There are No Short-Term Oscillations in Plant Transpiration. The Observed Weight Scale Signal Oscillations Were an Artifact

1. 1
There are No Short-Term Oscillations in Plant Transpiration.
The Observed Weight Scale Signal Oscillations Were an Artifact.
Leonid Asipov
Abstract
In order to avoid artifacts of measurement, control should be introduced. The study
"Development of synchronized, autonomous, and self-regulated oscillations in transpiration rate
of a whole tomato plant under water stress", which measured plant transpiration by weight loss
using an electronic weight scale, has focused on short-period fluctuations in the measured
weight loss rate. No control was introduced for case that the fluctuations are result of weight
scale measurement system noise. In the current study, a control is introduced. The control is
simultaneous measurement of transpiration by leaf gas-exchange. The results show lack of
short-term oscillations in the transpiration rate. Longer oscillations correlate with irradiation
levels in the greenhouse. The conclusion is that short-term fluctuations in the transpiration rate
seen in the weight scale signal are an artifact of the weight scale. A theoretic consequence of
this finding is that no central transpiration control mechanisms exist in plants. Plant cells
respond independently to changing environment.
Keywords: Oscillations, plant, transpiration, weight scale, weight loss, gas exchange

2. 2
Introduction
The authors of an article (Wallach et al. 2010) have measured plant transpiration by weight loss,
using an electronic weight scale and paid attention to short-term (3 minutes) fluctuations of the
transpiration rate. The weight scale measurement system, however, has its precision limitation
and substantial noise is abundant. The data, which is sampled every 10 seconds, has a large
random noise, resulting from analog to digital conversion and the transducer itself. To reduce
the noises, the data was averaged for a period of 3 minutes (18 points). After the averaging, the
noises were still about +- 1.6 grams. It is impossible to relate to a biological phenomenon
happening on short time periods of few minutes due to the large weight scale noise. The
significance of the data rises as we average it for longer time period. The writers of the article
(Wallach et. al 2010) have presented the short-term oscillations seen in the weight scale signal
as biological phenomena of short-term changes in stomata apertures. Since the oscillations are
seen on basis of a whole plant, the stomata change of aperture has to be centrally synchronized.
However, there was no control for case that the short-term oscillations are an artifact of the
weight scale. In the next section, a proper control is introduced: simultaneous measurement of
transpiration with leaf gas-exchange method.
Materials and Methods
Weight scale measurement: Tedea-Vishay 1040 transducer connected to an analog to digital
converter. The data was logged every 10 seconds and averaged every 2.5 minutes.
Gas-exchange measurement: LICOR LI6400 portable photosynthesis system. Clear-Top chamber
was used so the illumination in the chamber was the same as in the greenhouse. The H2O and
CO2 scrubbers were turned off, so the CO2 and humidity was the same as in the surroundings.
The data was logged every 2 minutes. No averaging was applied.
Plants: wt Solanum lycopersicum from Ailsa Claig strain of approximately 3 month old.

