I had never heard of root exudates before creating this website, but in fact their production is so important to plants that they “invest up to 20–40% of their photosynthetically fixed C”ref in this process.
Root exudates are basically substances created by root cells and sent out into the nearby environment – known as the rhizosphere. These can be waste products which diffuse across the cell wall, or manufactured compounds which serve a specific purpose in response to the environment.ref
There are many different exudates produced by plants, including carbohydrates (sugars), organic acids (such as acetic, citric or malic acid), amino acids, flavonols (molecules which can have protective effects on cells), enzymes (such as amylase, which helps to digest carbohydrates), plant growth regulators (substances which stimulate cell growth, such as auxins), phenolic acids (which have anti-oxidant properties), flavonoids, terpenoids, tannins, steroids and an assortment of other substancesref1,ref2.
The roles they play are just as diverse, including:
producing food in the form of carbon metabolites to support beneficial bacteria (such as nitrogen fixing bacteria), fungi, nemotodes and protozoaref1,ref2, which in turn assist with nutrient uptake and the production of their own compounds such as phytohormones
producing phytotoxins (plant poisons) such as terpenes (also found in conifer sap) to repel pathogenic microbes, invertebrate herbivores and parasitic plantsref1,ref2
detecting ‘kin’ (related plants) and avoiding competing plants which are not relatedref
changing the soil chemistry to allow for better nutrient uptake – for example exuding chelating substances which allow for better uptake of metallic micronutrients, or organic acids which enable better phosphorus uptakeref
So it seems that the roots of a tree can act like a sort of pharmacy, creating compounds that protect and nurture the tree and its beneficial partners via the rhizosphere. In my opinion what this means for bonsai is that firstly you don’t want to damage the tree’s ability to produce exudates, and secondly you don’t want to remove too much of the soil from a healthy root ball.
You can damage the tree’s ability to produce exudates by underwatering/drought – and this may not be recoverableref, but also by failing to provide all of the nutrients and micronutrients needed for healthy growth (by not fertilising enough).
What’s in the bonsai pot is clearly more than just roots and soil – it’s an entire ecosystem delicately managed by the tree itself. So perhaps being less heavy-handed during repotting would be a good idea – replacing a good amount of the soil back into the pot along with its microbes, exudates and adjusted chemical makeup.
Roots exist in a their own ecosystem along with soil, chemical compounds, microorganisms and variations in pH, humidity and temperature. This environment is known as the ‘rhizosphere’, a term created by Lorenz Hiltner in 1904, using the greek word for root ‘rhiza’.
The term refers to the area around the roots, and is broken into three parts. “The endorhizosphere includes portions of the cortex and endodermis in which microbes and cations can occupy the “free space” between cells (apoplastic space). The rhizoplane is the medial zone directly adjacent to the root including the root epidermis and mucilage. The outermost zone is the ectorhizosphere which extends from the rhizoplane out into the bulk soil.”ref
The rhizosphere is FULL of microbes – this articleref estimates there are 1000-2000 times the number which are found in non-rhizosphere soil. These include endomycorrhiza and ectomycorrhiza as well as beneficial (and pathogenic) bacteria. Below is an estimate of the number of genes represented in a sample rhizosphere across each type of organism (a list of the species included are in the research paperref)
Rather than passively respond to the rhizosphere, roots produce ‘exudates‘ – substances released from their cells – which are used both to sense the environment (such as, where competing roots are located and the presence of beneficial microbes and nutrients) and to alter it to the plant’s benefit. So the rhizosphere is a very dynamic place, teeming with life and being constantly manipulated by the tree for its own benefit. Below is a great image illustrating everything that’s going on – different mycorrhiza, bacteria and the roots interacting in the rhizosphere.
‘Mycorrhiza’ are fungi which have a symbiotic relationship with roots – they each provide something of value to the other party. The word comes from the Greek words for ‘fungus’ and ‘roots’ so one should strictly call them mycorrhiza and not mycorrhizal fungi since the latter is an example of ‘RAS syndrome’ (redundant acronym syndrome, which itself is also an example of RAS syndrome).
According to one study, “for efficient nutrient uptake, most land plants need to be associated with mycorrhizal fungi that supply minerals, increasing their productivity and conferring resistance to stress.”ref So these fungi are actually a critical part of life on earth, and necessary for healthy plant function.
