Results
Microclimate
The microclimate during the experiments was characterised by an air
temperature (± SE) of 22.95 ± 0.29 °C, a relative humidity of 94.61 ±
0.97 % and an average vapour pressure deficit of 0.15 ± 0.03 kPa inside
the exposure bag. Average transpiration rate during all experiments was
0.162 ± 0.014 mmol m−2 s−1, while
photosynthetic rate averaged around 2.139 ± 0.143 μmol
CO2 m-2 s−1.
PET images and 11C-tracer
dynamics
An example of strawberry fruits inside the FOV of the PET scanner can be
seen (Fig. 2A), accompanied by temporally integrated PET images of the
same fruits under the light intensity of 200 µmol m-2s-1 at leaf surface (Fig. 2B). The images were
integrated from 0 to 120 min, 120 to 150 min, and 150 to 180 min,
respectively from top to bottom and left to right. For the first 120 min
after the 11CO2 feeding, no11C-tracer was detected, while from then on,11C started entering the fruits and peduncle that were
placed inside the FOV. After that, the tracer uptake at fruits and
peduncle position increased over time. This was true for all of the
performed scans (see further).
PET image analysis of each strawberry fruit and subsequent normalisation
(with regard to photosynthetic activity) resulted in normalised TTCs.
Specifically, Fig. 3 shows an example of one fruit studied in four
different PET experiments (one for each PPFD). Averaged over all
normalised TTCs from each PET measurement per PPFD (i.e. 50, 100, 200
and 400 µmol m-2 s-1), the11C-tracer in the fruits began to increase after 132 ±
2.76, 133 ± 4.15, 113 ± 2.34 and 117 ± 2.02 min (± SE) of11CO2 feeding, respectively. In
addition, the total amount of recorded 11C-tracer in
the fruits during the PET measurement was between 5 and 32 times smaller
(depending on PPFD – data not shown) in large plants compared to small
plants.
Dynamic analysis of photosynthate
translocation
To investigate the performance of the source leaves during the
experiments, real-time photosynthetic activity was measured by analysing
the gas entering and leaving the exposure bag (grey circled in Fig. 4A).
The expected performance of the corresponding leaves was assessed by
obtaining light response curves (LRC) after taking the leaf out of the
exposure bag (white circles in Fig. 4A). The LRC showed that
photosynthetic rate rapidly increased as light intensity increased from
0 to 200 μmol m-2 s-1, while a
gradual increase was observed from 200 to 400 μmol m-2s-1. The light saturation point was reached at 400
μmol m-2 s-1. Inside the exposure
bag, photosynthetic rate was however lower compared with the LRC-derived
photosynthetic rate, reaching light saturation at 100 μmol
m-2 s-1 and resulted in a maximum
photosynthetic rate that was merely one fourth of the LRC one.
Relationship between PPFD and average relative rate of photosynthate
translocation is also shown in Fig. 4A (black circles). Relative
photosynthate translocation rate was highest under the light intensity
of 100 μmol m-2 s-1. It was 4.6, 1.1
and 3.0 times higher than those under the light intensity of 50, 200 and
400 µmol m-2 s-1, respectively. Fig.
4B shows the response of average relative translocation rate (black
circles in Fig. 4A) on the net photosynthetic rate of the source leaf
averaged per PPFD of each PET measurement (grey circles in Fig. 4A). A
linear relation between net photosynthetic rate and relative
photosynthate translocation rate would be expected but was not observed
for any of the experimental strawberries.
Whereas no relationship existed with photosynthetic rate, a clear
response of relative translocation rate into fruits and transpiration
rate was observed per fruit type (Fig. 5). There was a strong
correlation between transpiration rate of the source leaf and relative
photosynthate translocation rate. This was especially true for the
red-27mm-S, white-22mm-S and red-18mm-L having an R2value of 0.68, 0.62 and 0.83, respectively, which indicates that lower
transpiration rates lead to a higher relative photosynthate
translocation rate. With the green-18mm-S and red-27mm-L, a sufficient
correlation between transpiration rate and relative photosynthate
translocation rate into fruit was found with an R2value of 0.59 and 0.53, respectively.