Tag Archives: Stomata

Foliar Feeding

Some products advise spraying them on the leaves of your trees – a process known as foliar feeding. At first glance this makes no sense, as plants synthesise everything they need from nutrients obtained from the soil and air and these nutrients come up with water through the roots and xylem. And leaves haven’t evolved for nutrient uptake, they have evolved for photosynthesis.

But could this actually work? Well, in order for the nutrients in foliar feed to be useful to plant cells, they would need to both penetrate the leaf and enter the cell.

Can substances on a leaf surface enter the leaf itself? For the most part they can’t as leaves are covered by a protective waxy layer known as the cuticle (described in the post Leaf Structure). One of the main roles of the cuticle is to stop pathogens and other environmental stressors entering the leaf.

But as so often happens with systems of mind-bending complexity like plants – it’s not that simple. For one thing, we know leaves have stomata which allow gas to enter the leaf. But it also turns out that the rest of the cuticle isn’t completely impregnable. The cuticle has tiny pores in it at the base of trichomes (hairy projections) and glands – these range from 0.45 to 1.18nm in diameterref. One study did indeed find that dissolved nutrients can enter the cell through these pores in the cuticle: “penetration of ionic compounds can be fairly rapid, and ions with molecular weights of up to 800 g mol(-1) can penetrate cuticles that possess aqueous pores.” The key term here is ‘aqueous’ – the pores need to be wet in order for nutrients to enter through them. For example carnivorous plants use this process to bring nutrients in from their traps via the pores in glandsref.

A great article summarising the physics of nutrients entering a leaf is here – they conclude that it’s easier for positively charged ions (calcium, magnesium, potassium, ammonium-form nitrogen) to enter via the cuticle pores whilst it’s not as easy for negatively charged ions (phosphorous, sulfur, nitrate-form nitrogen). Similarly smaller molecules or those with a smaller positive charge are easier to translocate around the plant – including ammonium, potassium, and urea. Larger molecules will stay close to their point of entry, including calcium, iron, manganese , zinc and copper. Another study states that younger leaves are less able to transport nutrients out and so applying foliar feed to developing leaves may result in the nutrients staying within the leaf (which perhaps is an effect one might want to achieve?)ref

So it seems that some amount of foliar feed may be able to enter via the cuticle’s aqueous pores, and a subset of this may be able to move around the plant.

But what about the stomata? Previous studies have said that “the combination of cuticular hydrophobicity, water surface tension and stomatal geometry should prevent water droplets from infiltrating the stomata”.ref (ie. water can’t get through stomata) but apparently dissolved ions can in some circumstances, because the ions change the surface tension properties of the liquid. This study ‘confirmed the stomatal uptake of aqueous solutions’ref; but also said this depended on whether the aqueous solution was chaotropic (reducing water tension) or kosmotropic (increasing water tension). So it’s easier for the ions on the left to enter via the stomata, and harder for those on the right.

from: https://water.lsbu.ac.uk/water/kosmotropes_chaotropes.html

But once in the leaf, can nutrients be used by plant cells? It seems so, in some cases, but the evidence is extremely varied and there are many different variables to untangle.

A research study was conducted by ‘Christmas Tree Specialist’ Chad Landgren for the Oregon Department of Agriculture in 2009 comparing foliar feeding to other forms of nutrient applicationref. They tested a range of approaches on blue spruce, Atlas cedar and four varieties of fir (abies), in pots and in the ground, using application methods including “helicopters, mist blowers and various backpack sprayers”. Their conclusions were: “Each conifer species and site are potentially different with regard to nutrient needs and response. Blue spruce appears rather “immune” to foliar application… Nordmann fir appeared to pick-up some of the foliar fertilizer… on other sites, no treatment (soil or foliar) appeared to move the foliar nutrient content levels.”

In another paperref the author concludes that “foliar application of particular nutrients can be useful in crop production situations where soil conditions limit nutrient availability.” and that fruit can benefit from direct sprays, but also that “applying fertilizers to leaves (or the soil) without regard to actual mineral needs wastes time and money, can injure plant roots and soil organisms, and contributes to the increasing problem of environmental pollution.”