Mycorrhiza are usually divided into two groups – endomycorrhiza and ectomycorrhiza.
‘Endo’ comes from the Greek ‘endon’ meaning ‘within’ – and endomycorrhiza (known as Arbuscular Mycorrhiza or ‘AM’) have hyphae (fungal threads) which actually penetrate the plant’s root cells and establish an intracellular symbiosis with the plantref. AMs scavenge for nutrients such as Phosphorus and Nitrogen released by saprotrophic microbes (ie. bacteria which feed off dead material) and make these available to the plant.ref
‘Ecto’ comes from the Greek ‘ektos’ meaning ‘outside’ – and ectomycorrhiza (‘ECM’) form a thick mantle around root tips from which clusters of hyphae extend beyond the root zone.ref They ‘mine’ Nitrogen and Phosphorus from the soil by producing enzymes which digest soil organic matter – they can then make these available to the trees in return for carbon sources such as sugars.
Whether a particular species of tree is associated with endo- or ectomycorrhiza is detailed in this site. The trees we’re interested in from a bonsai perspective fall in each camp: Associated with ECM are oak, beech, hornbeam, birch, hazel, alder (actually with both), tilia (lime/linden), chestnut and all of the Pinaceae family (including fir, cedar, larch, spruce, pine & hemlock). Associated with endomycorrhiza (AM) are grapevine, Prunus (cherry, peach, plum etc), pyrancantha, magnolia, Ilex (holly), Araucariaceae, wisteria, ficus, mulberry, ash, olive, all maples, horse chestnut, poplar/aspen, willow, buddleja, yew, camellia, elm, podocarps, flowering quince, hawthorn, apple, cotoneaster and all of the Cupressaceae family (including Cryptomeria japonica, cypress, junipers, redwoods and thujas),
Aside from this, azaleas are associated with a different mycorrhiza called ericoid.
Fungi aren’t the only microbes in the rhizosphere – it’s also teeming with bacteria – ‘rhizobacteria’. Symbiotic bacteria in the rhizosphere – known as Plant Growth Promoting Rhizobacteria (‘PGPRs’) deliver a raft of benefits to their host plants – some of which they literally could not survive without. They improve a plant’s resistance to pathogenic fungi, bacteria, viruses and nematodes as well as abiotic (environmental) stress like drought or heavy metal pollution, they also fix nitrogen into root nodules, convert organic nitrogen into inorganic forms (NH4+ and NO3–) which are available for plants, improve the availability of phosphorus and iron, control other nutrients including sulphur, iron and manganese, and synthesise plant growth regulators which improve plant growth.ref1,ref2This study has a table showing some of the positive plant responses to specific bacteria in research studies.
They achieve these outcomes for their host plant partly by going about their task of decomposing organic matter, but crucially also by producing substances including siderophores which make iron available, enzymes which degrade the cell walls of pathogens, volatile compunds such as hydrogen cyanide, biosurfactants which lower the surface tension of liquids, antibiotics which target pathogenic bacteria and phytohormones which promote plant growth processes; all of these go into the soil and into roots.ref Bacteria are also able to remove toxic metals from the soil through several different mechanisms and pathways.ref
This is such a fascinating area – bacteria turn out to be tiny bespoke pharmacies available to plants to help them thrive. And plants are not just passive recipients of bacteria – they create root exudates which attract bacteria they specifically need at a point in time, they are able to manipulate the rhizosphere to meet their needs.ref Plant genotype (ie. it’s genetic makeup) and the soil type are two main drivers that shape the rhizosphere microbiome.ref pH is particularly important, with studies showing that bacterial diversity was highest in neutral soils and lower in acidic soils.ref
The different bacterial species which are associated with different benefits for plants include the followingref:
On a final note, bacteria can produce ‘bad’ substances as well, particularly in anaeroic (no oxygen) conditions, when they produce phytotoxic nitrates and hydrogen sulphide. So avoid your bonsai substrate becoming too enclosed without aeration.
Unlike animals, plants do not have a digestive system, although the sustainable food trust makes a good argument that ‘soil is the collective stomach of all plants’ref Trees synthesise all of the substances they need to live and grow from 17 nutrients. It’s important to understand that plants don’t ‘eat food’ in the sense of consuming sugars, fats or proteins like animals do. Aside from oxygen, carbon and hydrogen (which come from air and water), trees absorb nutrients through their roots.
