Winds from Africa, Part 2

by Jonathan Hull

The following essay is part two of a three part series that explores how something as commonplace as dust can profoundly effect natural systems.  Part one, which can be found here, explored its effect on the biosphere as a whole.  This second part takes a microscopic look at dust on plant leaves.  The third and final essay will be published next week.  This essay will take what has been discovered in parts one and two and consider practical applications, by way of foliar sprays, in managing fertility in Ohio gardens.  Foliar sprays have the potential of becoming one of our most powerful tools available in creating healthy gardens. 

I. Reorienting

Our biosphere is an incredibly complex, dynamic and ever adapting play of forces.  In the first part of this series, we met the main actors of this play in grand elemental forms: sun, earth, water and wind.  The title role is played by life; the adaptive tissue that knits these elemental forces into a global system. 

We learned that minerals in dust are essential to the function of the Amazon rainforest as well as numerous other ecosystems.  Changing distributions of this dust and its effects on life can even modulate changes in global climate patterns ultimately driven by the earth’s orientation to the sun.

In the next part of our investigation, these same elemental actors will reprise their roles in another complex and dynamic play of forces. Only it will play out not on a global scale but on a microscopic one: the surface of a leaf. 

Figure 6

[Figure 1: Microscopic detail of Leaf.  From: Stomata Of Lavendula Dentata, Sem Photograph

Recall that this whole investigation started when I realized that the role of dust in global nutrient cycling had implications for the use of foliar applications in the garden.  These are a broad range of techniques that attempt to boost plant health by changing the microscopic environment of its leaves.  One technique that seemed to work well in my garden was one that covers plant leaves with a spray of minerals in a solution.  I was a tentative proponent because I did not understand how this could work so well. 

When I learned how much dust moves around the globe and how important it is to any number of ecosystems; the results from foliar sprays no longer seemed incidental.  At the time it was pure speculation, but the thought occurred that it was entirely likely that plants adapted the ability to absorb minerals that were deposited on its leaves from atmospheric dust.  This led me to research if this was indeed true and in doing so I learned the features of several important processes that take place on the leaf.  These features will be incredibly important to my future use of foliar applications and to the realization of their maximum benefit.

II: When the Dust Settles

When you consider where plants first emerged in the history of life, it is not surprising that in some degree, they can all absorb nutrients through their leaves.  Plants first evolved in bodies of water and most aquatic plants use their leaves as the main sites for mineral uptake.

Once inside the leaf these minerals can move throughout the plant via the liquid sap that connects the various metabolic functions that happen inside of it.  This flow of minerals is critical to the function of a plant. For instance the electrical resonance of magnesium is essential to the structure and function of chlorophyll.  Now in the case of aquatic plants, their leaves are structured in a way that allows them to be relatively open to the flow of nutrients.  Their leaves can be open to this flow because the surrounding water buffers them from the effects of the sun and wind. 


[Figure 2:  Aquatic Plant.  From: … quality for aquatic plants and fish | Activities and Information]

However, when plants moved onto land they no longer had this luxury.  Separate and specialized structures were adapted for gathering the sunlight, nutrients and carbon dioxide needed for their metabolism.  Roots, protected underground from the harsh effects of the sun and wind, became the main site for mineral uptake in land plants.  Terrestrial plants still need to gather energy from the sun, so they adapted a waxy covering on their leaves called the cuticle.  This was done mainly to protect the plant  from the loss of too much water through its leaves by evaporation.  An interesting side note for this discussion: this layer also protects the plant from excessive leaching of nutrients from its leaves by rain.  In any case, this protective layer made leaves in terrestrial plants a lot less open to the flow of nutrients than those found in aquatic plants. 

Figure 8

[Figure 3: Water on a  Leaf. From: Beyond the Human Eye: Plant Cuticles]

This cuticle layer is the main barrier to the entry of nutrients.  It is by no means impenetrable and there are practical strategies for moving nutrients through it, but this mechanism is outside the scope of this essay.  Instead we will look at a more promising avenue.  You see, the cuticle layer is not continuous because leaves cannot be completely closed to the outside world: plants also use leaves to breathe.

Plants breathe, oxygen out and carbon dioxide in, through openings called stomata. (See Figure 1 above).  Thousands of these microscopic mouths can be found on the surface of leaves.  Stomata also serve as thermo-regulators by allowing the plant to transpire water through these openings to cool the plant when needed.  A self-regulating system exits within the leaf where stomata open and close depending on environmental conditions.  The system balances the trade-off between breathing and losing too much water to evaporation.    

When a stoma opens, it forms one tiny point of interface between the water inside the plant and the atmosphere.  It might seem obvious that the stoma would be a convenient avenue for the entry of foliar minerals.  But it is not that simple.  In fact, for some time it was considered impossible for nutrients to enter the plant in this way.  Although there are thousands of stomata on a leaf, added together they still comprise a tiny surface area.  The dust particle would have to land in just the right place to make contact.  Additionally, the vapor pressure from evaporating water flows out, which would seem to prevent the movement of anything in. 