And then of course it’s not just the leaves themselves. We now know that there is a phyllosphere – a symbiotic community of microbes in and on the leaves which perform a whole range of functions for their hosts, one of which includes producing cytokinins
that can be bioactive within the plant. If foliar feeding increases these bacteria, there may be effects throughout the plant not just on the leaf.ref

The message from all of these seems to be that foliar feeding may work for leaves or fruit with specific mineral deficiencies which need to be corrected in-situ, if the nutrient in question can get through the cuticle or stomata. Or for plants which have environmental reasons for not being able to access nutrients through their roots (like pH?). But there needs to be a specific requirement in a specific location on the tree for it to make a difference – and it will be dependent on the species, environment, nutrient etc. In most cases I would say it would be better to provide the roots with the requisite nutrients instead.


Stomata are “microscopic pores which mediate the uptake of CO2 and loss of water from terrestrial plant leaves”ref The pores exist in the cuticle of the leaf (refer back to Leaf Structure to learn about the cuticle). You can see a scanning electron microscope image of stomata below:


The stomata are the dark holes in the pictures, and each one is controlled by two guard cells. The guard cells bend or straighten to enlarge or close the hole, this controls the amount of air which can enter, and the amount of water vapour which can get out. The world of stomata is illustrated in beautiful detail on the Plant Stomata blog, and what you notice is just how symmetrical and perfect looking stomata are, even though they are only 20-70μm in size. In fact the creation of guard cells is choreographed by a gene known as MUTE – it triggers one round of cell division, then acts to stop any further division, resulting in one stomata with two guard cellsref.

Stomata are the interface between the inside of the leaf and the outside world. They are “typically fully open under conditions favouring photosynthesis, but close when water supply is limited.”ref They operate a control system which responds to several factors including CO2 and water levels – lower CO2 levels within the leaf space will open the stomata as will higher water levels and/or humidity. In most plants stomata close at night, since CO2 is not being used by photosynthesis, but some also operate on a circadian rhythm – opening before dawn or closing for a period at midday (Vogel).

Stomata control the most fundamental life-giving processes of plants, and as such are an ancient structure, found on plant fossils from 400 million years agoref, basically from when plants first grew on land. As a result, stomata patterns can be used for paleontology, and for genus (and sometimes species) identification.

Stomata are distributed on bottom of leaves, and sometimes on the top as well. The guard cells have different shapes, including crescent, rectangular, dome and triangularref, and in conifers they are often sunk into the leaf, surrounded by structures and/or contain wax plugs. They are arranged in different patterns as part of the overall epidermal structure, so appear in rows in certain species, and in between the pavement cells in different patterns in others.

This photographer (http://www.foto-vision.at/) produces amazing microscope images of leaf and stem cross-sections. Below is a pinus mugo needle – look closely at the cuticle and you can see dark spaces where the stomata are, surrounded by the guard cells stained in bright orange.

Guard cells work by inflating with water – since they are pinned at each end, and stiff (in conifers the guard cells often have lignin in them) – when water enters the cells they bend outwards. To inflate, they transport positively charged potassium ions inward – this attracts negatively charged ions (like chloride) and water then is attracted as well to dissipate the concentration of ions back to baseline levels. Pressures generated by guard cells are surprisingly high – from 2-40 atmospheres,or 16-320x the normal blood pressure generated by humans (Vogel).

So aside from providing enlightenment, how does knowing about stomata aid your bonsai practice? Well to start with, more stomata provide more photosynthesising capability and hence more growth potential (assuming water availability). The number of stomata created on a leaf is not just genetic, but is impacted by the environment -“in a number of species both light intensity and CO2 concentrations have been shown to influence the frequency at which stomata develop on leaves.”ref So putting your trees out in the sunlight will increase the number of stomata – this is determined by the mature leaves being in the sunlight – they use ‘long-distance signalling’ to developing leaves to produce more stomataref. Researchers hypothesise this signalling is probably mediated through plant hormones, but it’s not currently known exactly how.

