This section deals with all those aspects that are known and taught in the agronomic and biological background in relation to plant development. It also analyses the physics books that explain the influence of the Moon on our planet with the intention of clarifying whether they mention a possible relationship with plant development. Consequently, specific sections are devoted to those factors that could depend on or have an influence from the Moon, concretely in relation to the effect of gravity and the light reflected by it, both from the point of view of biology and physics. Likewise, the basic books on botany and plant physiology have been revised, paying special attention to those factors that are determining or causing stress to the development of plants.
The revision of the handbooks has been complemented with the information gathered from the analysis of different scientific articles published in data repositories, such as Web of Knowledge, Scopus or Google Scholar, using the keywords “Moon and plants” or “lunar and plants”.
3.1. What Handbooks Say from the Perspective of Physics and Biology
According to traditional beliefs, the influence of the Moon on plant growth is attributed, among other factors, to the attractive forces that the satellite exerts on the Earth and more specifically on its waters. The gravitational theory of the Moon could be attributed for the first time to Kepler (1571–1630), who claimed that the ocean tides were produced by a hidden force from the Moon. Kepler believed it was due to the affinity that the Moon had for water which was one of the four basic elements [48
] in [8
]. Gravity was also recognised as an agent of lunar influence with the publication of “Principia” by Newton (1643–1727).
The analyses of various physics textbooks (Table 4
) commonly used in science and engineering courses reveals that the term Moon appears in most of them linked to different concepts such as the distance from the Earth (as it was calculated in ancient times or as it is calculated today, with laser telemetry), the Moon’s gravity, tides, etc. With regard to the origin of tides, there are many possibilities: (i) it is not approached [49
]; (ii) it is introduced in a qualitative way [50
]; (iii) the exact dependence of R/r3
is provided where r is the distance Earth–Moon, and R is the size of the object on which the tides act—in the case of the oceans, the Earth’s radius [5
]; (iv) locally, high tides are shown as an effect of the resonance [52
] or tidal applications to produce energy are explained [53
]. In order to find correct demonstrations of the tides, we have to deal with books on Astronomy (e.g., [4
]) which, due to their extraordinary specificity, are beyond our general review.
Another factor that should be considered when approaching sap movement in plants is capillary action or capillarity, described as the spontaneous ability of a liquid to flow against gravity in a narrow space such as a thin tube or pipe (in plants, vascular tissues as xylem and phloem). This rising of liquid is the outcome of two opposing forces: cohesion (the attractive forces among similar molecules or atoms) and adhesion (the attractive forces among dissimilar molecules or atoms). In our case, the contact area between the particles of the liquid and the particles forming the tube. Capillarity is high when adhesion is greater than cohesion and vice versa. There is another important factor in capillarity, which is the contact area, dependent on the diameter of the tube (i.e., vascular tissue). Capillarity interacts with other forces, as gravity, which should be included when considering possible gravitational effects of the Moon on plants. In this sense, Jurin’s law is usually introduced, giving information on the height (h) reached when balancing the weight of the column of a liquid and the force h = 2 γ cos θ/ρgr, where γ is the surface tension (Nm−1), θ is the contact angle, ρ is the density of the liquid (kgm−3), g is the gravity acceleration (ms−2) and r is the radius of the pipe (m). Therefore, the Moon’s gravity would have to be subtracted from that of the Earth g = gE − gM, and since it is 288,000 times smaller, its effect on capillarity is negligible.
Physics books, even those studying applications of physics in biology [53
], do not deal with the Moon’s influence on plant growth. This may be due to the fact that the Moon’s gravity is, as we have seen in the Introduction (Section 1
), negligible compared to that of the Earth. Regarding illuminance, since it is a topic addressed in specialized books on optics [54
], it is not usually included in physics books (only one of them does, as shown in Table 4
), even less lunar illuminance.
