How Plant Nutrition affects Photosynthesis in Roses

by Gary Ritchie, Master Rosarian, Olympia and Tacoma Rose Society

Photosynthesis is the unimaginably complex biochemical process by which plants manufacture both the glucose (plant food) and oxygen that sustain all life on Earth. In roses, and other plants, the rate of photosynthesis is of seminal importance to growth, development, and flowering and is profoundly affected by the nutritional status of the plant. Before I launch into a discussion of how plant nutrient status affects photosynthesis however, it will be useful to review the fundamental steps involved in the photosynthetic reactions.

The process of photosynthesis comprises two primary biochemical reactions: the so-called “light reaction” and the Calvin Cycle (Reference 1) (Figure 1). In the light reaction, chlorophyll and other leaf pigments absorb some of the light energy that falls upon the leaf. This excites the chlorophyll molecule, causing one of its electrons to reach a high energy status. This excited electron then flows through a series of reactions known as an electron transport chain in which this light energy is miraculously converted into chemical energy in the form of two compounds called ATP and NADPH. The electron donated by the chlorophyll molecule is replaced by an electron that is torn out of a water molecule, releasing oxygen into the atmosphere as a by-product (2).

ABOVE: Figure 1

The energy and electrons contained in these chemicals are then used to power the Calvin Cycle. Atmospheric CO2, the raw material for the Calvin Cycle, is absorbed by the leaves through tiny pores called stomata (Figure 2) and converted into a simple sugar called glucose. These reactions are orchestrated in large measure by an enzyme called RuBisCo. RuBisCo is believed to be the oldest and most abundant enzyme on this planet. It is also the most important – without it, life could not exist. Amazingly, photosynthesis was invented around 3.4 billion years by tiny single-celled creatures called cyanobacteria that inhabited Earth’s primordial seas.

ABOVE: Figure 2

Photosynthesis occurs in tiny green organelles called chloroplasts that reside in plant cells (Figure 3). Chloroplasts are bound by outer and inner membranes and contain structures called granum that resemble stacks of pancakes. These are immersed in a fluid called the stroma. Each pancake is called a thylakoid and is bound within a membrane called the thylakoid membrane. The light reaction occurs on the thylakoid membrane, while the Calvin cycle occurs within the stroma (3).

ABOVE: Figure 3

Now, with this background, let’s outline how certain key plant nutrients impact this process. Plant physiologists recognize 17 nutrient elements as being essential for plant life. Here we will review six of these, all of which play important roles in photosynthesis and other processes in your rose plants. These are nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), sulfur (S), and iron (Fe).

Nitrogen.

Nitrogen is arguably the most important plant nutrient - making up about 2.8% of the above-ground dry weight of a rose plant. The effects of nitrogen nutrition have been carefully studied and modeled in hybrid tea roses (4). We know from this work that roses allocate a larger amount of nitrogen to the young upper leaves, which promotes a higher rate of photosynthesis in these leaves as opposed to the older, lower leaves. This is largely because these upper leaves exhibit a higher rate of electron transport in the light reaction, as well as enhanced formation of RuBisCo. Both of these processes are rate-limiting steps in photosynthesis.

Nitrogen is also an essential component of amino acids. Because amino acids are the building blocks of proteins, and all enzymes are proteins, it stands to reason that a nitrogen deficiency in roses will impede enzyme formation and cause an overall slowdown in the rate and efficiency of biochemical reactions–including those involved in photosynthesis.

Phosphorus.

The average rose plant may contain roughly 0.25% dry weight phosphorous, which is fairly typical of most plants. A normal phosphorus concentration range would be from 0.2% to 1.0%, with levels greater than 1% being toxic and below 0.2% inducing deficiency.

Phosphorus has many very important functions in the plant. When you think of phosphorus, think energy. After plants capture light energy, they convert it immediately into chemical energy during the light reaction. As we have learned, this chemical energy is stored in the form of a high-energy phosphate bond in the chemical ATP. This occurs in a reaction in the cell membrane where a P atom is added to ADP (adenosine diphosphate) converting it to ATP (adenosine triphosphate). This compound is often called the “energy currency” of the cell.

Potassium.

Potassium accounts for about 1.5% of a plant’s dry weight. An important property of Potassium ions (K+) is that they lower the osmotic potential of a solution. In plain English, this means that if a cell contains lots of K+ ions dissolved in the cell water, it can “suck” water in from its surroundings. Guard cells, the cells that surround and enclose stomatal pores, open and close as K+ concentrations increase and decrease - as cell turgor builds and ebbs. So, the ability of a leaf to absorb CO2 through its open stomata, and to feed it into the Calvin Cycle (Figure 1), is facilitated in large measure by the K+ concentration in the guard cells.

Potassium is also essential for protein synthesis and enzyme formation and activation. So, photosynthesis is affected by potassium concentration in many ways and a K deficiency greatly impedes photosynthesis.

Magnesium exists in plants at only about 0.15% dry weight, but nevertheless has many important functions in photosynthesis. Its most critical role may be as a key constituent of the chlorophyll molecule. Indeed, if you were to examine the structure of a chlorophyll molecule you would see that it is built around one Mg atom, which resides at its very center. Interestingly, the chlorophyll molecule is almost identical to the hemoglobin molecule, except that the central atom in hemoglobin is iron rather than magnesium (Figure 4).