One bonsai practice which relates to stomata is the use of anti-transpirants. This is sometimes used after collecting a yamadori. It’s promoted to ‘protect leaves’ from various environmental challenges (heat, dryness, wind) and to ‘reduce excessive transpiration’. The product is “a film-forming complex of polyethylenes and polyterpenes that when applied to foliage will reduce the moisture vapor transmission rate”ref – so basically you are spraying plastic onto the leaves and blocking the stomata.

My guess on this product is that most people are not spraying the bottoms of the leaves which is where the majority of stomata are located. This will indeed reduce transpiration (from the top of the leaf) but not prevent photosynthesis or gas exchange, because really most of the stomata are unaffected. I don’t really like the idea of spraying plastic on my trees though, and don’t think it should be necessary – if a plant is transpiring ‘excessively’ it needs more water, or it needs to be removed from the environment causing the transpiration (out of the wind or direct sun). Creating more humidity should have a similar effect (for example by covering with a plastic bag).

One situation where it may be justified might be when collecting yamadori, when more root has been removed than foliage, and the roots simply can’t keep up with the transpiration rate. Reducing the transpiration for a period of time would allow the roots to grow whilst keeping the foliage (otherwise in bonsai you would have to remove the foliage to match the root capability). But again, a plastic bag might work just as well, without the need for spray.


Rather naively, when setting up this website, I made a note to create a post on the topic of ‘leaves’. Several days of research later it’s clear, there can’t just be one post about leaves! Why? Well, leaves are the reason that complex life on earth exists. Their ability to photosynthesise – to turn energy from the sun into energy that living things can use – is fundamental to our planet. So – there is a lot going on inside a leaf. My starting point for this post is the wonderful book ‘The Life of a Leaf’ by Steven Vogel, and I am going to cover the purpose of leaves, respiration, transpiration, and how leaves deal with environmental challenges like heat, wind and cold. There is a lot more to know about leaves, to read this check out all the other posts tagged with ‘leaves’, or start with leaf structure.

So, what is the purpose of leaves? Simply, they are the energy generating mechanism for plants. Through the process of photosynthesis they use energy from the sun to ‘burn’ water, removing its oxygen atom and bolting the remaining hydrogen onto carbon dioxide to create sugars. These sugars are used to power the life and growth of the plant. And as a side-product, oxygen is created, for animals and humans to breathe. Photosynthesis only happens when a leaf is illuminated by the right kind of light – in nature, this is sunlight during the day. If you want to geek out with more detailed information about how photosynthesis actually works, try this post: photosynthesis

A key concept when it comes to leaves is transpiration. It turns out that up to 97% of the water used by plants is actually evaporated back into the air – a process known as transpiration. The reason for this is because plants need to get CO2 into their leaves in order to supply it to the chloroplasts (organelles which perform photosynthesis) and to make this possible they need to open holes on the leaf surface to allow CO2 molecules to enter – these holes are known as stomata. When the stomata are opened, water naturally evaporates from within the leaf – it is the negative pressure in the xylem caused by this evaporation which pulls the water (and nutrients) up from the roots. What this means for bonsai is that your trees need a lot more water than you might think based on their size – remember up to 97% of it will be evaporated out. But if you water on a sunny day, will you burn the leaves? Find out here.

Did you realise that individual trees have leaves of different shapes and sizes, depending on their position on the tree? Shade leaves are larger, thinner, darker green (containing more chlorophyll) and less lobed, with fewer stomata than sun leaves; conversely sun leaves are smaller, thicker, lighter yellowish green, more lobed, and have more stomata – each leaf is more efficient in their niche of light exposure (Thomas). In her book The Arbonaut, Meg Lowman describes a coachwood tree with each leaf having its own unique formula for success – the lower leaves are long-lived and better structured to harvest low amounts of filtered light, the upper leaves are short-lived “extremely high-powered chlorophyll factories…[which]…produce sugars that keep the entire tree alive, healthy and growing”. And between these, variations to suit the different light conditions. All of this is called ‘photomorphogenesis’, or a “developmental process in plants in which the incident light determines the growth of the plant”ref. One research team even discovered a gene which could force the growth of sun leavesref and resulted in 30% greater photosynthesis.