The analysis of reference handbooks and monographs dealing with plant growth and development in the background of biology, environmental sciences, forestry, and agronomy is a key issue to understanding the extent to which this is a question that is limited to agricultural practice and/or the scientific and training field. Table 5
shows a summary of six widespread and commonly used books on botany and plant physiology, making a synoptic review of the endogenous and exogenous factors that determine and modulate plant development. In particular, the focus has been placed on those Moon-dependent factors that could be beneficial or stressful for plants, specifically in relation to Moon gravity or to the light reflected by the Moon.
As mentioned in the Introduction (Section 1
) and reflected in Table 5
, plant growth and development are regulated by endogenous and exogenous factors. The possible effects of the Moon should be considered as abiotic external factors, either if the effect is considered to be due to the light reflected or to gravitation. Regarding light, we searched for possible quotes of the Moon when addressing light effects on seeds, plant development, phototropism, photoperiodism, phototaxis, photonasties and quantity and quality of light, etc. Focusing on gravitational influence, the search was made on different aspects of gravitropism.
Considering endogenous factors, we searched for possible interactions of Moon radiation and photoperiod, as well as gravity, on the transduction of the perception of those environmental stimuli as well as the possible determination by endogenous genetic components. An important internal process in plants, animals, fungi and cyanobacteria is that related to circadian rhythms that refer to any biological process that displays oscillation, driven by circadian clocks, synchronized with solar time. Plant circadian rhythms are related to seasons and determine, for example, when to flower to maximize the success of pollinator attraction. Circadian rhythms also determine leaf movement, growth, germination, gas exchange or photosynthetic activity, among others. All monographs reviewed mentioning circadian rhythms refer exclusively to synchronization with the light cycle of the surrounding environments of plants, considering the Sun as light source that can determine or influence these cycles.
This search in what is considered consolidated science and is incorporated to handbooks has revealed practically no mention of the Moon (Table 5
). We have only found an anecdotic reference, in relation to the possible influence of moonlight on flowering in Thomas and Vince-Prue [57
]. These authors explain the work of Salisbury [58
], who had indicated that the effective red-light threshold for flowering is higher than the amount of red light produced by the Moon. In addition, it is important to consider that the shade provided by the leaves of the plant itself can reduce the radiation received to 5–10% of the direct moonlight [59
]. Thomas and Vince-Prue [57
] state that it seems unlikely that full moon light can influence flowering, even in the most sensitive plants, highlighting the scarcity of research on this issue. In this book the authors mention the work of Kadman-Zahavi and Peiper [60
], who carried out research with Pharbitis nil
(L.) Roth —a very sensitive short-day species—which they exposed to moonlight or shielded for different periods. They concluded that, although it is possible that moonlight is perceived, it had no effect on the experience developed with a short-day species that is particularly sensitive to radiation. The difficulty of isolating the “Moon” factor was highlighted, pointing out the possible influence of shade treatment on plants in other environmental factors that could in turn have an effect on flowering [60
]. On the other hand, they indicated that the full moon was only present on very few days of the lunar cycle, so its effect should be negligible under natural conditions.
3.2. What Research Papers Say from the Perspective of Physics and Biology
We consider a reference and starting point for the review of scientific articles, the brief paper published in Nature
by Cyril Beeson in 1946, entitled “The Moon and Plant Growth” [61
]. In this paper, the author writes “Beliefs that phases of the Moon have a differential effect on the rate of development of plants are both ancient and world-wide” and concludes that the research carried out to that date had not been able to demonstrate a correlation between the Moon and vital processes of terrestrial plants pointing out that, if any research does, the relationship was so unclear that it has no implications for agriculture.
