the atmosphere over the same period of time, and for the same areas of vegetation, He et al. found a pronounced negative correlation between the two. When atmospheric pollution levels were high, SIF was lower, which situation was reversed when pollution levels were reduced. In terms of a week, SIF – a measure of photosynthesis – was higher at the weekends, over 64% of Europe. This relationship was uncovered using historic data from the years 2018 – 2021.

The particular form of atmospheric pollution examined by He et al. was aerosols (MO Andreae & D Rosenfeld, Earth-Science Reviews 89: 13-41, 2008; https://doi.org/10.1016/j.earscirev.2008.03.001), which are small particles or droplets (Karin Kirk; Gunnar Myhre et al. (2013). Nature Education Knowledge 4(5):7) rather than gases, and whose effect upon photosynthesis has been known for decades (Siegfried Gerstl & Andrew Zardecki, Nature 300: 436–437, 1982; https://doi.org/10.1038/300436a0). Although aerosol particles include such things as dust, pollen, and sea spray (He et al., 2024), a significant contribution to this category comes from human activity, e.g. the combustion of fossil fuels that are “burned in industry, power plants, vehicles, and residential heating” (MO Andreae & D Rosenfeld, Earth-Science Reviews 89: 13-41, 2008; https://doi.org/10.1016/j.earscirev.2008.03.001). Because they were able to distinguish aerosols from human sources from those of non-anthropogenic origin in their study, He et al. were able to conclude that the ‘photosynthetic-depression’ “is likely due to fewer human activities such as industrial and transportation emissions on weekends”.

In other words, a considerable amount of the atmospheric pollution was ascribed to aerosols produced by workers commuting to – and from – their places of work in motor vehicles. At the week-end, when many fewer fossil fuel-fuelled commuting journeys were undertaken, aerial pollution was considerable reduced, and the plants ‘breathed a sigh of relief’ [Indeed, they could ‘breathe’ (which is one way to envisage the uptake of CO₂ by plants – and is in keeping with statements that view the tropical rainforests – especially that in the Amazon – as “the lungs of the planet” (Javier Yanes, Frances Lopez)) much more easily than during the polluted days of the working week.

A major piece of evidence supporting the view that the driver for reduction in SIF was atmospheric aerosol pollution was what happened during the period(s) of lockdown during the COVID-19 pandemic in 2020****. This year was excluded from the studied data set of 2018 – 2021 because it showed “a less pronounced or even reversed weekly pattern” (He et al.), which was ascribed to the “lower level of aerosols due to decreased human activities, such as industrial and transportation activities, caused by the pandemic and related policies” (e.g. Zander S Venter et al., PNAS 117(32): 18984-18990, 2020; https://doi.org/10.1073/pnas.2006853117).

Whilst many pollutants in the atmosphere – such as NO_x [various oxides of nitrogen], and O₃ [ozone] – can cause direct harmful effects upon plants (e.g. Benjamin S Felzer et al., Comptes Rendus Geoscience 339: 784-798, 2007; https://doi.org/10.1016/j.crte.2007.08.008; David B Lobell et al., Science Advances 8, eabm9909 (2022); doi: 10.1126/sciadv.abm9909) – which may affect their ability to photosynthesise, aerosols act somewhat differently. Some aerosols absorb sunlight, whilst others scatter it in the atmosphere (Andrew Young). The effect of which is to reduce the amount of sunlight that reaches the plants at the Earth’s surface, which therefore reduces the amount of photosynthesis. As a species that is dependent upon crop plants – either directly e.g., as vegetable, fruits, and cereal products – or indirectly via the domesticated animals fed on cereals, etc. that we consume, this reduction in photosynthesis is of concern. But, it’s not just crops that are affected; this aerosol effect is likely to impact all photosynthetic organisms. And, since all terrestrial, and land-based aquatic, habitats are essentially fuelled by the photosynthetic efforts of plants or algae, the effects are ecosystem-wide.

