Non-nutrient Additives

Like any hobby, the bonsai world has its share of fads, snake oil and quackery. None more so than in the multitude of different potions and elixirs offering to bring a bonsai to perfection, for the right (expensive) price. Amusingly, many of these go to great pains to emphasise that they are NOT fertilisers (since actually fertiliser is cheap and relatively easy to buy). Not only that, but many fail to include ingredient lists, reference real data or otherwise explain how their product is supposed to work.

Of course plants need nutrients, these are usually elements and explained in the post Nutrients for Trees. But there are a range of other ingredients which may or may not support your trees’ health, so I thought I’d start a list to help you work out what a product might be seeking to achieve:

Blood meal – eugh! A non-ethical source of nitrogen. There really is no need to use animal blood when you can get nitrogen from any rotting organic matter/compost.
Charcoal – in the form of biochar – depending on what it’s made from, helps water retention, attracts and provides a home for benefical microbes and provides nutrients & micronutrients (acts as a fertiliser)
Cocoa bean shell mulch – supports endomycorrhizae and nitrogen-fixing bacteria for root growth BUT also contains theobromine which is toxic to dogs, cats & fish
Conifer oils – tend to be antimicrobial, insect repellent, antifungal etc
Ectomycorrhizae and endomycorrhizae – fungi which interact with roots to improve uptake of nutrients – whether a particular species of tree uses endo or ecto mycorrhizae is detailed in this site.
Feather meal – another non-ethical source of nitrogen.
Flavanoids, flavanols, flavanol glycosides, anthocyanins – support the symbiosis between roots and arbuscular mycorrhizal fungi (a type of endomycorrhizae), as well as with nitrogen-fixing bacteria^ref
Humic acid – ultra-dense organic matter, converts elements into forms available to plants^ref, nourishes microorganisms in the soil and may mimic the phytohormone auxin
Japanese Cedar (Cryptomeria japonica) oil – insect repellent, insecticide, antifungal, antimicrobial
Japanese Cypress (Chamaecyparis obtusa) oil – antibacterial, antifungal
Kaempferol – a flavonoid (see above)
Leonardite – source of humic acid (see above) – extracted via open-cut mines
Manure – animal manure is a source of organic matter; from omnivores (eating both plants and animals – eg. chickens or pigs) it is higher in total nitrogen and phosphorus than from herbivores (eating only plants – eg. horses, sheep or cows) which have manure higher in total carbon^ref
Pine oil – insecticide/insect repellent
Quercetin – a flavonoid (see above)
Saponin – an insect repellent
Seaweed – a fertiliser which includes micronutrients which don’t appear in standard fertilizer, such as sulphur, as well as plant metabolites which can support the growth of mycorrhizal fungi and nitrogen-fixing bacteria. Seaweed extracts have been known to promote plant growth.^ref
Succinic acid – helps reduce heavy metal contamination – as a component of Alar was used for improving fruit set in fruit tree (before Alar was banned as a carcinogen)
Vinasse – source of organic matter and potassium (can be chemically processed)

Read more about what some products contain here: HB-101 Analysis, SUPERthrive, Green Dream, Bioc h ar and Biogold.

Green Dream

Green Dream is one of those products that is spoken about in whispers as some kind of miracle elixir. It was created by UK bonsai artist Colin Lewis who now lives in the US – and sells the same product there from his website. In the UK it’s available from Kaizen Bonsai. So what’s in Green Dream? Let’s just say you don’t want to use this product if you are a vegetarian, vegan, or interested in animal welfare. In the FAQs on the UK supplier’s site, it lists the following:

Blood meal – a source of nitrogen. Personally I feel that animal blood is over the top when nitrogen can be found in compost or any other rotting organic matter. Most likely the blood is an abattoir side-product, and associated with animal cruelty.
Feather meal – also a source of nitrogen and a side-product of the poultry processing industry – not known for its animal welfare standards either.
Cocoa shells – are the husks from processing cacao beans for chocolate. In theory this might be positive for your trees since cocoa (and its shells – known as Cocoa Bean Shells or “CBS”) contain bioactive compounds such as polyphenols. The polyphenols in chocolate products “comprise mainly catechins, flavonol glycosides, anthocyanins and procyanidins”^ref – of these, flavonols and anthocyanins are both flavonoids, known to support the symbiosis of roots with arbuscular mycorrhizal fungi as well as nitrogen-fixing bacteria^ref
CBS also contain theobromine^ref, which is toxic to aquatic animals, cats and dogs at reasonably low levels, so please be careful if you are using this product where cats or dogs can access it.
Dried organic seaweed – since seaweed is a plant itself it contains all the nutrients plants require – including many of the micronutrients which don’t appear in standard fertilizer, such as sulphur, as well as plant metabolites which can support the growth of mycorrhizal fungi and nitrogen-fixing bacteria. Seaweed extracts have been known to promote plant growth.^ref
Vinasse – is the by-product of sugar ethanol production. It is a potassium source and a “soil fertility improver because it promotes deep root development, nutrient lixiviation and increases considerably the sugarcane yield”^ref however there has been controversy over its use due to environmental damage from the high organic content.^ref
Slow release compound fertiliser with an analysis of N.6% – P.5% – K.7%.
With added trace elements. Iron, Manganese, Zinc, Copper, Boron, Molybdenum (probably from the seaweed and vinasse)

Overall I won’t be using Green Dream Original as I am a vegetarian and don’t wish to use animal products from cruel farming practices for my bonsai. Also, I have a dog and don’t want to put him at risk.

