Abstract Background Tree stem diameters and electrical stem potentials exhibit rhythmic variations with periodicities of 24–25 h. Under free-running conditions of constant light or darkness these rhythms were suggested to be mediated by the lunisolar gravitational force. Scope To further unravel the regulation of tree stem diameter dilatations, many of the published time courses of diameter variations were re-evaluated in conjunction with the contemporaneous time courses of the lunisolar tidal acceleration. This was accomplished by application of the Etide program, which estimates, with high temporal resolution, local gravitational changes as a consequence of the diurnal variations of the lunisolar gravitational force due to the orbits and relative positions of Earth, Moon and Sun. In all instances investigated, it was evident that a synchronism exists between the times of the turning points of both the lunisolar tide and stem diameter variations when the direction of extension changes. This finding of synchrony documents that the lunisolar tide is a regulator of the tree stem diameter dilatations. Conclusions Under the described experimental conditions, rhythms in tree stem diameter dilations and electrical stem potentials are controlled by the lunisolar gravitational acceleration. Biological clock, circadian rhythm, gravitational force, lunisolar gravitational acceleration, Picea abies, rhythmic stem diameter dilatation, tidal acceleration, zeitgeber Many of the growth movements of plants (diurnal leaf movements, and perhaps stem dilatation cycles) initiate action potentials which are propagated within the plant body. Action potentials are then able to serve as informational signals that regulate further processes. Some movements appear to be regulated by turning points in the time-courses of the lunisolar tidal accelerative force, when the rate of accelerative change is zero. There are, in addition, other more constitutive bioelectrical phenomena in plants, such as electrical potential differences. These, also, are critically examined in relation to the lunisolar tide. Because of its ever present nature, it is difficult to analyse experimentally effects of this lunisolar tide on organic processes; nevertheless, it may be possible to take steps towards validating the Moon’s effect. This would take advantage of the predictability of the tidal acceleration profile and, hence, experiments could be devised to anticipate possible lunisolar tidal effects on biological events. Certain additional cosmic regulators of bioelectric patterns in plants, such as geomagnetic variations are also discussed, as are the effects of natural seismic events. Peter W. Barlow, 2012a INTRODUCTION Almost 20 years ago Zürcher et al. (1998) reported significant correlations between the natural, rhythmic variation of stem diameter (δD) of two trees of Picea abies(Norway spruce) and the timing and strength of the vertical component of the lunisolar tidal acceleration (δg), a partial and indirect measure of the continually varying lunar gravitational force experienced everywhere on Earth as a consequence of the relative positions of the Sun, Moon and Earth. Similar correlations were also mentioned in relation to stem-diameter variations of alder (Alnus sp.) trees (Zürcher et al., 1998; Zürcher, 2011). The two experimental spruce trees were grown in darkness within a controlled environment cabinet. The variations in stem diameter (dilatation rhythms) they displayed were therefore considered to have been independent of entraining influences from fluctuations of lighting, temperature and humidity. In the present Viewpoint I am going to outline major progress in the field of ultra-weak gravity perception that induced rhythmic stem diameter dilatations with a periodicity of 24.8 h in various tree species. Barlow (Barlow et al., 2010; Barlow, 2012a) gathered a huge set of published data and aligned the corresponding time series in rhythmic variation of stem diameter (δD) with the local lunisolar tidal profiles. The correlations and conclusions he provided are in support of the initial suggestion of Zürcher et al. (1998). RESULTS AND DISCUSSION Initial correlations between rhythmic stem diameter dilatation and the lunisolar gravity profile as described by Zürcher et al. (1998) were confirmed by Millet and Moallem (2001), who studied the growth rhythms and sap flow in mandarin orange trees (Citrus deliciosa). Millet and Moallem (2001) observed that the sap flow changed its period of 24 h under regular long-day conditions (14 h light/10 h darkness) to a 25-h period when maintained under conditions of constant light and constant temperature. In confirmation of the results derived by Zürcher et al. (1998), their findings revealed the same basic synodic lunar rhythm as for Picea, appearing under constant light as well as under constant darkness. In view of occasionally expressed reservations (Vesala et al., 2000) regarding a possible link between δD and the estimated lunisolar tidal