Any improvement in this aerosol-induced impairment of photosynthesis is therefore likely to have ‘knock-on‘ effects in terms of better ecosystem functioning more generally, to the benefit of all living things. Whilst Improvement in air quality is often discussed in terms of the health benefits to humans (Ioannis Manisalidis et al. (2020) Front. Public Health 8:14; doi: 10.3389/fpubh.2020.00014; Daniele Contini et al., Atmosphere 2021, 12(6), 730; https://doi.org/10.3390/atmos12060730; Regina MBO Duarte and Armando C Duarte, Atmosphere 2023, 14(2), 309; https://doi.org/10.3390/atmos14020309)****, it won’t hurt matters to broaden the conversation to include consideration of wider ecological benefits.

This depression of weekday photosynthesis is not trivial, either; He et al. estimate that an additional 41 Mt of CO₂ uptake by European ecosystems could be achieved if aerosol levels are reduced to that seen during the COVID-19 period in 2020. And, that’s just in Europe. Presumably extra CO₂ uptake could be achieved worldwide if the aerosol depression of P/S is a global phenomenon. Although, He et al. do point out that elevation of photosynthesis would only occur in areas that are light-limited – e.g. high latitude ecosystems – so any ‘bonus photosynthesis’ will not be evenly-spread, world-wide. Nevertheless, any ‘extra’ photosynthesis is arguably better than none.

Another important note of caution is made by He et al. in respect of proposals to ‘geoengineer’ global temperature increases, in particular the ‘stratospheric aerosol–based approach’ whereby fine particles [aerosols] released into the atmosphere would create a temporary shield that would reflect sunlight back into space and reduce the temperature of the atmosphere (Ken Caldeira et al., Annual Review of Earth and Planetary Sciences 41: 231-256, 2013; https://doi.org/10.1146/annurev-earth-042711-105548). Whilst such an approach should have an impact on global temperature, it’s also likely to have adverse consequences such as depression of crop yields because of reduced photosynthesis in mid-high latitudes. Furthermore, reduced photosynthesis will also mean lower uptake of CO₂, a major atmosphere-warming, greenhouse gas (Rebecca Lindsey). So, although He et al’s results “highlight that reducing aerosol pollution could have a significant positive impact on ecosystem productivity and carbon sequestration”, they caution that “These unforeseen results underscore the intricate nature and inherent uncertainties associated with manipulating the Earth system on a large scale, highlighting the need for cautious and comprehensive evaluations when considering geoengineering as a potential solution to global climate challenges.”

For scicomm versions of this study, see here, here, here, Chen Ly, and Chrissy Sexton.

* In the interests of balance, I should say that the more usual interpretation of this passage is in terms of clothing of the human body: ”Lilies of the field stand rooted in place and grow without doing any work or spinning any clothes for themselves. Still, as Jesus will say in the following verse, they are dressed in great beauty. They are clothed exactly as God the Father wants them to be”.

** OK, plants are still ‘working’ when they’re not performing their main ‘job’ of photosynthesis (and as well as when they are…) – just like human labourers during their main employment – and engaged upon all manner of metabolism, and physiology, growth and development at all times of the day. The focus in this post is on the main ‘job’ of plants, photosynthesis. Keeping with the work analogy, there is another plant-people employment parallel.

Whilst working humans may take a ‘siesta’ (Jay Summer & Anis Rehman; Shahabir Sami), a break from work around midday or early afternoon, plants often do, too. However, theirs isn’t out of choice, it’s largely enforced upon them because of stomatal closure resulting from water-supply-restrictions at that time. The stomata (Regina Bailey) are the orifices within the epidermis of various aerial surfaces of the plant through which gas exchange occurs, e.g. entry of CO₂ for photosynthesis, and exit of water vapour from transpiration (Melisa Petruzzella).