The product is labelled as an organic fertiliser but this doesn’t mean its ingredients are organically produced in the same way as food: What is organic fertiliser?

In researching this article I also looked into another product on Kaizen Bonsai’s website – Green Dream Rapeseed Meal. Rapeseed is also known as canola, it’s a Brassica vegetable whose seeds are grown used for oil, which is used as vegetable oil in cooking, and as a biofuel^ref Since 28.3 million metric tons of rapeseed oil was produced worldwide in 2020/21^ref, there is a heck of a lot of rapeseed meal to dispose of! Rapeseed meal is used as an animal feed, due to its relatively high protein content, and traditionally was used in China as a fertilizer. This is probably because China is the second largest producer and consumer of rapeseed oil and so has a lot of meal.

The meal has plant nutrients in it, because it’s made from plants, you can see a full breakdown here. Aside from the NPK listed on the label, according to the Canola Council it also contains almost every other nutrient a plant needs (see Nutrients for Trees), except for boron and nickel, so a bit like seaweed this can provide some of the trace elements that aren’t always available in the soil or in standard fertilizers.

There is a research paper^ref breaking down the compounds found in rapeseed meal, of these only one has any known effect on plant growth and that’s kaempferol – we came across it over in the HB-101 Analysis – it encourages the growth of endomycorrhizal fungi which aid nutrient uptake in roots.

One thing to know about rapeseed meal is that when it’s not dressed up as a bonsai fertilizer, it can purchased for quite a bit less. I found 20kg available for less than £14 (including delivery to UK mainland) at this site selling animal feed.

Will water drops on my trees burn the leaves?

This is one you sometimes hear in gardening circles – that you might burn the leaves of your plants if you water them in the heat of the day – because the drops of water act as a magnifying glass, focusing the light onto the plant and theoretically burning it. I have never observed water-droplet-shaped burns on a leaf which one would expect from this kind of behaviour. But what does the science say?

This article^ref references another one behind a paywall which says that although droplets can increase light 20x at their focal points, in most of the species tested a layer of leaf trichomes hold droplets above the leaf surface, and beyond the focal point.

In another study^ref “sunlit water drops on horizontal leaves without waxy hairs cannot cause sunburn regardless of solar elevation and drop shape.” – this is because “the focal region of water drops falls far below the leaf at higher solar elevations and can fall on to the leaf only at lower solar elevations, when the intensity of light from the setting sun is generally too small to cause sunburn.”

Unfortunately both papers conflict somewhat as the first says trichomes specifically hold water droplets too far away from a leaf to enable sunburn, and the second paper says only horizontal ‘hairy leaves’ can get sunburn (they found this could happen for floating fern but this plant has quite specific trichomes/hairs). In reality most bonsai trees do not have horizontal leaves, instead leaves are at a multitude of angles, and the water droplets if stuck to a leaf would probably be angled away from the midday sun. But do send me a picture if you ever see a bonsai tree burned by a water droplet!

SUPERthrive

SUPERthrive is another product claiming great results for plant health without being a fertiliser. I’m not sure why these companies find it so offensive for their products to be known as fertilisers! Fertiliser just means a product containing plant nutrients. Anyway, what is in SUPERthrive? Here is the ingredient list:

So to start with – it *is* a fertiliser (1:1:1). Aside from nitrogen, phosphorus and potassium it also contains another macronutrient – calcium, and a micronutrient – iron. I’ve taken the explanation of these below from my post What each nutrient does (x17).

Calcium is used for plant structure as it strengthens the cell walls in plants. Its presence (or absence) is also used for signalling of stresses to the plant, allowing it to activate defences against pathogens. There is twice as much calcium in this product than N P or K – so quite a lot.

Iron is present in a large number of different enzymes within plant cells, appearing in chloroplasts (where photosynthesis takes place), mitochondria (where energy is created) and the cell compartment. Iron is therefore a key nutrient for growth and survival in plants – in just the same way it is with humans and other forms of life. Iron is a component of so many enzymes that there is a specific name for them – ‘FeRE’ or iron requiring enzymes (Fe is the chemical symbol for iron).

Iron can be toxic if too much is present, so plants have evolved mechanisms to remove it when it gets too high. There is a much smaller amount of iron than other nutrients in SUPERthrive.

SUPERthrive also contains four varieties of mycorrhizae (fungus which integrates with plant roots). The types included are all arbuscular mycorrhizal fungi which are a form of endomycorrhizae – that is, the fungal cells enter the plant’s roots. This supports healthy root development, improved access to soil nutrients and healthier trees. You can find these mycorrhizae in soil, particularly in established forests. Whether or not a specific type of mycorrhizal fungus will benefit your tree depends on the species of tree. There is a list of which types work with with species on this site.

From a bonsai point of view this list helps us see that endomycorrhizae (ie. the fungi in SUPERthrive) are not associated with plants in the families Pinaceae (fir, cedar, larch, spruce, pine, hemlock) and Fagaceae (beech, chestnut, oak), neither should they work for lime trees (Tilia).