acceleration δg prevailing during the period when stem diameters were measured, Barlow et al. (2010) set out to renew examination of this proposed relationship. Their initial objective had been to examine the suggestion of Zürcher et al. (1998) that the naturally occurring variations in stem diameter of two experimental trees of Picea alba were related to near-simultaneous variations in the lunisolar tidal acceleration. The relationship was positive: lunar peaks were approximately synchronous with stem diameter peaks (Barlow et al., 2010). To extend the investigation of this putative relationship, additional data on stem diameter variations from six other tree species were gathered from published literature. Sixteen sets of data were analysed by Barlow et al. (2010) retrospectively using graphical representations as well as cosinor analysis, statistical cross-correlation and cross-spectral analysis, together with estimated values of the lunisolar tidal acceleration corresponding to the sites, dates and times of collection of the biological data. Positive relationships were revealed between the daily variations of stem diameter and the variations of the lunisolar tidal acceleration (Barlow et al., 2010). Time series of δD (compare Barlow et al., 2010: their Figs 2–10) nearly always demonstrated that each peak of δD followed a peak of δg after a short time delay. These delays (δD after δg), as judged from visual inspection, are indicated as positive values. Values of 0, +1 and +2 h were quite characteristic of the delay between δg and the following δD. Had the delays been more random, rather than putatively being related to the timing of the peak of δg, then they would have shown values between 5 and 12 h, these being the half-period between pairs of peaks A and B and major single peaks in δg (Barlow et al., 2010). Sets of data relating to δD and transpiration were originally collected by Cantiani (1978) and Cantiani and Sorbetti Guerri (1989) in their study of rhythmic stem diameter variation. In particular, studies of transpiration in two of the observed trees indicated that although this variable was not linked to stem diameter variation, it might be subject to lunisolar gravitational regulation (Barlow et al., 2010). Moreover, in three cases the geomagnetic Thule index (Troshichev et al., 1979) showed a weak but reciprocal relationship with stem diameter variation, as well as a positive relationship with the lunisolar tidal force. The Thule index, because of its universality, stands as a proxy for the geomagnetic activities that apply at the two sites (Vallombrosa and Firenze) of the previously performed biological investigations. Based on their statistical analyses, Barlow et al. (2010) concluded that lunar gravity alone could influence stem diameter variation and that, under certain circumstances, additional regulation may come from the geomagnetic flux. To provide further support for a lunisolar influence on rhythmic tree stem diameter variations as dependent on the environmental factors (controlled greenhouse versus outdoor), a high-sensitivity device was developed by Holzknecht and Zürcher (2006) to measure low-potential electric currents along the bole of two trees (adult P. abies/young Pinus cembra) growing under open conditions (in Radein, South Tyrol). This technical device represents the most recent development of the methodology initially developed by Burr (1947), who discovered lunar-correlated variations in tree potentials. Rhythmic variations of the (bio-)electric potentials were found by Holzknecht and Zürcher (2006), with mainly the usual photoperiod during the vegetation time, and with clearly lunar periods during the winter rest. These results constitute a synthesis of the formerly apparently diverging results on diameter measurements, and confirm specific conditions under which lunar rhythms effectively occur. Furthermore, Holzknecht and Zürcher (2006) performed a Fourier analysis of the electrical potential (EP) time-course from P. abies, using a longer recording period, from 10 November 2000 to 27 January 2001. The continuously recorded EP time-course revealed an oscillating pattern with a period corresponding to the synodic lunar month (~29.5 d). The minimal EP values occurred at New Moon, whereas maxima occurred around the time of Full Moon. Results of Gibert et al. (2006) are also pertinent. These authors published a time-course of EP recorded continuously from a single tree of Populus nigra (black polar), extending from 18 June 2004 to 19 July 2004. Amplitudes of EP were least just after the time of New Moon and greatest at Full Moon, on 3 July. This is similar to what Holzknecht and Zürcher (2006) had found, and this New Moon effect on EP amplitude is also evident in the results of Burr (1947; see his Fig. 2), who surveyed electrical potential difference (EPD) values over a period of more than a whole year, between late 1943 and early 1945. Moreover, from the results of Gibert et al. (2006) a statistically valid