Water and nutrients are transported around trees via the xylem, a network of narrow dead cells which act like a kind of pipe. Nutrients are dissolved in the water (‘solutes’) and travel with it in the form of ions (charged molecules). To get into the xylem in the first place, water is absorbed into the root tips.
In many species this is done through the root hairs. Root hairs are “long tubular extensions of root epidermal cells that greatly increase the root surface area and thereby assist in water and nutrient absorption.”ref According to Thomas most live only for a few hours, days or weeks, and are constantly replaced by new ones as the root growing tip elongates. Some conifers do not have root hairs and rely on mycorrhiza instead to assist nutrient and water absorption.
In order to absorb water, the root tips need to be in physical contact with it, so having root hairs that reach into the soil provides contact with more water (and nutrients). Nutrients in the form of ions are ‘pumped’ into root hairs (or cells, if the species has no root hairs) using a process called active transport, which uses some of the energy from photosynthesis. Because the root cells have dissolved nutrients in them, water is then attracted into the space by osmosis.
From the roots tips, water and solutes make their way to the ‘stele’ – this is the central part of the root which contains the vascular system (xylem & phloem, shown in blue and red respectively in the left hand diagram below). Surrounding the stele is the endodermis – seen below in orangey-brown cells with red lines through them.
The red lines represent cells known as ‘Casparian strips’. They are full of lignin and other hydrophobic molecules, which basically plug any gaps between the endodermis cells. This forces any water or solute to pass through the endodermis cells. After this they travel through the root parenchyma cells into the xylem.ref
The existence of Casparian strips leads to a pretty important insight, which suggests that most molecules entering the xylem from the outside world are actively invited in, and have to be able to traverse a cell membrane. So the tree can theoretically control or at least limit what can enter. Vogel says “the sap that rises up a tree trunk has to be nearly free of dissolved material. So much water gets transpired that the accumulation of dissolved solids, coming out of solution as water evaporated in the leaves, would make big trouble as the growing season advanced.” So this implies there aren’t a lot of non-nutrients dissolved in xylem sap. But in fact, xylem has a microbiome (it’s part of the endosphere) and literally thousands of dissolved molecules in it (described more in xylem), so obviously the Casparian strips are not a 100% barrier.
It’s not all down to the root hairs or root tips though, symbiotic fungus known as mycorrhiza play an important role in enabling root function, read more about this in The Microbiome and Symbiotic Microbes.
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.
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.
The roots of your tree are *just* as important as the above-ground parts, with a lot of responsibilities which aren’t immediately obvious. I’ve summarised the main ones here but there is a lot more detail in separate posts with links provided below. So why are roots so important?
they absorb water from the soil to meet all the tree’s needs (both for photosynthesis and transpiration)
they absorb all the nutrients that the tree needs from the soil (using a different process to water, hence a separate point)
they transport nutrients & water up to the above-ground parts of the plant, and photosynthates (the products of photosynthesis) down to the root tips
they produce exudates (secretions) which sense and control the rhizosphere (the environment in which the roots are growing)
they produce plant growth regulators for signalling and enabling growth within the plant
they store food for later use
they provide structural strength and stability for the tree by attaching it into the soil
Points 1 and 2 are fundamental to the health and growth of the tree – the roots are the mechanism for the tree to obtain all of the water and nutrients it requires (despite the mythic popularity of foliar feeding, this is only a way of augmenting nutrient absorption and not a primary mechanism). The mechanics of how they do this is described in more detail in how roots absorb water and nutrients – in summary it’s the fine roots and their root hairs which do the majority of the absorption since they have the closest and most expansive contact with the soil. There needs to be enough root surface area to supply the stems, shoots and leaves with the water and nutrients they require.
Point 3 reflects the fact that roots are part of a tree’s vascular system, that is to say, they transport the fluids necessary for growth around the tree. Above the ground the vascular system is present in stems, shoots and leaves, and below the ground it is present in the roots. Water and nutrients are transported up from the roots through the xylem and photosynthates (the products of photosynthesis) are transported down from the leaves and other storage organs in the tree via the phloem, to provide the energy and nutrients for the roots to grow and function. Do roots grow all year round? Find out here: when do roots grow?.
