Direct redox potential (Eh) recording in plant tissue
Benada J.
The aim this contribution is to show that the potato tuber is suitable object for redox
measurement in living plant tissues for the whole year.
Method
The redox potential (Eh) was measured with mV reading device Digital Multimeter
Metex M3650CR (10x106 Ohm input resistance). Other instruments with the same or
higher input resistance can be used, eg. various pH meters .
Electrodes: Analytical Pt plate electrode 5x5 mm in form of lancet or needle shaped
electrode and saturated calomel standard electrode (+244 mV). Ag/AgCl reference
electrode can be used as well. Both electrodes were separated, the combined
electrodes are not convenient, especially for measuring in aerobic conditions.
The mV values (RP) in results are direct readings not transformed to Eh of the
reference electrode.
1.Measurement in aerobic conditions:
Arrangement for aerobic measurement: A filter paper disc was put to the bottom of a
Petri dish with diameter 10 cm and moistened with deionized water. Then the
electrode holder was inserted in the Petri dish. The reference electrode was placed in
the holder so that it contacted the filter paper firmly by the gap.
Preparation of Pt electrode: common cleaning with wet sponge.
Preparation of potato tuber: a transverse slice with diameter cca 30 mm and 7 mm
thickness was excised from the tuber and put on filter paper.
Measurement: Both electrodes were connected with the voltmeter, then the Pt
electrode was inserted into the tissue. The Eh reading began to sink to a bottom turn
point and then began to increase. This turn point was declared as Eh of particular
place (e.g. +50 mV bottom turn point or +50 mV btp). By repeating the electrode
insertion into the different places of a slice we can obtain an Eh gradient It is
convenient when the Multimeter has a max/min data hold function. Before each new
measurement new cleaning with wet sponge is necessary.
When the Pt electrode is inserted into the surface tissues of potato tuber then we
obtain RP upper turn point (e.g. +100 mV utp).
It is recommended to omit the first reading when we begin measurement with new
sample of plants.
For measurement in rolled leaf it is recommended to repeat two times the piercing
into the selected tissue. Then the turn point may be obtained earlier.
2)Measurement in hypoxia
A potato tuber slice was put in a glass beaker and overflooded with deionised water
so that its level was about 5 mm above the slice surface . The connecting gap of the
reference electrode was situated beside the potato tissue. The Pt electrode was
either inserted into the tissue or beside them.
The given Eh values were simple readings without adjustment to the Eh of calomel
electrode.The velocity of Eh records are dependent of physiological state of plant
organs.
3) Results
3.1)Measurement in hypoxia
Trial No.1)
a) Pt electrode beside the tissue
First day:
Start at 7,45 h a.m. :+160 mV, in next minutes RP increased to +197 mV and then
decreased, at 9,00 h a.m. +54 mV, after agitation an increase to +135 mV. Then
interruption of measurement, all arrangement was left in the laboratory
Second day: start at 6.30 h a.m. reading -540 mV, after agitation -413 mV and then
within 2 minutes decreased to -542 mV.
b) Pt electrode from the previous trial was cleaned and inserted into the potato
tissue: Eh sank within few minutes to -550 mV and later on only small changes were
observed
Trial No 2)
a) Pt electrode beside the tissue
First day
Start at 7,30 h a.m.,: +220 mV , then slow sinking, at 8.30 h a.m. +40 mV, after
agitation an increase to +112 mV, at 9.00 h a.m. -10 mV, after agitation an increase
to +47 mV, at 9.05 h a.m. -33 mV. Then the interruption of measurement to the next
day.
Second day: start at 6,30 h a.m.reading – 521 mV, after agitation -360 mV, then
sinking during 2 minutes and Eh restored to -521 mV.
b) The Pt electrode from the previous trial was cleaned and inserted into the tuber:
start Eh value -463 mV and then RP decreased within 1 minute to -540 mV In next
hours only small changes..
Third day:Pt electrode was left in the potato tissue from the previous day: start
reading at 7.00 h a.m. – 470 mV..
Comment: when the tissue is dying, Eh is increasing.
3.2)Aerobic RP measurement in potato tuber, records of separated insertions
Trial No. 1)
The Pt sensor electrode inserted close to the slice centre,:
+45 mV, +88 mV, +32 mV, +73 mV, all values bottom turn points:
The Pt sensor electrode inserted close to the slice (with shale) border:
+109 mV, +113 mV, +145 mV, all values upper turn points
Trial No.2)
The Pt sensor electrode inserted close to the slice centre:
+64 mV, +43 mV, +11 mV, all values bottom turn points
The Pt sensor electrode inserted close to the slice (with shale) border:
+107 mV, +126 mV, +97 mV, all values upper turn points
Comment:
There are Eh gradients in different parts of a tuber, influenced by the physiological
activity of tissues (meristems), influence of previous warming, cooling etc.