Roots Exposed! A look down under!

It’s been a while since my last article and I have been soaking up the Australian sunshine as part of some well deserved rest and relaxation! While I was enjoying my holiday, a good friend (and no, not my friend Mr. Daniels!) and I were chatting about plants in general and what new technologies/theories were available that could substantially help growers. Of course with all the new found research by our R&D team on foliar spraying and foliar technologies, I immediately began relating the importance of foliar spraying as the new wave of performance gardening. “ But what about the roots?” he asked. “Aren’t they important too?” “Sure,” I replied, “but only for basic nutrition.” “Basic nutrition, are you sure?” my friend enquired. So I sat him down and we had a great discussion about a plant’s root system together with how the plant uses this system for uptake of certain elements and not others. After our little (well actually a very long) chat I thought this would make a great topic for my next article. So today we are going to take an exposed but brief look at the root system of your plants. We will review how and why it works, together with examples, of how it doesn’t work quite the way you might have thought. Along the way we are going to dispel a few things that we thought were true for a long time but which modern science and research have now shown to be false. The root system is actually one of my favourite plant sub systems (next to the leaves) as it is so complex and dynamic. If you remember in my last article, we compared a plant to a city, with the different parts of the plant representing corresponding parts of a city. In that article the roots were likened to the port of a city that receives a good portion of the incoming goods and raw materials that the city needs to function. I think this analogy is a good one as it best describes the basic function of a root system. Goods (nutrients and water) arrive at the port, are checked by customs (the Casparian Strip) for things that don’t belong and then the remaining goods are cleared and transported to the manufacturing centers (the leaves) for production. Ok, you get the basic idea, now let’s look a little more closely at the different parts of our (port) root system and their functions. As we look at the different parts of our root system, I am going to dispel a couple of long held myths about the root system, including one that was long held to be true but recently has been proven otherwise by some very interesting research. To assist us in our understanding of the root system and the different roles played by the various parts we are going to track the journey of two parcels of goods. One will be a package of nitrogen, the other a package of enzymes and proteins that are commonly added as part of a performance supplement. For ease of use we will refer to the nitrogen package as N and the enzyme and protein package as EP. Both packages (N and EP) arrive at the port to be unloaded at the dock. For many years it was thought that the actual dock (site of absorption) was almost exclusively located in the young, newly growing root tips and rootlets of the plant. The work of prominent plant researcher Paul J. Kramer has actually proved that this is not the case and that most absorption occurs several centimeters behind the root tips of the young newly growing roots. Professor Kramer used radiotracers to follow the uptake of selected ions to prove this. (Myth one dispelled: plants only absorb nutrients from the growing tips of the roots.) So our two packages have now been unloaded at the dock and are ready to proceed onwards to customs. In a real city, the main function of customs is to find and prohibit the import of illegal or undeclared goods. In the plant, a unique little barrier inside the root called “The Casparian Strip” performs this customs function. The main job of the Casparian Strip is to keep out or prevent the uptake of foreign bodies like fungal spores, bacteria, viruses, etc., as well as other compounds that may be harmful to the plant. The primary way the Casparian Strip performs this is through a form of size exclusion. By size exclusion we mean that this strip will not let particles or elements of any size through, thus forcing the uptake of elements to go via the symplastic pathway. Essentially there are two pathways; the apoplastic (between cells) and the symplastic (through the cells). The cell membranes in the symplastic pathway then act as the size exclusion filters. If we look at diagram one we can see how the Casparian Strip exits between all of the cells surrounding the vascular bundle (the phloem and xylem). This way all elements must pass through the cells on either side of the Casparian Strip before they can be up taken by the vascular system and delivered inside the plant.