Points 4 & 5 show that roots are very much an active participant in tree growth and not simply a set of supply pipes. They produce both cytokinin and auxin (read more in the post about plant growth regulators), they also produce a huge variety of substances known as exudates which both sense and control the rhizosphere (the environment in which the roots exist). Researchers believe that roots use exudates to “regulate the soil microbial community in their immediate vicinity, withstand herbivory, encourage beneficial symbioses, change the chemical and physical properties of the soil, and inhibit the growth of competing plant species”ref. Read more about exudates and how they are produced in root exudates.
Point 6 reflects the fact that roots are used to store food, in the same way that the trunk and branches do this aboveground (throughout the ‘woody skeleton’ (Ennos)). I was going to tell you that a lignotuber is an example of this and show you a lovely picture of my eucalyptus, but then I read “contrary to common assumptions…the lignotuber in young eucalypt trees did not appear to be a specialized starch storage organ. Rather, the lignotuber resembled an extension of the stem because its starch concentrations and temporal fluctuations mirrored that of the stem.”ref How roots store food and how much of a contribution to the plant’s overall storage capacity they make is debated. More on that in Root Food Storage (or, can I root prune before bud break?)
Finally as per point 7, the roots are responsible for physically holding the tree steady and stable against wind and gravity. They do this in many ingenious ways by adopting different root architectures – combining vertical taproots, lateral roots & sinker roots, creating ‘buttress’ roots, sending roots far from the trunk when needed and managing new root development in ways which stabilise the tree. More about this in root structure and architecture.
What all of this means from a bonsai perspective is that you need to pay just as much attention to the health and care of the roots of your tree as you do to the above-ground parts. Never mind developing a strong nebari for aesthetic purposes, you need to ensure that even though the roots of your bonsai trees are squashed into teeny-tiny pots, they are still able to perform the vital functions outlined above. Neglecting the roots will negatively affect the overall health of your tree.
Practically speaking, this is why you should aim to develop a well-ramified fine root ball, to provide the tree with lots of root surface area for nutrient & water uptake – taking into consideration the amount of biomass above-ground as this will determine how much root mass is needed.
The growing medium plays a huge role as well – this is your tree’s rhizosphere. It should provide the water, nutrients and micro-organisms the tree needs as well as (some) oxygen for root cell respiration, and ideally should not be disrupted so much so that exudates and microbes (fungi or bacteria) are lost. The risk associated with bare-rooting a tree (or excessive repotting) is that it destroys the rhizosphere all at once, leaving the tree vulnerable to pathogens and forcing it to regenerate exudates it has already created (which can use up to 40% of its stored carbon).
Your tree’s roots need to have a regular supply of nutrients, so they require fertiliser of some kind. Even if good compost is added during potting, the small size of bonsai pots will mean the nutrients won’t stay in there for very long. Trees will need added fertiliser – either home-made (for example, regular doses of diluted compost leachate), or purchased. And obviously – watering is critical. Given the role of symbiotic partners (such as fungi & bacteria), you can also add these to the soil – if your tree senses their presence and wants them to stick around – it will probably produce exudates to achieve this.
The cambium – or more precisely the vascular cambium – is a layer of cells underneath the outer and inner bark and outside the wood of a tree. It’s officially defined as a ‘meristem’ – that is, a region of cells capable of division and growthref. You may recall the ‘shoot apical meristem’ in the post about How Trees Grow – this is the part of the shoot which is actively dividing and creating new cells at the tip of the shoot (known as primary growth). The vascular cambium does something similar – it divides to create the vascular system – a layer of xylem cells on one side and a layer of phloem cells on the other. The vascular cambium is where part of the secondary thickening of a tree takes place, as the xylem layers become the wood of the tree, and the phloem layers become the inner bark (the outer bark has another meristerm – more here). Always in between there is a single-cell thickref layer of vascular cambium. See below for an image of the cambial zone, phloem and xylem cells.
Two types of cells exist in the cambium – vertically elongated ‘fusiform initials’ and horizontally oriented ‘ray cell initials’. The fusiform initials produce xylem and phloem cells and the ray cell initials produce rays (which cut across the tree connecting xylem and phloem).ref The ray cells can create gum and resin channels, which can also be activated when the cambium is wounded. You can see a resin duct cavity in the image above, as well as a ray.