Recent research has found that a unique process that occurs on the surface of leaves in the presence of dust changes how the stomata operate.  This process transforms stomata into a main avenue for the entry of nutrients.  To explain this phenomenon, we need to understand what happens on the surface of a leaf where dust collects.  How,we need to consider, does dust interact with a leaf.

The leaves of many plants have adapted structures to facilitate the collection of dust onto their surfaces.  These include microscopic ridges in leaves that trap dust and tiny hairs that grow out of the leaves, called trichomes, which change aerodynamics on a microscopic level to pull even more dust to the leaf surface.  With such microscopic adaptations, plant surfaces have evolved into the major sink for atmospheric dust.  To give you an idea of the amount, one study in Chicago found that, urban trees, which occupy 11% of the city area, remove about 234 tons of dust from the air per year!

        Figure 9

[Figure 4: Leaf Trichomes.  From: Nettle Leaf Trichomes, Sem Photograph]

These particles are deposited on the leaf from dry dust in the atmosphere but also from particles left by evaporated rain droplets.  Which come to think of it, you might wonder how much of this dust is washed off by rain.  One study found that although the large particles of dust were washed from the leaves of trees the majority of smaller particles remained in the canopy.   The study reported here, found that 75% of nitrogen that was deposited on a spruce forest never made it to the soil but was retained in the canopy!   

Most of the particles of dust deposited on leaves are of a certain type that readily absorb moisture from the atmosphere.  These are technically termed hygroscopic particles.  On the leaf’s surface, they function in much the same way as they do in the creation of raindrops in the atmosphere; they serve as the nuclei for the condensation of water vapor.  We might think of dust as dry, but in this case when considered  at their own scale, they are positively soggy.  Even at relatively low humidity levels, dust condenses water vapor  on leaves where it would not otherwise stick.   

Taken together, the moisture these particles gather produce the provocatively named “breath figures”: microscopically thin films of water that are invisible to the naked eye.  These thin films of water remain persistent on the surface of plant leaves even under surprisingly hot conditions.  Breath figures are not pure water, but a solution of the mineral particles that attract water from the air. 

Breath figures have enormous implications for uptake of the nutrients into the leaf – especially through the stomata.   In a newly opened “pristine” leaf, the liquid sap and all of the functions it connects are largely separate from anything that might occur on the leaf surface.  Stomata on such leaves are just a tiny ports to the atmosphere.  However, this new leaf quickly attracts dust particles, which, in turn, attract tiny films of water.  Now when the stomata open, the liquid sap inside the plant connects to breath figures.  The mineral can now land anywhere on the leaf and be connected to the stomata, and so connected to all of the functions that occur inside the leaf.

This continuous film of liquid that runs from the exterior of the leaf through the stomata and into the interior of the leaf is even thought to play a role in activating the opening of stomata themselves – a feedback loop that furthers the development on this continuous liquid connection.

Solutions always try to even themselves out.  If there are differences in concentrations within the solution, called concentration gradients, the liquid will move from low to high concentration and high to low.  The force of movement caused by concentration gradients can overcome the force of pressure moving water out of the stomata by evaporation. Thus, a flow of nutrients from plant exterior to interior becomes possible. ]

Plants have adapted different responses to the formation of these continuous liquid connections.  Certain plants are particularly reliant on  inter-leaf nutrient cycling.  In the dusty grasslands of South Africa researches have shown how trees can establish themselves in poor soils, even where grasses might otherwise have a competitive advantage..  As the trees grow taller, they collect more dust and thrive that much more.  The cycle reinforces itself in a positive feedback loop.

However, it can also be the case that plant leaves provide too much of a good thing.  If there are too many hygroscopic particles activating too many stomata, and so increasing evaporation through the plant,  it can significantly lower its ability to deal with drought.  In this case we have a negative feedback loop. 

Plants that live near the coast have had to adapt to this problem.  These plants can receive heavy deposits of sea minerals from salt spray that is carried off the ocean by winds.  These plants have adapted leaf structures that rapidly shed particles so that they are not desiccated by the formation of continuous liquid connections and the resultant opening of stomata.

One issue that I will briefly mention: plants have adapted to environments that over evolutionary timescales receive relatively consistent amounts of dust.   The problem is that in last 200 years there has been an 270% increase in particulate matter in the atmosphere – an increase caused by industrialization.  Plants that had adapted to collect a steady stream of dust may now be overloaded.   This increase in human produced particular matter, which has strong hydroscopic properties, has been theorized to be major factor in the decline of forests the world over.

We will consider all of these factors in the final essay in this series.  Here we will discuss the practical use of all this knowledge in creating healthy and vibrant gardens.  It should be noted, however,  that we have highlighted only one tiny portion of a huge set of interconnected processes that effect how plants absorb nutrients through their leaves. 

If one wants to master the use of foliar applications there is so much more that needs to be understood and even more that has yet to be discovered.  Perhaps a topic for future essays, but as such it is out of our scope.  However, what has been sketched out so far is a good foundation for incorporating this tool into a general gardening practice. 


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