relationship can be found between the amplitudes, Δ, of the daily EPs and those of the contemporaneous amplitudes of the daily δg values. Linear regression yielded a correlation coefficient r = 0.84 (P < 0.001; n = 22). In comparison, Burr’s early work (1945, 1947) with the maple tree consisted of recording the EPD between two silver electrodes inserted within the living bark and cambial cells of the trunk, and located 15 and 150 cm above ground level (Fig. 1). During a 3-d period of observation he found that the EPD, measured in millivolts, varied rhythmically. The maximum and minimum EPD values nevertheless varied from day to day. This pattern was found in both summer (August 1943) and late autumn (November 1943). At the later date the tree had presumably lost its leaves, had entered dormancy and was no longer transpiring. Fig. 1. View largeDownload slide Electrical potential difference (EPD, red line) and variation in trunk diameter (δD, blue line), measured with a dendrograph (original data from Burr, 1945). The corresponding lunisolar-derived gravimetric tidal profiles, δg, are also shown. Long vertical arrows indicate correspondences between turning points of δg and turning points in both the EPD and δD profiles. MR, moonrise; MS, moonset. Shaded horizontal bars indicate the dark period. Fig. 1. View largeDownload slide Electrical potential difference (EPD, red line) and variation in trunk diameter (δD, blue line), measured with a dendrograph (original data from Burr, 1945). The corresponding lunisolar-derived gravimetric tidal profiles, δg, are also shown. Long vertical arrows indicate correspondences between turning points of δg and turning points in both the EPD and δD profiles. MR, moonrise; MS, moonset. Shaded horizontal bars indicate the dark period. Nevertheless, many of Burr’s observations alerted him to the possibility that EPD values were subject to lunar modulation, and his later results from 1944 onwards also tend to suggest this possibility (Barlow, 2012a). It seems, therefore, that in the absence of any other contender, the gravimetric tide is more likely to be the regulator of EPD than the diurnal cycle of night and day. So, once again it seems likely that the biological variables and the daily change in trunk diameter, δD, are somehow ‘in tune with the Moon’. The modulation, by the Moon, of the stem diameter and bioelectric properties of trees over the course of a lunar month, suggests that rhythms of EP (or EPD) with longer cycles might be found. In fact, Burr (1947), when he examined his data concerning the value of EP, recorded at midnight on each day within his maple tree in New Haven, CT, USA, believed that he could discern such a prolonged rhythm. His observations took place from mid-October 1943 to mid-October 1944. He reached the preliminary conclusion that, in addition to monthly cycles of EPD, the details of which were apparently governed by the lunar phases, there was another, longer cycle of ~6 months. For Burr, this latter pattern did not seem to correspond to any growth feature of the tree, nor to any atmospheric or weather condition. Except for the lunar monthly cycle, he does not suggest any other cycle that could account for the 6-month EP cycle. However, one such cycle becomes evident when the distance, from Earth, of the Moon at perigee is subtracted from its distance at apogee. The temporal pattern of this difference, є, gives a measure of the constantly changing eccentricity of the lunar orbit. A typical range of є is 3.5–5.0 × 104 km. When the values of the monthly maximal EP recorded from Burr’s experimental maple tree (Burr, 1947) and the monthly values of є are plotted against time (days), two curves, one the inverse of the other, are obtained (data not shown): a maximum value of є coincides with a minimum of EP, and vice versa. The period of each cycle is ~220 d (~7.3 months). The cycle of perigee–apogee distances is constructed from a cycle of the daily values of δg, the amplitudes of which are greatest when the є values are greatest (Barlow, 2012a). Further evidence in favour of a role for the lunar cycle in influencing plant function came from a re-examination of data concerning the EP between two electrodes inserted into maple trees (Burr, 1945, 1947). Markson (1972) used spectrum analysis to examine whether a solar rotation of 27.3 d or a lunar cycle of 29.5 d could be distinguished in Burr’s tree potential data. In examining a time series consisting of the midnight tree potentials from 1953 to 1960, Markson calculated a fundamental frequency of 29.5 d. This is the average period of the Moon’s revolution with respect to the line joining the Sun and Earth. Rhythmic dilatations in tree stem diameters are not the only attributes of lunisolar gravitational changes that affect living organisms. In the past, many observations were made on the rhythmic movements of bean leaves, and from this and other evidence the idea of a ‘physiological clock’ was born (Bünning 1956, 1963). Fundamentally, however, these