Why Plants Don’t Get Sunburnt

For some reason, it’s a widespread belief that watering plants in full sun damages plants. Do droplets really act as tiny magnifying glasses, as some people claim? Rather than digging into professional research immediately to reveal the truth, how would we go out and find out for ourselves?

sherlockIf I go ahead and use a sprinkler on a plant in the midday sun to see what happens, it would be a good start. But I can’t take short cuts such as merely watering a single plant’s leaves. That won’t produce anything conclusive. If I report a change, someone skeptical of the magnifying-glass- hypothesis will justifiably suggest that perhaps some of the leaves were already damaged. If, on the other hand, no changes are observed, someone with the opposite viewpoint will wonder if no droplets persisted on the leaves, or if they were at the necessary angle to the sun.

It would also be better to water a variety of plants. After all, if I observe no burning upon watering only tomato plants, I could conclude that tomatoes are not vulnerable, but everyone will be left wondering if other types of plants are. And in general, whatever variables exist should be controlled by changing only one at a time. The experiments have to be repeated a number of times to be statistically meaningful.

Thinking about it alone won’t settle the question. For example, suppose I imagine that each drop resting on the leaf does act as a magnifying glass. That would imply that the concentrated spot of sunlight potentially burning the leaf would gradually heat up as it does when a convex lens is placed at the right distance above a newspaper. But in the leaf’s case, water is sitting on the hot spot. Isn’t water efficient at removing heat? And wouldn’t the transferred heat cause the little bit of water to evaporate away? Probably not—because if you use a magnifying glass to concentrate sunlight on a wet hand, as I did, you will still feel a burning sensation. The water doesn’t remove the heat quickly enough.

But what if the typical water droplet is not at the right distance from the surface of the leaf to concentrate sunlight? If the drops are too close as one would guess, there would be no magnifying glass-effect and no damage done. In fact, upon closer observation of droplets on a citrus plant early in the morning, I notice no focusing of light rays on the leaf’s surface. The bright, tiny points I see are reflection off the surface of droplets, no matter how I angle the leaf towards the sun.P1140287d

Actual researchers used computer modelling to check if droplets were at the right distance and then did tests on real leaves. Only some tropical plants with hairy leaves held the droplets at the required distance to focus sunlight, but in the field, the droplets evaporated before sunlight caused any damage. From the point of evolution, the conclusion is not startling. After a rainfall, given that the sun often appears before droplets have had a chance to dry off, a fair amount of damage would have ensued since the dawn of foliage, with or without gardeners’ advice.

But if there’s a grain of truth in every myth, the one involved here is that strong sunlight could potentially damage a plant but by an alternate mechanism. In humans, the ultraviolet portion of sunlight affects DNA, potentially leading  to the excess production prostaglandins and cytokines in just a few hours. These compounds stretch blood vessels, leading to redness and pain. If the DNA is damaged enough, the cells die, leading to peeling. In many dark-skinned humans this rarely happens because their skin contains high concentrations of pigments known as eumelanins. They absorb UV and through proton transfer, quickly convert UV to heat. Similarly plants could potentially be damaged by singlet oxygen, a more reactive form of reactive oxygen gas which can form from exposure to intense sunlight, but plants usually convert the latter to harmless infrared.

Zeaxanthin_molecule_spacefill

Zeaxanthin is the particular xanthophyll that’s formed from violaxanthin in bright conditions and helps protect plants from intense sunlight. This yellow pigment is also responsible for the colour of corn and is mixed with carotenes in paprika.

Fluorescence spectroscopy has helped unravel the protective mechanism in both plants and humans.. The melanin-like shield in plants is the light-harvesting complex (LHC). It’s a group of proteins that embed the chlorophylls and accessory pigments needed for photosynthesis. When sunlight becomes intense, increasing photosynthesis leads to further acidification. The pH-drop in turn leads to altered conformations of LHC proteins. This changes their position relative to xanthophylls pigments, drawing them closer after they too had undergone changes in highly bright conditions.   Xanthophylls, a type of carotenoid, are known for their ability to absorb photons of frequencies that chlorophylls are blind to.  But here xanthophylls have a second role. From the approaching LHC protein, xanthophylls pick up the potentially damaging energy and dissipate it by vibrating their long tail-like structure.

