lemon tree Citrus × limon (Rutaceae) and its fruits, the insecticidal and antibacterial effect of the latter, about endophytic organisms and the fallen and rotting fruits as a microhabitat for phoretic mites
About the lemon tree Citrus × limon (Rutaceae) and its fruits, the insecticidal and antibacterial effect of the latter, about endophytic microbiota and the fallen and rotting fruit as a small habitat for phoretic mites (Histiostomatidae, Astigmata).
About the origin of lemon trees
The lemon is a hybrid of bitter orange (Citrus × aurantium) and citron (Citrus medica). It is unknown when the lemon was first cultivated. Some researchers place the origin of the lemon in ancient times. But wall paintings from the ancient Roman city may actually depict the citron. Presumably originally from India, it can be proven with certainty that the lemon reached southern Europe as a cultivated plant in the 13th century, where it was first cultivated in Sicily and Spain.
Morphology of flowers and fruits
The lemon flowers can be formed all year round and consist of five free petals and have 20-40 stamens that are fused together in groups. The fruits, which are on the tree at the same time, consist of up to ten segments that are interspersed with so-called juice tubes, which must be botanically referred to as emergences because different tissue areas are involved in their formation.
Lemon trees as species communities and about endophytic microbiota that a transmitted via the seeds
Like other trees, lemon trees are not just an individual plant, but a complex species community that contains a variety of micro habitats for a wide variety of organisms. The root system, the leaf system, the bark, the shoot endosphere, organisms live everywhere, mainly from taxa of bacteria, fungi, but also mites, nematodes and insects.
Organisms found inside a living plant can enter either through the roots, through injuries to the plant body or through natural openings such as in the area of the leaves. But how else can endophytic organisms get into the plant? The authors T. Faddetta et al. (2021) for the first time looked at the endophytic microbiota on Citrus x limon that enter the new plant via the seeds of the mother tree.
The researchers isolated fungi of the genera Aspergillus, Quambalaria and Bjerkandera and bacteria of the genus Staphylococcus directly from the lemon seeds. The following taxa were also identified by DNA sequencing from sprouts from externally sterilized seeds and the seeds themselves: Cutibacterium and Acinetobacter as particularly common representatives of the bacteria, Cladosporium and Debaryomyces as the most conspicuous fungal representatives.
Beneficial aspects of endophytes for the plant and hypotheses about coevolution
According to the authors, the biological role of the endophytes mentioned has not yet been sufficiently clarified. However, there is evidence that some of the endophytes are beneficial for the establishment of the young tree by promoting growth and inhibiting pathogens. Growth is promoted through the release of plant growth-promoting (PGP) factors. These can be, for example, acetoin, indoleacetic acid (IAA) or 1-aminocyclopropane-1-carboxylate deaminase, which can increase the solubility of phosphates and support nitrogen binding, thereby optimizing the metabolism of the lemon plant. The authors therefore assume that there may be a coevolution of the lemon with its endophytes, as selection pressure is conceivable that favored the vertical transfer of pathogens from the mother plant to the next generation. The authors also assume that endophytes have the ability to suppress plant pathogens. In general, the endophytes could help the young lemon tree cope better with adverse environmental conditions.
Defensive compounds of the fruits, a decomposing fungus and about the biological importance of decomposition
The fruit that is still ripening on the tree chemically protects itself from parasite infestation. The juice tubes inside the lemon are acidic, the peel contains bitter substances and an oil that contains i.a. the insecticidal substance citral. In particular, the fungus Penicillium digitatum (Eurotiales, Ascomycota), which regularly occurs on lemon plantations, is considered to be an important primary destruent of ripening and fallen fruits. It seems to have tolerance to the fruit’s defensive substances. The acids and oils of the lemon are a hurdle for numerous other organisms that normally decompose fallen fruits quickly. This applies to insects, but also to mites and various fungi, for example. Since the decomposition process of organic substances and the organisms involved are of fundamental biological interest, and since the return of free nutrients to the soil and the biogeochemical cycle is also of applied interest, I will discuss my own acarological research approach in the last chapter of this blog article. First, the chemistry of the lemon fruit should be introduced in more detail.
Major components in plant parts of lemon trees
The authors R. Hartati et al. (2021) deal with the phytochemically active substances in the various plant parts of the lemon tree in the form of a comprehensive literature research. They also deal with the multiple spectrums of action of these substances in the pharmacological spectrum. Active ingredients of the lemon plant can, for example, be divided into flavanones, flavones, flavonols, and terpenoids such as limonene, carotenoids and limonoids. For example, flavones can have an anti-cancer, antioxidant or anti-inflammatory effect on the human organism. They have in the plant tissue an antibacterial and fungicide effect.
Returning of nutrients into the biogeochemical cycle and the connection with mites and other decomposing organisms
In connection with plants, in view of my own acarological research work, I am particularly interested in the biological development of dying parts of plants through to the return of nutrients into the general biogeochemical cycle. Of course, the activities of various destructive organisms such as bacteria, fungi, nematodes, mites and insects are particularly important. Their presence obviously depends on various aspects.
Conditions, in which I found mites of the Histiostomatidae
The organisms must first be able to get to the fallen fruit, in this case the fallen lemon. They must also have suitable living conditions there. During collecting excursions from 2006 onwards in the area of Sorrento (Italy), which is known for its lemon groves, I examined rotting lemons for Histiostomatidae mites and developed breeding approaches. The mites seemingly only appear, when the lemon is in an already continuing state of decomposition. There are also requirements for sun intensity (shaded), soil quality (rather sandy and slightly moist) and condition of the fallen fruit (partly moist, sometimes already earthy looking).
A mite of the Histiostomatidae that actively influences the microclimate of its microhabitat and that I used as model to study details of the feeding mechanism
I discovered the mite Histiostoma sp. in 2006. (H. feroniarum-complex, Histiostomatidae, Astigmata). The species arrives at the decomposing lemon fruit phoretically, i.e. it is transported there by a more mobile arthropod. The transporter is still unknown, but dipolopods might be possible transporters. The mite apparently has no problems on the lemon as there is little acid and oil left in the peel residue. Where mites of the Histiostomatidae occur, they take on a creative function for their microhabitat. This is because they can hyperphoretically introduce fungal spores into a habitat and have a chemical effect (L. Koller, S. F. Wirth, G. Raspotnig, 2012) on the growth of fungi that they have brought with them and those that are already present. Among other things, fungicides are formed in certain paired gland systems on the mite’s backsides. The mite was easy to breed under room conditions with the addition of potato pieces to enrich the growth of the food base (dying fungus with bacterial growth). At times I used the mite as a model organism, for example to analyze details of the food intake principle (S. F. Wirth, 2023).
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© StefanFWirth Berlin 2024
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Sources:
T. Faddetta et al. (2021):
https://doi.org/10.1038/s41598-021-86399-5
R. Hartati et al. (2021):
https://ijrps.com/home/article/view/270
L. Koller, S. F. Wirth, G. Raspotnig (2012):
http://dx.doi.org/10.1080/01647954.2012.662247
S. F. Wirth (2023) in FAO, page 102:
https://doi.org/10.4060/cc6728en
corresponding poster publication (2022):
https://wp.me/p2l6XU-1IJ