Phloem sap flow assessed by MRI
A. PrusovaA, F. J. VergeldtA, H. Van AsA
A Wageningen University, Laboratory of Biophysics, Department of Agrotechnology & Food Sciences, The Netherlands
Magnetic resonance imaging (MRI)
Diurnal flow profiles
Xylem and phloem vascular tissues are
extremely sensitive to experimental
manipulation. Therefore a truly noninvasive and repeatable technique like
MRI (a technique widely accepted as the
gold standard1) is suitable to study the
xylem and phloem sap flow velocities in
intact plants in relation to water content
in the surrounding tissues. Vascular
tissue plays a crucial role in the
distribution of water, nutrients and
carbohydrates along the plant (fig. 1).
Phloem sap is distributed from the
source leaves to the sinks.
watered every 48hrs 0.5L water, light 50mE m-2s-1 at the soil level
Phloem flow below the truss
Xylem flow below the truss
-0.12
0.01
-0.18
-0.03
-0.20
-0.04
-0.05
-0.22
-0.06
-0.24
-0.07
36
48
60
72
84
5.6
1.8
4.8
1.6
4.0
1.4
3.2
1.6
-0.10
0.8
0.8
-0.11
0.6
0.0
-0.09
24
2.0
1.0
-0.28
12
6.4
2.4
-0.08
0
7.2
2.2
1.2
-0.26
-0.30
Average linear velocity (mm/s)
-0.02
Volume flow (mm3/s)
Average linear velocity (mm/s)
-0.16
8.0
2.4
-0.01
8.8
Volume flow (mm3/s)
0.00
-0.14
Xylem
(water)
2.6
0
96
12
24
36
48
60
72
84
96
108
Time (hours)
Time (hours)
Figure 5. Xylem sap flow below the truss
Figure 4. Phloem sap flow below the truss
watered every 24hrs 0.5L water, light 300mE m-2s-1 at the soil level
Figure 1. Location and direction of
xylem and phloem vascular tissue
(image: Carel Windt, modified)
Phloem flow below the truss
-0.12
Xylem flow below the truss
0.00
9
2.6
-0.01
● Next xylem and phloem flowing masks (fig. 3) were obtained.
● Quantitative diurnal flow profiles were obtained on a per pixel
basis, resulting in average linear velocity and total volume flow of
xylem (fig. 5, 7, 9) and phloem (figs 4, 6, 8) sap flow.
● In xylem a typical day-night cycle of both parameters is clearly
observed. Low water supply (fig. 5) and (too?) high water supply
(fig. 7) conditions can be deduced from the xylem flow profile as
well.
● Phloem flow does not show any diurnal cycle below the truss while
above changes in sap flow velocity and volume flow can be
observed.
-0.06
-0.07
-0.24
-0.08
-0.26
-0.09
-0.28
-0.10
-0.30
-0.11
0
12
24
36
48
60
72
2.0
6
1.8
5
1.6
4
1.4
3
1.2
2
1.0
1
0.8
0.6
84
0
0
12
24
36
Time (hours)
48
60
72
84
Time (hours)
Figure 6. Phloem sap flow below the truss
Figure 7. Xylem sap flow below the truss
watered every 24hrs 0.25L water, light 300mE m-2s-1 at the soil level
Xylem
flow
above
the
truss
Phloem flow above the truss
3.5
7
0.00
-0.15
-0.02
-0.20
-0.04
-0.06
-0.25
-0.08
-0.30
-0.10
-0.35
-0.12
-0.40
-0.45
-0.14
-0.16
-0.50
-0.18
0
12
24
36
48
60
72
84
Time (hours)
96
108
120
132
Figure 8. Phloem sap flow above the truss
6
3.0
2.5
2.0
5
4
3
1.5
2
1.0
1
0.5
0
0
12
24
36
48
60
72
84
96
108
120
132
Time (hours)
Figure 9. Xylem sap flow above the truss
Conclusions
● Diurnal xylem flow profile is very informative about soil condition
(drought, (too) wet), and can be used to determine the total water
evaporation.
● The volume flow of phloem tends to increase at increasing light
intensity, average velocity does not show much dependence.
● Diurnal phloem flow profile clearly reflect sink function (fruits,
wounds).
References
1
2
BioSolar Cells
Project Office
P.O. Box 98
6700 AB Wageningen
[email protected]
T + 31 (0)317 48 10 96
www.biosolarcells.nl
Wageningen University - Laboratory of Biophysics,
Department of Agrotechnology & Food Sciences,
Dreijenlaan 3
6703 HA Wageningen
The Netherlands
[email protected]
T + 31 (0)317 482026
www.wageningenur.nl
Volume flow (mm3/s)
-0.22
Average inear velocity (mm/s)
-0.05
Volume flow (mm3/s)
-0.20
7
Van As H. et al., Journal of Magnetic Resonance, 2013, 229: 25–34.
Scheenen et al., Journal of Magnetic Resonance, 2000, 142:207–215.
Acknowledgement
Authors would like to thank to Edo Gerkema and John Philippi for technical help with
measurements and setup.
Volume flow (mm3/s)
Figure 2. Cross-sectional transversal slice Figure 3. Cross-sectional transversal slice
MRI image of the tomato main stem + MRI image of the tomato main stem with
reference tubes overlapped with the phloem
reference tubes.
(red) and xylem (blue) flowing masks.
-0.04
2.2
no watering
Sap flow in the main stem of tomato (Solanum lycopersicum L.) was
studied in relation to the sink function of fruits. The measurement was
performed below and above a truss with three unripe fruits. By use of
a 3T MRI system and pulsed field gradient turbo spin echo PFG-SETSE pulse sequence2 the cross-sectional images of the stem are
obtained (fig.2).
-0.18
Volume flow (mm3/s)
Average linear velocity (mm/s)
Results
Experiment
-0.03
8
2.4
-0.02
-0.16
no watering
Studying the xylem and phloem sap flow as a function of the sink
location, light intensity and soil water content.
Average linear velocity (mm/s)
Objective
Average linear velocity (mm/s)
-0.14