They are associated with plants in the families Cupressaceae (cypress, juniper, redwoods, thuja), as well as acers, ginkgos and most other flowering trees. So you might see a difference in effect depending on the species of tree with which you use this product.

The final ingredient in SUPERthrive is humic acid. Leonardite, the source of the humic acid in this product, “is an oxidized form of lignite, very enriched in HS [humic substances] and characterized by well-known auxin-like effects”^ref Lignite is a form of brown coal, which used to be peat but hasn’t become coal yet. Humic acid is a liquid made by dissolving leonardite, so it contains dissolved, concentrated organic matter (dead plants), effectively this is like super-concentrated liquid compost. Could it be similar to compost tea? In fact, yes, liquid from compost has also shown the same auxin-like effect^ref. Peat analysis shows a chemical composition of carbon, oxygen, hydrogen, nitrogen, sulphur in decreasing order.^ref But this doesn’t tell the whole story. Peat and its compressed descendants contain a multiplicity of nutrients as well as other components – basically it’s all the ingredients which go into plants, and are synthesised by plants, compressed and starting to decompose. So this component of SUPERthrive probably has most of the nutrients required for plant growth – although these aren’t necessarily bioavailable. This study found that “Leonardites did not affect significantly any measured variables in comparison to the control”^ref So my guess is that if you are fertilising your trees with a comprehensive fertiliser, and giving them some organic matter, the addition of humic acid may not make a difference.

Leonardite is mined in open-cut mines, and can be extracted using chemicals, so it’s not particularly environmentally friendly^ref, nor sustainable as claimed by this manufacturer^ref, since it takes 300 million years to create!

Ultimately this product is a fertiliser with extra calcium, mycorrhizae to promote healthy root development (but only for certain tree species), and concentrated liquid compost/organic matter. It probably provides beneficial compounds to bonsai trees – particularly those in families which benefit from endomycorrhizal fungae. But I’d argue these compounds could be obtained elsewhere – from a comprehensive fertiliser, compost tea and a handful of humus (dark, organic material that forms in soil when plant matter decays) from your local forest. It’s probably a useful product though if you don’t have the time or access to other additives for your bonsai.

HB-101 Analysis

Recently a member of my bonsai club was talking about this product, HB-101. It is apparently wildly popular in Japan and supposed to be fantastic for bonsai. It claims to be an “all-purpose natural plant vitalizer”. Since plants synthesise their own requirements for growth from the 17 nutrients, I couldn’t really see how this would work unless this product was a fertilizer, so wanted to dive a bit deeper into this product to work out what it does. Unfortunately the product website is pretty waffly, or possibly just poorly translated, so a bit of investigation was required.

According to the manufacturer HB-101 is made from “essences of such long-lived trees as cedars, Japanese cypress, and pines as well as from plantains.” Without knowing how they define ‘essences’ and which bit of the plantain they use, this doesn’t help much. But more info is in their submission to the United Nations Industrial Development Organisation: “HB-101 is synthesized from organic distillate, which is extracted from the heated raw material of cedars, Japanese cypress, pines, and plantains as raw materials”. So it contains substances from Cryptomeria japonica (Japanese Cedar), Chamaecyparis obtusa (Japanese Cypress) and Pinus thunbergii (Japanese Pine) as well as plantain grass (not plantains like bananas).

In their safety data sheet it shows the product has a pH: 3.0 ~ 4.5, and is toxic to fish, daphnia and other aquatic invertebrates within 48h at 1% concentration or more. The product explanation from the UN submission contains more useful data including a typical analysis chemical breakdown:

Kaempferol; 0.1 ~ 0.2 ppm
Water-Soluble Nitrogen (as N); 0.001 ~ 0.005 %,
Water-Soluble Phosphoric Acid (as P₂O₅); 0.0001~ 0.0005 %,
Water-Soluble Potassium (as K₂O); 0.0001 ~ 0.0005 %,
Total Sulfur (as S); 0.0001 ~ 0.001 %,
Calcium (Ca); 0.5 ~ 3 ppm,
Magnesium (Mg); 0.3 ~ 3 ppm,
Iron (Fe); 0.01 ~ 0.05 ppm,
Zinc (Zn); 0.01 ~ 0.05 ppm,
Silicon (Si); 1 ~ 5 ppm

Based on this breakdown, the product appears to be mainly a nitrogen fertilizer (NPK of 10:1:1) which includes all six macronutrients (nitrogen, phosphorus, sulphur, potassium, magnesium, calcium), and two micronutrients (zinc, iron). It also contains two non-nutrient ingredients – silicon and Kaempferol.

I was surprised to see silicon in the list as it’s not considered one of the 17 required plant nutrients. But a bit of digging and apparently it “activates plant defence mechanisms” and “increases the resistance of plants to pathogenic fungi”^ref. Interesting! Looks like I will need to write a new post on non-essential-but-benefical nutrients…there are bound to be others aside from silicon.

The other ingredient is Kaempferol. Kaempferol (in case you were wondering) is a flavonoid (substance synthesised by plants) which “has a role as an antibacterial agent, a plant metabolite, a human xenobiotic metabolite, a human urinary metabolite, a human blood serum metabolite and a geroprotector.”^ref It has a molecular formula of C₁₅H₁₀O₆. Flavonoids are “pigments that color most flowers, fruits, and seeds”^ref and they are actually produced by plants themselves, so it’s not obvious to me why adding them to a plant would do anything.

