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  • Rafael Saer

Hydroponic Science: A Closer Look at Silicon and Plant Stress



Introduction

In our previous blog post, I explored the potential role that silicon can play during the conditions of salt stress, and whether salt stress really is something to be concerned about in home or commercial hydroponics. I concluded that, under most cases, the salt (sodium) concentration in a water system never got high enough to negatively impact plants grown in hydroponic systems. Additionally, these salt levels were never high enough for the effects of silicon to manifest. So where did that leave us with the silicon story? Like I mentioned before, silicon is a bit of pain to supplement in a hydroponic system, however there is no shortage of scientific articles that point to the benefits of this element for plant growth, stress, and defenses. Ultimately, I want to better understand the conditions in which silicon supplementation plays a beneficial role in hydroponics. There are so many different hydroponic systems out there, existing in a variety of geographies and microclimates.

Silicon abundance and availability

Silicon is the second most abundant element in the Earth’s crust after oxygen. In your garden soil, silicon dioxide (SiO2) comprises about 50-70% of the soil mass. It’s safe to say that plants growing in the presence of silicon is the norm, unlike the situation in hydroponics. In order for this element to be taken up by plants, however, silicon must be available as silicic acid or mono silicic acid, and many soils differ in their availability of these compounds. Silicic acids are normally formed from silicates through gradual weathering in the presence of water. Like many chemical reactions, I’m sure that this process can be accelerated in the presence of heat, but that discussion is a detour that I’m better off not getting into. If you’re a soil grower, you’re fertilizing your plants with silicon whether you like it or not, and maybe this is something to keep in mind the next time you want to make comparisons between soil and hydroponic growing methods. I hasten to add here that many hydroponic substrates, like perlite and hydroton, have silicon in their chemical structures, but the bioavailability of silicic acid from these substrates is likely to be lower than what is present in soil.

Silicon uptake and transport in plants

A quick disclaimer. A lot of the research that I mention below was performed on Oryza sativa, aka rice.


Although some plants accumulate silicon more than others (most garden vegetable plants accumulate low amounts, whereas rice seems to uptake silicon like crazy), it appears that the transport of silicon from the external environment into the roots of plants is mediated by the same culprit—a transporter called Lsi1. For the more scientifically curious, this transporter belongs to a subfamily of aquaporin-type proteins (see below for a structure), which transport water and uncharged solutes across cell membranes. Nature likes to take a good design and tweak it to suit many purposes. The picture I used below is, in fact, an aquaporin from a cow (I couldn’t find structures of the silicon transporter available in the protein database, but nevertheless I wanted a pretty picture that wasn’t a stock photo this time). In rice, Lsi1 is present in higher quantities than, say, a tomato plant, and this results in a greater uptake of silicon from the soil. There does exist another silicon transporter, Lsi2, and this transporter seems to be more involved in pumping silicon out of plant cells. Solutes like silicon need to travel in and out of root cells in order to reach the xylem, and Lsi1 together with Lsi2 facilitate this. Once in the xylem tubes, silicon can passively cruise on over to the shoots of the plant.



Silicon uptake in plant roots is mediated by proteins like these, that mobilize silicic acid across a membrane. Left: view of the protein parallel to the plane of a cell membrane. Right: view perpendicular to the plane of the membrane. Image created with VMD.


Given the low toxicity of silicon, I’d be interested in seeing what would happen if you were to overexpress these transporters in a tomato or lettuce plant. Will it make my lettuce crunchier? Will it boost the stress tolerance of a tomato? Or will it be a detriment to the plant because it has to spend a ton of resources making a transporter that it doesn’t need? A more hydroponics-related question would be: what is the consequence of having this transporter expressed in an environment without silicon? In rice, this transporter is expressed constituitively, meaning all the time. Does the transporter start uptaking something else? Proteins are generally not perfectly specific for substrates, and they can often process other substrates if they’re lying around. For example, one of my colleagues in grad school found out that the protein that puts the iron into a heme molecule can also shove a zinc in there at a lower frequency. Does something like this occur with the silicon transporters in hydroponic systems? If you’re a researcher in this field, and you’re reading this, I’d love to discuss! Always a future direction in research I guess…


Silicon deposition in plant cells


Silicon is deposited all over the plant, including the roots as well as the leaves and stems. As it’s taken up by the roots, it doesn’t take much for it to get deposited into the underground portion of the plant. To get to the aerial portion, however, it needs to enter and exit the xylem as mentioned above. As silicon travels through the xylem, it’s still in the form of silicic acid. This makes sense because silicic acid is water-soluble, and so the silicon can piggyback on plant water channels in order for it to get to where it needs to go. To leave the xylem, a protein called Lsi6 is involved in xylem unloading. Lsi6 is a homolog of Lsi1 (the silicon uptake protein), meaning that it shares a common ancestry with Lsi1. It probably also looks like the protein structure above.

