Arabidopsis thaliana root elongation growth is sensitive to lunisolar tidal acceleration and may also be weakly correlated with geomagnetic variations

BACKGROUND

Correlative evidence suggests a relationship between the lunisolar tidal acceleration and the elongation rate of arabidopsis roots grown under free-running conditions of constant low light.

METHODS

Seedlings of Arabidopsis thaliana were grown in a controlled-climate chamber maintained at a constant temperature and subjected to continuous low-level illumination from fluorescent tubes, conditions that approximate to a 'free-running' state in which most of the abiotic factors that entrain root growth rates are excluded. Elongation of evenly spaced, vertical primary roots was recorded continuously over periods of up to 14 d using high temporal- and spatial-resolution video imaging and were analysed in conjunction with geophysical variables.

KEY RESULTS AND CONCLUSIONS

The results confirm the lunisolar tidal/root elongation relationship. Also presented are relationships between the hourly elongation rates and the contemporaneous variations in geomagnetic activity, as evaluated from the disturbance storm time and ap indices. On the basis of time series of root elongation rates that extend over ≥4 d and recorded at different seasons of the year, a provisional conclusion is that root elongation responds to variation in the lunisolar force and also appears to adjust in accordance with variations in the geomagnetic field. Thus, both lunisolar tidal acceleration and the geomagnetic field should be considered as modulators of root growth rate, alongside other, stronger and more well-known abiotic environmental regulators, and perhaps unexplored factors such as air ions. Major changes in atmospheric pressure are not considered to be a factor contributing to oscillations of root elongation rate.

Worldwide distribution of geomagnetic tides

Geomagnetic lunar-daily variations result from the Moon’s tidal action on the ionosphere and oceans. They provide information that bears on the general large-scale dynamics of the atmosphere, and can also add to our understanding of ionospheric processes. For these reasons the variations, though small, are worth determining from geomagnetic data. The data for this study are hourly mean values of the geomagnetic elements from 100 observatories for the interval 1957.5 to 1960.0. These are analysed by the Chapman-Miller method to obtain parameters of the principal lunar harmonics. For each observatory, the contribution of the sea tidal dynamo to these parameters is empirically determined; this contribution has been ignored in previous worldwide studies, but is shown here to be an appreciable proportion of the total lunar effect, and worthy of study in its own right. After removal of the ocean dynamo effect, the remaining geomagnetic lunar daily variation, ascribed to the ionospheric dynamo and its associated induced currents, is represented by a series of spherical harmonic coefficients, and separated into parts of internal and external origin. (In this context, the relative merits of spherical harmonic analysis and the Price-Wilkins method of analysis are discussed.) Internal and external current systems associated with the ionospheric dynamo effect are deduced. Similar analyses, for the solar daily geomagnetic variation and for the lunar semidiurnal tide in the atmospheric pressure, are appended.