The product explanation claims that Kaempferol ‘activates plant mitochondrial enzymes’. Looking into Kaempferol further, it is referenced in a wide variety of human disease research, including cancer^ref and brain injuries^ref. It does indeed appear to affect the function of mitochondria and provide protection to cells against injury – at least in humans. In this article Kaempferol is said to protect against oxidative stress and various forms of toxicity by affecting mitopaghy (the removal of damaged mitochondria), suppressing fission (cell reproduction) and apostosis (cell death). In humans it seems to have the almost magical property of helping healthy cells to survive while inducing the death of cancer cells^ref.

Admittedly plants (like all complex life) do contain mitochondria, which generate the energy needed to power cells. So if a substance affects mitochondria in people, it may well have similar effects in plants. The question is whether externally delivered Kaempferol can actually enter the plant and get into its cells, which it would need to do in order to make any difference. As flavonoids are synthesised by plants, and since plants don’t absorb this kind of substance – instead absorbing the raw materials to make it themselves – I don’t think Kaempferol can be having any impact on the plant mitochondria.

BUT – what it might be doing is having an effect on other living things that affect tree health. And hey presto, a bit of searching uncovered that Kaempferol has been shown to improve root development by supporting Arbuscular Mycorrhizal Fungi (AMF) ^ref – this is a type of fungi in soil which can help expand the volume of soil from which nutrients can be extracted (Thomas).

In one study a range of flavonols were tested for their contribution to the growth of mycorrhizal fungi and Kaempferol was shown to make a modest improvement^ref but another flavonoid called Quercetin was even better.

Kaempferol does come from conifers which are in this product’s ingredient list. But it’s also high in green leafy vegetables like spinach, kale and dill^ref. So my guess is that a lot of it comes from the plantain grass in this product. Quercetin, which was more effective for nurturing mycorrhizal fungi, is apparently found in red, green, and purple-pigmented plants – for example, red onions. Unfortunately though, it seems that Quercetin (and Kaempferol) are not very easy to extract – industrial methods (likely the one used for HB-101) require ethanol but if you’re keen here is a method using ‘subcritical water’ (liquid water under pressure at temperatures above usual boiling point, 100 °C (212 °F)

So what else might the cypress, pine and cryptomeria be contributing? The company’s product page gives a few other clues – specifically it mentions saponin, pine oil and succinic acid, although these are not listed on the chemical analysis data sheet.

Saponins are substances produced by plants which are “responsible for plant defense against antagonists; such as mollusks, pathogens and insects”^ref They are contained in a wide range of plants but do not appear to be present in conifers^ref. By contrast saponins are found in plantain grass (and actual plantains!)

Looking at what the conifers are providing to this product – pine oil has larvicidal and mosquito repellent properties^ref, Cryptomeria japonica oil is insect repellant and insecticidal^ref, antifungal against tree pathogenic fungi^ref as well as antimicrobial^ref and Japanese Cypress oil is antibacterial and antifungal^ref (and apparently also good for hair loss!).

And finally, succinic acid. This has been shown to improve tree tolerance (in Larix olgensis) to heavy metal contamination – specifically with Lead^ref and Cadmium^ref. This doesn’t seem to me a particularly useful attribute for bonsai, since we’re using inert bonsai medium. Dig a bit further and you find succinic acid was combined with 2,2-dimethylhydrazine to make a plant growth regulator called Daminozide (also known as Alar). It regulated the growth and set of fruit but has been banned due to concerns about cancer risk. This isn’t on the ingredient list so I don’t think it’s present in the product. I can’t find much useful information on what succinic acid might be contributing.

So what’s my overall analysis? Initially I was sceptical because I don’t like products that rely on fluffy advertising and don’t explain how they work. It annoyed me that the product doesn’t contain an ingredient list, and it doesn’t admit to largely being a fertiliser, which is most definitely is.

However one of the key features of this product is that parts of it are derived from a distillation process, which enables the extraction of beneficial compounds from the conifer leaves and wood as well as from the leafy plantain grass. These should help boost a tree’s defences to insects, pathogenic microbes and fungi, and improve the mycorrhizal activity in the pot to supports better root development (and thus healthier trees). It also has a lot of nitrogen, which as outlined in nutrients is hard for plants to obtain.

So somewhat grudgingly I have to say it might actually work.

Stomata

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 cells^ref.

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 ago^ref, 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 triangular^ref, 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 stomata^ref. 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.