So what eventually happens to all this silicic acid moving through the plant? Eventually it concentrates in the cell walls of plants. When it reaches a critical concentration, the silicic acid crashes out of solution, becoming silica particles. These particles grow in size, forming structures as they become larger and fuse with other growing particles. These amorphous formations have a fancy name: phytoliths. They are bound to various components of the cell wall. You can say that the silica eventually becomes part of the cell wall.


What’s the deal with all this silica?


By now you’re wondering: what’s the point of a plant spending resources to pump all this inert stuff into its cell walls? If you read up on what’s on the web, silicon is some sort of magic bullet against any and all bad things that come near your plants. How is this possible if all it’s doing is just hanging out on the fringes of plant cells? Thankfully, researchers have also been baffled by this, and Dr. Richard Bélanger’s group from the University of Laval in Quebec, Canada published a great article that addresses this issue. As a matter of fact, Devrim Coskun’s article goes into far more detail on all that I described above; it’s a great read for the more scientifically literate. I’ll mention here that much of what I mention below regarding plant stresses is taken from this great review. Coskun et al. proposed a model called the “apoplastic obstruction hypothesis”. Let’s break that down. Apoplast—the space outside a plant cell membrane where the cell wall resides. As the cell wall is “coarser” than the membrane, material can diffuse freely about this region, unless something is obstructing it. Silica gel becomes a roadblock in plant cell apoplasts, and this, the authors argue, provides a benefit via several mechanisms.


Plants can experience two types of stresses—biotic and abiotic. Biotic stresses include herbivores, bacterial and fungal infections, etc. Abiotic stresses include things like salt stress, nutrient deficiency, heavy metal toxicity, and so forth. How does silicon’s apoplastic blocking ability play a role in both these events?


Abiotic stress


Stresses like salt stress and heavy metal toxicity operate on the principle that something unwanted is getting inside the plant. Often times this leads to the generation of reactive oxygen species and subsequent oxidative damage. Many of these nasties infiltrate the plant through the apoplast (a pathway called the apoplastic bypass route). If big deposits of silica are in the way, these unwanted molecules get stuck, and can’t reach other parts of the plant. Furthermore, silica deposits help to reinforce a critical plant barrier called the casparian band. This barrier serves to regulate the flow of water and solutes through a plant by forcing these substances to go in and out of the root endodermal cells (i.e. through a cell membrane). This is like a security checkpoint in the plant roots that screens solutes prior to being allowed to get into the xylem. Any vulnerabilities in this casparian band will cause unwanted solutes to pass into the xylem and reach other parts of the plant. Ultimately, the effective barrier of silica in plant cell walls and the casparian band helps prevent the transport of unwanted solutes throughout the plant, which prevents downstream oxidative damage. This is super important because it’s a passive means by which the plant saves a lot of energy on damage control. Maybe sometime in the future I’ll talk about the photosynthetic machinery and how sensitive it is to this type of damage, and the ridiculous efforts that plant cells undertake in order to repair this damage (do a search on “photosystem II repair” if you’re curious).

It appears that silicon can also help with reducing water stress in plants. This is hardly an issue in a hydroponic system, lest you’ve quaffed a couple of drinks and feel like pumping a ton of polyethylene glycol into your nutrient reservoir. In rice and maize, silicon had an inhibitory effect on stomatal conductance. In other words, the plants were not transpiring as fast as in the absence of silicon. This, of course, is beneficial during times of water stress, as its in a plant’s best interest to conserve water.