ABOVE: Figure 4

In spite of their structural similarity, these molecules have very different functions. Hemoglobin combines with oxygen carrying it throughout the bodies of animals. In contrast, the functions of chlorophyll are to capture light energy and to donate essential electrons during the light reaction. From 5 to 50% of all the Mg found in a plant, depending on the type of plant and its nutritional status, resides within chlorophyll.

Magnesium also strongly affects the activity of RuBisCo in the Calvin Cycle. RuBisCo is formed by an enzyme called Rubisco activase, and becomes “activated” at pH of 9 or higher. Increasing magnesium increases pH, which enhances the activity of this enzyme, thus giving RuBisCo formation a boost.

Sulfur, also present at about 0.15% dry weight, is a critical constituent of four important amino acids, which the plant uses to build proteins. Sulfur plays a key role in the manufacture of the thylakoid membrane as well as in the manufacture of chlorophyll itself.

We mentioned above that a critical part of the light reaction is known as the electron transport chain – a series of complex molecules that transfer electrons from a high to a lower energy state enabling the plant to capture this energy and store it for later use to drive the Calvin Cycle. An important molecule in one of these chains is called ferredoxin. Sulfur is a critical structural component of ferredoxin.

Iron (present at only about 100 parts per million) combines with sulfur to form ferredoxin, so it is also essential for electron transport. In addition, iron is a vital component of an important group of proteins called “heme” proteins, which are necessary in the production of chloroplasts. Iron is also essential to the manufacture of chlorophyll. That’s why an iron deficiency causes yellowing (chlorosis) of leaves (Figure 5).

ABOVE: Figure 5

Also, and this is important, none of the photosynthetic reactions can occur without the help of innumerable enzymes. And enzymes rarely act alone – they need assistance from chemicals called cofactors. Many of these cofactors are ionic forms of metals, such as magnesium (Mg++), manganese (Mn++), and calcium (Ca++). Each of these is an essential plant nutrient.

These are just a few examples and I could go on and on. But you get the point – for photosynthesis to occur at a sustainable level in your rose plants, it must be adequately supplied with virtually all of the plant nutrients. A deficiency in any one element results in a reduction or cessation of photosynthesis with all of the attendant adverse consequences (5).

In conclusion, the message for rose growers is clear: keep your roses well supplied with nutrients, especially nitrogen, during the growing season. To assure that your soil contains adequate nutrients, testing your garden soil every few years is strongly recommended. But just because the nutrients are present in the soil doesn’t mean they are being taken up and transported throughout your rose plants. This requires that nutrient elements be dissolved in the soil moisture and that the plants be actively taking up moisture via transpiration. As soil dries, stomata close reducing transpiration and causing nutrient and CO2 uptake to slow markedly. So, we need to irrigate deeply and often during the growing season – especially as the soil dries throughout our Pacific Northwest summer and autumn seasons. Finally, nutrient uptake also depends on soil pH, with a desirable range for roses being between about 6.5 and 7.5. Soil pH should also be tested frequently and the soil amended with acid-forming fertilizer or lime as indicated by the test results.

References and notes:

(1) The Calvin Cycle was named for Dr. Melvin Calvin, a plant biochemist at the University of California, Berkeley, who worked out the main steps in the Calvin Cycle. He was awarded the Nobel Prize in Chemistry in 1961 for this research.

(2) The Earth’s atmosphere was devoid of oxygen until the appearance of photosynthetic organisms about 3.4 billion years ago. Our atmosphere currently contains roughly 21% oxygen – all of it a byproduct of photosynthesis.

(3) It is generally believed that chloroplasts originated as individual free-swimming organisms in primordial seas. They were subsequently enveloped by plants – giving them the ability to produce sugar. This theory is supported by the fact that chloroplasts have their own unique DNA, which is different from the DNA of organisms in which they are contained.

(4) Kim, S. H and J. H. Leith. 2003. A coupled model of photosynthesis, stomatal conductance and transpiration for a rose leaf (Rosa hybrida L.), Annals of Botany. 91(7): 771–781.

(5) More thorough discussions of photosynthesis and plant nutrition can be found in Chapters 7 and 11 of my book “Inside Plants: A Gardeners’ Guide to Plant Anatomy and Physiology”, available on Amazon.com.

Figure captions

Figure 1. Outline of the key steps of photosynthesis. Red arrows indicate inputs, green arrows are outputs and the blue arrow indicates a by-product. See text for explanation.

Figure 2. Stomatal pores (stoma) are found in great numbers on the lower surfaces of rose leaves. They provide the leaves with entry and exit ports for the exchange of carbon dioxide, oxygen and water vapor. Each pore is created when the surrounding guard cells absorb water and become turgid. As the leaves lose water, the guard cells become flaccid and the pore closes. There can be over 300 stomata per square millimeter of leaf surface.

Figure 3. Structure of a chloroplast, the organelle in which the process of photosynthesis takes place. The light reaction occurs on the thylakoid membrane, while the Calvin cycle occurs in the thylakoid lumen (Source: Wikimedia commons).

Figure 4. Diagram of the molecular structure of chlorophyll and hemoglobin (Source: https://www.pngwing.com/en/free-png-xemui/download).

Figure 5. Iron deficiency causes chlorosis (yellowing) of leaves because it results in the loss of chlorophyll, which gives leaves their green color (Photo by the author).