Leaf Structure

It’s useful for bonsai enthusiasts to understand how a leaf is structured, as this answers some questions about how water/air/nutrients/sugars get in and out of the leaf and therefore also the rest of the tree. Of course there are many different leaf types belonging to different trees in different environments, so there will be many differences between them. What’s important to know is the main structures which are common to most leaves. Below is a diagram of a leaf cross-section:

http://Scaling Functional Traits from Leaves to Canopies – Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/The-internal-structure-and-biochemistry-of-leaves-within-a-canopy-control-the-optical_fig2_342371888

The outside of the leaf is covered by the ‘cuticle’, which is the first line of cells between the leaf and the outside environment. This is not a passive line of cells, but instead “waxes, fatty acids, and aromatic components build chemically and structurally diverse layers with different functionality.”^ref Not only that, the cuticle changes as the leaf develops – building up its layers and constituents over time until the leaf has fully extended.^ref The above diagram only shows a cuticle on one side of the leaf – but apparently a cuticle “covers the outer epidermal surface of most above-ground tissues, such as leaves, fruit, and floral organs.”^ref

The main function of the cuticle is as a barrier. It protects the tissue beneath from mechanical damage by the elements, or from insects, and acts as the primary defence against pathogens.^ref It is composed of “the polyester cutin, containing oxygenated and unsubstituted fatty acids, glycerol, and phenolic acids, that is impregnated by waxes of very-long chain fatty acids (VLCFAs) and their derivatives.”^ref In another study the top layer of the cuticle was found to contain Kaempferol. This is a flavonol which is known to be an antifungal, antibacterial and antioxodant (see this article about HB-101).

Since the waxy cuticle is impermeable to water and CO₂, leaves have specially controlled holes distributed across it^ref – these are known as stomata (described below).

Underneath the cuticle is the epidermis – the upper epidermis at the top of the leaf and the lower epidermis at the bottom. Humans have an epidermis too – it’s the top layer of skin. Everything you could want to know about the plant epidermis is covered in an excellent article in ‘The Plant Cell’ journal from January 2022. The authors say “the epidermis plays many important roles including regulating the exchange of gases, water, and nutrients with the surroundings, responding to external threats such as pathogens, herbivores and abiotic stresses, resisting mechanical strain, detoxifying xenobiotics, and contributing to mechanical strength while allowing the flat and flexible shape necessary for maximum light capture.”

There are three main types of cells in the epidermis, and these develop into their final form starting from the leaf tip and gradually moving back towards the petiole until all of the cells are formed.

The first cell types are ‘pavement’ cells – so named because they interlock with one another and look like paving (sometimes it’s crazy paving – other times it’s very neat). Among the paving are stomatal guard cells – these “form microscopic valves in the leaf surface” so that gas can get in and out for photosynthesis. You may have heard of stomata – this is the name for the hole that is created and controlled by the stomatal guard cells. Basically plants breathe through their stomata – air comes in, oxygen from photosynthesis and CO₂ from respiration come out, and water vapour comes in and out as well. It’s actually reasonably easy to ‘see’ the stomata even with the naked eye – depending on the species of tree and the shape of the leaf. If you put adhesive tape on a leaf, and pull it off, you pull off some of the cuticle which shows the outlines of the stomata. There is quite a lot to say about these guys – see this post: Stomata.

Aside from the stomatal guard cells and the pavement cells, the epidermis can also have ‘trichomes’ which are hair-like protrusions from the surface. These can appear in lots of different forms, and can be ‘glandular’ (or not). If a trichome is glandular, it can “biosynthesize, store and secrete a large diversity of specialized metabolites including terpenoids, alkaloids, polysaccharides, and polyphenols” – such as the terpenoids that conifers exude to defend against insects^ref. This image from the journal nature shows the trichomes on white spruce:

https://www.nature.com/articles/s41598-020-69373-5

Anyway moving on past the cuticle and the epidermis, you come to the mesophyll. The mesophyll is “the parenchyma between the epidermal layers of a foliage leaf”^ref – OK great Merriam-Webster dictionary, now what is ‘parenchyma’? Parenchyma is “the essential and distinctive tissue of an organ”^ref which in the case of leaves means the cells which photosynthesise and store the products of photosynthesis. So the mesophyll is the engine room of the leaf.

Referring to the image at the top of this post, you will see there are two types of cell in the mesophyll – palisade cells and spongy mesophyll cells. The palisade cells face the light, and are located on the top of the leaf^ref. They are columnar cells (with the end of the column facing the light) and they are supposed to contain the majority of chloroplasts, which are the organelles responsible for photosynthesis. The spongy mesophyll cells are arranged in a lattice, with air gaps (like a sponge) to allow for the absorption of CO₂ – they also contain chloroplasts, but apparently not as many. Good luck trying to find a research paper which actually counts them! The best I could find was this data^ref looking at five species living in different light conditions, and the number of spongy mesophyll cells ranged from 40-50% of the total chloroplast count. Which isn’t exactly a minority.

The shape of these cells has evolved to improve photosynthesis. The palisade cells which are long and columnar, “act as light conduits”^ref distributing collimated (parallel) light to chloroplasts within the leaf. Internal light scattering also takes place, allowing photons of light to reach the chloroplasts in the spongy mesophyll cells. When a leaf has a different (usually lighter) colour on one side, this can keep light inside the leaf by reflection.

The final part of the leaf structure is the vascular bundle – this contains the water-transporting xylem and the sugar transporting phloem. See xylem & phloem.

Not to forget, leaves have their own microbiome, just like the roots. This is called the phyllosphere and contains many bacterial and fungal species in symbiotic relationships with the host plant.

You can dive even deeper into the structure of leaves by going into the plant cells themselves, looking at mitochondria, chloroplasts, vacuoles and the thousands of chemical reactions going on, but that’s a post for another day.