Nutrient imbalance is another abiotic stress and this is one that is perhaps more relevant to hydroponics. Given what we know about silicon’s role in regulating solute uptake, I’m curious if silicon supplementation can be useful in plant feeding. Here’s my thinking: your NPKs and micronutrients are cruising right alongside all the salt, silicon, and heavy metals through those plant apoplasts. Any holes or gaps in the walls will also affect how your plant nutrients get distributed throughout the roots and shoots of the plant. I can rationalize that my crops have, through evolution, figured out how best to regulate their nutrient uptake, and barriers such as the casparian band play a pivotal role in this (after all, food crops have evolved in a soil environment). If my hydroponic crops have gaps in their barriers, then perhaps my plants are not balancing their nutrient uptake as well as they could be. Furthermore, hydroponic fertilizer suppliers often have a variety of mixes optimized for different crops. As a home grower, I certainly don’t care to have 20 different mixes for all my plants. Again, I can ask: if my plants have a good infrastructure for regulating their nutrient uptake, can I get away with a general purpose nutrient (like those good ol’ 20-20-20 mixes) for all my plants if I spike in a bit of silicon for building up the barriers? Will this provide a practical benefit if I grow different kinds of veggies in a system that shares a common nutrient reservoir? Can a little silicon help in developing new hydroponic formulations? Is it worth the pain of dealing with silicon supplements?


Biotic stress


Let’s look at how silica in a plant cell wall can confer a resistance to a biotic stresses. First of all, no amount of silica is going to stop the neighborhood groundhog from chewing up your plants, unless you build a physical wall out of it (it’s probably cheaper to build a fence). Actually, this isn’t totally true, because silica can make some grasses more abrasive, which deters feeding from mammalian herbivores. However, I don’t think this terribly relevant for the crops that humans eat. When it comes to biotic stresses, think fungi, bacteria, and small insects. Silicon seems effective against plant-specific, biotrophic pathogens. Biotrophic pathogens are those that interact with living plant cells in order to derive their energy from them. This is in contrast to a necrotrophic organism, which quickly kills a plant cell in order to feed upon the remains. Biotrophic pathogens such as powdery mildew must release effectors—proteins or other bioactive compounds—into the apoplast in order to overcome the plant’s defenses. These effectors have to infiltrate the cell membrane to get inside the plant cell, and would you guess? The silica in the plant apoplast makes it hard for these effectors to reach their targets. It’s another case of the bad stuff getting stuck in transit. With the effectors not able to pierce the plant cell membrane, the plant is freely able to make a host of defensive compounds in order to fight off the pathogen. Some herbivorous insects, such as aphids, also rely on the use of secreted effectors to feed on the plant. In these cases the blockage of these effectors by silica results in the insects having a shorter meal. The idea here is that the plant can now produce compounds that are harmful against the feeding insect, so the insect can only feed as long as it can take the pain!


What about necrotrophic pathogens then? As these pathogens don’t rely on effectors, but rather bombard the plant cell with compounds that kill it (think hydrogen peroxide), silica doesn’t really do much to help here. These types of pathogens normally attack wounded or damaged tissue (gray mold on strawberries classic example of a necrotrophic host-pathogen interaction), so it’s more of an issue for plants that are already weakened, sick, or have been cut up and sitting in your fridge for too long.


Final thoughts


Ultimately, what I wanted to learn from this was: under what conditions should I be considering using that Armor Si bottle in my hydroponics grows? I don’t think I’ve fully answered that question, but I’ve certainly gained some insights. Do you suffer from humid summers and a lack of dehumidification? These conditions can promote pathogenesis from biotrophic fungi. Perhaps reach for that bottle of Si supplement. Are aphids always a problem? Maybe try some Si supplementation as part of your pest management strategy. Root rot? Try silicon. Worried about heavy metals? Maybe you should contact the city first. Should you always be using it? Probably not, though I would really like to know how it affects nutrient uptake. In my case, I’m glad to put the silicon to use in some hydroponics experiments that I can hopefully learn a lot from.


There’s a cost associated with silicon supplementation. Armor Si isn’t free, and the precipitates that silicon supplements leave on your hydro equipment are going to cost you time and effort to remove. It may pay off in the time and cost savings of your pest management strategy. It may make your leafy veggies taste better, and that might be worth all the effort. Should you be using silicon in your hydro setup? I can’t make that decision for you, but I’ll leave you with a quote from chef Marco Pierre White: “it’s your choice”.

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