In the 1950s, Frank A. Brown [22
] undertook different investigations in which he studied the possible lunar rhythmicity in organisms. Most of this research was carried out on marine organisms closely linked to the tides—such as algae, crustaceans, molluscs—he also studied the physiological aspects of terrestrial plants. Brown et al. [22
] studied the persistent rhythms of O2
-consumption in potatoes, carrots (Daucus carota
L.) and brown seaweed (Fucus
) and searched for a possible influence of barometric pressure rhythms of primary lunar frequency, noting that they are of much lower amplitude than the solar ones. The study was inconclusive in relation to what external rhythmic forces are involved in the rhythms of O2
-consumption, as many of them exhibit some degree of correlation with barometric pressure. In barometric pressure p = ρgh, as its expression depends on g, we would have the same case as with capillarity: the effect of gM
should be subtracted from gE
and, as we have seen, gM
is approximately 300,000 times lower than gM
, so the effect of the Moon on barometric pressure is negligible. The authors discuss the possibility that some of the responses attributed to external factors are due to endogenous rhythmic components. This connection between internal and external factors is supported by Wolfgang Schad [64
], who states that “all chrono-biological rhythms are always exo-endogenous, sharing their autonomous inner clock to some degree with the periodicity of the environment, both sides being connected by the long process of evolution”, remaining unanswered, the question of how the balance between endogenous and exogenous factors oscillates.
Some authors mention the influence of the lunar phases in a tangential way, without getting to clarify anything. One example explores the resistance of circadian clocks to transient fluctuations in night light levels in nature (i.e., change in cloud cover or stellar/lunar illumination) [65
]. Van Norman et al. [66
], when differentiating the circadian and infradian rhythms, indicate that the former are the best characterised with a period of around 24 h, while the infradians have periods of more than 24 h and can be due to the tides, lunar, seasonal, annual or longer. In other publications, the authors actively search, without finding them, for relationships between the Moon and some organisms. A paradigmatic case is the study conducted by Bitzand Sargent [67
], who unsuccessfully tries to relate the growth rate of the fungus Neurospora crassa
Shear & Dodge to the influence of a supposed lunar magnetic field (which, as we explain in detail in this article, is even more negligible than the gravitational field). Recently Mironov et al. [68
] mentioned a circalunar growth rhythm in a research carried out with genus Sphagnum
. They found an acceleration in the growth of the mosses studied near the new moon, and a slowdown in growth near the full moon.
Regarding biodynamic practices in agriculture, Hartmut Spiess carried out chronobiological investigations of crops grown under biodynamic management, developing experiments to test the effects of lunar rhythms on the growth of winter rye (Secale cereale
L.) and little radish (Raphanus sativus
L., cv. Parat) [69
]. Spiess [69
] tried to clarify some of the varying results that a number of studies conducted in the 1930s and 1940s had left unclear. This author also focused on studies made by M. and M.K. Thun [71
] establishing a relationship between the position of the Moon relative to the zodiac (sidereal rhythms), planting dates and crop growth, which served as a basis for the publication of calendars. Spiess’ [69
] results pointed out that the effects of lunar rhythms were weak, and especially the effects of the sidereal rhythms described by Thun and Thun were not apparent. In contrast to these papers, Kollerstrom and Staudenmaier [72
], pointed out that, although Spiess’ [69
] experiments were well designed, there was a lack of care in the data analysis. According to these authors, the results published to date of its publication suggested that lunar factors may have a practical significance for agriculture.
Without a doubt, one of the botanists who dedicated the most effort and publications to the search for relationships between the Moon and plants was Peter Barlow. Barlow [73
] devoted part of his research to decoding the influence of the Moon on biological phenomena. Specifically those aspects that take place in plants [73
], such as the movements of leaves [74
], stem elongation [77
], fluctuations in tree stem diameters [78
], the growth of roots [79
], biophoton emissions from seedlings [82
], and chlorophyll fluorescence [85
]. According to Barlow et al. [76
], and other works of the same author, at least in the cases analysed, the rhythm of leaf movements seem to have been developed or entrained in synchrony with the exogenous lunisolar rhythm experienced either on the Earth or in Space. Barlow [76
] believed that plant movements were related with water movements within the plant: as ocean tides are produced by lunisolar gravitational force, water movement in the pulvinus could be responsible for leaf movement, explanation that we have previously discussed.