Other source:

http://www.nature.com/nchem/journal/vaop/ncurrent/full/nchem.2818.html?foxtrotcallback=true

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Some Leading Questions About Lead

EiU9fU4Z2NV7CzJweYbiNxGnThe ancients were drawn to lead because of its combination of properties: low-melting, very malleable and resistant to corrosion. But it turned out to be very toxic. Although lead’s dark side was revealed a long time ago, lead poisoning is still not a thing of the past. The health of workers and children continue to be seriously compromised as precautions in dealing with lead are ignored domestically in several places and abroad. Hopefully this set of four questions and answers will help the reader become a more vigilant and enlightened citizen with regard to lead.

1. A while ago, one of my summer jobs as a student involved working for a lab in a copper refinery. Why did our blood have to be monitored for lead when lead is not an impurity of copper ore?

Although lead is not an impurity of copper ore, silver, platinum and gold are. After electrolysis causes copper to deposit on the anode, the precious metals precipitate at the bottom of the electrolyte solution. After gold is separated from the other metals, its purity is measured by an ancient but accurate technique know as fire assay. And that’s where lead(Pb) enters the picture. Pb is used in that type of analysis because of its ability to dissolve gold and leave impurities behind. Its melting point is lower so that it can subsequently be separated from the purified gold. The gold sample is weighed before and after the assay, completing the purity-calculation.

Before the 1980s, when precautions were not taken in our fire assaying lab, the technicians developed high levels of lead in their blood. They were given paid leave to recover. When they returned to work, they were provided with a lead-free area in which they kept their non-work clothes apart from the ones worn in the lab . They had to wear respirators during the analyses and ventilation was improved. Routine blood analyses also became the norm for all employees in case they were assigned to work in the fire assay department or in nearby labs.

Unfortunately due to low standards, lead is still a significant occupational hazard in China. As the Chinese authors of this 2010 study point out:

Lead (Pb) and its compounds remain the leading cause of chronic poisoning (51.83%), followed by benzene (13.74%) and trinitrotoluene (TNT) (11.05%); 82.97% of the cases were distributed in medium- and small-scale enterprises. Nowadays, the risk factors of occupational poisoning have been transferred rapidly from developed countries to our country, from urban to rural areas, from developed areas to less developed regions, and from the formal to the informal business sectors.

thecloudmindershd0263.jpg

In the 1969 Star Trek episode The Cloud Miners, the higher class consider themselves superior to the Troglytes who work in mines. With high conventional IQs but low ecological-IQs, the elite didn’t realize that the work environment of their servants exposed them to a poisonous gas that rendered them aggressive and mindless. Undoubtedly, this episode was inspired by the fact that children of poor neighborhoods and certain occupations are more exposed to lead and its serious consequences.


2. Why is lead so toxic?

Lead can damage through a variety of mechanisms, but we’ll look at the two most important ones.

A- Oxidative stress.

Flora_F1

Lead(Pb) increases free radical production(specifically, reactive oxygen species(ROS))  and lowers cells’ capacity to defend against free radicals. Diagram is from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3485653/

Organisms either deliberately or inadvertently create uncharged, short-lived molecules with unpaired electrons. These so-called highly reactive free radicals can come in handy, for example, as one of the immune system’s defensive mechanisms. But free radicals can also turn against the host. Lead’s presence causes an overproduction of free radicals while also interfering with our ability to repair the damage from free radicals. (see adjacent diagram)

B- Ionic mechanism. A common charge for the lead ion is +2. Its ionic radius is also similar enough for cells to confuse it for calcium +2. This so-called ionic mechanism is what makes it toxic neurologically. Disguised, lead ion sneaks in across the protective blood-brain-barrier, and accumulates in astrocytes, specialized cells that help keep brain chemistry in balance.  astrocyteThey can get Pb2+ out of the way with the help of binding proteins But the young developing brains of children have immature astrocytes that lack lead-binders. Lead’s unconfronted presence damages the specialized cells, interferes with the formation of the protective myelin sheath, and compromises the development of the blood-brain-barrier.