Photosynthesis

Another epic topic which has occupied scientists for the best part of 400 years, the equation for photosynthesis itself was not understood until the 1930s.^ref

Photosynthesis is the process of turning energy from sunlight into chemical energy, a process which famously is performed by plants, specifically by plant cells containing ‘chloroplasts’. Chloroplasts contain a substance called chlorophyll. Chlorophyll is often described as a ‘pigment’ – but this makes it sound like its only role is to colour things green! Which of course it isn’t – being green is just a result of its real function which is to absorb sunlight of a certain spectrum. Chlorophyll absorbs visible light in two regions, a blue band at around 430 nanometers and a red band around 680 – according to Vogel. Other sources state the bands are 680 nm and 700 nm^ref. Everything else is reflected, which looks green. Chloroplasts are believed to have originally been cyanobacteria which were incorporated into plant cells to provide photosynthetic capability (Lane, 2005). To see where chloroplasts and chlorophyll are present within a leaf, check out this post on leaf structure.

The sequence of reactions which happen during photosynthesis are known as the Calvin-Benson-Bassham cycle, after the scientists who discovered it. The simplified equation describing photosynthesis is as follows:
6CO₂ + 6H₂O –light energy–> C₆H₁₂O₆ + 6O₂.

This suggests that photosynthesis is the exact opposite of respiration, creating glucose instead of consuming it. But actually this equation is incorrect as photosynthesis does not create glucose. The Calvin et al cycle produces a molecule called G3P which is a 3 carbon sugar which can be used to create other molecules. Anyway, that’s probably not super-relevant for bonsai! The main insight from the equation above is that light energy is needed for plant survival and growth – this is why trees do not like being inside unless they have a suitable artificial light source.

Another point to note is that plant cells, like all living cells, respire. Plant cell respiration is like animal respiration, that is, cells consume oxygen and glucose to produce energy, while emitting carbon dioxide and water. Cells use respiration to generate the energy they need for metabolism (basically, keeping the cell alive and functioning). All cells in a tree respire, including the leaves, roots and stems, 24 hours a day. This means trees offset some of their photosynthesis by respiring – in particular at night when no photosynthesis occurs.

On balance, leaves produce a lot more sugars and oxygen through photosynthesis than they use during respiration – and this provides the energy they need to maintain themselves and grow. In fact the point at which they *don’t* produce more than they use through respiration is as low as 1% of full sunlight (Vogel). Note that this low level can be reached if a leaf is entirely shaded by 2 other leaves, since the typical light transmission through a leaf is only 5%.

Another interesting fact about photosynthesis is that leaves can only use about 20% of full sunlight before the photosynthetic system saturates (Vogel). This number actually varies depending on the species and leaf type. So leaves below the top layer (likely to be ‘shade leaves’ described in this post: Leaves) can still get enough light from partial exposure, if they are slightly shaded or lit for only part of the day, to saturate and max out their photosynthetic capability. The majority (99%) of the energy absorbed is used to maintain the leaf itself, so only 1% is released for growth.

But – back to the photosynthesis equation – which is an extremely simplified version of what is actually happening. Photosynthesis requires 150 discrete steps involving a similar number of genes^ref. A good reference is this Nature article.

Photosynthesis takes place in two stages. First the ‘light dependent’ reactions happen. Light energizes electrons within the chlorophyll, and these electrons are harnessed to produce ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate) which are molecules used by cells for energy and as an electron source. The chlorophyll replaces its lost electrons by taking some from water – effectively they ‘burn’ or oxidise water, leaving the oxygen behind as a waste product (Al-Khalili).

After this the ‘light independent’ reactions happen. These are facilitated (sped up) by an enzyme known as RuBisCO. Using the energy sources created in the first step (ATP and NADPH), RuBisCO ‘bolts’ together the hydrogen from the water, and the carbon and oxygens atom from the carbon dioxide, along with phosphorus, to make the 3 carbon sugar glyceraldehyde-3-phosphate (C₃H₇O₇P) (Lane, 2005). This is known as carbon fixation.

In reality there is also a third stage where the ingredients for the cycle are regenerated so that it can continue running.

The full equation (the Calvin-Benson-Bassham cycle) – is:
3 CO₂ + 6 NADPH + 6 H⁺ + 9 ATP + 5 H₂O → C₃H₇O₇P + 6 NADP⁺ + 9 ADP + 8 P_i
(P_i = inorganic phosphate, C₃H₇O₇P = glyceraldehyde-3-phosphate or G3P)

Sorry for geeking out for a minute there!

An important point is that photosynthesis doesn’t just go up with increasing sunlight, it has a set of limiting factors which were described by Blackman in 1905 in his article^ref “Optima and Limiting Factors”. He said “When a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the slowest”. The limiting factors he described for photosynthesis included light intensity, carbon dioxide and water availability, the amount of chlorophyll and the temperature in the chloroplast.

The amount of sun needed to max out photosynthesis from a light intensity point of view is not as high as you would imagine. Vogel says a leaf “absorbs about 1000W per square metre from an overhead sun shining through a clear sky” and photosynthesis only consumes 5% of this – 50W per square metre. Obviously this is going to vary depending on the species and whether a leaf is a sun leaf or a shade leaf.