When the plant is sufficiently hydrated the stomata can remain open. Under that condition, the plant has ready access to CO₂, even though that is seen to be at the expense of loss of water in transpiration. However, if the supply of water [which is essential for the plant’s survival] becomes short – e.g. due to high temperatures from strong sunlight drying out the soil, which is the main source of water for plants – then mechanisms within the plant come into play that result in closure of the stomata (Melissa Ha et al.; Hadi Pirasteh-Anosheh et al.). This reduction in diameter of the stomatal pore – and it need not be complete, partial will be enough – impairs gas exchange between the plant and the atmosphere. In practice this means that it not only restricts further transpirational water loss from the plant through the stomata [which self-preservation mechanism helps to prevent the plant dehydrating and dying], but also reduces uptake of CO₂ for photosynthesis. Hence, there may be a midday-depression in photosynthesis (Shin-ichi Sawada, Japan. Jour. Crop Sci. 47(1): 18-24, 1978; Eri Maai et al., Plant Production Science 23: 172-181, 2020; doi: 10.1080/1343943X.2019.1673666; Xing Li et al., Science Advances 9, eadi0775(2023); doi: 10.1126/sciadv.adi0775) by the plant, or even a halt to that process. Although that may look a little like human’s siesta time, it’s rather more serious than that; if sustained for long enough the plant may die from a combination of ‘starvation’ and dehydration. [Although, if a human fails to return to work after a siesta s/he may lose their job and the wherewithal to pay for the necessities of life, and enter in a downward spiral that may result in their demise…].

*** More on SIF (solar-, or sun-, induced fluorescence), and its relevance as a ‘proxy’ [something that can be measured, but which isn’t the thing you want to really want to measure – the amount of photosynthesis in this case, which can be used as an alternative to the thing you’re really trying to measure…] for photosynthesis can be found here, and in articles by Jonathan O’Brien, Lianhong Gu et al. (New Phytologist 223: 1179-1191, 2019; https://doi.org/10.1111/nph.15796), Troy S. Magney et al. (PNAS 116(24): 11640-11645, 2019; https://doi.org/10.1073/pnas.1900278116), Gina H Mohammed et al. (Remote Sensing of Environment 231, 15 September 2019, 111177; https://doi.org/10.1016/j.rse.2019.04.030); Meng Guo et al. (Remote Sens. 2022, 14(6), 1329; https://doi.org/10.3390/rs14061329); Xingan Chen et al. (Sci Data 9, 427 (2022); https://doi.org/10.1038/s41597-022-01520-1); Yao Zhang & Josep Peñuelas (Journal of Remote Sensing 19 Sep 2023 Vol 3 Article ID: 0085; doi: 10.34133/remotesensing.0085); Ying Sun et al. (Global Change Biology 29: 2893-2925, 2023; https://doi.org/10.1111/gcb.16646); Ying Sun et al. (Global Change Biology 29: 2926-2952, 2023; https://doi.org/10.1111/gcb.16634).

**** It’s nice to think that there may actually have been a ‘COVID benefit’ The only other one that I’m aware of is the discovery of ‘homeworking’, the realisation that – some – employees can actually do work at, or from, home, i.e. remotely (Susan Lund et al.), instead of commuting to and from the workplace. Whilst WFH helped to reduce pollution during the lockdowns [as did employees at home on furlough (Harriet Clark)], its persistence amongst the work-force post-lockdown (Susan Lund et al.) will presumably contribute to an on-going effect in reducing atmospheric pollution. Which wouldn’t be a bad COVID legacy.

And mindful of the words of Liyin He “There’s less traffic and industrial activities on the weekend. But during the weekdays, the air is dirtier, so we see a strong weekly cycle” [quoted in Chen Ly], got me thinking about another type of cycle, the bicycle (Frank Berto). If employees who still need to commute could be persuaded to do so using bicycles rather than cars – or even walk – that would give us less-polluted air, for the benefit of plants and people. Not only should enhanced photosynthesis draw-down more of the excess CO₂ in the atmosphere that’s contributing to global warming/climate change (Sarah Fecht; Rebecca Lindsey), but increased crop productivity should mean more food. The improved air quality should be better for human health, and the exercise from cycling or walking will provide further health benefits. All of which now looks something like, Plants 1: People 3(!)*****.

***** However, there is no room for complacency nor time to rest upon one’s laurels because the World Health Organization [WHO] estimated in 2022 that “Almost the entire global population (99%) breathes air that exceeds WHO air quality limits, and threatens their health”. The magnitude of which issue is graphically underlined by the words of Dr Maria Neira, WHO Director, Department of Environment, Climate Change and Health, “After surviving a pandemic, it is unacceptable to still have 7 million preventable deaths and countless preventable lost years of good health due to air pollution. That’s what we’re saying when we look at the mountain of air pollution data, evidence, and solutions available. Yet too many investments are still being sunk into a polluted environment rather than in clean, healthy air”. Maybe it’s time for us all to get on our bikes?