So our two packages (N and EP) are ready to proceed through customs and then onwards to the leaves via the plants highway (Phloem and Xylem). N (our nitrogen package) is the first to go through, and as its size (as the main exclusion criteria) is small, it is allowed to pass freely through and into the plant to complete its journey to the leaves. A job well handled and performed by our port (root system). Now our second package, EP (enzymes and proteins) is ready to proceed to the Casparian Strip and the symplastic pathway (customs). But wait, our enzyme and proteins package is not allowed to pass. It’s being held back, why? The plant is going to need this package (it’s a performance supplement) and will do good things if allowed inside. Unfortunately, the size of this package is preventing it from passing via the symplastic pathway. So our plant’s customs department is doing its job very well by keeping out what is perceived to be a foreign body that may be harmful to the plant. So if this package can’t go through, what size molecules can the symplastic pathway allow through? In recent studies conducted by our Canadian Dutch Master R&D team we looked at this unique barrier and what sized molecules could pass through and which were excluded. In our experiments conducted by The University of British Columbia we attached or conjugated a special dye tracer to a range of carbohydrate supplements ranging from 2,000 Daltons through to 20,000 Daltons. Now 20,000 Daltons may sound large but in reality it’s very small. If you look at figure 2 you can see the relative sizes of common elements and supplements that are applied to plants. Using a special type of laser scanning microscope called a fluorescence microscope we examined cross sections of roots that were exposed to these dye tagged supplements. What we found was that even the smallest carbohydrate supplement tested at 2,000 Daltons was effectively blocked from uptake. The roots took up the dye alone at 450 Daltons, so we can effectively say the cut off point lies somewhere between 450 and 2,000 Daltons. I have included some great pictures of this (Figure 3) for reference purposes. Now this poses some very interesting questions that my friend brought up earlier in our rather lengthy (and informative) discussion while I was on my holidays. If the plant can’t take up molecules or compounds larger than 2,000 Daltons, then what about those supplements that contain molecules or compounds larger than this? Truth of the matter is, they can’t, or at least not with any efficiency. The plant’s own, unique defense system, the Casparian Strip and the symplastic pathway (their customs dept.) prevents this from happening. (Myth two dispelled: Plants can take up large molecule compounds (like complex carbohydrates, most enzymes, etc., through their root systems). My friend then countered with a very good question. “But I’ve used some of those supplements that contain these enzymes, proteins, etc., and they seemed to work, or at least they seemed to a little bit.” What my friend was probably experiencing was a slight (very slight) increase in plant performance due to some of those enzymes, etc., promoting a healthier or more abundant root system. The more roots, the more nutrients are absorbed, but not the uptake of larger-sized enzymes and proteins. So if those compounds are proven to work once inside the plant, how do we get them there? The answer to that is very simple. Use a good foliar spraying program that includes the use of a specialized delivery agent. This is the only way that larger-sized molecules like most enzymes, proteins and complex carbohydrates can be introduced to where they are actually needed. The other topic that needs to be covered here, is as it is very relevant, is organic nutrients and additives. Organics are being utilized more and more by growers in both hydroponic and soil based applications as they strive to improve the flavor or size of their crop. Now that we have actually looked at some aspects regarding the way a root system functions, albeit in a very simple way, we can apply that knowledge to put organic-based nutrients into proper perspective. Knowing, as we now do, that the Caspsarian Strip prevents any molecules or compounds from being absorbed into the plant via the apoplastic pathway (between cells) we can apply this principle to organics. Organic nutrients are particularly great in soil-based applications because they provide to the plant a steady release of nutrients over time. They do this by being repeatedly broken down a little more each time until they release their constituent elements. A molecule of nitrogen from an organic source is identical to a molecule of nitrogen from a chemical source. They are exactly the same thing to the plant and in chemistry. So our knowledge of the Casparian Strip and the symplastic pathway together with how it prevents uptake of larger-based molecules should invariably lead us to the correct conclusion: that the larger organic based molecules cannot be taken up by the plant until they are broken down into their constituent elements. As mentioned before this steady and slow release is great for soil-based systems but does leave some rather large question marks about its performance in hydroponics as the plant must first wait for the organic compounds to be broken down before they are accessible to the plant. This is in comparison to chemical-based nutrients, which are immediately available to plants. Lynette Morgan, a leading scientist and regular contributor to this magazine, recently conducted a series of experiments regarding the comparative performance of organic vs. chemical nutrients in hydroponics. It was seen that the chemical-based nutrients, on average, produced larger crops due, in large part, I believe, to their immediate availability. So now we have seen a little about how our root system works and the types of elements it can or cannot uptake for distribution to the rest of the plant, including organics. So what can we take away from this quick little journey into the world of roots? 1. We learned that not everything we apply to our plant’s root system would be effectively absorbed for use by the plant in spite of what we may have been told. 2. That to get the best performance from many of the larger molecule-based supplements (carbohydrates, enzymes and proteins supplements, etc.) we have to go through the leaves instead of the roots. Well, now you’ve had the same little chat with me that my friend in Australia did. I hope that, like him, you can take something away from this that will help you grow and enjoy your indoor garden a little bit more. Until next time take care and happy gardening.