But what does it mean for bonsai? Well – you might have spotted some other elements in the photosynthesis equation. NADPH (formula C₂₁H₂₉N₇O₁₇P₃) contains nitrogen and phosphorus, as does ATP (formula C₁₀H₁₆N₅O₁₃P₃). RuBisCO relies on a magnesium ion to perform its role as a catalyst. There are a bunch of other enzymes and co-factors which are required to support photosynthetic reactions as well – which explains why certain nutrients (including Nitrogen, Phosphorus, Magnesium, Potassium, Chlorine, Copper, Manganese and Zinc) are critical for trees in varying amounts – more here: Nutrients for Trees.

And if you’re concerned about whether your trees have enough light for photosynthesis, you can see exactly how much energy is arriving on your outside bonsai at this site. Find your location and download the PDF report and you’ll see the irradiation levels – which can be useful in understanding how your bonsai trees will fare. As an example, the max in my location is 328Wh/m² in April which explains why the olive trees in London are so unhappy looking – if you look at irradiance in Greece, where they thrive, it barely ever goes under 328Wh/m² even in winter! Trees in Greece receive nearly double the amount of irradiance than those in South West London.

One final point of interest with admittedly little relevance to bonsai – some plants (nineteen different plant families, independently of each other) evolved an improved photosynthetic process which is known as C4 photosynthesis. This concentrates CO₂ nearer to the RuBisCO enzyme, reducing its error rate. Unfortunately most trees use C3 photosynthesis with its associated inefficiency – other than Euphorbia, which are apparently “exceptional in how they have circumvented every potential barrier to the rare C₄ tree lifeform”^ref.

Growth Types Table

Fixed Growth (determinate) (leaves formed inside the bud before opening)	Free Growth (indeterminate) (leaves and buds continue forming throughout season)	Rhythmic Growth (a bit of both)
Ash Beech Hornbeam Oak Hickory Walnut Horse chestnut Pine Spruce Ginkgo short shoots	Elm Lime/linden Cherry Birch Poplar Willow Sweet gum/ liquidambar Alder Apple Larch Juniper Western Red Cedar Coastal Redwood Ginkgo long shoots Maple	Loblolly pine Shortleaf pine Monterey pine Caribbean pine Cocoa Rubber tree Avocado Mango Tea Lychee Citrus Olive Pinus radiata

From Thomas (2018) and RNET^R

Plant Growth Regulators (or Phytohormones)

You’ve probably heard of rooting hormone powder, or auxin, or gibberellins – these are all ‘Plant Growth Regulators’. Plant Growth Regulators used to be known as ‘phytohormones’ which means plant hormones. This has been quite a contentious topic among plant biologists.

A hormone in an animal is a chemical messenger, a substance which acts as a signalling or control molecule to cause an action to take place. “Hormones carry out their functions by evoking responses from specific organs or tissues that are adapted to react to minute quantities of them”^ref. In animals, hormones are produced at a specific site (a gland, like the pancreas), work at low concentrations, and have a predictable dose-response. That is, an increase in hormone will result in the more of the related action (eg. more insulin leads to more sugar being taken up by the liver).

There *are* substances synthesised by plants which are involved in regulating growth – plant growth regulators – but they don’t work in the same way as animal hormones. It’s said that the assumption that they did sidetracked plant researchers for decades.^ref Plant ‘hormones’ are synthesised in multiple sites in a plant (and potentially in every cell^ref), have multiple actions on different cells (they don’t act on just one organ or tissue), don’t exhibit predictable dose-response behaviour like animal hormones and are involved in significant interactions or feedback loops with each other.^ref

What this means is that it’s quite hard to unpick what they do and how they work. The roles and mechanisms of plant growth regulators are still very much current research topics, as can be seen at two of the research groups at Cambridge University’s Sainsbury lab^ref1,^ref2 Some of the early theories about them were comprehensively demolished in a seminal article by Anthony Trewavas^ref (in particular the theory of auxin-derived apical dominance which was later proven wrong as explained below).

We now know that plant growth regulators act in concert with genes and the proteins they express, and not as an independently acting substance (one of the genes involved in cytokinin synthesis is known as LONELY GUY…).

So what are the plant growth regulators, where and how are they made? There are nine main plant growth regulators you may come across in your reading:

Auxin^ref – classically called ‘the growth hormone’ and a signal for division, expansion, and differentiation throughout the plant life cycle – involved in root formation, branching, the tropic responses, fruit development, shoot meristem function, the development of cotyledons and senescence. The most common form is Indole-3-acetic acid (IAA). Auxin acts in a ‘ying-yang’ relationship with cytokinin (see below)^ref as well as with gibberellins. More about auxin below.
Cytokinins (CK)^ref1 ^ref2 – involved in cell division, shoot initiation and growth (including maintaining the stem cell niche), nutritional signaling, root proliferation, phyllotaxis, vascular bundles, leaf senescence, branching and nodulation, seed germination, nutrient uptake, and biotic and abiotic stress responses. 6-BAP or 6-Benzylaminopurine is a synthetic cytokinin which is used in micropropagation and agriculture. Coconut water (not milk) has been found to be a natural source of cytokinins.^ref
Brassinosteroids^ref – involved in a wide spectrum of physiological effects, including promotion of cell elongation and division, enhancement of tracheary element differentiation, retardation of abscission, enhancement of gravitropic-induced bending, promotion of ethylene biosynthesis, and enhancement of stress resistance.
Gibberellins (GA)^ref – involved in multiple processes including seed germination, stem elongation, leaf expansion and flower and fruit development.
Strigolactones (SL)^ref – induce hyphal branching of arbuscular mycorrhizal fungi and are shoot branching inhibitors.
Abscisic Acid (ABA)^ref – involved in the induction and maintenance of seed dormancy, stomatal closure, and response to biotic and abiotic stresses.
Jasmonates (JA)^ref – shown to be inhibitors of growth but also involved in development of flowers and defense responses against herbivores and fungal pathogens
Salicylic Acid (SA)^ref – associated with disease resistance
Ethylene^ref – multiple effects on plant development including leaf and flower senescence, fruit ripening, leaf abscission, and root hair growth.