leaf movements are expressions of a ‘lunar clock’ (Barlow, 2007; Klein, 2007): they are initiated upon the turning of the lunar tide (Barlow et al., 2008; Barlow and Fisahn, 2012; Fisahn et al., 2012, 2017, 2015a, b; Barlow, 2015; Zajączkowska and Barlow, 2017). Unequivocal support of this causality was recently provided by novel experiments performed by J. Fisahn, O. Francis and E. Klingele (unpubl. res.). In particular, leaves of bean plants were exposed to gravitational changes that exactly mimic the daily modulations exerted by the lunisolar gravitational force. As expected, the bean leaves were able to sense these artificially impinged modulations in the ultra-weak gravitational acceleration and initiated a response in leaf movement progression that could be recorded by time-lapse video imaging. Circadian rhythms were found also for the spontaneous, ultra-weak photon emission (UPE) from developing wheat seedlings (Gallep et al., 2012, 2014). Spontaneous ultra-weak light emissions from wheat seedlings are rhythmic and synchronized with the time profile of the local gravimetric tide. This spontaneous light emission is related to the time-course and activity of metabolic processes during germination and early seedling growth, when rapid reproduction occurs promoting new tissues for leaflets and radicles; it can be used as a non-invasive, real-time probe of the living state (Gallep et al., 2012, 2014). Lunisolar tidal variations cause changes in the electrical field of the ionosphere that affect the geomagnetic field (Tarpley, 1970; Minorsky and Bronstein, 2006; Barlow et al., 2013). However, the fundamental questions of (1) whether or not plants perceive the Earth’s magnetic field, (2) the physical nature of the magnetic receptor(s) and (3) whether or not the geomagnetic field has any bearing on the physiology and survival of plants remain largely unanswered (Minorsky, 2007). The data presented by Barlow et al. (2013) provide support for effects upon biological material of both the lunar tide and variations in the magnetic field of the Earth. Although co-ordination, or co-operation, between the lunisolar tide and geomagnetic variation may sometimes be apparent, this does not necessarily indicate a close coupling between these parameters in the regulation of growth (Barlow et al., 2013). Nevertheless, it suggests that biological material may perceive geomagnetic force variations, in addition to guidance received from the lunisolar tide, thereby maintaining a rhythmic pattern of growth (Barlow, 2012b). A general conclusion derived from the considerations of δD–δg interrelations described here might be summed up in the words of Burr (1945): ‘It is, therefore, not at all impossible that the lunar cycle produces, in some as yet undiscovered way, tides in the tree’. All of the published re-evaluations of the δD–δg interrelations were performed by Dr Peter W. Barlow (Barlow et al., 2010, Barlow, 2012a, b) and provide unequivocal support for the words of Burr (1945). It goes without saying that, besides the internal milieu of plants, the external biosphere, semiosphere, as well as the magnetosphere, ionosphere and cosmosphere, are exceedingly complex domains, and all of them envelop and penetrate the Earth’s phytogeodermal domain. In the areas of human health and physiology, which together comprise arguably the most advanced area of biological research, it is becoming realised that the biospheric and cosmospheric domains impinge upon the workings of the human organism and its social dynamics. This was foreseen some years before NI Vasil’eva drew attention to some of the correlations which exist between interacting cycles of solar activity on biological, geophysical and social rhythms. And this interactive cyclicity is also penetrated by the frequencies and phases of the harmonics of the cosmospheric system. Although such considerations may be aimed at a very long-range view of life and of life processes, an appropriate starting point is nevertheless the day-to-day activities of organisms. However, any set of short-term diurnal events has the possibility of being a pre-condition for amplification over time and, in the longterm, of becoming a set of post-conditional states. Conditional states may be evidenced in those features which, by directed gene mutations or epigenetic modifications, are forwarded into future generations of organisms. The persistent bioelectricity initiation and transmission of bioelectric potentials may be one way in which lunar cycles not only feed into the day-to-day vital processes of plants but also make their effects felt in plant survival strategies and evolution. Peter W. Barlow, 2012a LITERATURE CITED Barlow PW. 2007. Foreword. In: Klein G, ed. Farewell to the internal clock. A contribution in the field of chronobiology . New York: Springer, vii– xx. 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Annals of Botany – Oxford University Press
Published: Jan 24, 2018
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