Like a lot of growth in a plant, the activity of the cambium meristem involves plant growth regulatorsref. Auxin levels peak in the middle of the cambial zone, where cells are dividing, cytokinin peaks in the developing phloem cells and gibberellin peaks in the developing xylem cells. A study in Populus found that increased local biosynthesis of cytokinin led to increased trunk biomass and radial size (width).ref ‘Local’ biosynthesis in this case meant a tree which had been transgenically modified to produce more cytokinin.
The vascular cambium slows down or stops completely during winter in temperate zones, this depends on the tree’s phenology. More detailed information about the vascular cambium can be found in this book.
From a bonsai point of view the main takeaway is that the vascular cambium tissue underneath the bark is critical for your tree’s growth so avoid damaging it.
The word ‘phloem’ comes from the word for bark in ancient Greek. It is a parallel system to the xylem which transports water and nutrients up from the roots, but instead transports the products of photosynthesis (‘photosynthates’) from the leaves to the rest of the tree. A big callout to The International Association of Wood Anatomists for the images in this post, contained in this open-access publication.
One of the main photosynthates produced by trees’ leaves is sucrose (maple syrup anyone?), but others found in phloem include fructose and glucose, sugar alcohols and the raffinose family of oligosaccharides (RFOs). A sugar alcohol known as ‘D-pinitol’ has been found in substantial amounts in gymnospermsref and is believed to be the main carbon transport molecule for Scots pine. In addition to sugars, the phloem system is used for signalling and defence throughout the tree (as is the xylem), so plant growth regulators (including auxin, cytokinin and salicylic acid), proteins, minerals and RNA travel in the phloem sap as well. If a foliar insecticide/herbicide/fungicide has been applied and is able to penetrate the pores or stomata (see foliar feeding), and is able to get into the phloem vs staying inside adjacent cells, it will translocate throughout the plant.ref As a result I would not be eating non-organic maple syrup (previously paraformaldehyde was used to reduce microbial attacks on maple trees for syrup product, but this was banned by 1989).ref
There still seems to be quite a bit that’s unknown about how phloem actually works – an article published in 2014 said “Because of the difficulties in measuring phloem function, particularly in trees, we lack a basic natural history and phenomenology of tree phloem”ref and another published as recently as 2021 said “phloem loading strategies in gymnosperm trees have been only tested in three species: P. sylvestris , Pinus mugo and Ginkgo biloba.”ref
But the basic principle is that sugars are created by the process of photosynthesis, ‘loaded’ into the phloem cells (with assistance from adjacent cells) and transported to places in the plant where they are needed, then ‘unloaded’ (but even the mechanism for transportation of sugars in phloem is debated – a famous theory involving ‘osmotically generated pressure gradients’ has dominated but many recent articles point out the lack of data to support it.ref) According to one account, sugars are loaded from leaves into phloem companion cells by active transport (a process which consumes energy) and then diffuse into the sieve tube elements through the plasmodesmata (cytoplasm which is shared between cells via small pores between them). Water then moves by osmosis into these cells (creating the phloem sap), and sugars translocate (move) when sinks (areas of the plant consuming energy) remove sugar and reduce its concentration in the phloem sap.ref
Phloem is also believed to translocate (move from one place in the plant to another) sugars even when photosynthesis is not taking place – eg. in winter in deciduous species.ref In this case the sugars are coming from storage tissues in the rays and roots.
The cells which make up the phloem system in gymnosperms are different to those in angiosperms (similarly to the difference in xylem), but the basic structure for both is that tubular cells, known as sieve cells (gymnosperms) or sieve tube elements (angiosperms), are connected together via pores in their end walls, and the phloem sap ‘flows’ through these sieve cells/tubes.ref
Below is an image of pine sieve cells. The side and end walls are structurally similar, unlike the sieve tubes of angiosperms. The phloem sap flows from cell to cell downwards, through the pores. Many studies reference the fact that sieve cells & tubes contain material which would appear to create a barrier to flow, which calls into question the abovementioned ‘osmotically generated pressure gradients6’ theory.ref
If you’ve read the post about the cambium, you’ll know that there is a constant process of creating new xylem and phloem cells, and in the case of phloem, the most recent does the conducting.refThe conducting phloem usually lasts for one season, but can remain ‘functional’ for one-two years (ie. the cell is still alive, even if it’s not conducting phloem any more). Like xylem, phloem rings are created – see the image to the right of pinus strobus – all of the dark cells are the annual phloem sieve cells which are now non-conducting. The conducting cells are in the lower purple region.