What this means for bonsai is unclear as it’s not obvious from the research at what scale these differences can manifest. In theory there will be a layer of leaves around the canopy of the bonsai tree which could be sun leaves, and leaves below and within it which could be shade leaves. If it is literally only the sun exposure on a bud which determines what type of leaf is grown, then theoretically these two different leaf types could appear on a bonsai. If so, then removing the top layer of leaves – as is often recommended in bonsai, to ‘let light into the tree’ may not be the best course of action. If the leaves below have grown as shade leaves they may struggle to tolerate their new position in full sun. Anyone noticed sun & shade leaves on their bonsai? Let me know at info@bonsai-science.com.

‘Leaf morphology’ – or the shapes and sizes of leaves – is another rabbit hole you can joyfully enter via Google if you so desire, but to sum things up, leaves have differing physical attributes, each lending the leaves different properties – “leaf traits may reflect the adaptation mechanisms of plants to the environment.”ref

A key measure related to leaves is the ‘specific leaf area’ – the ratio of total leaf area (ie. the total amount of leaf surface) to total leaf dry mass (the total weight of the leaves without water). SLA is used as a measure of the overall health of a plant, as it reflects the efficiency of carbon gain relative to water lossref.

What is observed is that different trees have different strategies for optimising specific leaf area – in addition to photosynthesis mentioned above. Three interrelated attributes include leaf size & shape, leaf venation (how the xylem & phloem ‘veins’ are structured) and stomatal conductance (the number and distribution of stomata on the leaf).ref The model in this article suggests several predictions related to these attributes.

One of these is that a leaf with a larger width to length ratio can support higher carbon production factors, stomatal conductance, and leaf area than a leaf with smaller width to length ratio. So long, narrow leaves are less efficient and have higher ‘stomatal resistance’ – ie. they are less efficient at transporting gas and water vapour in and out of their leaves. This also applies for thick leaves, which have higher stomatal resistance than thin leaves.

That’s not all bad though, because higher stomatal resistance is correlated with smaller xylem, which are more resistant to embolism. These tradeoffs help different trees thrive in different conditions and drive evolutionary adaptation.

The implications of this model that on average narrow leaved species will do better in more arid conditions with less available water, since they transpire more slowly and are less at risk of embolism, and as water becomes more available the broadleaved trees will do better. So conifers may do better on a mountain top, and angiosperms such as beech will do better in the valleys. Translated to bonsai medium, this suggests that a narrow leaved tree (including most conifers) will prefer a more well-drained medium (hence the advice that conifers don’t tend to like getting ‘their feet wet’). Your tropical trees on the other hand – like a Ficus – may be perfectly happy with extremely wet conditions (as long as they don’t become anoxic – ie. so wet they prevent any oxygen from entering).

Leaves have more to contend with than ‘just’ photosynthesising and transpiring – they also have to deal with extremely large temperature variations – “on windless days, ordinary leaves on ordinary trees quite commonly run around 10°C, nearly 20°F, above the air that surrounds them. Under exceptional (exceptionally bad) circumstances they can reach twice that.” So explains Vogel, who then describes in detail the different strategies used by leaves to cool down – starting with evaporation but also including reradiation (radiating heat back into the air) and convection (transferring heat to the air from leaf surfaces and edges). He suggests that smaller, more lobed leaves are better at cooling than large wide leaves – and that large wide leaves avoid horizontal positioning in order to reduce overheating. What this means for bonsai is probably just that the natural leaf shape and positioning of a particular species helps it achieve a temperature balance (as well as photosynthesis) – and also perhaps that highly ramified trees with lots of smaller leaves may have improved temperature regulation (due to improved convection).