The ionic mechanism interferes with neurotransmitters, which lowers the intellectual quotients of children who have been exposed to lead. Children, when compared to adults, retain 3 to 6 times as much of the ingested toxin.


3. Why are there no excuses for the existence of lead pipes in our cities?

Considering the deadly nature of lead poison and the fact that so many natural waters dissolve this metal, it is certainly in the cause of safety to avoid, as far as possible, the use of lead pipe for the carrying of water which is to be used for drinking.

Amazingly, the above is from an 1845 report on water supplies for the city of Boston. Despite the warning, cities went ahead and used lead pipes in building water-distribution networks in the late 19th and early 20th centuries. Currently over 100 000 citizens in Montreal  are served by lead pipes.  Only in 2006 did they introduce a 20- year plan to completely remove them. In the meantime city officials advise households in affected neighborhoods with pregnant women and young children to install charcoal filters.

Luckily, leaching of lead is reduced by the introduction of phosphate ion during the water treatment phase. It causes a protective scale to build up inside the lead pipe, keeping most of the lead out of the drinking water. But such a procedure is a temporary, albeit, vital solution. When that chemical treatment has been ignored, tragedies such as the one in Flint, Michigan in 2014-15 have ensued. Meanwhile other American cities with ageing infrastructures continue to violate recommended concentrations of lead in drinking water.


 4. Were lead levels in some Flint, Michigan homes so bad that they could have been detected by a simple wet-chemistry test?

Having often shown students how lead ion’s presence can be revealed by adding iodide ion and creating a startling canary-yellow precipitate, I wondered if an informed citizen of Flint would have been able to see such a product from carrying out the test on their drinking water.

p1060146During the crisis in Flint Michigan, homes had as much as 13 200 ppb (part per billion) or 13.2 mg /L or 13 ppm of lead. The simple iodide test has a   threshold of 20 ppm, so the answer is no; the wet-chemistry test is not sensitive enough, but astonishingly, the concentration of lead in their tap water wasn’t far off from the wet-chemistry’s detection limit.  Instead, atomic absorption was required to detect concentrations in the range of 0.005 mg/L  (= 5 ppb = recommended maximum) to 20 mg/L. It was thanks in part to the inspirational volunteer lab work of the Virginia Tech Research Team that the lead problem in Flint was brought to light.

Other Sources:

Casarett & Doull’s Toxicology: The Basic Science of Poisons. McGraw &Hill

Brent JA. Review of: “Medical Toxicology” Clin Toxicol. 2006;44:355–355.

Bellinger DC. Lead. Pediatrics. 2004;113:1016–1022. [PubMed]——

Click to access water-corrosion-eau-eng.pdf

http://www.healthycanadians.gc.ca/health-system-systeme-sante/consultations/lead-drinking-water-plomb-eau-potable/document-eng.php#a1

C and Eng Volume 94 Issue 7 | pp. 26-29 | Latest News Web Date: February 11, 2016

Arsenic Still Leaching from Old Decks

ccabnrIn January of 2004, Canada and the United States started to phase out the manufacture and sale of wood treated with copper, chromium and arsenic (CCA). CCA is used in pressure-treated wood to reduce rotting from insects and microbial
agents. The Healthy Building Network had lobbied government agencies, basing their position partly on sound research by David Stilwell of the Connecticut Agricultural Experimental Station. His work had demonstrated that after leaching out of CCA-treated wood decks, arsenic, a known carcinogen (group 1), was consequently accumulating in nearby garden soil and plants. The toxin can also transfer to skin and become ingested if children play on such decks.

As I explained in a letter this morning to my local member of parliament, the issue is still relevant because many neighbourhoods have CCA-treated decks built before 2004. Many of these have not been well-maintained and stained regularly, and acidic precipitation causes arsenic to be released from treated wood. ( It’s long been known that arsenic is a problem, so I wonder if the original tests, on what was then a newly treated wood-product, were carried out with tap water. At a higher pH, far less arsenic would have leached out. But as David Stillwell indicated, outside of the laboratory, decks are subjected to acidified precipitation, which is capable of dissolving more of the carcinogen.)