From all external factors, the perception of light plays a significant role as it can modify biosynthesis by photostimulation and act as a trigger initiating the different stages of development (Table 6
). Reversive responses of plant to changes in light conditions can allow them to adjust their leaf or flower position (photonastic and heliotropic movements, respectively) to modulate the incoming radiation. Germination is also severely affected in some plants by light exposition. In fact, some seeds only germinate when they are exposed to a particular red to far-red ratios (660/730 nm), and in a particular moment [14
Despite light being crucial for plant life, just a few studies have explored the effect of moonlight on plant physiology and their results are not conclusive. Kolisko [88
] observed that the period and percentage of germination and subsequent plant growth was influenced by the phase of the Moon at sowing time. And according to Bünning and Moser [59
], light intensities as low as 0.1 lx, which correspond approximately to moonlight intensities (see Table 1
), may influence photoperiodism in plants and animals whose threshold values of photoperiodic time-measurement is on the order of 0.1 lx. They suggest that light intensity may reach 0.7 lx or even 1 lx when the altitude of the Moon is at 60° or higher altitudes in tropical and subtropical regions (respectively), clearly influencing photoperiodic reactions. However, they observed that in short-day plants such as Perilla ocymoides
L. and Chenopodium amaranticolor
H.J.Coste & Reyn., light intensities similar to those of the full moon favoured rather than inhibited flowering [59
]. They justified the circadian leaf movements observed in Glycine
plants as an adaptive mechanism to reduce the intensity of full moon received in the upper surface of the leaf avoiding plant misinterpretations of confounding full moonlight as it would be long day [59
]. However, Kadman-Zahavi and Peiper [60
] rejected this hypothesis concluding “that in the natural environment moonlight may have at most only a slight delaying effect on the time of flower induction in short-day plants” (p. 621). Furthermore, Raven and Cockell [89
] suggested that photosynthesis on Earth can occur in the photosynthetically active radiation (PAR) range of (10−8
–8 × 10−3
) mol of photons m−2
, and PAR values of moonlight at full moon goes from (0.5–5) × 10−9
mol of photons m−2
, suggesting that moonlight is not a significant source of energy for photosynthesis on Earth.
Recently, Breitler et al. [90
] described that the photoreceptors present in Coffea arabica
L. plants are able to perceive full moonlight and this full moonlight PAR is inadequate for photosynthetically supported growth. Plants perceive it as blue light with a very low R/FR ratio, yet this weak light has a great impact on numerous genes. In particular, it affects up to 50 genes related to photosynthesis, chlorophyll biosynthesis and chloroplast machinery at the end of the night. Moreover, full moonlight promotes the modification of the transcription of major rhythmic redox genes, many heat shock proteins and carotenoids genes suggesting that the moonlight seems to be perceived as a stress factor by the plant.
In other cases, full moonlight is correlated with a successful pollination of Ephedra
species. Rydin and Bolinder [91
] observed a correlation between pollination and the phases of the Moon on the gymnosperm Ephedra foeminea
Forssk., specifically with the full moon of July. During that period, non-mature cones secreted enough pollination drops to apparently attract pollinators that can use the full moon to navigate and also be attracted to the glittering drops in the full moonlight. According to the authors, when insects are not used as pollinators, as it happens in other species of Ephedra
, the adaptive value of correlating pollinating with the full moon is lost.
