When you look at some leafy plants, it’s easy to get confused about what colour their leaves are. Red is one common colour that can be seen on many different types of leaves – but why would a plant have these coloured leaves and how does this affect its ability to perform photosynthesis?
Plants often grow in environments where there isn’t enough sunlight for them to absorb all the energy from light waves through photosynthesis. This means that when the sun shines brightly over a field full of wheat, only part of those plants will receive enough solar radiation to meet their daily carbon requirements. The rest will rely on other sources of carbon such as methane or bacteria-producing enzymes called ‘carbon monoxide dehydrogenase’.
In order to help some of these less sunny plants access more carbon, certain species’ leaves take on a variety of colours, including red. But if the leaves themselves don’t contain any photosynthetic pigment (like chlorophyll), then how exactly do they capture photons of light and turn them into chemical energy? Let’s find out…
How Does A Plant With Red Leaves Carry Out Photosynthesis?
The process by which plants photosynthesize involves three main steps: absorbing, using and releasing CO2. During each step, the molecules involved need to pass between two membranes – the thylakoid membrane and the plastid envelope – before they reach their final destination. The first stage requires an electron acceptor molecule, also known as oxygenic photosystem II (PSII). In the case of most land plants, O2 is used instead.
Oxygenic PSII has several important functions during this initial phase of photosynthesis. It acts like an antennae for capturing incoming photons of light, converting them into electrons. These electrons travel along special structures within the cell called ‘thylakoids’, eventually reaching NADPH oxidoreductase enzyme complexes embedded in the thylakoid lumen. Here, the electrons combine with hydrogen ions released by water splitting reactions to form ATP, the basic unit of biological energy storage.
Once the electrons have been captured, they must now cross another membrane to enter the site of carbon fixation. Photosystem I (PSI) uses ferredoxin to transfer electrons across the thylakoid lumen and then combines them with protons to release glucose 1,6-bisphosphate, the precursor of glyceraldehyde 3-phosphate, ribulose 1,5-diphosphate and finally Ribonucleotides. Glyceraldehyde 3-phosphates is converted into trioses by transketolases, while ribonucleotides become amino acids. Glucose 6-phosphate produced by both PSI and PSII supplies the Calvin cycle reaction, which produces carbohydrates.
Photosystem I transfers electrons directly to ferredoxin, allowing the system to function even under low light conditions. However, due to the presence of large amounts of iron atoms in the protein complex, photosystem I cannot work efficiently in aerobic organisms, limiting them to anaerobic respiration. On the contrary, photosystem II works best under aerobic conditions because iron atoms are absent from the proteins present in this particular structure.
During oxidation, oxygenic PSII captures six electrons per second. Each electron travels down eight separate chains, passing through four centres made up of manganese, calcium, zinc and magnesium atoms. At least four water molecules act as proton donors as well as mediators of charge separation. As the number of absorbed photon increases, so too does the rate of energy conversion, ultimately leading to formation of molecular oxygen.
After completing the above processes, the products of photosynthesis move out of the light-harvesting centre via channels formed by integral membrane proteins. They are then transported to various cellular locations, mainly the stroma and mitochondria.
As mentioned earlier, some plants use non-oxygenic cyanobacteria to produce sugars. Cyanobacteria have no capacity for oxygenic photosynthesis, thus making them ideal candidates for sugar production. Their cells consist entirely of water and semi-permeable walls, similar to algal cells. Instead of capturing and transferring electrons, cyanobacteria collect and store energy by forming carbohydrate reserves inside their cells. When needed, the stored energy is transferred to specific sites on the wall surface, namely carboxysomes. Carboxysome contains bound oxaloacetate that serves as a substrate for gluconeogenesis. Once again, the same mechanism occurs thanks to the action of hydroxymethylglutaryl coenzyme A reductase and malate synthase enzymes located on the outer cytoplasmatic layer of carboxysomes [2].
So far we’ve looked at the process of photosynthesis in general terms. Now let us see how plants with red leaves fit into the picture. What happens if the leaves themselves are lacking in the typical green pigmentation found in higher plants? How can they still perform photosynthesis? Read below to discover the answer…
Can Plants Photosynthsize Without Chlorophyll?
Chloroplasts are organelles containing porphyrin ring compounds responsible for carrying out photosynthesis. Chlorophyll is considered the key component required for photosynthesis because it absorbs blue/green light wavelengths and helps excite nearby pigments. Without it, plants wouldn’t be able to convert sunlight into usable energy.
However, although chlorophyll is essential for performing photosynthesis, it is not strictly necessary for the process itself. Plants that live in shaded areas may sometimes exhibit symptoms of damage caused by a lack of sufficient chlorophyll content. Some examples include necrotic lesions on petals, loss of photosynthetic activity, reduced growth rates and poor fruit development.
It was previously thought that plants with red leaves could not perform photosynthesis because they lacked the green pigmented chlorophylls. Although this might seem true based upon our current understanding of the process, new research suggests otherwise.
According to Dr Robert Gourdet from the University of Massachusetts Medical School, scientists discovered that the red leaves of Arabidopsis arenaria plants were capable of performing photosynthesis despite having little chlorophyll. The reason behind this unexpected finding is likely linked to the fact that these plants have developed unique mechanisms to overcome the absence of chlorophyll. One of these mechanisms includes an alternative pathway for transporting electrons.
One interesting discovery showed that red-leaved Arabidopsis arenarias had evolved a way to make their own chlorophyll. Similar findings were recorded in experiments involving tobacco plants [4]. Scientists believe that these plants synthesised chlorophyll molecules that contained extra heme groups attached to the existing tetrapyrrole ring. According to researchers, these added components allowed the modified chlorophyll molecules to bind metals better than chlorophyll itself, thereby improving the efficiency of photosynthesis.
But Why Did These Plants Evolve To Create Their Own Chlorophyll?
There are three possible explanations:
A shortage of available nitrogenous bases causes problems with DNA synthesis and results in lower concentrations of chlorophyll pigments. Thus, plants compensate for this deficiency by increasing levels of chlorophyll precursors.
Another possibility is that the appearance of red colouration in these plants enables them to compete against neighbouring plants by utilising additional resources. For example, plants near roads tend to develop reddish foliage to avoid being eaten by insects attracted to bright yellow flowers. Such behaviour allows these red-leafed plants to absorb nutrients from roadside ditches, particularly phosphorus and nitrogen, which serve as base materials for chlorophyll synthesis.
Finally, plants may adapt to environmental changes. If temperatures decrease significantly, the concentration of total chlorophyll decreases simultaneously. Consequently, chlorophyll becomes scarcer, forcing the plant to reduce its amount of photochemistry and increase absorption of visible spectrum. Therefore, it seems plausible that plants with red leaves may be responding to climate change by reducing the amount of chlorophyll in their tissues in anticipation of future environmental challenges.