3. 3
Results
Simultaneous measurement from electronic scales and gas-exchange
To prove that short-term oscillations result from measurement noise, an additional transpiration
measurement method (leaf gas-exchange) was used simultaneously with the weight scale
(Fig.1). Short term oscillations were seen only at the electronic scales measurement. During the
day, there were fluctuations in irradiation levels and transpiration was well correlated to them
(Fig. 1). These oscillations are longer-term (12-30 minutes) and seen on the gas-exchange and
the smoothed weight-scale measurement (Fig. 2). In the transpiration measured with the
weight scale (Fig. 1) there were short-term (3 minutes) oscillations during the day and the night.
We would expect the daily oscillations to be larger since transpiration is larger. However, they
were not different. The short-term oscillations exist only in weight scale measurement and they
are day/night unspecific. No nocturnal oscillations (short or long term) were seen in gas-
exchange.
Comparison of weight scale signals from a plant and from a bowl of water.
For control purposes, during weight-scale measurement of plants, a simple bowl of water,
exposed to air, was measured (Fig. 3). The short-term oscillations exist as well in the bowl of
water signal. The noise from the plant and from the bowl of water was similar by its pattern.
The amplitude of the oscillations seen in the bowl of water signal is slightly less, possibly due to
the smaller weight of the bowl of water.
Measurement of transpiration by weight-scale and gas-exchange in controlled environment.
In controlled environment, the light is constant and no longer-term (related to light) oscillations
are expected. Plant transpiration was measured by weight-scale and by gas-exchange to explain
the nature of the short-term oscillations seen in the weight scale signal.
Large short-term oscillations in the weight-scale signal were seen (Fig. 4).
The transpiration signal measured by gas-exchange is stable (Fig. 5). No oscillations seen, short
or long term.
The response speed of LI-6400 to changes in humidity of the air.
To make sure that the LI-6400 Portable Photosynthesis System responds quickly enough to
changes in humidity to potentially track the short-term oscillations seen in the weight-scale
measurement, humidity was suddenly changed during continuous measurement. The built-in
H2O absorber absorbs humidity from the air. Turing it on or off, changes the humidity suddenly.
It is possible to see that the response time of LI-6400 to sudden changes in humidity is about 10-
15 seconds (Fig. 6). If short-term oscillations were really happening they would have been
tracked by the gas-exchange measurement.

4. 4
Discussion
No short-term oscillations were seen in gas exchange measurement (Fig. 1, 5). Therefore, the
short term oscillations of the weight scale are an artifact resulting from measurement noise of
the weight scale. The observed short-term oscillations are similar during the day and the night,
without relation to the extent of the transpiration rate (Fig. 1). If they were a real phenomenon,
we would have seen differences in the extent of the oscillations between the day and the night
and also they would have appeared on the gas-exchange measurement.
Longer oscillations result from irradiation changes. They are synchronized by abiotic factors. The
plant cells respond to environment independently and stomata aperture is changed according to
the turgor pressure of the guard cells. There is no point in speculating an existence of a "central
control" to transpiration while the normal explanation of "no central control" is sufficient.
Short-term oscillations are observed in the bowl of water evaporation (Fig. 3). This is the reason
why the plant transpiration's short-term oscillations are not a result of biological phenomenon.
The controlled room results support the finding that the short-term oscillations are an artifact.
No oscillations were found in the gas-exchange signal (Fig. 5).
The response time of LI-6400 to changes in humidity is quick (Fig. 6) and the short-term
oscillations would have been tracked by the gas-exchange if they were really happening.
The reported short-term oscillations (Wallach et. al 2010) seen in weight scale measurement of
transpiration are an artifact of the weight scale and not a result of biological phenomena.
References
(1)
Wallach R, Da-Costa N, Raviv M, Moshelion M. 2010. Development of synchronized,
autonomous, and self-regulated oscillations in transpiration rate of a whole tomato plant under
water stress. Journal of Experimental Botany 61, 3439-3449 .

5. 5
Figure Legends
Fig. 1
Simultaneous measurement of transpiration by gas-exchange and weight scale. Irradiation (first),
transpiration measured by gas-exchange (second) and transpiration measured by weight scale
(third) is plotted relative to time
Fig. 2
Smoothing of the transpiration measured by weight scale. Moving average of two points at each
side, twice. The initial data is the same as in Fig. 1. Irradiation (first) and smoothed transpiration
measured with a weight scale (second) is plotted relative to time
Fig. 3
Weight loss signal of a plant vs bowl of water. The weight data is unscaled (the units are
change of volts in three minutes). Gray is plant and black is bowl of water relative to time
Fig. 4
Measurement of transpiration by weight-scale in controlled environment. Light sensor (first),
raw weight (second), weight loss rate (third) is plotted relative to time
Fig. 5
Measurement of transpiration by gas-exchange in controlled environment. Illumination (first)
and transpiration (second) is plotted relative to time
Fig. 6
LI-6400 response time to sudden changes in humidity. Arrows indicate changes in the mode
of the H2O absorber. When it is turned on, water is absorbed from the air. Relative
humidity of the chamber is plotted vs time

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Figures
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