In the literature review carried out, some works were found that deal with two different topics that could have relationship with the Moon: polarization and magnetism. According to Semmens [92
] during certain periods, moonlight is partially polarised, “the maximum effect being with the oblique reflexion of half-moon, or somewhat later for the waxing and earlier for the waning moon” and that polarised light can favour the diastase, which catalyses the hydrolysis, first of starch into dextrin and immediately afterwards into sugar or glucose, to favour germination, as he observed in crushed mustard seeds in the presence of this polarised light. Macht [95
] studied the effect of (not lunar) polarized light on seeds of Lupinus albus
L. and his results were consistent with previous findings of the action of diastase on starch. However, as far as we know, apart from those works no other research papers have been focused on the role of lunar polarized light. Despite, a full body of evidence supports that polarized moonlight has a biological significance in the vision and orientation of nocturnal animals [96
]. Although we are at the very beginning of understanding the extent to which and why nocturnal animals use the lunar polarization, we do know that the land area over which it is viewable in pristine form is relentlessly shrinking due to human activity. In this sense, Kyba et al. [98
] showed that urban skyglow has a great degree of linear polarization and confirmed that its presence diminishes the natural lunar polarization signal. They also observed that the misalignment between the polarization angles of the skyglow and scattered moonlight could explain the reduction of the degree of linear polarization as the Moon rises. Regarding nocturnal animal navigation systems based on perceiving polarized scattered moonlight, these authors highlighted the necessity of considering polarization pollution models in highly light-polluted areas. In any case, there is almost no doubt that the level of polarization of moonlight would be extremely small: so minimal, that its effect would be completely negligible in plants [98
On the other hand, some studies suggest an influence of the lunar magnetic field. There is evidence that some animals, fungi, some protists and some bacteria seem to be able to react to the variation of the Earth’s magnetic field [99
]. The question that arises is whether plants are also able to respond to these fields and whether the Moon is capable of producing some magnetic field that plants can respond. There is abundant literature discussing magnetoreception in plants [102
], but no conclusive results have been reported with direct application to agriculture.
Our planet has a magnetic field, called geomagnetic field, with an intensity of approximately (25–65) × 10−6
T, ridiculously small compared to a commercial magnet (about 0.01 T) or a 0.2 T neodymium magnet. Although there are studies that argue that billions of years ago the Moon generated a magnetic field probably even stronger than the current magnetic field of the Earth, the lunar dynamo ended around one billion years ago [107
]. The intensity of the present-day magnetic field on the lunar surface is <0.2 × 10−9
T, indicating that the Moon currently does not have a global magnetic field [109
]. A magnetic field of this numerical value is approximately 225,000 times less than the Earth’s, and if divided by the distance Earth–Moon (3.84 × 1010
m), we can easily conclude that the possible effect of a hypothetical lunar magnetic field on the Earth would be much more negligible than that of the gravitational field.
Other theories claim that it is not the lunar magnetic field that affects, but the disturbance in the Earth’s electromagnetic field caused by the lunar gravitational changes that take place during the full moon [4
]; or also that Moon effects to the Earth’s magnetosphere [110
]. In both cases, the assumed effects would be (as we have seen in the calculations for the gravity case) completely insignificant.
A general analysis of the above-mentioned literature highlights the heterogeneity in the information sources regarding year of publication and discipline of the journal. On the one hand, there are very recent papers [68
] but also literature from more than half a century ago [61
]. On the other hand, there are peer-reviewed papers indexed in the Q1 of JCR in specific publications on Plant Science discipline, as Annals of Botany
], BMC Plant Biology
], Frontiers in Plant Science
], Journal of Plant Research
], New Phytologist
], Physiologia Plantarum
], Plant Cell
] or Plant Physiology
], with a long and consolidated trajectory in the field and with a pool of reviewers with solid expertise. Other articles are published in the Q2–Q3 of JCR
in the same category as Plant Biology
] and Protoplasma
], or in other categories as Horticulture or Agronomy (e.g., Biological Agriculture and Horticulture
]). Other papers included belong to other disciplines: Astrobiology
], Biology Letters
], Philosophical Transactions of the Royal Society B: Biological Sciences
], indexed in Q1–Q2 JCR
lists. Nevertheless, there are also some papers not included in JCR
lists but in other repositories as Communicative and Integrative Biology
], Earth, Moon and Planets
] and Star and Furrow
This analysis also raises the question of the extent to which the authors have a good basis in the physics behind all these phenomena, given that to date Moon has not been proved to affect plant biology regarding consolidated physics.