A key difference between xylem and phloem is that phloem cells are living cells. This means that phloem sap must pass through living cells and their membranes in order to flow and this articleref suggests that this mechanism provides a high degree of control for the plant in managing what gets into and out of the phloem system. The phloem passes through holes in the sieve cells known as sieve plates (see pics below both of ficus species, the left hand side shows a transverse section and the right hand side a lateral section).)
In order to create the space for the phloem sap, sieve cells and tubes are missing quite a bit of the normal cell machinery, including a nucleus, vacuole and ribosomes – so they can’t control their metabolism or make proteins. Although they still have some specific proteins (P-proteins – apparently previously known as ‘slime’!ref), mitochondria, endoplasmic reticulum, and sieve element plastids.ref Both types of sieve cells have helper cells alongside which metabolise on their behalf – companion cells in angiosperms and Strasburger cells in gymnosperms.
Since phloem is full of delicious sugar-rich fluid, it can be a magnet for insects, which in turn introduce microbial pathogens including bacteria and viruses.ref Plants produce metabolites to defend themselves against these pathogens, and also induce sieve plate occlusion – basically blocking up the sieve cell or tube where the pathogen is located to avoid it spreading.ref
Both the active phloem and the old phloem which no longer transports photosynthates are together known as the inner bark. Outside these phloem layers is the ritidome or outer bark. You can read more about bark here.
For bonsai there’s really not a lot you need to worry about with respect to phloem, unless you are wiring super tight and cutting off the phloem (but by then your wire will be well embedded in the outer bark).
Biogold is another popular bonsai fertiliser, which may or may not be cagey about its ingredients since the packaging is all in Japanese which I cannot read. Deploying google translate on their website, and searching online yielded some information:
It’s a fertiliser with N:P:K ratio NPK 5.5:6:3
It contains micronutrients iron 0.12%, copper 50mg/kg, molybdenum 27 mg/kg, sulfur 0.5% and also magnesium and calcium (in unspecified amounts)
It contains chicken manure fermented using bacterial processes
Chicken manure isn’t used directly on plants because the organic matter will ferment and generate heat, usually it is fermented separately along with plant matter such as straw, leaves, cardboard etc, and requires turning or mixing to ensure exposure to air (this is aerobic fermentation which requires oxygen).
So it’s likely that Biogold contains some other kind of plant matter which is unspecified. When looking at other products, plant matter (particularly green or coloured leaves or skins) provided substances which helped the microbial communities in the soil flourish, enhancing root growth and nitrogen uptake. One study found a “positive effect of BioGold® and Compost in increasing the soil microbial population by providing nutritive sources for the growth of soil microbes”ref
Chicken manure is a good source of nitrogen, contains humic acidref, which is a concentrated form of organic matter (also contained in the coal precursors leonardite and peat), and was found to have better growth potential for plants than cow manure.ref
A researcher growing coconut compared BioGold with other fertilisers in this studyref – and found “There were no significant differences (P> 0.005) between treatments in any of the growth parameters tested after a period of six months after planting.” ie. the plants tested had similar outcomes from all the fertilisers tested (which included inorganic fertiliser, cattle manure, ‘Kochchikade biofertilizer’ and compost).
So overall, hard to say, this product appears to be a good fertiliser with micronutrients and humic acid/concentrated organic matter. But since it doesn’t disclose all the ingredients it’s impossible to fully assess it.
The word ‘organic’ in terms of fertiliser does not mean the same thing as ‘organic’ when it comes to food.
Organic food follows principles of production which in general do not permit soluble fertilisers and synthetic pesticidesref to be used during the food production.
Organic fertiliser means “any substance composed of animal or vegetable matter used alone or in combination with one or more nonsynthetically derived elements or compounds which are used for soil fertility and plant growth.”ref This does not imply that the animal or vegetable matter itself was not produced using chemicals or is organic in a food sense.
As an example, rapeseed meal can be called organic fertiliser if the oil has been extracted using cold pressing methods, but this doesn’t mean that the rapeseed itself was grown using organic farming methods. If the oil has been extracted using a solvent, I think it’s doubtful that this rapeseed meal should be called organic.
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 bacteriaref
Humic acid – ultra-dense organic matter, converts elements into forms available to plantsref, 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 carbonref
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)