Many residents are still not aware of this problem, and I hope our mp will use her influence to help enlighten the public. She is a former colleague, so I hope that it will make a difference. The previous member of parliament did not carry out a much needed and recommended information campaign.

Thanks to a bank sponsor, a water-testing kit was freely distributed to Canadian high schools in 2008, and I used it to get a taste of Stilwell’s work. My report was published in University of Waterloo’s newsletter for chemistry educators.  If teachers obtained class sets of the kit, they too could have easily create meaningful labs like the one that I’m about to describe.

Since the kit is designed to test the amount of arsenic (other tests include manganese, sulphate, chloride, etc.) in water samples, I took about 5 grams of a 5-year old unstained sample of treated wood. Using pliers I cut it into several 1-cm to 2-cm pieces and let soak for 5 minutes in about 100 mL of tap water. After decanting the water into a thin bottle, I successively applied the three reagents of the modified Gutzeit test, which I will describe in detail later.
I also repeated the procedure using 1 mL of vinegar diluted a hundredfold to simulate the approximate average pH of rain samples in Eastern Canada. Of course, to make sure that the arsenic was not coming from the tap water itself, I ran a blank by applying the arsenic test to tap water that had not come into contact with treated wood.
The  results are summarised in the following table.

arsenicCCA

Admittedly, chopping the treated wood exaggerated the surface area actually exposed in the field, but a mere 5 minutes of soaking greatly underestimated the actual amount of leaching and physical displacement that occurs over the lifetime of a wooden deck or Hydro pole. For instance, David Stilwell found, next to 8 utility poles in Canada, an average of 262 mg of arsenic per kg of soil.

Here’s the chemistry behind the analysis, the so-called modified Gutzeit test.
The first reagent that’s added to 100 mL of sample contains tartaric acid with enhancers. In step 3 it will be evident why the acid is needed. Reagent #2, an oxidizer, is then added to remove hydrogen sulfide interference. Finally, with the addition of pure zinc powder reagent, inorganic arsenic ions (As3+ and As5+) are reduced to As3-, which, in the presence of acid, forms arsine (AsH3) gas:
(in the presence of As3+)
As2O3 + 6 Zn + 12 HCl → 2 AsH3 + 6 ZnCl2 + 3 H2O
(in the presence of As5+)
H3AsO4 + 4 Zn + 8 HCl → AsH3 + 4 ZnCl2 + 4 H2O
The arsine gas comes out of solution and reacts with a test strip
containing mercury (II) bromide (HgBr2), forming mixed arsenic
mercury halogenides:
x AsH3 + y HgBr2 → AsH2HgBr + As(HgBr)3 + other variants
Colours ranging from white to shades of yellow or brown depend on the amount of halogenides formed, which are proportional to the amount of arsenic ions in the sample.
Once the reaction is completed, the test strip is removed and matched to a colour chart to obtain the exact number of ppm (mg/L) of arsenic in the tested sample.
Safety Tips
The reactions should be carried out in a fume hood in case any arsine escapes and the reagent on the mercury strips should not be touched with bare hands. Any readers who are concerned about exposure to arsenic from decks constructed prior to the CCA ban should be relieved to learn that leaching can be greatly diminished by oil-based, semi-transparent wood stains, the kind that soak into the wood. In fact, Stilwell found significantly lower concentrations of arsenic in the soil next to CCA decks that were well maintained.

References
1. http://www.epa.gov/oppad001/reregistration/cca/
2. Stilwell’s Research, D. Stilwell and K.D. Gorny, Contamination of
Soil with Copper, Chromium, and Arsenic Under Decks Built From
Pressure Treated Wood. Bull. Environ. Contam. Toxicol., 58, 22-29
(1997).

3. Legal Issues,
http://www.ct.gov/CAES/cwp/view.asp?a=2815&q=376678,
http://www.healthybuilding.net/arsenic/index.html
(accessed July 23, 2008).
4. Health and Welfare Canada’s fact sheet on CCA:

Click to access fs_cca-e.pdf

(accessed July 23, 2008).
5. Chemistry of modified Gutzeit Test
http://inspectusa.com/product_info.php/products_id/833,
http://www.patentstorm.us/patents/6576143/description.html
(accessed July 23, 2008).