Slightly maddeningly none of these substances do just one thing – they’re all involved throughout the plant!

So how and where they are made in a plant? This isn’t simple either. In fact local biosynthesis is thought to be critical for plants, whereby plant growth regulators are made at the site where they are needed. For example both auxin and cytokinin are synthesised in leaves *and* in roots^ref, and can be made by chloroplasts and mitochondria, organelles which occur throughout the plant.^ref Chloroplasts can make precursors to auxin, abscisic acid, jasmonates and salicylic acid.^ref

Even though plant growth regulators don’t act in a predictable dose-response way like animal hormones, they still have a role in shaping plant growth in tissues which are sensitised to respond to them. Theoretically by understanding these responses we can manipulate a tree’s growth. And this is what they do in plant tissue culture (more on that below).

You may have heard the theory of auxin-controlled ‘apical dominance’, which holds that auxin produced by leaves at the apex inhibits lateral buds. This theory was strongly criticised by Trewavas in 1981: “The only hypothesis of apical dominance which has retained some measure of support is the nutritional one. A number of plants placed under conditions of reduced nutritional status adopt a growth pattern of strict apical dominance.” His point of view was further supported by a 2014 study which found that “sugar demand, not auxin, is the initial regulator of apical dominance”.^ref The researchers found that after removal of the shoot tip, sugars were rapidly redistributed over large distances and accumulated in axillary buds within a timeframe that correlated with those buds releasing. But auxin didn’t travel fast enough to be responsible for bud release. So basically they found that apical dominance arises because the main shoot is greedy for sugars, and due to its position at the end of the vascular system it can prevent lateral buds from taking the sugars needed to release and grow.

Auxin does play some role though, and the theory is that its role related to the fact that it’s the only plant growth regulator which displays polar transport. That is – it moves from the apex to the base of the plant, via the phloem, and can travel the entire length of the plant, ending up in the roots. This gives auxin a special role related to the spatial aspects of growth, and auxin ‘maxima’ (locations where auxin accumulates) are sites where new buds, flowers or lateral roots emerge. In fact, auxin and cytokinin work in concert throughout the plant, from shoots to roots, with apparently opposite effects in each location “like yin and yang”.^ref

An excellent reference point for this subject is the world of plant tissue culturing. This is where small pieces of plant tissue are sterilised and cultured in a medium containing plant growth regulators, which cause the tissue to grow into a ‘plantlet’ (sometimes in a test tube, if the source material is small). Further steps multiply the plantlet into several plantlets, which are then encouraged to create roots, multiplied again and/or planted out as seedlings to harden off. This process is used for industrial plant cloning where large numbers are required, and in the aquarium trade to avoid contamination with snails and other microbes (see Tropica’s website).

In plant tissue culturing, plant growth regulators are used to induce the relevant growth stage, which ones work for each species in which stage is documented in the ‘protocol’ for that species. In all cases specific ratios of cytokinin:auxin (and sometimes gibberellins) lead to different developmental stages – shoot growth, lateral shoot growth and root growth.^ref1,^ref2 To give you a bonsai-oriented example, one study determined a protocol for the micropropagation of Prunus Mume^ref. They were able to multiply fresh prunus mume shoots in a petri dish using a 4:1 ratio of cytokinins to auxins, and then root them using auxin.

So – apologies for the rather long read, it is quite a complicated subject! What can we take from all this for our bonsai practice? Firstly we can stop the brain-bending trying to understand how auxin controls apical dominance because it doesn’t – access to sugars does this instead.

Also we can use the yin-yang rule – high cytokinin:auxin encourages buds & shoots, and high auxin:cytokinin encourages roots. So I’m going to start adding some auxin rooting gel into my air layers and cuttings. Cuttings have never worked for me in the past so maybe this will be the secret sauce I need. I’m also going to try some cytokinin gel to encourage lateral budding on my trees.

If you are looking for products to give this a try, make sure the product actually contains the plant growth regulator you want. For example, Clonex contain auxin, and some of the orchid budding pastes such as Keiki paste contain Kinetin (a cytokinin). Many other ‘rooting hormones’ or plant hormones products on the market have no ingredient list at all, so avoid those. You can also find these products online in shops dedicated to hydroponics, where cloning and plant tissue culturing is a technique used by practitioners, or in lab supply shops such as microscience or Phillip Harris in the UK. You can even make your own hormone gels following these instructions.

Another trick you can use is that gibberellic acid can be used to break dormancy in seeds, if you really don’t have the patience to wait for natural dormancy to break. Or give coconut water a try, this has been found to have a similar effect in a range of species. For hard coated seeds in particular, usually it’s best to search Google Scholar for a researcher who has experimented with different approaches, since what works is very species-dependent.