Another environmental challenge faced by leaves is freezing temperatures through the winter. One obvious strategy for avoiding these is to be deciduous, but there are plenty of evergreen trees out there and many of them thrive in extremely cold conditions (eg. the boreal forest where snow cover lasts for months and “gymnosperms such as Abies, Larix, Pinus, and Picea dominate”ref). Of the various strategies deployed to avoid damage from freezing (four at least are described in detail by Vogel, as well as in this paperref), the strategy most relevant to bonsai is that the tree generates substances within its cells and organs – such as dissolved sugars, resins and anti-freeze proteins – which reduce the freezing temperature in cells, reduce ice crystal formation and prevent ice crystals from growing within cells. The reason this is relevant is because one of these mechanisms – the creation of anti-freeze proteins – requires a gradual acclimation to the cold. This allows the tree to turn on the genes which produce the anti-freeze proteins, and gives it enough time to accumulate them. So it is a bad idea to move a conifer from a warm/protected area directly into the freezing cold. But if that tree has been in the same place as the seasons change, it will likely have time to build its defences against freezing temperatures.

The final environmental challenge described in fascinating detail by Vogel is the wind. Leaves are adapted to deal with wind so that they can continue to photosynthesise but also not tear, or build up enough resistance to pull the tree over. They do this by bending with the wind into aerodynamic shapes which minimise drag – Vogel’s research includes a great photo illustrating how four different groups of leaves move into cone shapes as the wind increases. One key point for the bonsai enthusiast though – leaves are more resistant to the wind once they have hardened off – if they are new spring leaves they are more prone to being wind damaged.

It’s not just the leaves which contend with all these environmental challenges though, it’s all the microbes on the leaves, in its phyllosphere.

I could write pages and pages more on this fascinating aspect of tree biology but for now I am going to leave it there.

The water system of a tree

One of the first topics you come across when starting to study trees is the question of how they manage to lift water all the way to the leaves at the top of the canopy.

Different organs play their part in this system, starting with the roots where water is absorbed into the xylem. Xylem is a network of interconnected cells, which die quickly after birth, so that the cell contents is eliminated leaving a large space for water to enter. New xylem is constantly being created in the roots, trunk, branches and leaves, and this is all connected so that water can pass from one to the other.

But what causes it to rise up towards the leaves? The phenomenon is well described in pretty much any tree biology book you care to pick up (see references page). The answer (as is beautifully described in Ennos’s book ‘Trees’) is that it is pulled from above.

The force which pulls up the water actually starts at the leaves. Cells in leaves need gases to photosynthesise and respire (carbon dioxide and oxygen), and the waxy epidermis (outer layer) is impermeable to gas. So, leaves have small holes called stomata which are pores in the epidermis allowing gas to enter the leaf interior. These holes also allow water vapour to escape from the leaf, and as this water vapour evaporates from the leaf it pulls up the water underneath it by hydrostatic force. Water is strongly attracted to its own molecules (a force known as cohesion), and when they move upwards by evaporation it creates tension pulling more water up. This is known as the ‘cohesion-tension’ theory (Smith et al) and the process is known as transpiration. This is why trees need far more water than their size would suggest – the majority is evaporated from the leaves during transpiration.

As most bonsai enthusiasts know, when you cut a branch, water does not spurt out. So it’s obviously not being pumped from the roots. But you can make water spurt out, if you put a cut branch in a pressure vessel and apply pressure which is equal to the tension that the water was under. Experimentally this has shows stretching forces of over 20 atmospheres (294 p.s.i) (Ennos), evidence which has supported the cohesion-tension theory. There are those who disagree with this as the exclusive mechanism for water movement against gravity – one paper argues that there is an “interplay of several forces including cohesion, tension, capillarity, cell osmotic pressure gradients, xylem-phloem re-circulation, and hydrogel-bound gradients of the chemical activity of water”.ref

Whatever the nuances of the forces involved, the transpiration flow is essential for other processes within the tree – it helps maintain cell turgor (stiffness), maintains solute levels in cells which are needed for metabolism, draws nutrients, plant growth regulators and metabolites up through the tree from the roots via the xylem sap, cools leaves via evaporative cooling, and supplies water to the top of the phloem for the transportation of photosynthates (Smith et al).