To understand interveinal chlorosis in a plant, it is useful to review what the veins of a leaf look like and what they do:
The veins of most leaves are generally obvious as they look much like our own veins. They provide physical support for the leaf and transport water and nutrients through the leaf. Leaf veins do look much different when comparing monocots and dicots, however. With a dicot, there are larger leaf veins feeding smaller ones across the face of the leaf.
In a monocot, the veins run parallel and may not seem so obvious. Palm trees are monocots (monocotyledon) while tomatoes are dicots (dicotyledon), and while we won’t get into the differences between these here, the appearance of the leaf veins is very different. This discussion will be referring to dicot plants.
Symptoms of Interveinal Chlorosis
When a leaf displays interveinal chlorosis, the veins in that leaf will look a normal shade of green while the leaf tissue in between the veins (called mesophyll) is a pale green, or even yellow when the chlorosis is more severe. Chlorophyll is the chemical or biomolecule within a plant that absorbs sunlight and uses its energy to create carbohydrates. These carbohydrates then become a source of energy for the plant. The process that creates them using CO2 and water is called photosynthesis. Chlorophyll is the heart and soul of photosynthesis, which in turn is the powerhouse of the plant. This molecule is key to plant health, and we need to be aware of it if our plant begins to exhibit interveinal chlorosis.
Chlorophyll has magnesium as its central metal ion and the organic molecule that it bonds to is called a porphyrin. There are several steps (biosynthesis) within the plant that must occur to create a chlorophyll biomolecule. Ultimately, these steps lead to the creation of either a hemeprotein or chlorophyll. The science behind the evolution of the chlorophyll molecule is complex, but suffice it to say that manganese, iron, and zinc all have key roles in its creation. Manganese is an enzyme activator during chlorophyll production and a structural component of the chloroplasts where photosynthesis occurs. Zinc is a key nutrient in supporting the creation of chlorophyll, and a lack of iron will interfere with it as well. There are debates regarding the symptoms and determination of which nutrient deficiency has caused a plant’s chlorosis. These yellowing symptoms look very similar. Other symptoms such as necrotic leaf spots caused by low zinc levels can help determine which may be the culprit.
A key point to remember when evaluating chlorosis is that the lack of iron, zinc, or manganese may cause the chlorotic condition in a plant. Very often, it is assumed that only the lack of iron is causing this problem, and so iron is added as a cure. Sometimes, the result is only a very slight change in symptom or no change at all because it is one of the other micronutrients that are deficient for the adequate production of chlorophyll. Then again, there may be an issue with the root system that is causing the micronutrients to fail assimilation.
A thorough analysis of all the factors that may contribute to chlorosis is the goal. We have a good idea of the most likely suspects, so let’s see how this malnutrition condition might get its start. Understanding the most accurate and comprehensive cause of a plant malady leads us ultimately to the proper treatment and cure.
Starting at the roots, our growing medium is our fist place to look. Is it capable of holding an adequate level of the three vital micronutrients that affect this condition? Looking at soils, there are conditions that will make them unable to hold or transport these minerals.
We need to consider the pH of that soil. It turns out that each of the nutrients we are discussing are adversely affected by high pH. Iron easily reacts with oxygen in the presence of high pH, or very aerobic soils. When this happens, it is quickly converted to iron oxide, which then is not plant available. You can see that in grow medias with high pH or those with high oxygen levels, much of the available iron may not be useful. Overly wet or cold soils will also contribute to this issue, as will a soil with high-clay content.
Environmental Stress and Micronutrients
Environmental stress on the plant will also affect the plant’s root system and its ability to uptake micronutrients. It is due to the plant’s immune or self-protection system that this occurs, as an overabundance of these minerals can be toxic. This response allows the plant to reduce what it thinks may be a nutrient overload. The presence of higher levels of cadmium or cobalt can also play a role in increasing iron deficiency in plants.
Manganese (Mn) is present in soils in three different states: Mn+2, Mn+3, and Mn+4. It is this first state, Mn+2, that is plant available. When this form of manganese is released in the soil and present at the root surface, it is easily diffused into the root. In the presence of high pH, however, manganese can precipitate into other chemical forms. In the presence of very low pH soils (below 5), manganese can become too available and hence become toxic to the plant. High organic matter can also lead to trouble with the availability of this mineral as the manganese becomes easily chelated. In this case, this process makes it less available as dry soils reduce the availability of manganese. Just to demonstrate the complexity of maintaining availability of this and many nutrients, a wet and cold soil will reduce mineralization (needed to produce Mn+2) of soil organic matter as well as reduce root growth and the metabolic activity of the roots. It becomes apparent how the grow media will affect availability of these important minerals.
Zinc reacts similarly to soil pH conditions as do the other two minerals we have looked at. The common symptoms for zinc deficiency are: Chlorosis, necrotic spots, and bronzing of the leaves.
Chlorosis can be caused by a deficiency of any of the three of the vital minerals for our crop or even a combination of all of them. Transportation of these minerals through the soil is a key aspect toward nutrient availability and it is not only high pH that can make this transport difficult but so too can heavy clay soil.
Since chlorosis is easy to spot, this makes it easier to initiate corrective action. Don’t forget good-quality foliar fertilizers. These can get to solving plant nutrition issues quickly as they are absorbed directly into the leaf tissue. A deficiency of these minerals will not only cause chlorosis, but will also produce other negative effects in the plant.
If our evaluation pointed to probable soil issues, that should be a first part of our program for correction. Most native soils have an abundance of manganese in them, but not all.
If the soil is highly porous, contains a high percentage of clay, or holds a high pH level, this could be the problem. If you are growing in a high-clay, cold, or an overly wet soil, correct this if possible. If the issue is clay, the best place to start is to remove an appropriate per cent of the native soil, then replace it with silt and sand in addition to some good organic matter before the next crop. Check a soil pyramid chart for the percentages of these three particles that represent a good loamy soil. You can do jar tests to find out your current ratio of these.
Plants grown in hydroponic systems won’t have a clay issue, but may have pH issues as rapid drainage tends to promote leaching of these nutrients.
Hydroponic grow medias are also generally aerobic and contain lots of oxygen that supports mineral conversion. This can make them prone to requiring the addition of these micronutrients much more often. Due to these issues, the addition of a complete nutrient formula, one containing an appropriate balance of macronutrients and micronutrients, should be used and applied more often than one would do in conventional soil.
Check the nutrients that you are providing, and if some of the micronutrients are missing there are many excellent mineral products available to add and fill that void. Pay attention to labeling on all the nutrients you use. Re-establish the proper nutrient level and condition of the soil or media and remember, for a quick plant pick-me-up, foliar feeding is very effective. Interveinal chlorosis is very treatable, but ignoring the symptoms will minimize the yield of you crop.